The Role of Inverse Compton Scattering in Solar Coronal Hard X-ray and Gamma-ray Sources

Coronal hard X-ray (HXR) and continuum gamma-ray sources associated with the impulsive phase of solar flares have been the subject of renewed interest in recent years. They have been interpreted in terms of thin-target, nonthermal bremsstrahlung emission. This interpretation has led to rather extreme physical requirements in some cases. For example, in one case, essentially all of the electrons in the source must be accelerated to nonthermal energies to account for the coronal HXR source. In other cases, the extremely hard photon spectra of the coronal continuum gamma-ray emission suggest that the low energy cutoff of the electron energy distribution lies in the MeV energy range. Here we consider the role of inverse Compton scattering (ICS) as an alternate emission mechanism in both the ultra- and mildly relativistic regimes. It is known that relativistic electrons are produced during powerful flares; these are capable of up-scattering soft photospheric photons to HXR and gamma-ray energies. Previously overlooked is the fact that mildly relativistic electrons, generally produced in much greater numbers in flares of all sizes, can up-scatter EUV/SXR photons to HXR energies. We also explore ICS on anisotropic electron distributions and show that the resulting emission can be significantly enhanced over an isotropic electron distribution for favorable viewing geometries. We briefly review results from bremsstrahlung emission and reconsider circumstances under which nonthermal bremsstrahlung or ICS would be favored. Finally, we consider a selection of coronal HXR and gamma-ray events and find that in some cases the ICS is a viable alternative emission mechanism

Combined STEREO/RHESSI study of CME acceleration and particle acceleration in solar flares

Using the potential of two unprecedented missions, STEREO and RHESSI, we study three well observed fast CMEs that occurred close to the limb together with their associated high energy flare emissions in terms of RHESSI HXR spectra and flux evolution. From STEREO/EUVI and STEREO/COR1 data the full CME kinematics of the impulsive acceleration phase up to 4 Rs is measured with a high time cadence of less equal 2.5 min. For deriving CME velocity and acceleration we apply and test a new algorithm based on regularization methods. The CME maximum acceleration is achieved at heights h < 0.4 Rs, the peak velocity at h < 2.1 Rs (in one case as small as 0.5 Rs). We find that the CME acceleration profile and the flare energy release as evidenced in the RHESSI hard X-ray flux evolve in a synchronized manner. These results support the standard flare/CME model which is characterized by a feed-back relationship between the large-scale CME acceleration process and the energy release in the associated flare.Comment: accepted for Ap

Drift-Kinetic Modeling of Particle Acceleration and Transport in Solar Flares

Based on the drift-kinetic theory, we develop a model for particle acceleration and transport in solar flares. The model describes the evolution of the particle distribution function by means of a numerical simulation of the drift-kinetic Vlasov equation, which allows us to directly compare simulation results with observations within an actual parameter range of the solar corona. Using this model, we investigate the time evolution of the electron distribution in a flaring region. The simulation identifies two dominant mechanisms of electron acceleration. One is the betatron acceleration at the top of closed loops, which enhances the electron velocity perpendicular to the magnetic field line. The other is the inertia drift acceleration in open magnetic field lines, which produces antisunward electrons. The resulting velocity space distribution significantly deviates from an isotropic distribution. The former acceleration can be a generation mechanism of electrons that radiate loop-top nonthermal emissions, and the latter be of escaping electrons from the Sun that should be observed by in-situ measurements in interplanetary space and resulting radio bursts through plasma instabilities.Comment: 32 Pages, 11 figures, accepted by Ap

Solar wind density turbulence and solar flare electron transport from the Sun to the Earth

Solar flare accelerated electron beams propagating away from the Sun can interact with the turbulent interplanetary media, producing plasma waves and type III radio emission. These electron beams are detected near the Earth with a double power-law energy spectrum. We simulate electron beam propagation from the Sun to the Earth in the weak turbulent regime taking into account the self-consistent generation of plasma waves and subsequent wave interaction with density fluctuations from low frequency MHD turbulence. The rate at which plasma waves are induced by an unstable electron beam is reduced by background density fluctuations, most acutely when fluctuations have large amplitudes or small wavelengths. This suppression of plasma waves alters the wave distribution which changes the electron beam transport. Assuming a 5/3 Kolmogorov-type power density spectrum of fluctuations often observed near the Earth, we investigate the corresponding energy spectrum of the electron beam after it has propagated 1 AU. We find a direct correlation between the spectrum of the double power-law below the break energy and the turbulent intensity of the background plasma. For an initial spectral index of 3.5, we find a range of spectra below the break energy between 1.6-2.1, with higher levels of turbulence corresponding to higher spectral indices.Comment: 9 pages, 9 figures, to be published in Ap

The Solar X-ray Limb

We describe a new technique to measure the height of the X-ray limb with observations from occulted X-ray flare sources as observed by the RHESSI (the Reuven Ramaty High-Energy Spectroscopic Imager) satellite. This method has model dependencies different from those present in traditional observations at optical wavelengths, which depend upon detailed modeling involving radiative transfer in a medium with complicated geometry and flows. It thus provides an independent and more rigorous measurement of the "true" solar radius, meaning that of the mass distribution. RHESSI's measurement makes use of the flare X-ray source's spatial Fourier components (the visibilities), which are sensitive to the presence of the sharp edge at the lower boundary of the occulted source. We have found a suitable flare event for analysis, SOL2011-10-20T03:25 (M1.7), and report a first result from this novel technique here. Using a 4-minute integration over the 3-25 keV photon energy range, we find $R_{\mathrm{X-ray}} = 960.11\ \pm\ 0.15 \pm 0.29$ arcsec, at 1 AU, where the uncertainties include statistical uncertainties from the method and a systematic error. The standard VAL-C model predicts a value of 959.94 arcsec, about 1$\sigma$ below our value.Comment: 12 pages, 5 figures, accepted for publication in Ap

Onsets and spectra of impulsive solar energetic electron events observed near the Earth

Impulsive solar energetic electrons are often observed in the interplanetary space near the Earth and have an attractive diagnostic potential for poorly understood solar flare acceleration processes. We investigate the transport of solar flare energetic electrons in the heliospheric plasma to understand the role of transport to the observed onset and spectral properties of the impulsive solar electron events. The propagation of energetic electrons in solar wind plasma is simulated from the acceleration region at the Sun to the Earth, taking into account self-consistent generation and absorption of electrostatic electron plasma (Langmuir) waves, effects of non-uniform plasma, collisions and Landau damping. The simulations suggest that the beam-driven plasma turbulence and the effects of solar wind density inhomogeneity play a crucial role and lead to the appearance of a) spectral break for a single power-law injected electron spectrum, with the spectrum flatter below the break, b) apparent early onset of low-energy electron injection, c) the apparent late maximum of low-energy electron injection. We show that the observed onsets, spectral flattening at low energies, and formation of a break energy at tens of keV is the direct manifestation of wave-particle interactions in non-uniform plasma of a single accelerated electron population with an initial power-law spectrum.Comment: 4 pages, 4 figures, to be published in ApJ Letter