Bouncing motions of fast electrons using Nobeyama Radioheliograph

Nugget
Number:	459
1st Author:	Keitarou MATSUMOTO
2nd Author:
Published:	November 6, 2023
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Previous Nugget:	Impact of nanoflare heating in the lower solar atmosphere

Introduction

When a solar flare occurs, many particles are accelerated. It is noteworthy that approximately fifty percent of the total energy discharged by solar flares may appear in the kinetic energy of fast electrons. However we do not observe the accelerated electrons directly, and remain unclear about the acceleration mechanism that produces them. Several models for the acceleration mechanism have been proposed by various authors, and these may differ in terms of the pitch-angle distribution of the electrons they energize.

The non-thermal radiations from the accelerated electrons are important to understand the acceleration mechanisms, and fortunately solar flares emit strong non-thermal microwave gyro synchrotron radiation. This depends strongly on the pitch angle of the electrons. That's one of the reasons why non-thermal radiation is essential to understand the acceleration mechanism.

The Nobeyama Radio Heliograph (NoRH) observed solar flares at two microwave frequencies (17 and 34GHz). The time resolution was 0.1 seconds, meaning that it could capture the short time scales of the solar flares, even following the motion of the electrons. An early investigation (Ref. [1]) did study the high-speed propagation of the microwave sources using NoRH and discussed the pitch-angle distributions of the accelerated electrons. However, before the launch of the SDO satellite with its high-resolution EUV imaging, this work could not be put in the context of the coronal magnetic field properly. We have now analyzed another flare event from the SDO era and discuss the physics of this flare taking advantage of the better time resolution.

Observations

We analyze the GOES M8.7-class flare SOL2014-10-22, from the very productive NOAA Active Region (AR) 12192. The analysis uses NoRH, the Solar Dynamics Observatory (SDO), RHESSI, the Fermi Gamma-ray Burst Monitor (GBM), and magnetic-field extrapolations via an NLFFF model as shown in Figure 1.

Figure 1: (a) A 17 GHz image observed with NoRH at 01:45:57.984 UT, with color showing a logarithmic scale of brightness temperature. (b) SDO/HMI data at 01:34:15 UT. The same image as (a) is shown as black contours. (c) SDO/AIA 94Å image (logarithmic scale) at 01:57:120 UT. The microwave image from (a) is shown as red contours. (d) The magnetic field via the NLFFF algorithm, with blue lines showing saturated loops from (c) and white lines others. (e) AIA 1600Å image at 01:46:06.710 UT. The red and green contours are RHESSI data at 15-25 and 30-50 keV respectively, with blue showing the NoRH 17GHz data. (f) Punch diagram of microwave loop and hard X-ray loop. The green and orange regions represent the footpoints of the loop radiating hard X-rays and microwaves, respectively.

Figure 2 shows the slit along the coronal loop interconnecting Points A and D and time variation of the brightness temperature at 17 GHz along the slit. Comparing with Figure 1 (c) and (d), we conclude that Points A and D are the footpoints of the same coronal loop shown as blue lines in Figure 1 (d).

Figure 2: Analysis results for NoRH 17 GHz. (a) The purple line denotes the slit along the apparent microwave propagation. (b) The time variation of the brightness temperature at each point along the slit. The base time of Figure 3(b) is 01:45:54.084 UT (t=0). The dotted line may show a high speed microwave propagation from Point A to Point B as discussed in Ref. [2].

Periodic enhancement of the microwave radiation at the footpoints

The right panels in Figure 3 show cyclical brightenings observed at both footpoints (Points A and D). The peak times were 4.0 s (1A) and 6.1 s (2A) at Point A. For Point D, the peak times were 4.6 s (1D) and 6.7 s (2D). We suggest that the injection of accelerated electrons occurred somewhere in this loop.

Using the information on the coronal magnetic field of the loop in the NLFFF model and some assumptions, we conclude that this periodic enhancement may be caused due to the bouncing motion of the accelerated electrons at the footpoints.

Figure 3: For the all panels, time starts at 01:45:54.084 UT. The left five panels show the time variation of brightness temperature and the time variation of hard X-ray photon counts detected by the Fermi/GBM sun-directed detector in the energy range from 49.0 keV to 101.4 keV. The right four panels show the Brightness Temperature Difference (BTD) from the Point A to Point D in (Ref. [2]).

Conclusions

We discuss the possibility for explaining the enhancement of the microwave radiation at the footpoints after considering the pitch angle of the electrons and the size of the loss cone at the footpoints. This analysis improves upon that of Ref. [1] owing to the availability of SDO observations and the use of an NLFFF model. It is likely that the bouncing motion of the accelerated electrons can explain this periodic enhancement. Please see Ref. [2] for full detail.

References

[1] "Microwave observations of the rapid propagation of nonthermal sources in a solar flare by the nobeyama radioheliograph"

[2] "Characteristics of the accelerated electrons moving along the loop derived from cyclical microwave brightenings at the footpoints"