1,120 research outputs found
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 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 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
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