Simulations of Gyrosynchrotron Microwave Emission from an Oscillating 3D Magnetic Loop

Abstract

Radio observations of solar flares often reveal various periodic or quasi-periodic oscillations. Most likely, these oscillations are caused by magnetohydrodynamic (MHD) oscillations of flaring loops which modulate the emission. Interpretation of the observations requires comparing them with simulations. We simulate the gyrosynchrotron radio emission from a semi-circular (toroidalshaped) magnetic loop containing sausage-mode MHD oscillations. The aim is to detect the observable signatures specific to the considered MHD mode and to study their dependence on the various source parameters. The MHD waves are simulated using a linear three-dimensional model of a magnetized plasma cylinder; both standing and propagating waves are considered. The curved loop is formed by replicating the MHD solutions along the plasma cylinder and bending the cylinder; this model allows us to study the effect of varying the viewing angle along the loop. The radio emission is simulated using a three-dimensional model and its spatial and temporal variations are analyzed. We consider several loop orientations and different parameters of the magnetic field, plasma, and energetic electrons in the loop. In the model with low plasma density, the intensity oscillations at all frequencies are synchronous (with the exception of a narrow spectral region below the spectral peak). In the model with high plasma density, the emission at low frequencies (where the Razin effect is important) oscillates in anti-phase with the emissions at higher frequencies. The oscillations at high and low frequencies are more pronounced in different parts of the loop (depending on the loop orientation). The layers where the line-of-sight component of the magnetic field changes sign can produce additional peculiarities in the oscillation patterns.

Sausage oscillations in a plasma cylinder with a surface current

“…QPP of the microwave, hard X-ray and gamma-ray emission of a single flaring loop, with the period about 40 s has been found to be consistent with a sausage oscillation by Nakariakov et al (2010). This interpretation is supported by the forward modelling of the observational manifestation of a sausage oscillation in the gyrosynchrotron emission performed by Kuznetsov et al (2015). One-second quasi-periodic wiggles of microwave zebra patterns during a solar flare (Yu et al, 2013;Kaneda et al, 2018) have also been associated with sausage oscillations on the basis of forward modelling (Yu et al, 2016).…”

Quasi-periodic Pulsations in Solar and Stellar Flares: An Overview of Recent Results (Invited Review)

“…However, with forward modelling, it was shown that the intensity variations may be small (Gruszecki, Nakariakov, and Van Doorsselaere, 2012), and strongly dependent on the spectral line (Antolin and Van Doorsselaere, 2013). Also in the gyrosynchrotron radio emission, an unintuitive dependence on the radio wavelength (Reznikova, Antolin, and Van Doorsselaere, 2014;Kuznetsov, Van Doorsselaere, and Reznikova, 2015) was found, in contrast to earlier analytical predictions (Mossessian and Fleishman, 2012).…”

Forward Modeling of Standing Slow Modes in Flaring Coronal Loops

“…It opposes previous findings of Mossessian & Fleishman (2012), which did not consider the inhomogeneity of the emitting source along the LOS. Kuznetsov et al (2015) used a semi-torus model to forward model the gyrosynchrotron radio emission of both propagating and standing slow modes in a curved magnetic structure.…”