Showing 1–50 of 78 results for author: Choudhuri, R

To Copilot and Beyond: 22 AI Systems Developers Want Built

Authors: Rudrajit Choudhuri, Christian Bird, Carmen Badea, Anita Sarma

Abstract: Developers spend roughly one-tenth of their workday writing code, yet most AI tooling targets that fraction. This paper asks what should be built for the rest. We surveyed 860 Microsoft developers to understand where they want AI support, and where they want it to stay out. Using a human-in-the-loop, multi-model council-based thematic analysis, we identify 22 AI systems that developers want built… ▽ More Developers spend roughly one-tenth of their workday writing code, yet most AI tooling targets that fraction. This paper asks what should be built for the rest. We surveyed 860 Microsoft developers to understand where they want AI support, and where they want it to stay out. Using a human-in-the-loop, multi-model council-based thematic analysis, we identify 22 AI systems that developers want built across five task categories. For each, we describe the problem it solves, what makes it hard to build, and the constraints developers place on its behavior. Our findings point to a growing right-shift burden in AI-assisted development: developers wanted systems that embed quality signals earlier in their workflow to keep pace with accelerating code generation, while enforcing explicit authority scoping, provenance, uncertainty signaling, and least-privilege access throughout. This tension reveals a pattern we call "bounded delegation": developers wanted AI to absorb the assembly work surrounding their craft, never the craft itself. That boundary tracks where they locate professional identity, suggesting that the value of AI tooling may lie as much in where and how precisely it stops as in what it does. △ Less

Submitted 9 April, 2026; originally announced April 2026.

Comments: Companion paper to "AI Where It Matters": arXiv:2510.00762

Thinking Less, Trusting More: GenAI's Impacts on Students' Cognitive Habits

Authors: Rudrajit Choudhuri, Christopher Sanchez, Margaret Burnett, Anita Sarma

Abstract: Objectives: When students use generative AI in coursework, what are its persistent effects on their intellectual development? We investigate (RQ1-How) how students' trust in and routine use of genAI affect their cognitive engagement habits in STEM coursework, and (RQ2-Who) which students are particularly vulnerable to cognitive disengagement. Method: Drawing on dual-process, cognitive offloading… ▽ More Objectives: When students use generative AI in coursework, what are its persistent effects on their intellectual development? We investigate (RQ1-How) how students' trust in and routine use of genAI affect their cognitive engagement habits in STEM coursework, and (RQ2-Who) which students are particularly vulnerable to cognitive disengagement. Method: Drawing on dual-process, cognitive offloading, and automation bias theories, we developed a statistical model explaining how and to what extent students' trust-driven routine genAI use affected their cognitive engagement -- specifically, reflection, the need for understanding, and critical thinking in coursework, and how these effects differed across students' cognitive styles. We empirically evaluated this model using Partial Least Squares Structural Equation Modeling on survey data from 299 STEM students across five North American universities. Results: Students who trusted and routinely used genAI reported significantly lower cognitive engagement. Unexpectedly, students with higher technophilic motivations, risk tolerance, and computer self-efficacy -- traits often celebrated in STEM -- were more prone to these effects. Interestingly, students' prior experience with genAI or academia did not protect them from cognitively disengaging. Implications: Our findings suggest a potential cognitive debt cycle where routine genAI use weakens students' intellectual habits, potentially driving and escalating over-reliance. This poses challenges for curricula and genAI system design, requiring interventions that actively support cognitive engagement. △ Less

Submitted 10 April, 2026; v1 submitted 29 January, 2026; originally announced January 2026.

AI Where It Matters: Where, Why, and How Developers Want AI Support in Daily Work

Authors: Rudrajit Choudhuri, Carmen Badea, Christian Bird, Jenna Butler, Rob DeLine, Brian Houck

Abstract: Generative AI is reshaping software work, yet we lack clear guidance on where developers most need support and how to design it responsibly. We report a large-scale, mixed-methods study of N=860 developers examining where, why, and how they seek or limit AI help across SE tasks. Using cognitive appraisal theory, we provide the first empirically validated mapping of developers' task appraisals to A… ▽ More Generative AI is reshaping software work, yet we lack clear guidance on where developers most need support and how to design it responsibly. We report a large-scale, mixed-methods study of N=860 developers examining where, why, and how they seek or limit AI help across SE tasks. Using cognitive appraisal theory, we provide the first empirically validated mapping of developers' task appraisals to AI adoption patterns and Responsible AI (RAI) priorities. Appraisals predict AI openness and use, revealing distinct patterns: strong current use and demand for improvement in core work (e.g., coding, testing); high demand to reduce toil (e.g., documentation, operations); and clear limits for identity- and relationship-centric work (e.g., mentoring). RAI priorities vary by context: reliability and security for systems-facing tasks; transparency, alignment, and steerability to maintain control; and fairness and inclusiveness for human-facing work. Our results offer concrete, contextual guidance for delivering AI where it matters to developers and their work. △ Less

Submitted 2 January, 2026; v1 submitted 1 October, 2025; originally announced October 2025.

Comments: ICSE-SEIP'26

"Maybe We Need Some More Examples:" Individual and Team Drivers of Developer GenAI Tool Use

Authors: Courtney Miller, Rudrajit Choudhuri, Mara Ulloa, Sankeerti Haniyur, Robert DeLine, Margaret-Anne Storey, Emerson Murphy-Hill, Christian Bird, Jenna L. Butler

Abstract: Despite the widespread availability of generative AI tools in software engineering, developer adoption remains uneven. This unevenness is problematic because it hampers productivity efforts, frustrates management's expectations, and creates uncertainty around the future roles of developers. Through paired interviews with 54 developers across 27 teams -- one frequent and one infrequent user per tea… ▽ More Despite the widespread availability of generative AI tools in software engineering, developer adoption remains uneven. This unevenness is problematic because it hampers productivity efforts, frustrates management's expectations, and creates uncertainty around the future roles of developers. Through paired interviews with 54 developers across 27 teams -- one frequent and one infrequent user per team -- we demonstrate that differences in usage result primarily from how developers perceive the tool (as a collaborator vs. feature), their engagement approach (experimental vs. conservative), and how they respond when encountering challenges (with adaptive persistence vs. quick abandonment). Our findings imply that widespread organizational expectations for rapid productivity gains without sufficient investment in learning support creates a "Productivity Pressure Paradox," undermining the very productivity benefits that motivate adoption. △ Less

Submitted 28 July, 2025; originally announced July 2025.

A bound for plany Kakeya sets in $\mathbb{F}_q^4$ using the planebrush method

Authors: Izabella Łaba, Mukul Rai Choudhuri, Joshua Zahl

Abstract: Katz and Zahl used a planebrush argument to prove that Kakeya sets in $\mathbb{R}^4$ have Hausdorff dimension at least 3.059. In the special case when the Kakeya set is plany, their argument gives a better lower bound of 10/3. We give a nontechnical exposition of the Katz-Zahl argument for plany Kakeya sets in the finite field setting. Katz and Zahl used a planebrush argument to prove that Kakeya sets in $\mathbb{R}^4$ have Hausdorff dimension at least 3.059. In the special case when the Kakeya set is plany, their argument gives a better lower bound of 10/3. We give a nontechnical exposition of the Katz-Zahl argument for plany Kakeya sets in the finite field setting. △ Less

Submitted 13 July, 2025; originally announced July 2025.

Comments: 15 pages, 4 figures

Journal ref: Res Math Sci 13, 13 (2026)

What Needs Attention? Prioritizing Drivers of Developers' Trust and Adoption of Generative AI

Authors: Rudrajit Choudhuri, Bianca Trinkenreich, Rahul Pandita, Eirini Kalliamvakou, Igor Steinmacher, Marco Gerosa, Christopher Sanchez, Anita Sarma

Abstract: Generative AI (genAI) tools promise productivity gains, yet miscalibrated trust and usage friction still hinder adoption. Moreover, genAI can be exclusionary, failing to adequately support diverse users. One such aspect of diversity is cognitive diversity, which leads to diverging interaction styles (e.g., a risk-averse developer may gate genAI outputs behind tests/review; a risk-tolerant one may… ▽ More Generative AI (genAI) tools promise productivity gains, yet miscalibrated trust and usage friction still hinder adoption. Moreover, genAI can be exclusionary, failing to adequately support diverse users. One such aspect of diversity is cognitive diversity, which leads to diverging interaction styles (e.g., a risk-averse developer may gate genAI outputs behind tests/review; a risk-tolerant one may prototype directly/fix issues post-hoc). When an individual's cognitive styles are unsupported, it creates additional usability barriers. Thus, to design tools that developers trust and use, we must first understand which factors shape their trust and intentions to use genAI at work? We developed a theoretical model of developers' trust and adoption of genAI through a large-scale survey (N = 238) conducted at GitHub and Microsoft. Using Partial Least Squares-Structural Equation Modeling (PLS-SEM), we found aspects related to genAI's system/output quality (e.g., presentation, safety/security, performance), functional value (e.g., educational/practical benefits), and goal maintenance (ability to sustain alignment with task goals) significantly influence trust, which, alongside developers' cognitive styles (i.e., risk tolerance, technophilic motivations, computer self-efficacy), affect adoption. An Importance-Performance Matrix Analysis (IPMA) identified high-importance factors where genAI underperforms, revealing targets for design improvement. We bolster these findings by qualitatively analyzing developers' reported challenges and risks of genAI use to uncover why these gaps persist in development contexts. We offer practical guidance for designing genAI tools that support effective, trustworthy, and inclusive developer-AI interactions. △ Less

Submitted 14 November, 2025; v1 submitted 22 May, 2025; originally announced May 2025.

Comments: Journal extension of ICSE 2025 paper (arXiv:2409.04099)

Measuring SES-related traits relating to technology usage: Two validated surveys

Authors: Chimdi Chikezie, Pannapat Chenpaiseng, Puja Agarwal, Sadia Afroz, Bhavika Madhwani, Rudrajit Choudhuri, Andrew Anderson, Prisha Velhal, Patricia Morreale, Christopher Bogart, Anita Sarma, Margaret Burnett

Abstract: Software producers are now recognizing the importance of improving their products' suitability for diverse populations, but little attention has been given to measurements to shed light on products' suitability to individuals below the median socioeconomic status (SES) -- who, by definition, make up half the population. To enable software practitioners to attend to both lower- and higher-SES indiv… ▽ More Software producers are now recognizing the importance of improving their products' suitability for diverse populations, but little attention has been given to measurements to shed light on products' suitability to individuals below the median socioeconomic status (SES) -- who, by definition, make up half the population. To enable software practitioners to attend to both lower- and higher-SES individuals, this paper provides two new surveys that together facilitate measuring how well a software product serves socioeconomically diverse populations. The first survey (SES-Subjective) is who-oriented: it measures who their potential or current users are in terms of their subjective SES (perceptions of their SES). The second survey (SES-Facets) is why-oriented: it collects individuals' values for an evidence-based set of facet values (individual traits) that (1) statistically differ by SES and (2) affect how an individual works and problem-solves with software products. Our empirical validations with deployments at University A and University B (464 and 522 responses, respectively) showed that both surveys are reliable. Further, our results statistically agree with both ground truth data on respondents' socioeconomic statuses and with predictions from foundational literature. Finally, we explain how the pair of surveys is uniquely actionable by software practitioners, such as in requirements gathering, debugging, quality assurance activities, maintenance activities, and fulfilling legal reporting requirements such as those being drafted by various governments for AI-powered software. △ Less

Submitted 7 February, 2025; originally announced February 2025.

Insights from the Frontline: GenAI Utilization Among Software Engineering Students

Authors: Rudrajit Choudhuri, Ambareesh Ramakrishnan, Amreeta Chatterjee, Bianca Trinkenreich, Igor Steinmacher, Marco Gerosa, Anita Sarma

Abstract: Generative AI (genAI) tools (e.g., ChatGPT, Copilot) have become ubiquitous in software engineering (SE). As SE educators, it behooves us to understand the consequences of genAI usage among SE students and to create a holistic view of where these tools can be successfully used. Through 16 reflective interviews with SE students, we explored their academic experiences of using genAI tools to complem… ▽ More Generative AI (genAI) tools (e.g., ChatGPT, Copilot) have become ubiquitous in software engineering (SE). As SE educators, it behooves us to understand the consequences of genAI usage among SE students and to create a holistic view of where these tools can be successfully used. Through 16 reflective interviews with SE students, we explored their academic experiences of using genAI tools to complement SE learning and implementations. We uncover the contexts where these tools are helpful and where they pose challenges, along with examining why these challenges arise and how they impact students. We validated our findings through member checking and triangulation with instructors. Our findings provide practical considerations of where and why genAI should (not) be used in the context of supporting SE students. △ Less

Submitted 20 December, 2024; originally announced December 2024.

Comments: 12 pages, Accepted by IEEE Conference on Software Engineering Education and Training (CSEE&T 2025)

Journal ref: IEEE Conference on Software Engineering Education and Training (CSEE&T 2025)

Investigating the Impact of Interpersonal Challenges on Feeling Welcome in OSS

Authors: Bianca Trinkenreich, Zixuan Feng, Rudrajit Choudhuri, Marco Gerosa, Anita Sarma, Igor Steinmacher

Abstract: The sustainability of open source software (OSS) projects hinges on contributor retention. Interpersonal challenges can inhibit a feeling of welcomeness among contributors, particularly from underrepresented groups, which impacts their decision to continue with the project. How much this impact is, varies among individuals, underlining the importance of a thorough understanding of their effects. H… ▽ More The sustainability of open source software (OSS) projects hinges on contributor retention. Interpersonal challenges can inhibit a feeling of welcomeness among contributors, particularly from underrepresented groups, which impacts their decision to continue with the project. How much this impact is, varies among individuals, underlining the importance of a thorough understanding of their effects. Here, we investigate the effects of interpersonal challenges on the sense of welcomeness among diverse populations within OSS, through the diversity lenses of gender, race, and (dis)ability. We analyzed the large-scale Linux Foundation Diversity and Inclusion survey (n = 706) to model a theoretical framework linking interpersonal challenges with the sense of welcomeness through Structural Equation Models Partial Least Squares (PLS-SEM). We then examine the model to identify the impact of these challenges on different demographics through Multi-Group Analysis (MGA). Finally, we conducted a regression analysis to investigate how differently people from different demographics experience different types of interpersonal challenges. Our findings confirm the negative association between interpersonal challenges and the feeling of welcomeness in OSS, with this relationship being more pronounced among gender minorities and people with disabilities. We found that different challenges have unique impacts on how people feel welcomed, with variations across gender, race, and disability groups. We also provide evidence that people from gender minorities and with disabilities are more likely to experience interpersonal challenges than their counterparts, especially when we analyze stalking, sexual harassment, and doxxing. Our insights benefit OSS communities, informing potential strategies to improve the landscape of interpersonal relationships, ultimately fostering more inclusive and welcoming communities. △ Less

Submitted 3 November, 2024; originally announced November 2024.

Comments: Accepted by 47th IEEE/ACM International Conference on Software Engineering (ICSE 2025)

An improved bound on the Hausdorff dimension of sticky Kakeya sets in $\mathbb{R}^4$

Authors: Mukul Rai Choudhuri

Abstract: Kakeya sets are compact subsets of $\mathbb{R}^n$ that contain a unit line segment pointing in every direction. The Kakeya conjecture states that such sets must have Hausdorff dimension $n$. The property of stickiness was first discovered by Katz-Łaba-Tao in their 1999 breakthrough paper on the Kakeya problem. Then Wang-Zahl formalized the definition of a sticky Kakeya set, and proposed a special… ▽ More Kakeya sets are compact subsets of $\mathbb{R}^n$ that contain a unit line segment pointing in every direction. The Kakeya conjecture states that such sets must have Hausdorff dimension $n$. The property of stickiness was first discovered by Katz-Łaba-Tao in their 1999 breakthrough paper on the Kakeya problem. Then Wang-Zahl formalized the definition of a sticky Kakeya set, and proposed a special case of the Kakeya conjecture for such sets. Specifically this conjecture states that sticky Kakeya sets in $\mathbb{R}^n$ have Hausdorff dimension $n$ and Wang-Zahl went on to prove the conjecture for $n=3$. A planebrush is a geometric object which is a higher dimensional analogue of Wolff's hairbrush. Using the planebrush argument, Katz-Zahl showed that Kakeya sets in $\mathbb{R}^4$ have Hausdorff dimension at least 3.059. If we restrict our attention to sticky Kakeya sets, we can improve upon this bound by combining the planebrush result with additional stickiness property. To be precise, we will show in this paper that sticky Kakeya sets in $\mathbb{R}^4$ have dimension at least 3.25. △ Less

Submitted 30 October, 2024; originally announced October 2024.

Comments: 31 pages, 8 figures

The Interaction of Matter and Radiation: The Physics of C.V. Raman, S.N. Bose and M.N. Saha. Part 2: Physics Highlights

Authors: Arnab Rai Choudhuri

Abstract: Three extraordinary physics discoveries were made from colonial India, which did not have any previous tradition of research in modern physics: Saha ionization equation (1920), Bose statistics (1924), Raman effect (1928). All the three discoverers were founding faculty members of the new small physics department of Calcutta University, which started functioning from 1916. These discoveries were al… ▽ More Three extraordinary physics discoveries were made from colonial India, which did not have any previous tradition of research in modern physics: Saha ionization equation (1920), Bose statistics (1924), Raman effect (1928). All the three discoverers were founding faculty members of the new small physics department of Calcutta University, which started functioning from 1916. These discoveries were all in the general topic of interaction between matter and radiation. In Part 1, we have described the social and the intellectual environment in which these discoveries were made. Now, in Part 2, we shall first give a background of the revolutionary developments taking place in physics at that time. Then we shall provide a non-technical account of the three discoveries and point out the kind of impact these discoveries made in the subsequent development of physics. △ Less

Submitted 13 September, 2024; originally announced October 2024.

Comments: 13 pages, 8 figures. Accepted for publication in "Resonance Journal of Science Education", Indian Academy of Sciences

The Interaction of Matter and Radiation: The Physics of C.V. Raman, S.N. Bose and M.N. Saha. Part 1: Historical Background

Authors: Arnab Rai Choudhuri

Abstract: Three extraordinary physics discoveries were made from colonial India, which did not have any previous tradition of research in modern physics: Saha ionization equation (1920), Bose statistics (1924), Raman effect (1928). All the three discoverers were founding faculty members of the new small physics department of Calcutta University, which started functioning from 1916. These discoveries were al… ▽ More Three extraordinary physics discoveries were made from colonial India, which did not have any previous tradition of research in modern physics: Saha ionization equation (1920), Bose statistics (1924), Raman effect (1928). All the three discoverers were founding faculty members of the new small physics department of Calcutta University, which started functioning from 1916. These discoveries were all in the general topic of interaction between matter and radiation. In Part 1 of this article, we describe the social and the intellectual environment in which these discoveries were made. Part 2 will focus on the science involved in these discoveries. △ Less

Submitted 13 September, 2024; originally announced October 2024.

Comments: 11 pages, 4 figures. Accepted for publication in "Resonance Journal of Science Education", Indian Academy of Sciences

What Guides Our Choices? Modeling Developers' Trust and Behavioral Intentions Towards GenAI

Authors: Rudrajit Choudhuri, Bianca Trinkenreich, Rahul Pandita, Eirini Kalliamvakou, Igor Steinmacher, Marco Gerosa, Christopher Sanchez, Anita Sarma

Abstract: Generative AI (genAI) tools, such as ChatGPT or Copilot, are advertised to improve developer productivity and are being integrated into software development. However, misaligned trust, skepticism, and usability concerns can impede the adoption of such tools. Research also indicates that AI can be exclusionary, failing to support diverse users adequately. One such aspect of diversity is cognitive d… ▽ More Generative AI (genAI) tools, such as ChatGPT or Copilot, are advertised to improve developer productivity and are being integrated into software development. However, misaligned trust, skepticism, and usability concerns can impede the adoption of such tools. Research also indicates that AI can be exclusionary, failing to support diverse users adequately. One such aspect of diversity is cognitive diversity -- variations in users' cognitive styles -- that leads to divergence in perspectives and interaction styles. When an individual's cognitive style is unsupported, it creates barriers to technology adoption. Therefore, to understand how to effectively integrate genAI tools into software development, it is first important to model what factors affect developers' trust and intentions to adopt genAI tools in practice? We developed a theoretically grounded statistical model to (1) identify factors that influence developers' trust in genAI tools and (2) examine the relationship between developers' trust, cognitive styles, and their intentions to use these tools in their work. We surveyed software developers (N=238) at two major global tech organizations: GitHub Inc. and Microsoft; and employed Partial Least Squares-Structural Equation Modeling (PLS-SEM) to evaluate our model. Our findings reveal that genAI's system/output quality, functional value, and goal maintenance significantly influence developers' trust in these tools. Furthermore, developers' trust and cognitive styles influence their intentions to use these tools in their work. We offer practical suggestions for designing genAI tools for effective use and inclusive user experience. △ Less

Submitted 2 December, 2024; v1 submitted 6 September, 2024; originally announced September 2024.

Comments: 13 pages, Accepted by 47th IEEE/ACM International Conference on Software Engineering (ICSE 2025)

Journal ref: 47th IEEE/ACM International Conference on Software Engineering (ICSE 2025)

"Gene": A personal tribute to the Life and Science of Eugene Newman Parker

Authors: Arnab Rai Choudhuri

Abstract: This review provides a brief account of the life of Eugene Parker (1927-2022) and discusses his contributions to plasma astrophysics. Growing up in Michigan, he went to graduate school at Caltech and then worked at the University of Utah before shifting to the University of Chicago, where he spent the rest of his illustrious career. Parker's most important scientific works are discussed in the con… ▽ More This review provides a brief account of the life of Eugene Parker (1927-2022) and discusses his contributions to plasma astrophysics. Growing up in Michigan, he went to graduate school at Caltech and then worked at the University of Utah before shifting to the University of Chicago, where he spent the rest of his illustrious career. Parker's most important scientific works are discussed in the context of the historical development of plasma astrophysics. In the study of the Sun, he made enormous contributions both to the MHD of the solar convection zone (including the formulation of turbulent dynamo theory) and to the understanding of the outer solar atmosphere (including the theory of coronal heating and the prediction of the solar wind). Parker's non-solar contributions include the Parker instability in the interstellar gas and the Parker limit of magnetic monopoles. We also try to convey an idea of Parker's highly individualistic personality and his very unique way of doing science. △ Less

Submitted 4 March, 2024; originally announced March 2024.

Comments: In press with Reviews of Modern Plasma Physics; 54 Pages, 17 Figures

How Far Are We? The Triumphs and Trials of Generative AI in Learning Software Engineering

Authors: Rudrajit Choudhuri, Dylan Liu, Igor Steinmacher, Marco Gerosa, Anita Sarma

Abstract: Conversational Generative AI (convo-genAI) is revolutionizing Software Engineering (SE) as engineers and academics embrace this technology in their work. However, there is a gap in understanding the current potential and pitfalls of this technology, specifically in supporting students in SE tasks. In this work, we evaluate through a between-subjects study (N=22) the effectiveness of ChatGPT, a con… ▽ More Conversational Generative AI (convo-genAI) is revolutionizing Software Engineering (SE) as engineers and academics embrace this technology in their work. However, there is a gap in understanding the current potential and pitfalls of this technology, specifically in supporting students in SE tasks. In this work, we evaluate through a between-subjects study (N=22) the effectiveness of ChatGPT, a convo-genAI platform, in assisting students in SE tasks. Our study did not find statistical differences in participants' productivity or self-efficacy when using ChatGPT as compared to traditional resources, but we found significantly increased frustration levels. Our study also revealed 5 distinct faults arising from violations of Human-AI interaction guidelines, which led to 7 different (negative) consequences on participants. △ Less

Submitted 18 December, 2023; originally announced December 2023.

Comments: 13 pages, ICSE 2024

Journal ref: ACM/IEEE 46th International Conference on Software Engineering (ICSE 2024)

The Near-surface Shear Layer (NSSL) of the Sun: A Theoretical Model

Authors: Arnab Rai Choudhuri, Bibhuti Kumar Jha

Abstract: We present a theoretical model of the near-surface shear layer (NSSL) of the Sun. Convection cells deeper down are affected by the Sun's rotation, but this is not the case in a layer just below the solar surface due to the smallness of the convection cells there. Based on this idea, we show that the thermal wind balance equation (the basic equation in the theory of the meridional circulation which… ▽ More We present a theoretical model of the near-surface shear layer (NSSL) of the Sun. Convection cells deeper down are affected by the Sun's rotation, but this is not the case in a layer just below the solar surface due to the smallness of the convection cells there. Based on this idea, we show that the thermal wind balance equation (the basic equation in the theory of the meridional circulation which holds inside the convection zone) can be solved to obtain the structure of the NSSL, matching observational data remarkably well. △ Less

Submitted 7 November, 2023; originally announced November 2023.

Comments: Proceedings of IAUS 365; 4 Pages, 2 Figures

Journal ref: Proc. IAU 19 (2023) 59-62

Mean field models of flux transport dynamo and meridional circulation in the Sun and stars

Authors: Gopal Hazra, Dibyendu Nandy, Leonid Kitchatinov, Arnab Rai Choudhuri

Abstract: The most widely accepted model of the solar cycle is the flux transport dynamo model. This model evolved out of the traditional $αΩ$ dynamo model which was first developed at a time when the existence of the Sun's meridional circulation was not known. In these models, the toroidal magnetic field (which gives rise to sunspots) is generated by the stretching of the poloidal field by solar differenti… ▽ More The most widely accepted model of the solar cycle is the flux transport dynamo model. This model evolved out of the traditional $αΩ$ dynamo model which was first developed at a time when the existence of the Sun's meridional circulation was not known. In these models, the toroidal magnetic field (which gives rise to sunspots) is generated by the stretching of the poloidal field by solar differential rotation. The primary source of the poloidal field in the flux transport models is attributed to the Babcock--Leighton mechanism, in contrast to the mean-field $α$-effect used in earlier models. With the realization that the Sun has a meridional circulation, which is poleward at the surface and is expected to be equatorward at the bottom of the convection zone, its importance for transporting the magnetic fields in the dynamo process was recognized. Much of our understanding about the physics of both the meridional circulation and the flux transport dynamo has come from the mean field theory obtained by averaging the equations of MHD over turbulent fluctuations. The mean field theory of meridional circulation makes clear how it arises out of an interplay between the centrifugal and thermal wind terms. We provide a broad review of mean field theories for solar magnetic fields and flows, the flux transport dynamo modeling paradigm and highlight some of their applications to solar and stellar magnetic cycles. We also discuss how the dynamo-generated magnetic field acts on the meridional circulation of the Sun and how the fluctuations in the meridional circulation, in turn, affect the solar dynamo. We conclude with some remarks on how the synergy of mean field theories, flux transport dynamo models, and direct numerical simulations can inspire the future of this field. △ Less

Submitted 18 February, 2023; originally announced February 2023.

Comments: 43 pages, 8 figures, submitted to Space Science Reviews

The emergence and growth of the flux transport dynamo model of the sunspot cycle

Authors: Arnab Rai Choudhuri

Abstract: The sunspot cycle is the magnetic cycle of the Sun produced by the dynamo process. A central idea of the solar dynamo is that the toroidal and the poloidal magnetic fields of the Sun sustain each other. We discuss the relevant observational data both for sunspots (which are manifestations of the toroidal field) and for the poloidal field of the Sun. We point out how the differential rotation of th… ▽ More The sunspot cycle is the magnetic cycle of the Sun produced by the dynamo process. A central idea of the solar dynamo is that the toroidal and the poloidal magnetic fields of the Sun sustain each other. We discuss the relevant observational data both for sunspots (which are manifestations of the toroidal field) and for the poloidal field of the Sun. We point out how the differential rotation of the Sun stretches out the poloidal field to produce the toroidal field primarily at the bottom of the convection zone, from where parts of this toroidal field may rise due to magnetic buoyancy to produce sunspots. In the flux transport dynamo model, the decay of tilted bipolar sunspot pairs gives rise to the poloidal field by the Babcock--Leighton mechanism. In this type of model, the meridional circulation of the Sun, which is poleward at the solar surface and equatorward at the bottom of the convection zone, plays a crucial role in the transport of magnetic fluxes. We finally point out that various stochastic fluctuations associated with the dynamo process may play a key role in producing the irregularities of the sunspot cycle. △ Less

Submitted 30 December, 2022; originally announced December 2022.

Comments: 26 Pages, 14 Figures; Chandrasekhar Prize lecture to be submitted to Reviews of Modern Plasma Physics

A Theoretical Model of the Near Surface Shear Layer of the Sun

Authors: Bibhuti Kumar Jha, Arnab Rai Choudhuri

Abstract: The Sun has a Near-Surface Shear Layer (NSSL), within which the angular velocity decreases rapidly with radius. We provide an explanation of this layer based on the thermal wind balance equation. Since convective motions are not affected by solar rotation in the top layer of the convection zone, we argue that the temperature falls at the same rate at all latitudes in this layer. This makes the the… ▽ More The Sun has a Near-Surface Shear Layer (NSSL), within which the angular velocity decreases rapidly with radius. We provide an explanation of this layer based on the thermal wind balance equation. Since convective motions are not affected by solar rotation in the top layer of the convection zone, we argue that the temperature falls at the same rate at all latitudes in this layer. This makes the thermal wind term very large in this layer and the centrifugal term has also to become very large to balance it, giving rise to the NSSL. From the values of differential rotation $Ω(r<r_c, θ)$ at radii less than a radius $r_c$, we can calculate the temperature difference $ΔT (r, θ)$ with respect to the standard solar model at different points of the convection zone, by making use of the thermal wind balance equation. Then we again use this equation in the top layer to calculate $Ω(r>r_c, θ)$ there from $ΔT (r, θ)$. We carry on this exercise using both an analytical expression of the differential rotation and the actual data provided by helioseismology. We find that our theoretical results of the NSSL match the observational data reasonably well for $r_c \approx 0.96~{\rm R_{\odot}}$, giving an estimate of the radius till which the convective motions are affected by the solar rotation. △ Less

Submitted 29 May, 2021; originally announced May 2021.

Comments: 11 pages, 12 figures, under review in MNRAS

M.K. Das Gupta, the first Indian radio astronomer, and his connection with the 2020 Physics Nobel Prize

Authors: Arnab Rai Choudhuri, Ritaban Chatterjee

Abstract: Half of the 2020 Nobel Prize is awarded for discovering a super-massive black hole at the centre of our Galaxy. One of the first indications of the existence of a black hole at the centre of a galaxy was found by Jennison and Das Gupta in 1953 while carrying on a radio observation of the source Cygnus A. Mrinal Das Gupta, who was doing his PhD at Manchester University at the time of this discovery… ▽ More Half of the 2020 Nobel Prize is awarded for discovering a super-massive black hole at the centre of our Galaxy. One of the first indications of the existence of a black hole at the centre of a galaxy was found by Jennison and Das Gupta in 1953 while carrying on a radio observation of the source Cygnus A. Mrinal Das Gupta, who was doing his PhD at Manchester University at the time of this discovery, spent the major part of his professional career at Calcutta University. We give an outline of Das Gupta's life and explain the scientific significance of his discovery. △ Less

Submitted 2 December, 2020; originally announced December 2020.

Comments: Invited paper submitted to Science and Culture

The Meridional Circulation of the Sun: Observations, Theory and Connections with the Solar Dynamo

Authors: Arnab Rai Choudhuri

Abstract: The meridional circulation of the Sun, which is observed to be poleward at the surface, should have a return flow at some depth. Since large-scale flows like the differential rotation and the meridional circulation are driven by turbulent stresses in the convection zone, these flows are expected to remain confined within this zone. Current observational (based on helioseismology) and theoretical (… ▽ More The meridional circulation of the Sun, which is observed to be poleward at the surface, should have a return flow at some depth. Since large-scale flows like the differential rotation and the meridional circulation are driven by turbulent stresses in the convection zone, these flows are expected to remain confined within this zone. Current observational (based on helioseismology) and theoretical (based on dynamo theory) evidences point towards an equatorward return flow of the meridional circulation at the bottom of the convection zone. Assuming the mean values of various quantities averaged over turbulence to be axisymmetric, we study the large-scale flows in solar-like stars on the basis of a 2D mean field theory. Turbulent stresses in a rotating star can transport angular momentum, setting up a differential rotation. The meridional circulation arises from a slight imbalance between two terms which try to drive it in opposite directions: a thermal wind term (arising out of the higher efficiency of convective heat transport in the polar regions) and a centrifugal term (arising out of the differential rotation). To make these terms comparable, the poles of the Sun should be slightly hotter than the equator. We discuss the important role played by the meridional circulation in the flux transport dynamo model. The poloidal field generated by the Babcock--Leighton process at the surface is advected poleward, whereas the toroidal field produced at the bottom of the convection zone is advected equatorward. The fluctuations in the meridional circulation (with coherence time of about 30--40 yr) help in explaining many aspects of the irregularities in the solar cycle. Finally, we discuss how the Lorentz force of the dynamo-generated magnetic field can cause periodic variations in the large-scale flows with the solar cycle. △ Less

Submitted 21 August, 2020; originally announced August 2020.

Comments: Invited review submitted to Science China: Physics, Mechanics & Astronomy

A theoretical estimate of the pole-equator temperature difference and a possible origin of the near-surface shear layer

Authors: Arnab Rai Choudhuri

Abstract: Convective motions in the deep layers of the solar convection zone are affected by rotation, making the convective heat transport latitude-dependent, but this is not the case in the top layers near the surface. We use the thermal wind balance condition in the deeper layers to estimate the pole-equator temperature difference. Surface observations of this temperature difference can be used for estim… ▽ More Convective motions in the deep layers of the solar convection zone are affected by rotation, making the convective heat transport latitude-dependent, but this is not the case in the top layers near the surface. We use the thermal wind balance condition in the deeper layers to estimate the pole-equator temperature difference. Surface observations of this temperature difference can be used for estimating the depth of the near-surface layer within which convection is not affected by rotation. If we require that the thermal wind balance holds in this layer also, then we have to conclude that this must be a layer of strong differential rotation and its characteristics which we derive are in broad agreement with the observational data of the near-surface shear layer. △ Less

Submitted 7 August, 2020; originally announced August 2020.

Comments: Submitted to Solar Physics

A New Formula for Predicting Solar Cycles

Authors: Gopal Hazra, Arnab Rai Choudhuri

Abstract: A new formula for predicting solar cycles based on the current theoretical understanding of the solar cycle from flux transport dynamo is presented. Two important processes---fluctuations in the Babcock-Leighton mechanism and variations in the meridional circulation, which are believed to be responsible for irregularities of the solar cycle---are constrained by using observational data. We take th… ▽ More A new formula for predicting solar cycles based on the current theoretical understanding of the solar cycle from flux transport dynamo is presented. Two important processes---fluctuations in the Babcock-Leighton mechanism and variations in the meridional circulation, which are believed to be responsible for irregularities of the solar cycle---are constrained by using observational data. We take the polar field near minima of the cycle as a measure of the randomness in the Babcock-Leighton process, and the decay rate near the minima as a consequence of the change in meridional circulation. We couple these two observationally derived quantities into a single formula to predict the amplitude of the future solar cycle. Our new formula suggests that the cycle 25 would be a moderate cycle. Whether this formula for predicting the future solar cycle can be justified theoretically is also discussed using simulations from the flux transport dynamo model. △ Less

Submitted 4 June, 2019; v1 submitted 4 November, 2018; originally announced November 2018.

Comments: 12 pages, 6 figures, Accepted for publication in ApJ

How the Saha Ionization Equation Was Discovered

Authors: Arnab Rai Choudhuri

Abstract: Although the Saha ionization equation is a standard topic in advanced statistical physics and most professional physicists would have some knowledge of it, the exact nature of Meghnad Saha's contributions in this subject is not widely known. Based on an analysis of Saha's original papers and other relevant papers by contemporary scientists, as well as other source materials such as letters exchang… ▽ More Although the Saha ionization equation is a standard topic in advanced statistical physics and most professional physicists would have some knowledge of it, the exact nature of Meghnad Saha's contributions in this subject is not widely known. Based on an analysis of Saha's original papers and other relevant papers by contemporary scientists, as well as other source materials such as letters exchanged between scientists, I discuss how Saha used the theory of thermal ionization to solve some of the most important astrophysics problems of that era. △ Less

Submitted 25 October, 2018; originally announced October 2018.

Comments: To be published in "Physics News", journal of Indian Physics Association

The Sun's polar magnetic field: datasets, proxies and theoretical issues

Authors: Arnab Rai Choudhuri

Abstract: The polar magnetic field of the Sun is a manifestation of certain aspects of the dynamo process and is a good precursor for predicting a sunspot cycle before its onset. Although actual synoptic measurements of this field exist only from the mid-1970s, it has now been possible to determine its evolution from the beginning of the twentieth century with the help of various proxies. The recently devel… ▽ More The polar magnetic field of the Sun is a manifestation of certain aspects of the dynamo process and is a good precursor for predicting a sunspot cycle before its onset. Although actual synoptic measurements of this field exist only from the mid-1970s, it has now been possible to determine its evolution from the beginning of the twentieth century with the help of various proxies. The recently developed 3D kinematic dynamo model can study the build-up of the Sun's polar magnetic field more realistically than the earlier surface flux transport model. △ Less

Submitted 12 October, 2018; originally announced October 2018.

Comments: To be published in the Proceedings of IAU Symposium 340

My Life and My Journey through Solar Physics

Authors: Arnab Rai Choudhuri

Abstract: This is the talk I gave at the workshop "Solar-Stellar Magnetism: Past, Present and Future" held in Jaipur on 18 February 2018 on the occasion of my 60th year. This is the talk I gave at the workshop "Solar-Stellar Magnetism: Past, Present and Future" held in Jaipur on 18 February 2018 on the occasion of my 60th year. △ Less

Submitted 26 August, 2018; originally announced September 2018.

Comments: Presentation at the workshop "Solar-Stellar Magnetism: Past, Present and Future"

Explaining the variation of the meridional circulation with the solar cycle

Authors: Gopal Hazra, Arnab Rai Choudhuri

Abstract: The meridional circulation of the Sun is observationally found to vary with the solar cycle, becoming slower during the solar maxima. We explain this by constructing a theoretical model in which the equation of the meridional circulation (the $φ$ component of the vorticity equation) is coupled with the equations of the flux transport dynamo model. We find that the Lorentz force of the dynamo-gener… ▽ More The meridional circulation of the Sun is observationally found to vary with the solar cycle, becoming slower during the solar maxima. We explain this by constructing a theoretical model in which the equation of the meridional circulation (the $φ$ component of the vorticity equation) is coupled with the equations of the flux transport dynamo model. We find that the Lorentz force of the dynamo-generated magnetic fields can slow down the \MC\ during the solar maxima in broad conformity with the observations. △ Less

Submitted 18 September, 2018; originally announced September 2018.

Comments: 4 pages, 2 figures, to appear in the Proceedings of IAU Symposium No. 340, 2018

The Sun as a Laboratory for Plasma Physics

Authors: Arnab Rai Choudhuri

Abstract: Several phenomena connected with the magnetic field of the Sun (the cool sunspots, the hot corona, solar flares, the solar wind) are collectively known as solar activity. This paper discusses how one uses the MHD equations to understand how the magnetic field of the Sun is produced by the dynamo process and then gives rise to these diverse activities, making the Sun the best laboratory for plasma… ▽ More Several phenomena connected with the magnetic field of the Sun (the cool sunspots, the hot corona, solar flares, the solar wind) are collectively known as solar activity. This paper discusses how one uses the MHD equations to understand how the magnetic field of the Sun is produced by the dynamo process and then gives rise to these diverse activities, making the Sun the best laboratory for plasma physics in the limit of high magnetic Reynolds number (defined at the end of the Introduction). △ Less

Submitted 30 August, 2018; originally announced August 2018.

Comments: To be published in the M.N. Saha 125th anniversary issue of "Science and Culture"

Flux Transport Dynamo: From Modelling Irregularities to Making Predictions

Authors: Arnab Rai Choudhuri

Abstract: The flux transport dynamo, in which the poloidal magnetic field is generated by the Babcock--Leighton mechanism and the meridional circulation plays a crucial role, has emerged as an attractive model for the solar cycle. Based on theoretical calculations done with this model, we argue that the fluctuations in the Babcock--Leighton mechanism and the fluctuations in the meridional circulation are th… ▽ More The flux transport dynamo, in which the poloidal magnetic field is generated by the Babcock--Leighton mechanism and the meridional circulation plays a crucial role, has emerged as an attractive model for the solar cycle. Based on theoretical calculations done with this model, we argue that the fluctuations in the Babcock--Leighton mechanism and the fluctuations in the meridional circulation are the most likely causes of the irregularities of the solar cycle. With our increased theoretical understanding of how these irregularities arise, it can be possible to predict a future solar cycle by feeding the appropriate observational data in a theoretical dynamo model. △ Less

Submitted 26 August, 2018; originally announced August 2018.

Journal ref: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 176, p. 5-9 (2018)

Predicting a solar cycle before its onset using a flux transport dynamo model

Authors: Arnab Rai Choudhuri

Abstract: We begin with a review of the predictions for cycle~24 before its onset. After summarizing the basics of the flux transport dynamo model, we discuss how this model had been used to make a successful prediction of cycle~24, on the assumption that the irregularities of the solar cycle arise due to the fluctuations in the Babcock--Leighton mechanism. We point out that fluctuations in the meridional c… ▽ More We begin with a review of the predictions for cycle~24 before its onset. After summarizing the basics of the flux transport dynamo model, we discuss how this model had been used to make a successful prediction of cycle~24, on the assumption that the irregularities of the solar cycle arise due to the fluctuations in the Babcock--Leighton mechanism. We point out that fluctuations in the meridional circulation can be another cause of irregularities in the cycle. △ Less

Submitted 26 August, 2018; originally announced August 2018.

Comments: To be published in the Proceedings of IAU Symposium 335: Space Weather of the Heliosphere: Processes and Forecasts

A Modular Supersonic Ping Pong Gun

Authors: Mark French, Rajarshi Choudhuri, Jim Stratton, Craig Zehrung, Davin Huston

Abstract: A vacuum-powered device that shoots ping pong balls at high subsonic speeds has been used for physics demonstrations for more than a decade. It uses physics that are easily understood by students, even though its operation is not immediately intuitive. The addition of a pressure plenum and nozzle results in muzzle velocities exceeding Mach 1.5. Balls are readily fired through ping pong paddles and… ▽ More A vacuum-powered device that shoots ping pong balls at high subsonic speeds has been used for physics demonstrations for more than a decade. It uses physics that are easily understood by students, even though its operation is not immediately intuitive. The addition of a pressure plenum and nozzle results in muzzle velocities exceeding Mach 1.5. Balls are readily fired through ping pong paddles and sheets of plywood up to 12.7mm (1/2 inch) thick. Popular reaction to the device indicates that it is an effective way to spark interest in physics and engineering. △ Less

Submitted 14 February, 2018; originally announced February 2018.

Comments: 13 pages, 11 figures

A theoretical model of the variation of the meridional circulation with the solar cycle

Authors: Gopal Hazra, Arnab Rai Choudhuri

Abstract: Observations of the meridional circulation of the Sun, which plays a key role in the operation of the solar dynamo, indicate that its speed varies with the solar cycle, becoming faster during the solar minima and slower during the solar maxima. To explain this variation of the meridional circulation with the solar cycle, we construct a theoretical model by coupling the equation of the meridional c… ▽ More Observations of the meridional circulation of the Sun, which plays a key role in the operation of the solar dynamo, indicate that its speed varies with the solar cycle, becoming faster during the solar minima and slower during the solar maxima. To explain this variation of the meridional circulation with the solar cycle, we construct a theoretical model by coupling the equation of the meridional circulation (the $φ$ component of the vorticity equation within the solar convection zone) with the equations of the flux transport dynamo model. We consider the back reaction due to the Lorentz force of the dynamo-generated magnetic fields and study the perturbations produced in the meridional circulation due to it. This enables us to model the variations of the meridional circulation without developing a full theory of the meridional circulation itself. We obtain results which reproduce the observational data of solar cycle variations of the meridional circulation reasonably well. We get the best results on assuming the turbulent viscosity acting on the velocity field to be comparable to the magnetic diffusivity (i.e. on assuming the magnetic Prandtl number to be close to unity). We have to assume an appropriate bottom boundary condition to ensure that the Lorentz force cannot drive a flow in the subadiabatic layers below the bottom of the tachocline. Our results are sensitive to this bottom boundary condition. We also suggest a hypothesis how the observed inward flow towards the active regions may be produced. △ Less

Submitted 17 August, 2017; originally announced August 2017.

Comments: 15 pages, 11 figures, accepted for publication in MNRAS

The golden age of Calcutta physics: Difficulties in reconstructing the history

Authors: Arnab Rai Choudhuri

Abstract: Classes started in the newly established Physics Department of Calcutta University Science College in 1916. Raman, Bose and Saha were three young members of the small physics faculty consisting of barely half a dozen faculty members. Within about one decade, three extraordinary discoveries came from these young men---Saha ionization equation in 1920, Bose statistics in 1924, Raman effect in 1928.… ▽ More Classes started in the newly established Physics Department of Calcutta University Science College in 1916. Raman, Bose and Saha were three young members of the small physics faculty consisting of barely half a dozen faculty members. Within about one decade, three extraordinary discoveries came from these young men---Saha ionization equation in 1920, Bose statistics in 1924, Raman effect in 1928. However, fortunes of Calcutta University quickly got intertwined with India's freedom struggle led by Mahatma Gandhi exactly at the same time and the physics group got tragically disrupted. Indian physics never succeeded in reaching that height again. This paper discusses the difficulties in reconstructing a critical history of this Calcutta school of physics during the very short epoch of unmatched brilliance. △ Less

Submitted 8 December, 2016; originally announced December 2016.

Comments: 24 pages, invited paper for CU Physics 100 (conference to commemorate the centenary of Physics Department, Calcutta University)

Starspots, Stellar Cycles and Stellar Flares: Lessons from Solar Dynamo Models

Authors: Arnab Rai Choudhuri

Abstract: In this review, we discuss whether the present solar dynamo models can be extrapolated to explain various aspects of stellar activity. We begin with a summary of the following kinds of data for solar-like stars: (i) data pertaining to stellar cycles from Ca H/K emission over many years; (ii) X-ray data indicating hot coronal activity; (iii) starspot data (especially about giant polar spots); and (… ▽ More In this review, we discuss whether the present solar dynamo models can be extrapolated to explain various aspects of stellar activity. We begin with a summary of the following kinds of data for solar-like stars: (i) data pertaining to stellar cycles from Ca H/K emission over many years; (ii) X-ray data indicating hot coronal activity; (iii) starspot data (especially about giant polar spots); and (iv) data pertaining to stellar superflares. Then we describe the current status of solar dynamo modelling---giving an introduction to the flux transport dynamo model, the currently favoured model for the solar cycle. While an extrapolation of this model to solar-like stars can explain some aspects of observational data, some other aspects of the data still remain to be theoretically explained. It is not clear right now whether we need a different kind of dynamo mechanism for stars having giant starspots or producing very strong superflares. △ Less

Submitted 8 December, 2016; originally announced December 2016.

Comments: 30 pages, 16 figures, invited review published in Science China - Physics, Mechanics & Astronomy

Journal ref: Sci. China-Phys. Mech. Astron. 60, 019601 (2017)

A theoretical study of the build-up of the Sun's polar magnetic field by using a 3D kinematic dynamo model

Authors: Gopal Hazra, Arnab Rai Choudhuri, Mark S. Miesch

Abstract: We develop a three-dimensional kinematic self-sustaining model of the solar dynamo in which the poloidal field generation is from tilted bipolar sunspot pairs placed on the solar surface above regions of strong toroidal field by using the SpotMaker algorithm, and then the transport of this poloidal field to the tachocline is primarily caused by turbulent diffusion. We obtain a dipolar solution wit… ▽ More We develop a three-dimensional kinematic self-sustaining model of the solar dynamo in which the poloidal field generation is from tilted bipolar sunspot pairs placed on the solar surface above regions of strong toroidal field by using the SpotMaker algorithm, and then the transport of this poloidal field to the tachocline is primarily caused by turbulent diffusion. We obtain a dipolar solution within a certain range of parameters. We use this model to study the build-up of the polar magnetic field and show that some insights obtained from surface flux transport (SFT) models have to be revised. We present results obtained by putting a single bipolar sunspot pair in a hemisphere and two symmetrical sunspot pairs in two hemispheres.We find that the polar fields produced by them disappear due to the upward advection of poloidal flux at low latitudes, which emerges as oppositely-signed radial flux and which is then advected poleward by the meridional flow. We also study the effect that a large sunspot pair, violating Hale's polarity law would have on the polar field. We find that there would be some effect---especially if the anti-Hale pair appears at high latitudes in the mid-phase of the cycle---though the effect is not very dramatic. △ Less

Submitted 25 January, 2017; v1 submitted 9 October, 2016; originally announced October 2016.

Comments: 18 pages, 18 figures, published in ApJ

Journal ref: The Astrophysical Journal, Volume 835, Issue 1, article id. 39, 16 pp. (2017)

The treatment of magnetic buoyancy in flux transport dynamo models

Authors: Arnab Rai Choudhuri, Gopal Hazra

Abstract: One important ingredient of flux transport dynamo models is the rise of the toroidal magnetic field through the convection zone due to magnetic buoyancy to produce bipolar sunspots and then the generation of the poloidal magnetic field from these bipolar sunspots due to the Babcock-Leighton mechanism. Over the years, two methods of treating magnetic buoyancy, a local method and a non-local method… ▽ More One important ingredient of flux transport dynamo models is the rise of the toroidal magnetic field through the convection zone due to magnetic buoyancy to produce bipolar sunspots and then the generation of the poloidal magnetic field from these bipolar sunspots due to the Babcock-Leighton mechanism. Over the years, two methods of treating magnetic buoyancy, a local method and a non-local method have been used widely by different groups in constructing 2D kinematic models of the flux transport dynamo. We review both these methods and conclude that neither of them is fully satisfactory, presumably because magnetic buoyancy is an inherently 3D process. We also point out so far we do not have proper understanding of why sunspot emergence is restricted to rather low latitudes. △ Less

Submitted 12 November, 2015; originally announced November 2015.

Comments: 23 pages, 3 figures. Submitted to a special issue of Advances in Space Research on "Solar Dynamo Frontiers"

An Incentivized Approach for Fair Participation in Wireless Ad hoc Networks

Authors: Arka Rai Choudhuri, Kalyanasundaram S, Shriyak Sridhar, Annappa B

Abstract: In Wireless Ad hoc networks (WANETs), nodes separated by considerable distance communicate with each other by relaying their messages through other nodes. However, it might not be in the best interests of a node to forward the message of another node due to power constraints. In addition, all nodes being rational, some nodes may be selfish, i.e. they might not relay data from other nodes so as to… ▽ More In Wireless Ad hoc networks (WANETs), nodes separated by considerable distance communicate with each other by relaying their messages through other nodes. However, it might not be in the best interests of a node to forward the message of another node due to power constraints. In addition, all nodes being rational, some nodes may be selfish, i.e. they might not relay data from other nodes so as to increase their lifetime. In this paper, we present a fair and incentivized approach for participation in Ad hoc networks. Given the power required for each transmission, we are able to determine the power saving contributed by each intermediate hop. We propose the FAir Share incenTivizEd Ad hoc paRticipation protocol (FASTER), which takes a selected route from a routing protocol as input, to calculate the worth of each node using the cooperative game theory concept of 'Shapley Value' applied on the power saved by each node. This value can be used for allocation of Virtual Currency to the nodes, which can be spent on subsequent message transmissions. △ Less

Submitted 4 March, 2015; originally announced March 2015.

Comments: 6 pages, 4 figures, published in the International Journal of Recent Development in Engineering and Technology

Journal ref: IJRDET 2, no. 3 (2014): 117-121

Correlation between Decay Rate and Amplitude of Solar Cycles as Revealed from Observations and Dynamo Theory

Authors: Gopal Hazra, Bidya Binay Karak, Dipankar Banerjee, Arnab Rai Choudhuri

Abstract: Using different proxies of solar activity, we have studied the following features of solar cycle. (i) A linear correlation between the amplitude of cycle and its decay rate, (ii) a linear correlation between the amplitude of cycle $n$ and the decay rate of cycle $(n - 1)$ and (iii) an anti-correlation between the amplitude of cycle $n$ and the period of cycle $(n - 1)$. Features (ii) and (iii) are… ▽ More Using different proxies of solar activity, we have studied the following features of solar cycle. (i) A linear correlation between the amplitude of cycle and its decay rate, (ii) a linear correlation between the amplitude of cycle $n$ and the decay rate of cycle $(n - 1)$ and (iii) an anti-correlation between the amplitude of cycle $n$ and the period of cycle $(n - 1)$. Features (ii) and (iii) are very useful because they provide precursors for future cycles. We have reproduced these features using a flux transport dynamo model with stochastic fluctuations in the Babcock-Leighton $α$ effect and in the meridional circulation. Only when we introduce fluctuations in meridional circulation, we are able to reproduce different observed features of solar cycle. We discuss the possible reasons for these correlations. △ Less

Submitted 7 May, 2015; v1 submitted 31 October, 2014; originally announced October 2014.

Comments: 22 pages, 10 figures and 2 tables, accepted in Solar Physics

Report number: NORDITA-2014-122

A Critical Assessment of the Flux Transport Dynamo

Authors: Arnab Rai Choudhuri

Abstract: We first discuss how the flux transport dynamo with reasonably high diffusion can explain both the regular and the irregular features of the solar cycle quite well. Then we critically examine the inadequacies of the model and the challenge posed by some recent observational data about meridional circulation, arriving at the conclusion that this model can still work within the bounds of observation… ▽ More We first discuss how the flux transport dynamo with reasonably high diffusion can explain both the regular and the irregular features of the solar cycle quite well. Then we critically examine the inadequacies of the model and the challenge posed by some recent observational data about meridional circulation, arriving at the conclusion that this model can still work within the bounds of observational data △ Less

Submitted 18 August, 2014; originally announced August 2014.

Comments: 11 pages, 2 figures, Journal of Astronomy and Astrophysics (invited article for the special issue "MHD Waves and Dynamical Processes in the Magnetised Solar and Space Plasmas")

Polar Network Index as a magnetic proxy for the solar cycle studies

Authors: Muthu Priyal, Dipankar Banerjee, Bidya Binay Karak, Andres Munoz-Jaramillo, B. Ravindra, Arnab Rai Choudhuri, Jagdev Singh

Abstract: The Sun has a polar magnetic field which oscillates with the 11 year sunspot cycle. This polar magnetic field is an important component of the dynamo process which is operating in the solar convection zone and produces the sunspot cycle. We have systematic direct measurements of the Sun's polar magnetic field only from about mid 1970s. There are, however, indirect proxies which give us information… ▽ More The Sun has a polar magnetic field which oscillates with the 11 year sunspot cycle. This polar magnetic field is an important component of the dynamo process which is operating in the solar convection zone and produces the sunspot cycle. We have systematic direct measurements of the Sun's polar magnetic field only from about mid 1970s. There are, however, indirect proxies which give us information about this field at earlier times. The Ca K spectroheliograms taken in Kodaikanal Solar Observatory during 1904 - 2007 have now been digitized with the 4k x 4k CCD and have higher resolution (0.86 arcsec) than the other available historical datasets. From these Ca-K spectroheliograms, we have developed a completely new proxy (Polar Network Index, PNI) for the Sun's polar magnetic field. We calculate the PNI from the digitized images using an automated algorithm and calibrate our measured PNI against the polar field as measured by the Wilcox Solar Observatory for the period of 1976 - 1990. This calibration allows us to estimate polar fields for the earlier period up to 1904. The dynamo calculations done with this proxy as input data reproduce the Sun's magnetic behavior for the past century reasonably well. △ Less

Submitted 18 July, 2014; originally announced July 2014.

Comments: 19 pages, 5 figures Accepted for publication in APJL

Report number: NORDITA-2014-88

A dynamo model of magnetic activity in solar-like stars with different rotational velocities

Authors: Bidya Binay Karak, Leonid L. Kitchatinov, Arnab Rai Choudhuri

Abstract: We attempt to provide a quantitative theoretical explanation for the observations that Ca II H/K emission and X-ray emission from solar-like stars increase with decreasing Rossby number (i.e., with faster rotation). Assuming that these emissions are caused by magnetic cycles similar to the sunspot cycle, we construct flux transport dynamo models of $1M_{\odot}$ stars rotating with different rotati… ▽ More We attempt to provide a quantitative theoretical explanation for the observations that Ca II H/K emission and X-ray emission from solar-like stars increase with decreasing Rossby number (i.e., with faster rotation). Assuming that these emissions are caused by magnetic cycles similar to the sunspot cycle, we construct flux transport dynamo models of $1M_{\odot}$ stars rotating with different rotation periods. We first compute the differential rotation and the meridional circulation inside these stars from a mean-field hydrodynamics model. Then these are substituted in our dynamo code to produce periodic solutions. We find that the dimensionless amplitude $f_m$ of the toroidal flux through the star increases with decreasing rotation period. The observational data can be matched if we assume the emissions to go as the power 3-4 of $f_m$. Assuming that the Babcock-Leighton mechanism saturates with increasing rotation, we can provide an explanation for the observed saturation of emission at low Rossby numbers. The main failure of our model is that it predicts an increase of magnetic cycle period with increasing rotation rate, which is the opposite of what is found observationally. Much of our calculations are based on the assumption that the magnetic buoyancy makes the magnetic flux tubes to rise radially from the bottom of the convection zone. On taking account of the fact that the Coriolis force diverts the magnetic flux tubes to rise parallel to the rotation axis in rapidly rotating stars, the results do not change qualitatively. △ Less

Submitted 11 July, 2014; v1 submitted 8 February, 2014; originally announced February 2014.

Comments: Accepted for publication in ApJ

Report number: NORDITA 2014-15

Journal ref: ApJ, 791, 59 (2014)

The irregularities of the sunspot cycle and their theoretical modelling

Authors: Arnab Rai Choudhuri

Abstract: The 11-year sunspot cycle has many irregularities, the most promi- nent amongst them being the grand minima when sunspots may not be seen for several cycles. After summarizing the relevant observational data about the irregularities, we introduce the flux transport dynamo model, the currently most successful theoretical model for explaining the 11-year sunspot cycle. Then we analyze the respective… ▽ More The 11-year sunspot cycle has many irregularities, the most promi- nent amongst them being the grand minima when sunspots may not be seen for several cycles. After summarizing the relevant observational data about the irregularities, we introduce the flux transport dynamo model, the currently most successful theoretical model for explaining the 11-year sunspot cycle. Then we analyze the respective roles of nonlinearities and random fluctuations in creating the irregularities. We also discuss how it has recently been realized that the fluctuations in meridional circula- tion also can be a source of irregularities. We end by pointing out that fluctuations in the poloidal field generation and fluctuations in meridional circulation together can explain the occurrences of grand minima. △ Less

Submitted 12 December, 2013; originally announced December 2013.

Comments: Invited paper for the C.V. Raman 125th birth anniversary issue of Indian Journal of Physics

Is a deep one-cell meridional circulation essential for the flux transport Solar Dynamo?

Authors: Gopal Hazra, Bidya Binay Karak, Arnab Rai Choudhuri

Abstract: The solar activity cycle is successfully modeled by the flux transport dynamo, in which the meridional circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the meridional circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallow… ▽ More The solar activity cycle is successfully modeled by the flux transport dynamo, in which the meridional circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the meridional circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a meridional circulation with such a shallow return flow can still retain the attractive features of the flux transport dynamo (such as a proper butterfly diagram, the proper phase relation between the toroidal and poloidal fields). We consider additional cells of the meridional circulation below the shallow return flow---both the case of multiple cells radially stacked above one another and the case of more complicated cell patterns. As long as there is an equatorward flow in low latitudes at the bottom of the convection zone, we find that the solar behavior is approximately reproduced. However, if there is either no flow or a poleward flow at the bottom of the convection zone, then we cannot reproduce solar behavior. On making the turbulent diffusivity low, we still find periodic behavior, although the period of the cycle becomes unrealistically large. Also, with a low diffusivity, we do not get the observed correlation between the polar field at the sunspot minimum and the strength of the next cycle, which is reproduced when diffusivity is high. On introducing radially downward pumping, we get a more reasonable period and more solar-like behavior even with low diffusivity. △ Less

Submitted 4 February, 2014; v1 submitted 11 September, 2013; originally announced September 2013.

Comments: 12 pages, 13 figures

Journal ref: ApJ, 782, 93 (2014)

Studies of grand minima in sunspot cycles from a flux transport solar dynamo model

Authors: Bidya Binay Karak, Arnab Rai Choudhuri

Abstract: We propose that grand minima in solar activity are caused by simultaneous fluctuations in the meridional circulation and the Babcock-Leighton mechanism for the poloidal field generation in the flux transport dynamo model. We present the following results: (a) fluctuations in the meridional circulation are more effective in producing grand minima; (b) both sudden and gradual initiations of grand mi… ▽ More We propose that grand minima in solar activity are caused by simultaneous fluctuations in the meridional circulation and the Babcock-Leighton mechanism for the poloidal field generation in the flux transport dynamo model. We present the following results: (a) fluctuations in the meridional circulation are more effective in producing grand minima; (b) both sudden and gradual initiations of grand minima are possible; (c) distributions of durations and waiting times between grand minima seem to be exponential; (d) the coherence time of the meridional circulation has an effect on the number and the average duration of grand minima, a coherence time of about 30 years being consistent with observational data. We also study the occurrence of grand maxima and find that the distributions of durations and waiting times between grand maxima are also exponential, like the grand minima. Finally we address the question whether the Babcock-Leighton mechanism can be operative during grand minima when there are no sunspots. We show that an alpha-effect restricted to the upper portions of the convection zone can pull the dynamo out of the grand minima and can match various observational requirements if the amplitude of this alpha-effect is suitably fine-tuned. △ Less

Submitted 28 June, 2013; v1 submitted 23 June, 2013; originally announced June 2013.

Comments: To appear in Research in Astronomy and Astrophysics

Journal ref: Research in Astronomy and Astrophysics 13 (2013) 1339-1357

Fluctuations in the Alpha-Effect and Grand Solar Minima

Authors: S. V. Olemskoy, A. R. Choudhuri, L. L. Kitchatinov

Abstract: Parameters of a special kind of α-effect known in dynamo theory as the Babcock-Leighton mechanism are estimated using the data of sunspot catalogs. The estimates evidence the presence of the Babcock-Leighton α-effect on the Sun. Fluctuations of the α-effect are also estimated. The fluctuation amplitude appreciably exceeds the mean value, and the characteristic time for the fluctuations is comparab… ▽ More Parameters of a special kind of α-effect known in dynamo theory as the Babcock-Leighton mechanism are estimated using the data of sunspot catalogs. The estimates evidence the presence of the Babcock-Leighton α-effect on the Sun. Fluctuations of the α-effect are also estimated. The fluctuation amplitude appreciably exceeds the mean value, and the characteristic time for the fluctuations is comparable to the period of the solar rotation. Fluctuations with the parameters found are included in a numerical model for the solar dynamo. Computations show irregular changes in the amplitudes of the magnetic cycles on time scales of centuries and millennia. The calculated statistical characteristics of the grand solar minima and maxima agree with the data on solar activity over the Holocene. △ Less

Submitted 12 May, 2013; originally announced May 2013.

Comments: To appear in Astronomy Reports, 20 pages, 9 figures

Can Superflares Occur on Our Sun?

Authors: Kazunari Shibata, Hiroaki Isobe, Andrew Hillier, Arnab Rai Choudhuri, Hiroyuki Maehara, Takako T. Ishii, Takuya Shibayama, Shota Notsu, Yuta Notsu, Takashi Nagao, Satoshi Honda, Daisaku Nogami

Abstract: Recent observations of solar type stars with the Kepler satellite by Maehara et al. have revealed the existence of superflares (with energy of 10^33 - 10^35 erg) on Sun-like stars, which are similar to our Sun in their surface temperature (5600 K - 6000 K) and slow rotation (rotational period > 10 days). From the statistical analysis of these superflares, it was found that superflares with energy… ▽ More Recent observations of solar type stars with the Kepler satellite by Maehara et al. have revealed the existence of superflares (with energy of 10^33 - 10^35 erg) on Sun-like stars, which are similar to our Sun in their surface temperature (5600 K - 6000 K) and slow rotation (rotational period > 10 days). From the statistical analysis of these superflares, it was found that superflares with energy 10^34 erg occur once in 800 years and superflares with 10^35 erg occur once in 5000 years on Sun-like stars. In this paper, we examine whether superflares with energy of 10^33 - 10^35 erg could occur on the present Sun through the use of simple order-of-magnitude estimates based on current ideas relating to the mechanisms of the solar dynamo. △ Less

Submitted 6 December, 2012; originally announced December 2012.

Comments: Accepted by Publ. Astron. Soc. Japan on Dec. 6, 2012 (to be published on PASJ vol. 65, No. 3, (2013) June 25)

Flux-transport and mean-field dynamo theories of solar cycles

Authors: Arnab Rai Choudhuri

Abstract: We point out the difficulties in carrying out direct numerical simulation of the solar dynamo problem and argue that kinematic mean-field models are our best theoretical tools at present for explaining various aspects of the solar cycle in detail. The most promising kinematic mean-field model is the flux transport dynamo model, in which the toroidal field is produced by differential rotation in th… ▽ More We point out the difficulties in carrying out direct numerical simulation of the solar dynamo problem and argue that kinematic mean-field models are our best theoretical tools at present for explaining various aspects of the solar cycle in detail. The most promising kinematic mean-field model is the flux transport dynamo model, in which the toroidal field is produced by differential rotation in the tachocline, the poloidal field is produced by the Babcock--Leighton mechanism at the solar surface and the meridional circulation plays a crucial role. Depending on whether the diffusivity is high or low, either the diffusivity or the meridional circulations provides the main transport mechanism for the poloidal field to reach the bottom of the convection zone from the top. We point out that the high-diffusivity flux transport dynamo model is consistent with various aspects of observational data. The irregularities of the solar cycle are primarily produced by fluctuations in the Babcock--Leighton mechanism and in the meridional circulation. We summarize recent work on the fluctuations of meridional circulation in the flux transport dynamo, leading to explanations of such things as the Waldmeier effect. △ Less

Submitted 2 November, 2012; originally announced November 2012.

Comments: IAU Symposium 294: Solar and Astrophysical Dynamos and Magnetic Activity. Editors: A.G. Kosovichev, E.M. de Gouveia Dal Pino & Y.Yan

Journal ref: Proceedings of the International Astronomical Union, IAU Symposium, 294, 37 (2013)

Modelling grand minima of solar activity using a flux transport dynamo model

Authors: Bidya Binay Karak, Arnab Rai Choudhuri

Abstract: The occurrence of grand minima like the Maunder minimum is an intriguing aspect of the sunspot cycle. We use the flux transport dynamo model to explain the grand minima, showing that they arise when either the poloidal field or the meridional circulation falls to a sufficiently low value due to fluctuations. Assuming these fluctuations to be Gaussian and determining the various parameters from the… ▽ More The occurrence of grand minima like the Maunder minimum is an intriguing aspect of the sunspot cycle. We use the flux transport dynamo model to explain the grand minima, showing that they arise when either the poloidal field or the meridional circulation falls to a sufficiently low value due to fluctuations. Assuming these fluctuations to be Gaussian and determining the various parameters from the data of the last 28 cycles, we carry on a dynamo simulation with both these fluctuations. The results are remarkably close to the observational data. △ Less

Submitted 22 July, 2013; v1 submitted 1 November, 2012; originally announced November 2012.

Comments: IAU Symposium 294: Solar and Astrophysical Dynamos and Magnetic Activity. Editors: A.G. Kosovichev, E.M. de Gouveia Dal Pino, & Y.Yan

Journal ref: Proceedings of the International Astronomical Union Volume 8 Symposium S294, 2013, pp 433-438

The origin of grand minima in the sunspot cycle

Authors: Arnab Rai Choudhuri, Bidya Binay Karak

Abstract: One of the most striking aspects of the 11-year sunspot cycle is that there have been times in the past when some cycles went missing, a most well-known example of this being the Maunder minimum during 1645-1715. Analyses of cosmogenic isotopes (C14 and Be10) indicated that there were about 27 grand minima in the last 11,000 yr, implying that about 2.7% of the solar cycles had conditions appropria… ▽ More One of the most striking aspects of the 11-year sunspot cycle is that there have been times in the past when some cycles went missing, a most well-known example of this being the Maunder minimum during 1645-1715. Analyses of cosmogenic isotopes (C14 and Be10) indicated that there were about 27 grand minima in the last 11,000 yr, implying that about 2.7% of the solar cycles had conditions appropriate for forcing the Sun into grand minima. We address the question how grand minima are produced and specifically calculate the frequency of occurrence of grand minima from a theoretical dynamo model. We assume that fluctuations in the poloidal field generation mechanism and the meridional circulation produce irregularities of sunspot cycles. Taking these fluctuations to be Gaussian and estimating the values of important parameters from the data of last 28 solar cycles, we show from our flux transport dynamo model that about 1-4% of the sunspot cycles may have conditions suitable for inducing grand minima. △ Less

Submitted 25 September, 2012; v1 submitted 20 August, 2012; originally announced August 2012.

Comments: Accepted for publication in Physical Review Letters

Journal ref: Physical Review Letters, 109, 171103 (2012)

Back-reactions of dynamo-generated magnetic fields: Torsional oscillations and variations in meridional circulation

Authors: Arnab Rai Choudhuri

Abstract: The periodically varying Lorentz force of the periodic solar magnetic field generated by the solar dynamo can induce two kinds of motions: torsional oscillations and periodic variations in the meridional circulation. Observational evidence now exists for both these kinds of motions. We discuss our ongoing effort in theoretically studying the variations of the meridional circulation. Then we presen… ▽ More The periodically varying Lorentz force of the periodic solar magnetic field generated by the solar dynamo can induce two kinds of motions: torsional oscillations and periodic variations in the meridional circulation. Observational evidence now exists for both these kinds of motions. We discuss our ongoing effort in theoretically studying the variations of the meridional circulation. Then we present our theoretical model of torsional oscillations, which addresses the question why these oscillations start before sunspot cycles at latitudes higher than where sunspots are seen. △ Less

Submitted 10 November, 2011; originally announced November 2011.

Comments: 6 pages, 2 figures. Proceedings of the 1st Asia Pacific Solar Physics Meeting

Journal ref: First Asia-Pacific Solar Physics Meeting ASI Conference Series, 2011, Vol. 2, pp 131-136 [Edited by Arnab Rai Choudhuri & Dipankar Banerjee]