Engineering topics

Fleishman, Gregory D

Publications and source records attributed to Fleishman, Gregory D.

5 records

DOE OSTI2012

MODELING OF GYROSYNCHROTRON RADIO EMISSION PULSATIONS PRODUCED BY MAGNETOHYDRODYNAMIC LOOP OSCILLATIONS IN SOLAR FLARES

A quantitative study of the observable radio signatures of the sausage, kink, and torsional magnetohydrodynamic (MHD) oscillation modes in flaring coronal loops is performed. Considering first non-zero order effect of these various MHD oscillation modes on the radio source parameters such as magnetic field, line of sight, plasma density and temperature, electron distribution function, and the source dimensions, we compute time-dependent radio emission (spectra and light curves). The radio light curves (of both flux density and degree of polarization) at all considered radio frequencies are then quantified in both time domain (via computation of the full modulation amplitude as a function of frequency) and in Fourier domain (oscillation spectra, phases, and partial modulation amplitude) to form the signatures specific to a particular oscillation mode and/or source parameter regime. We found that the parameter regime and the involved MHD mode can indeed be distinguished using the quantitative measures derived in the modeling. We apply the developed approach to analyze radio burst recorded by Owens Valley Solar Array and report possible detection of the sausage mode oscillation in one (partly occulted) flare and kink or torsional oscillations in another flare.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

THREE-DIMENSIONAL SIMULATIONS OF GYROSYNCHROTRON EMISSION FROM MILDLY ANISOTROPIC NONUNIFORM ELECTRON DISTRIBUTIONS IN SYMMETRIC MAGNETIC LOOPS

Microwave emission of solar flares is formed primarily by incoherent gyrosynchrotron radiation generated by accelerated electrons in coronal magnetic loops. The resulting emission depends on many factors, including pitch-angle distribution of the emitting electrons and the source geometry. In this work, we perform systematic simulations of solar microwave emission using recently developed tools (GS Simulator and fast gyrosynchrotron codes) capable of simulating maps of radio brightness and polarization as well as spatially resolved emission spectra. A three-dimensional model of a symmetric dipole magnetic loop is used. We compare the emission from isotropic and anisotropic (of loss-cone type) electron distributions. We also investigate effects caused by inhomogeneous distribution of the emitting particles along the loop. It is found that the effect of the adopted moderate electron anisotropy is the most pronounced near the footpoints and it also depends strongly on the loop orientation. Concentration of the emitting particles at the looptop results in a corresponding spatial shift of the radio brightness peak, thus reducing effects of the anisotropy. The high-frequency ({approx}> 50 GHz) emission spectral index is specified mainly by the energy spectrum of the emitting electrons; however, at intermediate frequencies (around 10-20 GHz), the spectrum shape is strongly dependent on the electron anisotropy, spatial distribution, and magnetic field nonuniformity. The implications of the obtained results for the diagnostics of the energetic electrons in solar flares are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

THREE-DIMENSIONAL STRUCTURE OF MICROWAVE SOURCES FROM SOLAR ROTATION STEREOSCOPY VERSUS MAGNETIC EXTRAPOLATIONS

We use rotation stereoscopy to estimate the height of a steady-state solar feature relative to the photosphere, based on its apparent motion in the image plane recorded over several days of observation. The stereoscopy algorithm is adapted to work with either one- or two-dimensional data (i.e., from images or from observations that record the projected position of the source along an arbitrary axis). The accuracy of the algorithm is tested on simulated data, and then the algorithm is used to estimate the coronal radio source heights associated with the active region NOAA 10956, based on multifrequency imaging data over seven days from the Siberian Solar Radio Telescope near 5.7 GHz, the Nobeyama Radio Heliograph at 17 GHz, as well as one-dimensional scans at multiple frequencies spanning the 5.98-15.95 GHz frequency range from the RATAN-600 instrument. The gyroresonance emission mechanism, which is sensitive to the coronal magnetic field strength, is applied to convert the estimated radio source heights at various frequencies, h(f), to information about magnetic field versus height B(h), and the results are compared to a magnetic field extrapolation derived from photospheric magnetic field observations obtained by Hinode and Michelson Doppler Imager. We found that the gyroresonant emission comes from heights exceeding the location of the third gyrolayer irrespective of the magnetic extrapolation method; implications of this finding for coronal magnetography and coronal plasma physics are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

A COLD, TENUOUS SOLAR FLARE: ACCELERATION WITHOUT HEATING

We report the observation of an unusual cold, tenuous solar flare, which reveals itself via numerous and prominent non-thermal manifestations, while lacking any noticeable thermal emission signature. RHESSI hard X-rays and 0.1-18 GHz radio data from OVSA and Phoenix-2 show copious electron acceleration (10{sup 35} electrons s{sup -1} above 10 keV) typical for GOES M-class flares with electrons energies up to 100 keV, but GOES temperatures not exceeding 6.1 MK. The imaging, temporal, and spectral characteristics of the flare have led us to a firm conclusion that the bulk of the microwave continuum emission from this flare was produced directly in the acceleration region. The implications of this finding for the flaring energy release and particle acceleration are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

FAST GYROSYNCHROTRON CODES

Radiation produced by charged particles gyrating in a magnetic field is highly significant in the astrophysics context. Persistently increasing resolution of astrophysical observations calls for corresponding three-dimensional modeling of the radiation. However, available exact equations are prohibitively slow in computing a comprehensive table of high-resolution models required for many practical applications. To remedy this situation, we develop approximate gyrosynchrotron (GS) codes capable of quickly calculating the GS emission (in non-quantum regime) from both isotropic and anisotropic electron distributions in non-relativistic, mildly relativistic, and ultrarelativistic energy domains applicable throughout a broad range of source parameters including dense or tenuous plasmas and weak or strong magnetic fields. The computation time is reduced by several orders of magnitude compared with the exact GS algorithm. The new algorithm performance can gradually be adjusted to the user's needs depending on whether precision or computation speed is to be optimized for a given model. The codes are made available for users as a supplement to this paper.

79 ASTRONOMY AND ASTROPHYSICS↗