Modeling of Gyrosynchrotron Radio Emission Pulsations Produced by Magnetohydrodynamic Loop Oscillations in Solar Flares

Abstract

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.

Sources of Quasi-periodic Pulses in the Flare of 18 August 2012

“…The loop lengths are taken from the EUV image (Figure 4). To calculate periods we used formulaes (9 -12) from Mossessian and Fleishman (2012). It is seen that the main observed period of 10 seconds can be excited in the small loop as sausage, kink, and torsional modes.…”

“…Aschwanden, 1987;Grechnev, White, and Kundu, 2003). A quantitative study of the observable MW signatures of the MHD oscillation modes in the coronal loops was performed by Mossessian and Fleishman (2012). On the other hand the efficiency of particle acceleration mechanisms may strongly depend on changes of plasma parameters in acceleration sites, and the X-ray and microwave emission can therefore clearly respond to relatively weak disturbances associated with the MHD ones (e.g.…”

Perturbations of gyrosynchrotron emission polarization from solar flares by sausage modes: forward modeling

“…Observational evidence of the sausage mode of a single flare loop has mostly come from radio instruments: the Nobeyama Radioheliograph (Stepanov 2004;Melnikov et al 2005;Reznikova et al 2007;Inglis et al 2008) and the Owens Valley Solar Array (Mossessian & Fleishman 2012). Microwave emission from solar flares detected by these instruments is mainly generated by gyrosynchrotron (GS) radiation of trapped nonthermal electrons.…”

“…For many years, phase relations between modulation of the low-( f < f peak ) and high-( f > f peak ) frequency radiation were considered to be an important criterion of mode identification. Previous theoretical studies of this process predicted a phase difference of π between the oscillation in the optically thin and optically thick regime for negligible Razin suppression, as well as an in-phase behavior of low-and high-frequency radiation for strong Razin suppression (e.g., see Kopylova et al 2002;Reznikova et al 2007;Mossessian & Fleishman 2012). However, these studies were made without spatial resolution: models assumed that the magnetic field and density oscillate in phase within the entire source.…”

“…However, when such measurements are available, they can be used to analyze the quasi-periodic pulsations of microwave emission as well. Thus, modeling the sausage wave without spatial resolution, Mossessian & Fleishman (2012) predicted that the high-(low-) frequency polarization is in phase with the high-(low-) frequency intensity and, therefore, the oscillating components at low ( f < f peak ) frequencies are out of phase by the amount of π relative to the high ( f > f peak ) frequencies.…”