1,097 research outputs found
The Crimean Solar Maximum Year Workshop, selected reports
Problems associated with the transport of energy and acceleration of charged particles in solar flares are considered. Existing theories are compared with observation with a view to either discriminating between rival theories (such as whether hard X-rays are emitted by thermal or nonthermal bremsstrahlung), constraining existing theories (such as deduction of the number of nonthermal electrons present from spectroscopic diagnostics in the soft X-ray part of the spectrum), or suggesting theories (such as attempting to explain the observed spatial structure of microwave emission relative to alpha)
On the variation of solar flare coronal x-ray source sizes with energy
Observations with {\em RHESSI} have enabled the detailed study of the
structure of dense hard X-ray coronal sources in solar flares. The variation of
source extent with electron energy has been discussed in the context of
streaming of non-thermal particles in a one-dimensional cold-target model, and
the results used to constrain both the physical extent of, and density within,
the electron acceleration region. Here we extend this investigation to a more
physically realistic model of electron transport that takes into account the
finite temperature of the ambient plasma, the initial pitch-angle distribution
of the accelerated electrons, and the effects of collisional pitch-angle
scattering. The finite temperature results in the thermal diffusion of
electrons, that leads to the observationally-inferred value of the acceleration
region volume being an overestimate of its true value. The different directions
of the electron trajectories, a consequence of both the non-zero injection
pitch-angle and scattering within the target, cause the projected propagation
distance parallel to the guiding magnetic field to be reduced, so that a
one-dimensional interpretation can overestimate the actual density by a factor
of up to . The implications of these results for the determination of
acceleration region properties (specific acceleration rate, filling factor,
etc.) are discussed.Comment: 45 pages, 9 figures, accepted for publication in Ap
Collisional relaxation of electrons in a warm plasma and accelerated nonthermal electron spectra in solar flares
Extending previous studies of nonthermal electron transport in solar flares
which include the effects of collisional energy diffusion and thermalization of
fast electrons, we present an analytic method to infer more accurate estimates
of the accelerated electron spectrum in solar flares from observations of the
hard X-ray spectrum. Unlike for the standard cold-target model, the spatial
characteristics of the flaring region, especially the necessity to consider a
finite volume of hot plasma in the source, need to be taken into account in
order to correctly obtain the injected electron spectrum from the
source-integrated electron flux spectrum (a quantity straightforwardly obtained
from hard X-ray observations). We show that the effect of electron
thermalization can be significant enough to nullify the need to introduce an
{\it ad hoc} low-energy cutoff to the injected electron spectrum in order to
keep the injected power in non-thermal electrons at a reasonable value. Rather
the suppression of the inferred low-energy end of the injected spectrum
compared to that deduced from a cold-target analysis allows the inference from
hard X-ray observations of a more realistic energy in injected non-thermal
electrons in solar flares.Comment: accepted for publication in Ap
Do managed clinical networks improve quality of diabetes care? : Evidence from a retrospective mixed methods evaluation
Peer reviewedPostprin
High-Energy Aspects of Solar Flares: Overview of the Volume
In this introductory chapter, we provide a brief summary of the successes and
remaining challenges in understanding the solar flare phenomenon and its
attendant implications for particle acceleration mechanisms in astrophysical
plasmas. We also provide a brief overview of the contents of the other chapters
in this volume, with particular reference to the well-observed flare of 2002
July 23Comment: This is the introductory article for a monograph on the physics of
solar flares, inspired by RHESSI observations. The individual articles are to
appear in Space Science Reviews (2011
Determination of the total accelerated electron rate and power using solar flare hard X-ray spectra
Solar flare hard X-ray spectroscopy serves as a key diagnostic of the
accelerated electron spectrum. However, the standard approach using the
collisional cold thick-target model poorly constrains the lower-energy part of
the accelerated electron spectrum, and hence the overall energetics of the
accelerated electrons are typically constrained only to within one or two
orders of magnitude. Here we develop and apply a physically self-consistent
warm-target approach which involves the use of both hard X-ray spectroscopy and
imaging data. The approach allows an accurate determination of the electron
distribution low-energy cutoff, and hence the electron acceleration rate and
the contribution of accelerated electrons to the total energy released, by
constraining the coronal plasma parameters. Using a solar flare observed in
X-rays by the {\em RHESSI} spacecraft, we demonstrate that using the standard
cold-target methodology, the low-energy cutoff (and hence the energy content in
electrons) is essentially undetermined. However, the warm-target methodology
can determine the low-energy electron cutoff with 7\% uncertainty at the
level and hence permits an accurate quantitative study of the
importance of accelerated electrons in solar flare energetics.Comment: Accepted for publication in the Astrophysical Journal, 18 pages, 5
figure
Global Energetics of Thirty-Eight Large Solar Eruptive Events
We have evaluated the energetics of 38 solar eruptive events observed by a
variety of spacecraft instruments between February 2002 and December 2006, as
accurately as the observations allow. The measured energetic components
include: (1) the radiated energy in the GOES 1 - 8 A band; (2) the total energy
radiated from the soft X-ray (SXR) emitting plasma; (3) the peak energy in the
SXR-emitting plasma; (4) the bolometric radiated energy over the full duration
of the event; (5) the energy in flare-accelerated electrons above 20 keV and in
flare-accelerated ions above 1 MeV; (6) the kinetic and potential energies of
the coronal mass ejection (CME); (7) the energy in solar energetic particles
(SEPs) observed in interplanetary space; and (8) the amount of free
(nonpotential) magnetic energy estimated to be available in the pertinent
active region. Major conclusions include: (1) the energy radiated by the
SXR-emitting plasma exceeds, by about half an order of magnitude, the peak
energy content of the thermal plasma that produces this radiation; (2) the
energy content in flare-accelerated electrons and ions is sufficient to supply
the bolometric energy radiated across all wavelengths throughout the event; (3)
the energy contents of flare-accelerated electrons and ions are comparable; (4)
the energy in SEPs is typically a few percent of the CME kinetic energy
(measured in the rest frame of the solar wind); and (5) the available magnetic
energy is sufficient to power the CME, the flare-accelerated particles, and the
hot thermal plasma
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 sub-arcsecond hard X-ray structure of loop footpoints in a solar flare
The newly developed X-ray visibility forward fitting technique is applied to
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data of a limb
flare to investigate the energy and height dependence on sizes, shapes, and
position of hard X-ray chromospheric footpoint sources. This provides
information about the electron transport and chromospheric density structure.
The spatial distribution of two footpoint X-ray sources is analyzed using
PIXON, Maximum Entropy Method, CLEAN and visibility forward fit algorithms at
nonthermal energies from to keV. We report, for the first
time, the vertical extents and widths of hard X-ray chromospheric sources
measured as a function of energy for a limb event. Our observations suggest
that both the vertical and horizontal sizes of footpoints are decreasing with
energy. Higher energy emission originates progressively deeper in the
chromosphere consistent with downward flare accelerated streaming electrons.
The ellipticity of the footpoints grows with energy from at keV to at keV. The positions of X-ray emission are in
agreement with an exponential density profile of scale height ~km.
The characteristic size of the hard X-ray footpoint source along the limb is
decreasing with energy suggesting a converging magnetic field in the footpoint.
The vertical sizes of X-ray sources are inconsistent with simple collisional
transport in a single density scale height but can be explained using a
multi-threaded density structure in the chromosphere.Comment: 7 pages, 7 figures, submitted to Ap
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