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

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 ($\eta$) in localized dissipation regions where the magnetic twist \nabla \times \bhat exceeds a given threshold. Associated with $\eta > 0$ is a parallel electric field ${\bf E} = \eta {\bf j}$ 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 $z$ = 0 are associated with hard X rays, and electrons which escape at height $z$ $\sim$ 3$\cdot 10^4$ 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 $\eta$ 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 $z$ = 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 $z$ = 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

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 (∼20%) 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

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

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 ($\sim$ 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 0.69$\pm$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 emission

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 $k$-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 $k$-space filters and for diffusion processes exhibiting correlations between different mass scales

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

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 $\alpha$ 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&