Study of Particle Acceleration using Fine Structures and Oscillations in Microwaves from Electron Cyclotron Maser

Abstract: The accelerated electrons during solar flares produce radio bursts and nonthermal X-ray signatures. The quasi-periodic pulsations (QPPs) and fine structures in spatial-spectral-temporal space in radio bursts depend on the emission mechanism and the local conditions, such as magnetic fields, electron density, and pitch angle distribution. Radio burst observations with high frequency-time resolution imaging provide excellent diagnostics. In converging magnetic field geometries, the radio bursts can be produced via the electron-cyclotron maser (ECM). Recently, using observations made by the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, \cite{Yu2023} reported a discovery of long-lasting auroral-like radio bursts persistent over a sunspot and interpreted them as ECM-generated emission. Here, we investigate the detailed second and sub-second temporal variability of this continuous ECM source. We study the association of 5-second period QPPs with a concurrent GOES C1.5-class flare, utilizing VLA's imaging spectroscopy capability with an extremely high temporal resolution (50 ms). We use the density and magnetic field extrapolation model to constrain the ECM emission to the second harmonic o-mode. Using the delay of QPPs from X-ray emission times, combined with X-ray spectroscopy and magnetic extrapolation, we constrain the energies and pitch angles of the ECM-emitting electrons to $\approx$4-8 keV and $>26^{\circ}$. Our analysis shows that the loss-cone diffusion continuously fuels the ECM via Coulomb collisions and magnetic turbulence between a 5 Mm--100 Mm length scale. We conclude that the QPP occurs via the Lotka-Volterra system, where the electron from solar flares saturates the continuously operating ECM and causes temporary oscillations.