66 research outputs found
Visibility-Based Demodulation of Rhessi Light Curves
The Reuven Ramaty High Energy Spectroscopic Solar Imager (RHESSI) uses the
rotational modulation principle to observe temporally, spatially, and
spectrally resolved hard X ray and gamma ray images of solar flares. In order
to track the flare evolution on time scales that are commensurate with
modulation, the observed count rates must be demodulated at the expense of
spatial information. The present paper describes improvements of an earlier
demodulation algorithm, which decomposes the observed light curves into
intrinsic source variability and instrumental modulation.Comment: 6 pages, 3 figure
The structure of the magnetic reconnection exhaust boundary
The structure of shocks that form at the exhaust boundaries during
collisionless reconnection of anti-parallel fields is studied using
particle-in-cell (PIC) simulations and modeling based on the anisotropic
magnetohydrodynamic equations. Large-scale PIC simulations of reconnection and
companion Riemann simulations of shock development demonstrate that the
pressure anisotropy produced by counterstreaming ions within the exhaust
prevents the development of classical Petschek switch-off-slow shocks (SSS).
The shock structure that does develop is controlled by the firehose stability
parameter epsilon=1-mu_0(P_parallel-P_perpendicular)/ B^2 through its influence
on the speed order of the intermediate and slow waves. Here P_parallel and
P_perpendicular are the pressure parallel and perpendicular to the local
magnetic field. The exhaust boundary is made up of a series of two shocks and a
rotational wave. The first shock takes epsilon from unity upstream to a plateau
of 0.25 downstream. The condition epsilon =0.25 is special because at this
value the speeds of nonlinear slow and intermediate waves are degenerate. The
second slow shock leaves epsilon=0.25 unchanged but further reduces the
amplitude of the reconnecting magnetic field. Finally, in the core of the
exhaust epsilon drops further and the transition is completed by a rotation of
the reconnecting field into the out-of-plane direction. The acceleration of the
exhaust takes place across the two slow shocks but not during the final
rotation. The result is that the outflow speed falls below that expected from
the Walen condition based on the asymptotic magnetic field. A simple analytic
expression is given for the critical value of epsilon within the exhaust below
which SSSs no longer bound the reconnection outflow.Comment: 13 pages, 5 figure
Gamma-Ray Burst Polarization: Limits from RHESSI Measurements
Using the RHESSI satellite as a Compton polarimeter, a recent study claimed
that the prompt emission of GRB021206 was almost fully linearly polarized. This
was challenged by a subsequent reanalysis. We present an novel approach,
applying our method to the same data. We identify Compton scattering candidates
by carefully filtering events in energy, time, and scattering geometry. Our
polarization search is based on time dependent scattering rates in
perpendicular directions, thus optimally excluding systematic errors. We
perform simulations to obtain the instrument's polarimetric sensitivity, and
these simulations include photon polarization. For GRB021206, we formally find
a linear polarization degree of 41% (+57% -44%), concluding that the data
quality is insufficient to constrain the polarization degree in this case. We
further applied our analysis to GRB030519B and found again a null result.Comment: 39 pages, 11 figures, accepted for publication by the Astrophysical
Journa
Gyrokinetic electron acceleration in the force-free corona with anomalous resistivity
We numerically explore electron acceleration and coronal heating by
dissipative electric fields. Electrons are traced in linear force-free magnetic
fields extrapolated from SOHO/MDI magnetograms, endowed with anomalous
resistivity () in localized dissipation regions where the magnetic twist
\nabla \times \bhat exceeds a given threshold. Associated with is
a parallel electric field which can accelerate runaway
electrons. In order to gain observational predictions we inject electrons
inside the dissipation regions and follow them for several seconds in real
time. Precipitating electrons which leave the simulation system at height =
0 are associated with hard X rays, and electrons which escape at height
3 km are associated with normal-drifting type IIIs at the
local plasma frequency. A third, trapped, population is related to
gyrosynchrotron emission. Time profiles and spectra of all three emissions are
calculated, and their dependence on the geometric model parameters and on
is explored. It is found that precipitation generally preceeds escape by
fractions of a second, and that the electrons perform many visits to the
dissipation regions before leaving the simulation system. The electrons
impacting = 0 reach higher energies than the escaping ones, and
non-Maxwellian tails are observed at energies above the largest potential drop
across a single dissipation region. Impact maps at = 0 show a tendency of
the electrons to arrive at the borders of sunspots of one polarity. Although
the magnetograms used here belong to non-flaring times, so that the simulations
refer to nanoflares and `quiescent' coronal heating, it is conjectured that the
same process, on a larger scale, is responsible for solar flares
Cosmic Mass Functions from Gaussian Stochastic Diffusion Processes
Gaussian stochastic diffusion processes are used to derive cosmic mass functions. To get analytic relations previous studies exploited the sharp -space filter assumption yielding zero drift terms in the corresponding Fokker-Planck (Kolmogorov's forward) equation and thus simplifying analytic treatments significantly (excursion set formalism). In the present paper methods are described to derive for given diffusion processes and Gaussian random fields the corresponding mass and filter functions by solving the Kolmogorov's forward and backward equations including nonzero drift terms. This formalism can also be used in cases with non-sharp -space filters and for diffusion processes exhibiting correlations between different mass scales
Temporal Correlation of Hard X-rays and Meter/Decimeter Radio Structures in Solar Flares
We investigate the relative timing between hard X-ray (HXR) peaks and
structures in metric and decimetric radio emissions of solar flares using data
from the RHESSI and Phoenix-2 instruments. The radio events under consideration
are predominantly classified as type III bursts, decimetric pulsations and
patches. The RHESSI data are demodulated using special techniques appropriate
for a Phoenix-2 temporal resolution of 0.1s. The absolute timing accuracy of
the two instruments is found to be about 170 ms, and much better on the
average. It is found that type III radio groups often coincide with enhanced
HXR emission, but only a relatively small fraction ( 20%) of the groups
show close correlation on time scales 1s. If structures correlate, the HXRs
precede the type III emissions in a majority of cases, and by 0.690.19 s
on the average. Reversed drift type III bursts are also delayed, but
high-frequency and harmonic emission is retarded less. The decimetric
pulsations and patches (DCIM) have a larger scatter of delays, but do not have
a statistically significant sign or an average different from zero. The time
delay does not show a center-to-limb variation excluding simple propagation
effects. The delay by scattering near the source region is suggested to be the
most efficient process on the average for delaying type III radio emission
Polarization from GRB021206: No constraints from reanalysis of RHESSI data
The determination of a polarization signal in Gamma Ray Bursts (GRBs) would give new information about their nature and mechanism. Using the RHESSI satellite as a Compton polarimeter, Coburn W. and Boggs S. E. (Nature, 423 (2003) 415) reported that GRB021206 was highly linearly polarized. This was contradicted by Rutledge R. E. and Fox D. B. (Mon. Not. R. Astron. Soc., 350 (2004) 1288) who found about 10 times less scattering events suitable for measuring
polarization. Applying our own method to thesamedata weconfirm them uch lower number of suitable scattering events. But we obtain three times smaller errors by using better selection criteria. Comparison with our Monte Carlo simulations shows that from the RHESSI data of GRB021206 we cannot distinguish between no and full polarization within less than 2 standard deviations. We also applied our method
to other GRBs observed by RHESSI. This shows that the probability to observe a GRB suitable for polarization search with such an instrument is small
X-Ray Polarization of Solar Flares Measured with Rhessi
The degree of linear polarization in solar flares has not yet been precisely determined despite multiple attempts to measure it with different missions. The high energy range, in particular, has very rarely been explored, due to its greater instrumental difficulties. We approached the subject using the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study six X-class and 1 M-class flares in the energy range between 100 and 350 keV. Using RHESSI as a polarimeter requires the application of strict cuts to the event list in order to extract those photons that are Compton scattered between two detectors. Our measurements show polarization values between 2 and 54%, with errors ranging from 10 to 26% in 1σ level. In view of the large uncertainties in both the magnitude and direction of the polarization vector, the results can only reject source models with extreme propertie
A nanoflare model of quiet Sun EUV emission
Nanoflares have been proposed as the main source of heating of the solar
corona. However, detecting them directly has so far proved elusive, and
extrapolating to them from the properties of larger brightenings gives
unreliable estimates of the power-law exponent characterising their
distribution. Here we take the approach of statistically modelling light curves
representative of the quiet Sun as seen in EUV radiation. The basic assumption
is that all quiet-Sun EUV emission is due to micro- and nanoflares, whose
radiative energies display a power-law distribution. Radiance values in the
quiet Sun follow a lognormal distribution. This is irrespective of whether the
distribution is made over a spatial scan or over a time series. We show that
these distributions can be reproduced by our simple model.Comment: 13 pages, 18 figures, accepted for publication by A&
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