Radio and X-ray Observations of Short-lived Episodes of Electron Acceleration in a Solar Microflare
by R. Sharma et al.

In a solar flare, the plasma is locally heated and particles are accelerated to energies from a few tens of keV to MeVs. X-ray bremsstrahlung emission and radio gyrosynchrotron emission are highly complementary and provide diagnostics of the timing, location and spectral properties of flare-accelerated electrons in a broad energy range.

Here we present comprehensive observations of multiple individual bursts during a GOES B1.7-class (back-ground subtracted) microflare observed jointly in radio by the VLA, in X-rays by RHESSI, and in the ExtremeUltraviolet (EUV) by the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012). The observations are indicative of multiple co-temporal acceleration episodes during the impulsive phase of a solar microflare. The X-ray and radio burst sources likely originate from separate electron distributions in different magnetic loops.

Figure 1 – Temporal evolution of radio and X-ray emission during the B1.7-class flare on 2012 February 25 (SOL2012-02-25T20:50:34). Top: VLA dynamic spectrum showing the total flux computed from the radio images for each frequency-time pixel in the observation. Each pixel has a size of 4 MHz and 1 s in frequency and time respectively. The second panel shows the frequency averaged VLA spectrum from 1.65 GHz to 2.03 GHz. The inset shows 5 distinct radio bursts marked by letters. The third panel shows X-ray light curves from RHESSI and GOES.

Spatial-temporal Evolution of Radio and X-ray Sources

The radio lightcurves show six distinct short-lived radio bursts (labeled A, B, C, D, E, and F in Figure 1) associated with increased X-ray emission. The flare shows distinct spatial-temporal features at EUV, X-rays and radio wavelengths. The EUV 193 Å images show two separate ribbons (Figure 2A). The radio emission shows six distinct short-lived radio bursts (labeled A, B, C, D, E, and F in Figure 1). During the radio bursts, the observed compact clustering suggests a bright common radio source for all shown radio frequencies (Figure 2B). Both X-ray sources and radio bursts A, B and C appear near the northern ribbon, but they are not co-spatial with each other. The location of these burst sources projected onto the flare ribbon suggests a low coronal origin of the emission. Using a magnetic extrapolation model, the altitude of the radio sources is inferred to be ~2700 km.

Figure 2 – Panel (A): HMI magnetogram at 20:51:45 UT. The red and blue contours are emission observed by AIA 304 Å at 20:51:32 UT for northern and southern ribbons, respectively. The contour level shown is at 18% of the maximum brightness. Panel (B): Evolution of the AIA 94 Å EUV ribbons (black and white image, color table inverted), X-ray RHESSI sources (magenta and blue contours), and VLA radio centroid (crosses) positions. The RHESSI contour levels are at 65%, 75%, 85%, and 95% w.r.t map’s peak. Panel (C): The spectra of the brightness temperature for the three fitted, individual bursts (labeled A, B, and C) are shown in blue, green and red respectively. The solid lines shows the optimum MCMC fit represented in their colors.

RHESSI and Gyro-Synchroton Fits

RHESSI X-ray and VLA gyrosynchrotron fits provide the properties of the accelerated electron spectra, like spectral index, total electron flux, and low-energy cutoff. The RHESSI spectrum was fit between 20:47:00 UT and 20:47:28 UT. Note that radio bursts A, B, and C occurred during this time. A weak nonthermal component is present in the spectrum up to about 20 keV (Figure 3A), with an electron spectral index $\delta$ of 8.6 ± 3.2 and a low-energy cutoff ($E_{low}$ ) of 13.5 keV.

The radio spectra for burst A to C developed a positive slope, indicative of optically thick nonthermal gyrosynchrotron radiation (see, e.g., Dulk 1985). For calculating the model gyrosynchrotron spectrum, the user-friendly and computationally inexpensive fast GS code (Fleishman & Kuznetsov 2010) was used. Further, we adopted the MCMC method described in Chen et al. (2020) for the spectral fitting. Here, we assumed a homogeneous nonthermal source. We treated magnetic field strength ($B$), nonthermal density ($n_{nth}$), thermal density ($n_{th}$), electron spectral index ($\delta$), and low-energy cutoff ($E_{low}$) as free parameters. We note that the low-energy cutoff of the nonthermal electron distribution Elow is smaller than that inferred from the RHESSI X-ray spectrum by a factor of ∼4, while the nonthermal density from the gyrosynchrotron fit is two orders of magnitude higher than the RHESSI estimates. At the same time, the total electron density, and as a result, the total electron flux from the gyrosynchrotron fit, is a factor of 10 to 100 higher than from the X-ray fit. This discrepancy in Elow and nnth is present in burst A through C, a possible indication that the two instruments observe two different electron populations.

Figure 3 – Left: RHESSI spectrum along with the fitted thermal and nonthermal components between 20:47:00 UT and 20:47:28 UT. The black crosses show the RHESSI spectrum, while the green curve shows the sum of the thermal (blue line) and nonthermal (purple line) components. The yellow histogram shows the background spectrum. Middle: nonthermal power as a function of low-energy cutoff for the spectral parameters inferred from the X-ray fit (RHESSI) and from the gyrosynchrotron fits of bursts A to C. The symbols mark the nonthermal power calculated from the observed low-energy cutoff. Right: model electron spectra inferred from X-rays (RHESSI) and radio bursts A to C. The vertical lines give the position of the low-energy cutoff.


Our observations support a scenario with multiple acceleration events, possibly in different magnetic loops, since even though the X-ray emission was observed co-temporally with the radio bursts, the observed spectra are very different. Two factors contribute to this interpretation:
1. The spectral parameters inferred from the X-ray observations are different from the properties inferred from the radio observations to an extent that cannot be explained by uncertainties or by the fact that the X-ray spectrum was time-integrated over the 28 s during which bursts A to C were observed. For a given spectral index δ, total electron flux $F_e$ ($s^{−1}$), and cutoff energy $E_{low}$ in erg, the nonthermal power can be found as $P = (\delta-1)F_e E_{low}/(\delta-2)$. Figure 3B shows the nonthermal power inferred from the X-ray fit is a factor of 10 lower. However, both values lie within the range of nonthermal powers found in a statistical analysis of sub-C class flares by Hannah et al. (2008).
2. Both X-ray and radio sources show a remarkable footpoint asymmetry. Such asymmetries are unfeasible in a standard single reconnecting magnetic loop scenario. A possible explanation would be the presence of different electron distributions in separate magnetic loops.

Overall, the contrasting spectral properties and spatial displacements suggest two distinct electron populations. This microflare study demonstrates that even microflares can exhibit complex characteristics and behaviors.

This nugget is based on the paper by Sharma, R., Battaglia, M., Luo, Y., Chen, B., Yu, S., 2020 ApJ 904 94


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*Full list of authors: Rohit Sharma, Marina Battaglia, Yingjie Luo, Bin Chen and Sijie Yu