Navdeep K. Panesar1,2, Alphonse C. Sterling1, Ronald L. Moore1,3, Sanjiv K. Tiwari4,5, Bart De Pontieu4,6,7, and Aimee A. Norton2
1. NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
2. W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305, USA
3. Center for Space Plasma and Aeronomic Research (CSPAR), UAH, Huntsville, AL 35805, USA
4. Lockheed Martin Solar and Astrophysics Laboratory, Org. A021S, Bldg. 252, 3251 Hanover St., Palo Alto, CA 94304, USA
5. Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA
6. Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo, Norway
7. Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo, Norway
Solar coronal jets are magnetically channeled narrow eruptions that shoot up into the corona. Recent observations show that coronal jets are frequently driven by the eruption of mini-filaments (base width ~18,000km), and are analogous to the typical filament eruptions that make CMEs. In our recent studies of 23 on-disk coronal jets (in quiet regions and coronal holes), we found that the mini-filament magnetic field is built and triggered by flux cancelation at the neutral line underneath the mini-filament[2, 3, 4]. Here we report that flux cancelation triggers the smaller-scale coronal jets, known as jetlets, (base width ~4,000km), that erupt from edges of the magnetic network.
We randomly selected an on-disk coronal hole magnetic network region covered by IRIS on 2016 March 19. We found five jetlets at five different neutral lines. Simultaneously, we studied the same network region using AIA 171Å images and found five more jetlets within the IRIS FOV but outside the IRIS observation time (all 10 jetlets and their measured parameters are listed in Table 1 of Ref. ). We examined the magnetic setting of these 10 on-disk jetlets using IRIS data and AIA and HMI data from SDO.
Figure 1| IRIS Si IV Slit-jaw images of jetlet-C3 of Ref. . The white arrows point to Si IV brightenings at the base of 171 Å flare loops. The green arrows point to the locations of the feet of the bright loops in 171 Å images.
Figure 1 shows an example jetlet. Figures 2a and 2b display the photospheric magnetic field of the jetlet-base region. We observed three homologous jetlets from the same neutral line (the times marked in Figure 2c). During the example jetlet onset, at 19:15 UT, we observed brightenings at its base (Figures 1a and 1b). Concurrently, brightenings also appear in AIA 171Å images (Figure 3 of Ref. ). These base brightenings appear to include a miniature version of a jet bright point. These jetlets originate from the neutral line between a leading-polarity (positive) network flux lane and a merging following-polarity (negative) flux patch (Figure 2).
Figure 2| HMI magnetograms of jetlet-C3 of Ref. . Panels (a)–(b), show the magnetic field near the base of jetlets. The orange box in Figure 2a shows the region measured for the magnetic flux (negative flux) time plot in Figure 2c. The blue line in Figure 2c is a least-square fit from before to after the jetlet. The dashed lines show the three jetlet onset times.
Figure 2c shows the negative-flux plot of the jetlet-base region over four hours. We only measured the negative-polarity flux patch because it is well isolated within the orange box (Figure 2a). The negative flux decreases, which is a clear indication to flux cancelation at the base of the jetlet. We surmise that the continuous flux cancelation over ∼6 hr prepares and eventually triggers each of the three sequential eruptions. After the first jetlet, a significant amount of flux still remains, and that flux continues to cancel before the second homologous eruption and further cancelation apparently prepares and triggers a third homologous eruption (the negative-flux bump at 19:17 UT is from a coalescence of weak flux grains unrelated to the jetlet). The average rate of flux decrease using the best-fit line in Figure 2c and is ∼1.7 × 1018 Mx hr-1. Mini-flaments in sequential larger coronal jets have also been observed to erupt and reform/reappear at the same neutral line due to flux cancelation.
All 10 of our jetlets occur at the edges of magnetic network flux lanes, all but one of them at a site of apparent magnetic flux cancelation. The average flux-cancelation rate for our nine jetlets having obvious cancelation is ∼1.5 × 1018 Mx hr-1, which is similar to that for larger coronal jets in quiet regions (∼1.5 × 1018 Mx hr-1). and coronal holes (∼0.6 × 1018 Mx hr-1), whereas active-region coronal jets have higher flux cancelation rates ∼1.5 × 1019 Mx hr-1.
In summary, our observations of 10 jetlets suggest that flux cancelation is usually a necessary condition for the buildup and triggering of UV/EUV coronal network jetlets and that they usually stem from the edges of magnetic network flux lanes. Network jetlets are therefore plausibly small-scale versions of the larger coronal jets, and of still-larger CME events.
For more details of this study see Ref. .
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