Particle Acceleration in Two Coronal Jets

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	and discussed the particle acceleration locations in those events.		and discussed the particle acceleration locations in those events.
-	[[File:445f1.png|800px|thumb|center|<b>Figure 1:</b> <i>	+	[[File:445f1.png|700px|thumb|center|<b>Figure 1:</b> <i>
	RHESSI contours in 3-12 keV overlaid on the AIA 94 &Acirc; images.		RHESSI contours in 3-12 keV overlaid on the AIA 94 &Acirc; images.
	Panel (a): an image of the earlier jet. Panels (b)-(j): time slices		Panel (a): an image of the earlier jet. Panels (b)-(j): time slices
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	temporal evolution of the RHESSI HXR sources.		temporal evolution of the RHESSI HXR sources.
-	[[File:445f2.png|800px|thumb|center|<b>Figure 2:</b> <i>	+	[[File:445f2.png|400px|thumb|center|<b>Figure 2:</b> <i>
	Comparison of the AIA-alone DEM and the joint DEM for the		Comparison of the AIA-alone DEM and the joint DEM for the
	region at the top of the earlier jet. The bottom panel shows the		region at the top of the earlier jet. The bottom panel shows the
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	base.		base.
-	[[File:445f3.png|800px|thumb|center|<b>Figure 3:</b> <i>	+	[[File:445f3.png|500px|thumb|center|<b>Figure 3:</b> <i>
	he relation between collisional stopping distances and		he relation between collisional stopping distances and
	ambient plasma densities for electrons of certain energies. Red		ambient plasma densities for electrons of certain energies. Red

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Revision as of 09:56, 18 February 2023

Nugget
Number:	445
1st Author:	Yixiang ZHANG
2nd Author:
Published:	February 27, 2023
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Contents 1 Introduction 2 Differential emission measure analysis 3 Accelerated electron distributions 4 Particle acceleration locations 5 Conclusion 6 References

Introduction

Solar coronal jets occur ubiquitously and provide a mechanism for plasma and particles to enter interplanetary space. Many coronal jets are associated with hard X-ray (HXR) emissions, which are usually observed near the jet base where magnetic reconnection may occur. However, for two recurrent jets on November 13, 2014 (SOL2014-11-13T17:20 and SOL2014-11-13T20:47), HXR emissions were observed at several locations, including near the far end of the jet spire (i.e. the jet “top”) (Figure 1). In our recent work (Ref. [1]), we investigated those two events in great detail using data from the space observatories SDO/AIA, RHESSI, Hinode/XRT, and IRIS, and discussed the particle acceleration locations in those events.

Differential emission measure analysis

The observations characterize these events by determining the temperature distributions of the sources. This means a "differential emission measure (DEM)" analysis for the four hard X-ray (HXR) sources using two different data selections: (a) EUV images (AIA) only; and (b) a combination of the same set of AIA data, the RHESSI HXR measurements in 4-5, 5-6, and 6-7 keV bands, and soft (few keV) X-ray images from XRT if available. An example of the DEM solutions is plotted in Figure 2. We noticed that the AIA-only DEM failed to predict the HXR spectrum well. Specifically, the AIA-predicted HXR fluxes were consistently lower than the RHESSI fluxes and had much stronger line emissions in the 6-7 keV energy bin over the continuum as compared to the actual RHESSI observation. To obtain a DEM solution that could successfully predict both the HXR continuum and the line feature simultaneously, we found that a cross-calibration factor of ~3.5 between AIA and RHESSI was required. After applying that factor, the HXR spectrum predicted by the joint DEM had a much better agreement with the RHESSI measurements. In addition, by examining the DEM maps at different times, the evolution of the hot plasma showed consistent results with the location and temporal evolution of the RHESSI HXR sources.

Accelerated electron distributions

The spectra for the four HXR sources could all be fitted well with an emission model combining an isothermal component, plus bremsstrahlung from weakly relativistic electrons. Interestingly, the power-law spectral distributions required for the non-thermal electrons were similar, and needed low-energy cutoffs at around 9 keV. This suggests that the jet physics produces only mild particle acceleration. However, the non-thermal energy in those accelerated electrons reaches up to 1030 erg for each HXR source, which is more than one order of magnitude higher than the thermal energy of the source and the kinetic/gravitational energy of the jet.

Particle acceleration locations

Since the HXR sources share similar electron distributions, one may ask: were the HXR emissions in the later event produced by the same population of accelerated electrons that traveled to different locations, or were they produced by different groups of accelerated electrons individually? To answer this question, we calculated the collisional stopping distance for non-thermal electrons as a function of plasma density (Figure 3). For these HXR sources, the average electron energies are about 10 keV and the stopping distances are 10-30 arcsec. Furthermore, even along the jet body, the stopping distances are still 20-30 arcsec, a few times smaller than the distance from the jet base to the jet top (80-90 arcsec). Therefore, the HXR source at the top of each jet was likely produced by electrons that were accelerated very close to the source, rather than electrons that traveled far from the primary energy-release site at the jet base.

Conclusion

We have reported observations of coronal HXR emissions near the jet top for two recurrent jet events, in addition to the typical HXR source observed near the jet base. We found that those different HXR sources were likely to have been produced by groups of electrons that were accelerated separately near each source location. This suggests that there could be more than one particle acceleration site in a single jet event.

References

[1] [https://ui.adsabs.harvard.edu/abs/2023ApJ...943..180Z "Observations of Magnetic Reconnection and Particle Acceleration Locations in Solar Coronal Jets"]