936 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
Suppression of energetic electron transport in flares by double layers
During flares and coronal mass ejections, energetic electrons from coronal
sources typically have very long lifetimes compared to the transit times across
the systems, suggesting confinement in the source region. Particle-in-cell
simulations are carried out to explore the mechanisms of energetic electron
transport from the corona to the chromosphere and possible confinement. We set
up an initial system of pre-accelerated hot electrons in contact with ambient
cold electrons along the local magnetic field, and let it evolve over time.
Suppression of transport by a nonlinear, highly localized electrostatic
electric field (in the form of a double layer) is observed after a short phase
of free-streaming by hot electrons. The double layer (DL) emerges at the
contact of the two electron populations. It is driven by an ion-electron
streaming instability due to the drift of the back-streaming return current
electrons interacting with the ions. The DL grows over time and supports a
significant drop in temperature and hence reduces heat flux between the two
regions that is sustained for the duration of the simulation. This study shows
transport suppression begins when the energetic electrons start to propagate
away from a coronal acceleration site. It also implies confinement of energetic
electrons with kinetic energies less than the electrostatic energy of the DL
for the DL lifetime, which is much longer than the electron transit time
through the source region
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
Electron Acceleration by Multi-Island Coalescence
Energetic electrons of up to tens of MeV are created during explosive
phenomena in the solar corona. While many theoretical models consider magnetic
reconnection as a possible way of generating energetic electrons, the precise
roles of magnetic reconnection during acceleration and heating of electrons
still remain unclear. Here we show from 2D particle-in-cell simulations that
coalescence of magnetic islands that naturally form as a consequence of tearing
mode instability and associated magnetic reconnection leads to efficient
energization of electrons. The key process is the secondary magnetic
reconnection at the merging points, or the `anti-reconnection', which is, in a
sense, driven by the converging outflows from the initial magnetic reconnection
regions. By following the trajectories of the most energetic electrons, we
found a variety of different acceleration mechanisms but the energization at
the anti-reconnection is found to be the most important process. We discuss
possible applications to the energetic electrons observed in the solar flares.
We anticipate our results to be a starting point for more sophisticated models
of particle acceleration during the explosive energy release phenomena.Comment: 14 pages, 12 figures (degraded figure quality), 1 table. Accepted for
publication in ApJ
Charge-exchange limits on low-energy α-particle fluxes in solar flares
This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Lyα line of He II at 304 Å, as originally suggested for hydrogen by Orrall and Zirker. Via this mechanism a primary α particle that penetrates into the neutral chromosphere can pick up an atomic electron and emit in the He II bound-bound spectrum before it stops. The Extreme-ultraviolet Variability Experiment on board the Solar Dynamics Observatory gives us our first chance to search for this effect systematically. The Orrall-Zirker mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV nucleon<sup>–1</sup>. We study 10 events in total, including the γ-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles. The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broadband signatures, which could correspond to continua from other origins, but these do not have the spectral signatures expected from the Orrall-Zirker mechanism
Energy Distribution of Micro-events in the Quiet Solar Corona
Recent imaging observations of EUV line emissions have shown evidence for
frequent flare-like events in a majority of the pixels in quiet regions of the
solar corona. The changes in coronal emission measure indicate impulsive
heating of new material to coronal temperatures. These heating or evaporation
events are candidate signatures of "nanoflares" or "microflares" proposed to
interpret the high temperature and the very existence of the corona. The energy
distribution of these micro-events reported in the literature differ widely,
and so do the estimates of their total energy input into the corona. Here we
analyze the assumptions of the different methods, compare them by using the
same data set and discuss their results.
We also estimate the different forms of energy input and output, keeping in
mind that the observed brightenings are most likely secondary phenomena. A
rough estimate of the energy input observed by EIT on the SoHO satellite is of
the order of 10% of the total radiative output in the same region. It is
considerably smaller for the two reported TRACE observations. The discrepancy
can be explained partially by different thresholds for flare detection. There
is agreement on the slope and the absolute value of the distribution if the
same method were used and a numerical error corrected. The extrapolation of the
power law to unobserved energies that are many orders of magnitude smaller
remains questionable. Nevertheless, these micro-events and unresolved smaller
events are currently the best source of information on the heating process of
the corona
Penetrating particle ANalyzer (PAN)
PAN is a scientific instrument suitable for deep space and interplanetary
missions. It can precisely measure and monitor the flux, composition, and
direction of highly penetrating particles (100 MeV/nucleon) in deep
space, over at least one full solar cycle (~11 years). The science program of
PAN is multi- and cross-disciplinary, covering cosmic ray physics, solar
physics, space weather and space travel. PAN will fill an observation gap of
galactic cosmic rays in the GeV region, and provide precise information of the
spectrum, composition and emission time of energetic particle originated from
the Sun. The precise measurement and monitoring of the energetic particles is
also a unique contribution to space weather studies. PAN will map the flux and
composition of penetrating particles, which cannot be shielded effectively,
precisely and continuously, providing valuable input for the assessment of the
related health risk, and for the development of an adequate mitigation
strategy. PAN has the potential to become a standard on-board instrument for
deep space human travel.
PAN is based on the proven detection principle of a magnetic spectrometer,
but with novel layout and detection concept. It will adopt advanced particle
detection technologies and industrial processes optimized for deep space
application. The device will require limited mass (~20 kg) and power (~20 W)
budget. Dipole magnet sectors built from high field permanent magnet Halbach
arrays, instrumented in a modular fashion with high resolution silicon strip
detectors, allow to reach an energy resolution better than 10\% for nuclei from
H to Fe at 1 GeV/n
The RHESSI Microflare Height Distribution
We present the first in-depth statistical survey of flare source heights
observed by RHESSI. Flares were found using a flare-finding algorithm designed
to search the 6-10 keV count-rate when RHESSI's full sensitivity was available
in order to find the smallest events (Christe et al., 2008). Between March 2002
and March 2007, a total of 25,006 events were found. Source locations were
determined in the 4-10 keV, 10-15 keV, and 15-30 keV energy ranges for each
event. In order to extract the height distribution from the observed projected
source positions, a forward-fit model was developed with an assumed source
height distribution where height is measured from the photosphere. We find that
the best flare height distribution is given by g(h) \propto exp(-h/{\lambda})
where {\lambda} = 6.1\pm0.3 Mm is the scale height. A power-law height
distribution with a negative power-law index, {\gamma} = 3.1 \pm 0.1 is also
consistent with the data. Interpreted as thermal loop top sources, these
heights are compared to loops generated by a potential field model (PFSS). The
measured flare heights distribution are found to be much steeper than the
potential field loop height distribution which may be a signature of the flare
energization process
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