17 research outputs found
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 0.1 s. 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 (âŒ20%) of the groups show close correlation on time scales < 1 s. If structures correlate, the HXRs precede the type III emissions in a majority of cases, and by 0.69 ± 0.19 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 emissio
Particle Acceleration in Multiple Dissipation Regions
The sharp magnetic discontinuities which naturally appear in solar magnetic
flux tubes driven by turbulent photospheric motions are associated with intense
currents. \citet{Par83} proposed that these currents can become unstable to a
variety of microscopic processes, with the net result of dramatically enhanced
resistivity and heating (nanoflares). The electric fields associated with such
``hot spots'' are also expected to enhance particle acceleration. We test this
hypothesis by exact relativistic orbit simulations in strong random phase
magnetohydrodynamic (MHD) turbulence which is forming localized super-Dreicer
Ohm electric fields ( = ) occurring in 2..15 % of
the volume. It is found that these fields indeed yield a large amplification of
acceleration of electrons and ions, and can effectively overcome the injection
problem. We suggest in this article that nanoflare heating will be associated
with sporadic particle acceleration.Comment: 12 pages, 5 figures, to appear in ApJ
Are Coronae of Magnetically Active Stars Heated by Flares? III. Analytical Distribution of Superimposed Flares
(abridged) We study the hypothesis that observed X-ray/extreme ultraviolet
emission from coronae of magnetically active stars is entirely (or to a large
part) due to the superposition of flares, using an analytic approach to
determine the amplitude distribution of flares in light curves. The
flare-heating hypothesis is motivated by time series that show continuous
variability suggesting the presence of a large number of superimposed flares
with similar rise and decay time scales. We rigorously relate the amplitude
distribution of stellar flares to the observed histograms of binned counts and
photon waiting times, under the assumption that the flares occur at random and
have similar shapes. Applying these results to EUVE/DS observations of the
flaring star AD Leo, we find that the flare amplitude distribution can be
represented by a truncated power law with a power law index of 2.3 +/- 0.1. Our
analytical results agree with existing Monte Carlo results of Kashyap et al.
(2002) and Guedel et al. (2003). The method is applicable to a wide range of
further stochastically bursting astrophysical sources such as cataclysmic
variables, Gamma Ray Burst substructures, X-ray binaries, and spatially
resolved observations of solar flares.Comment: accepted for publication in Ap
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
The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence
We investigate the influence of coherent structures on particle acceleration
in the strongly turbulent solar corona. By randomizing the Fourier phases of a
pseudo-spectral simulation of isotropic MHD turbulence (Re ), and
tracing collisionless test protons in both the exact-MHD and phase-randomized
fields, it is found that the phase correlations enhance the acceleration
efficiency during the first adiabatic stage of the acceleration process. The
underlying physical mechanism is identified as the dynamical MHD alignment of
the magnetic field with the electric current, which favours parallel
(resistive) electric fields responsible for initial injection. Conversely, the
alignment of the magnetic field with the bulk velocity weakens the acceleration
by convective electric fields - \bfu \times \bfb at a non-adiabatic stage of
the acceleration process. We point out that non-physical parallel electric
fields in random-phase turbulence proxies lead to artificial acceleration, and
that the dynamical MHD alignment can be taken into account on the level of the
joint two-point function of the magnetic and electric fields, and is therefore
amenable to Fokker-Planck descriptions of stochastic acceleration.Comment: accepted for publication in Ap
Accretion and outflow-related X-rays in T Tauri stars
We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the "XMM-Newton Extended Survey of the Taurus Molecular Cloud.â X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a "soft excessâ in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion stream
ïżœ 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A. PARTICLE ACCELERATION IN MULTIPLE DISSIPATION REGIONS
The sharp magnetic discontinuities that naturally appear in solar magnetic flux tubes driven by turbulent photospheric motions are associated with intense currents. Parker proposed that these currents can become unstable to a variety of microscopic processes, with the net result of dramatically enhanced resistivity and heating (nanoflares). The electric fields associated with such âhot spots â are also expected to enhance particle acceleration. We test this hypothesis by exact relativistic orbit simulations in strong random phase magnetohydrodynamic 2 5 turbulence that is forming localized super-Dreicer Ohm electric fields ( 10 †E /E †10) occurring in 2%â15% of the volume. It is found that these fields indeed yield a large amplification of acceleration of electrons and ions and can effectively overcome the injection problem. We suggest in this article that nanoflare heating will be associated with sporadic particle acceleration. Subject headings: acceleration of particles â turbulence Understanding the mechanisms behind the dissipation of magnetic energy in the solar atmosphere is a key ingredient for the solution of several problems related to coronal heating