Abstract
Abstract The origin of hard X-rays and γ-rays emitted from the solar atmosphere during occulted solar flares is still debated. The hard X-ray emissions could come from flaring loop tops rising above the limb or coronal mass ejection shock waves, two by-products of energetic solar storms. For the shock scenario to work, accelerated particles must be released on magnetic field lines rooted on the visible disk and precipitate. We present a new Monte Carlo code that computes particle acceleration at shocks propagating along large coronal magnetic loops. A first implementation of the model is carried out for the 2014 September 1 event, and the modeled electron spectra are compared with those inferred from Fermi Gamma-ray Burst Monitor (GBM) measurements. When particle diffusion processes are invoked, our model can reproduce the hard electron spectra measured by GBM nearly 10 minutes after the estimated on-disk hard X-rays appear to have ceased from the flare site.
Highlights
Krucker et al (1999) has presented a good correlation between electron beams detected by Wind and extreme ultraviolet (EUV) waves observed by the Solar and Heliospheric Observatory (SoHO) spacecraft
In order to explain the origin of the hard X-ray emissions measured by Fermi Gamma-ray Burst Monitor (GBM) on 2014 September 1 we address a number of fundamental questions: (1) How efficiently are electrons accelerated by the mechanism of Shock-Drift Acceleration (SDA) during the event? (2) If SDA is insufficient, what other mechanisms or combination of mechanisms could energise electrons to several hundred keVs or even tens of MeVs? (3) If we assume that electrons are accelerated to high energies in the corona, what fraction of these particles can reach the solar surface to produce non-thermal emissions?
Our model roughly reproduces the very hard electron spectrum and the number of electrons inferred from Fermi GBM
Summary
By means of detailed 3-D coronal modeling Plotnikov et al (2017) showed that the onset of the hard-X ray and γ-ray emissions occurred when the CME-driven coronal shock became magnetically connected to the visible disk. This suggested that electrons and protons accelerated at the coronal shock had a means to propagate toward the visible disk and impact the chromosphere to produce high-energy radiation. For this event, the shock is a strong candidate for particle acceleration. The flaring site was on the far side of the Sun as viewed from Earth (W132◦)
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