Magnetic Flux Cancellation as the Trigger Mechanism of Solar Coronal Jets

Solar coronal jets are narrow, short-lived coronal features that occur frequently throughout the entire solar magnetic cycle (Shimojo et al. 1998; Wang et al. 1998; Savcheva et al. 2007; Hong et al. 2011; Raouafi et al. 2016). These events have been observed in all regions of the solar surface: in active regions (Shibata et al. 1992; Innes et al. 2011; Panesar et al. 2016; Sterling et al. 2016, 2017), quiet regions (Hong et al. 2011; Innes et al. 2016), and coronal holes (Cirtain et al. 2007; Nisticò et al. 2009; Pucci et al. 2013; Sterling et al. 2015; Panesar et al. 2018a). In addition to being wide-spread across the solar surface, they also occur very frequently; Savcheva et al. (2007) found that jets in polar coronal holes occur at an average rate of 60 jets per day. Coronal jets are most often observed in extreme ultraviolet (EUV) (Culhane et al. 2007; Zheng et al. 2013; Adams et al. 2014) and X-ray emission (Canfield et al. 1996; Yokoyama 1998; Alexander & Fletcher 1999; Cirtain et al. 2007; Glesener et al. 2012).

Coronal jets are characterized by a bright point (also known as the jet bright point, or JBP) at an edge of the base of the jet (Shen et al. 2012; Young & Muglach 2014; Sterling et al. 2015; Panesar et al. 2016) and a jet spire of collimated outward moving plasma (Shimojo et al. 1996; Shen et al. 2012; Young & Muglach 2014). Additionally, recent studies (e.g., Hong et al. 2011; Shen et al. 2012; Sterling et al. 2015) have noted the presence of a minifilament that sits at the site of the forthcoming JBP and erupts to drive the jet. Minifilaments are usually seen to begin rising earlier (≤1 minute) than the appearance of the JBP (Moore et al. 2018). Some earlier studies argued (based on observations with older data sets) that flux emergence may lead to jets (Shibata et al. 1992, 2007; Moreno-Insertis et al. 2008). More recent on-disk jet studies have found in many cases consistent evidence of magnetic flux cancellation prior to and during jet onset (Hong et al. 2011; Huang et al. 2012; Young & Muglach 2014; Panesar et al. 2016). Panesar et al. (2016) investigated the magnetic trigger mechanism of 10 randomly selected on-disk quiet region jets and found that flux cancellation was the trigger of each of these jets. In addition to larger coronal jets, small-scale jets (Panesar et al. 2018b) are also observed to erupt from neutral lines at which flux is canceling. As shown in Panesar et al. (2017), jets typically occur following the formation of a cool-plasma minifilament residing over the magnetic neutral line between canceling positive-polarity and negative-polarity flux. Thus, coronal jets appear to be miniature versions of larger-scale eruptions that make coronal mass ejections (Sterling et al. 2018).

Figure 1 is a diagram of the inferred jet-production process from Panesar et al. (2016), based on their Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) observations. Figure 1(a) shows schematically two neighboring bipoles having adjacent negative-polarity ends on the photosphere, the smaller of the two containing a highly sheared and twisted field of a flux rope that holds a minifilament. The minifilament sits directly over the magnetic neutral line between the positive- and negative-polarity flux patches. As the opposite-polarity flux patches continue to approach each other and cancel (Figure 1(b)), the minifilament field becomes increasingly unstable due to unleashing of the flux rope through slow tether-cutting flux cancellation driven by the converging photospheric flows. The magnetic pressure in the increasingly unleashed flux rope pushes the minifilament upwards, and the field starts to erupt. This results in runaway tether-cutting internal reconnection (Moore & Roumeliotis 1992) at a current sheet within the legs of the field enveloping the erupting minifilament flux rope. As this field erupts outwards it reconnects on its outside with the neighboring far-reaching oppositely directed field lines (“external reconnection”). The low lying red loop in Figure 1(b) represents the JBP that appears at the location where the minifilament originally resided. The external reconnection creates new magnetic connections in two places: closed field lines over the large, left-most bipole (Figure 1(c)) that causes external brightenings to appear in and around the base of the jet in EUV images, and reconnected open or far-reaching field lines (Figure 1(c)). Minifilament plasma escapes out along the reconnected open or far-reaching field to form part of the jet spire.

Zoom In Zoom Out Reset image size

The abovementioned papers have each analyzed only one event or a limited number of jet events, so the triggering and driving mechanisms of coronal jets are not yet fully established. In this paper, we test with a larger sample of non-active-region coronal jets whether flux cancellation and minifilament eruption usually cause coronal jets. To this end, we look for minifilament eruptions in and analyze the magnetic field evolution of 60 randomly selected solar coronal jets in quiet regions and coronal holes. We use Solar Dynamics Observatory (SDO)/AIA EUV animations to follow the jet emission, and use SDO/HMI line-of-sight magnetograms to track the magnetic flux over time at the jet-base region. With the high resolution and high cadence HMI magnetograms we are able to see flux cancellation that might not have been visible in other studies that used magnetograms with lower quality. Our study of 60 solar coronal jets indicates that minifilament eruption is usually the driver and flux cancellation is usually the trigger mechanism of solar coronal jets.

This study uses multithermal (171, 193, 211, and 304 Å) EUV images from SDO/AIA to study the eruption of the minifilament and jet spire. SDO/AIA takes high resolution (06 pixel⁻¹) and high temporal cadence (12 s) full-Sun images in seven EUV wavelengths (Lemen et al. 2012). We primarily used 171 and 193 Å images for this study, as we found pre-jet minifilaments to be best seen in 171 Å (0.6 MK, Lemen et al. 2012) and coronal hole jets to be easily seen in 193 Å (peak temperatures of 1.5 MK, Lemen et al. 2012). We used both 193 Å data and 211 Å (peak temperatures of 2 MK) data to confirm the presence of a jet in coronal holes and the quiet Sun, but used 171 Å data to measure base width and jet lifetime.

To analyze the magnetic field evolution, we used line-of-sight magnetograms from SDO/HMI. SDO/HMI produces high-resolution (05 pixel⁻¹) magnetograms with a temporal cadence of 45 s that allow us to examine closely the photospheric magnetic field around the jet-base region (Schou et al. 2012). We use magnetogram data to track the evolution of the photospheric magnetic flux in the jet-base region from approximately 6 hr prior to jet onset to approximately 1 hr after the jet.

For this study we used JHelioviewer (Müller et al. 2017) to randomly find 60 coronal jets in zoomed-in quiet regions and zoomed-in coronal holes, 30 jets in each type of region. We downloaded SDO/AIA and SDO/HMI for 200″ × 150″ areas surrounding each of the 60 jets from the Joint Science Operations Center cutout service⁶ (JSOC). We downloaded AIA data at a 12 s cadence and HMI data at a 45 s cadence to observe the structure and magnetic evolution of each jet. All of the data were derotated to the middle time of each data set in order to remove the drift from solar rotation. With the derotated data, we selected a smaller field of view focused closely on the jet-base region to perform measurements and to make animations of each jet. All jets and their parameters can be found in Table 1.

3.1. Overview

3.2. Coronal Hole Jets

We randomly selected 30 EUV coronal hole jets from 2016 September to 2017 May (see Table 1). We present two detailed examples of these coronal hole jets in Sections 3.2.1 and 3.2.2.

3.2.1. Coronal Hole Jet on 2017 January 2 (Event 39)

A coronal hole jet was observed starting at approximately 16:09 on 2017 January 2 (see Table 1 for details). A pre-jet minifilament was present in the jet-base region (Figure 2(a), red arrow; see the corresponding animation) residing above the magnetic neutral line between majority (black) and minority (white) flux patches (Figures 2(b), (f)). This minifilament is visible in Figure 2(a), almost an hour prior to the jet. The JBP (Figure 2(b)) then appeared at the site of the minifilament as the minifilament slowly began to rise, and then erupted into the jet spire (Figures 2(c), (d)). This progression appears analogous to that of typical filament eruptions, where the flare arcade grows over the neutral line in the wake of the erupting filament. The JBP is a miniature flare arcade made by internal reconnection of the legs of the erupting minifilament field. The duration of the jet spire was 11 ± 3 minutes, and the jet had a base width of approximately 12,600 km. We also observed new loop brightenings extending to majority flux patches (e.g., Figure 2(d), green arrow in Figure 2(h)).

Download figure:

Video Standard image High-resolution image

Download figure:

Video Standard image High-resolution image

Figures 2(e)–(h) shows the HMI magnetogram data for the jet observed on 2017 January 2. Initially, about 5 hr prior to the jet, the positive (white) and negative (black) flux patches were far apart (see the corresponding animation available). With time, the flux patches approached each other and eventually started canceling at the magnetic neutral line between them. This flux cancellation occurred continuously for 5 hr, until the minifilament erupted, producing the jet. We measured the isolated negative majority flux for 5.5 hr in the region indicated by the white box in Figure 2 panels (e)–(h). Figure 3 shows the total negative flux in the box as a function of time. It shows an overall decrease over the 5 hr prior to the jet, providing clear evidence of flux cancellation in the jet-base region leading to the eruption of the minifilament and jet onset (red line in Figure 3). This flux evolution is not monotonic, however, as there is a hump in the curve between ∼13:00 and 14:00 UT. Inspection of magnetograms over this period indicates that the increase is due to coalescence of negative flux in the jet-base region over this time (between Figures 2(e) and (f)), resulting in weak negative flux rising above the noise level. Even so, there is still an overall downward trend in the negative-polarity flux plot, consistent with cancellation dominating the field changes over the period.

Zoom In Zoom Out Reset image size

3.2.2. Coronal Hole Jet on 2017 March 22 (Event 11)

A coronal hole jet was observed on 2017 March 22 (see Table 1 for measurements). Similar to the jet discussed in Section 3.2.1, we observed a minifilament over the magnetic neutral line between the majority-polarity flux patch (negative) and the minority-polarity flux patch (positive, Figure 4). However, in this case, two jets occurred in close proximity from different magnetic neutral lines. We measured the larger jet, but shortly after it occurred, at 4:56 UT, a smaller jet occurred at 5:07 UT. We did not measure the smaller, second jet because its proximity to the larger jet made it difficult to obtain accurate measurements. The pre-jet minifilament (preceding the first jet) was visible at least 1 hr prior to the jet spire occurring (see the corresponding animation). At ∼04:50 UT, the minifilament began to rise (Figure 4(a)) and the JBP appeared (Figures 4(b), (f)) at the location (neutral line) of the minifilament before the eruption. The growth duration of the spire was 15 ± 2 minutes. One minute prior to formation of the jet spire, we measured the jet base to be 8700 km wide.

Download figure:

Video Standard image High-resolution image

Download figure:

Video Standard image High-resolution image

Figure 5 shows the measured positive minority flux from ∼5 hr prior to the jet at 04:56UT, to 30 minutes after jet onset. The measured positive flux is located in the jet-base region indicated by the white box in Figure 4 panels (e)–(h). As with the jet discussed in Section 3.2.1, on average there is an overall decrease in measured flux leading up to the jet (red line, Figure 5) that occurs as the opposite-polarity flux patches approach the magnetic neutral line and cancel.

Zoom In Zoom Out Reset image size

3.3. Quiet Region Jets

We observed 30 EUV coronal jets in quiet regions from 2016 September to 2017 May (see Table 1). We present two examples of these quiet region jets in Sections 3.3.1 and 3.3.2.

3.3.1. Quiet Region Jet on 2016 November 25 (Event 51)

A quiet region coronal jet was observed on 2016 November 25. In this jet region there are two jet-producing minifilament eruptions from the same neutral line: a partial minifilament eruption producing a weak jet at 06:21 UT at 06:21 UT (and there is an even weaker precursor to this jet at 06:00 UT), which we have not listed in Table 1; and a complete minifilament eruption producing a final jet at 07:11 UT (see the animated Figure 6). As with the previously discussed events, pre-jet cancellation occurred at the base of both of these jets, and the continued cancellation led to both jets. Similar homologous jets from the same neutral line have also been observed by Panesar et al. (2017) in their study of quiet region jets (and in Sterling et al. 2017 in active region jets). One minute prior to the final jet onset, the width of the jet-base region was approximately 8100 km. A minifilament starts to rise at 07:01:22 UT (Figure 6(a) and the corresponding animation). At 07:12 UT we observed a JBP (Figure 6(c), white arrow) at the site of minifilament formation as well as an external brightening at a nearby patch of positive majority flux (Figure 6(c), green arrow in 6(g)). After the rise of the minifilament, the jet spire started to extend and grew for 4 ± 2 minutes (Figure 6(d)). These observations follow behavior similar to that of the coronal hole jet discussed in Section 3.2.1.

Download figure:

Video Standard image High-resolution image

Download figure:

Video Standard image High-resolution image

We measured the minority (negative polarity in this case) flux prior to and during the jet in the region indicated by the white box in Figure 6 panels (e)–(h). We present this measured flux as a function of time in Figure 7. Starting at 03:30 UT, about 4 hr prior to the jet, we see a continuous decrease in the overall negative flux in the jet-base region leading up to the jet at 07:09 UT. Prior to this time, flux was coalescing, which caused the increase in measured flux at 3:00 UT (Figure 7). Similar to the jets discussed in Section 3.2, this decrease in flux occurs as opposite-polarity flux patches were approaching the magnetic neutral line and canceling with each other.

Zoom In Zoom Out Reset image size

3.3.2. Quiet Region Jet on 2017 April 10 (Event 5)

We observed a quiet region coronal jet at 16:56 UT on 2017 April 10. As with the jets discussed in Section 3.2, we observed a minifilament (Figure 8(a)) that lifted off from a magnetic neutral line, followed by the appearance of a JBP (Figure 8(b)) at that same location. During the eruption onset, the minifilament slowly rose to produce the jet spire (Figure 8(c)). One minute before the appearance of the jet spire, the jet-base region was approximately 8300 km in diameter. Upon eruption of the minifilament, the jet spire grew for 10 ± 1 minutes. In the AIA 171 images we see some smaller flows starting at 16:39 UT that precede the main jet at 16:56 UT. These smaller flows are accompanied by brightenings in the jet-base region, leading us to believe that they are homologous weaker jets resulting from the same magnetic neutral line (like those discussed in Section 3.3.1).

Download figure:

Video Standard image High-resolution image

Download figure:

Video Standard image High-resolution image

We measured the negative minority flux in the jet-base region and plotted it as a function of time in Figure 9. The flux was measured in the region bounded by the white box in Figures 8(e)–(h). Starting two hours prior to the jet, the positive- and negative-polarity flux patches approached the magnetic neutral line and canceled. This behavior is reflected in the continuous overall decrease of negative flux that we see in Figure 9 leading up to the jet at 16:56 UT. Prior to ∼15:30 UT, negative flux coalesced in the measured region, causing the measurable increase in flux visible at 15:30 UT in Figure 9.

Zoom In Zoom Out Reset image size

3.4. Results for 60 Jets

We have examined 60 solar coronal jets in quiet regions and coronal holes with EUV images from SDO/AIA and magnetogram data from SDO/HMI. In the large majority of instances, the jet was preceded by a minifilament eruption prepared and triggered by continuous flux cancellation at the magnetic neutral line within the jet base. This is consistent with the findings and proposed picture by Sterling et al. (2015) and Panesar et al. (2016). Additionally, for our 60-jet sample we find a mean jet spire-growth duration of 9 ± 4 minutes and a mean jet-base width of 8800 ± 3100 km.

Our jet spire-growth-duration measurements are very similar to the EUV jets in Panesar et al. (2016; ∼12 minutes) and the X-ray jets of Savcheva et al. (2007; ∼10 minutes). Our jet-base width is nearly the same as the jet widths of Savcheva et al. (2007; 8000 km) and the mean erupting minifilament size of Sterling et al. (2015). However, our mean jet-base width is roughly half that of Panesar et al. (2016; ∼17,000 km) plausibly because Panesar et al. (2016) found their jets from viewing JHelioviewer animations of full-disk AIA 171 Å coronal EUV images, whereas we found our jets by viewing JHelioviewer animations of zoomed-in partial disk AIA 171 Å coronal EUV images of quiet regions and coronal holes. That is, the Panesar et al. (2016) study may have preferentially selected jets of larger size scale compared to the jets selected in this study.

The large majority of minifilament eruptions we observed are in agreement with Sterling et al. (2015) and Panesar et al. (2016), as shown in Figure 1. In each case, prior to the jet, a minifilament resides between opposite-polarity flux patches at the magnetic neutral line. As these flux patches approach the magnetic neutral line they continuously cancel and the minifilament eventually begins to erupt outwards. This eruption results in runaway internal reconnection within the erupting minifilament-carrying field as well as runaway external reconnection with the encountered outward-reaching field. The runaway internal reconnection creates the JBP that we observe in Figures 2(b), 4(c), 6(c), and 8(b). The runaway external reconnection produces new field lines, including the far-reaching field line along which the jet spire forms (Figures 2(d), 4(d), 6(d), and 8(d)). The runaway external reconnection also produces external brightenings between the minority-polarity patch of the pair of canceling flux patches and a nearby patch of majority-polarity flux (Figures 1(c), 2(b), and 6(c)).

In conclusion, we report observations of 60 random on-disk coronal jets in quiet regions and coronal holes. From our observations, we find that at least for the great majority of these jets, minifilament eruption is the driver and magnetic flux cancellation is the trigger mechanism for coronal hole and quiet region coronal jets.

R.A.M. gives special thanks to Dr. Navdeep Panesar for her time and mentorship throughout the research process, and to the NASA Marshall Space Flight Center, the Center for Space Plasma and Aeronomic Research at the University of Alabama in Huntsville, and the National Science foundation for supporting this study. R.A.M. was supported by the Research Experience for Undergraduates (REU) program at UAH and MSFC over summer 2017, which was funded by the National Science Foundation under grant No. AGS-1460767. N.K.P.acknowledges current support from NASA’s SDO/AIA contract (NNG04EA00C) to LMSAL and previous support by NPP at the NASA/MSFC, administered by USRA under contract with NASA. A.C.S. and R.L.M. acknowledge the support from the NASA HGI program, and by the Hinode Project. We acknowledge the use of SDO data. AIA is an instrument onboard the Solar Dynamics Observatory, a mission for NASA’s Living With a Star program.