First Frequency-Time-Resolved Imaging Spectroscopy Observations of Solar Radio Spikes

TL;DR

This study presents high-resolution imaging spectroscopy of solar radio spikes, revealing their spatial, spectral, and temporal properties, and suggests their origin involves anisotropic density turbulence and small-scale energy release in the solar corona.

Contribution

First frequency-time-resolved imaging spectroscopy observations of solar radio spikes, providing insights into their spatial structure, motion, and emission mechanisms.

Findings

Radio spikes have superluminal centroid motion parallel to the solar limb.

Spike emission regions are approximately 10^8 cm with brightness temperatures up to 10^13 K.

Scattering by anisotropic density turbulence influences spike time profiles and suggests shorter energy release times.

Abstract

Solar radio spikes are short duration and narrow bandwidth fine structures in dynamic spectra observed from GHz to tens of MHz range. Their very short duration and narrow frequency bandwidth are indicative of sub-second small-scale energy release in the solar corona, yet their origin is not understood. Using the LOw Frequency ARray (LOFAR), we present spatially, frequency and time resolved observations of individual radio spikes associated with a coronal mass ejection (CME). Individual radio spike imaging demonstrates that the observed area is increasing in time and the centroid positions of the individual spikes move superluminally parallel to the solar limb. Comparison of spike characteristics with that of individual Type IIIb striae observed in the same event show similarities in duration, bandwidth, drift rate, polarization and observed area, as well the spike and striae motion in the image plane suggesting fundamental plasma emission with the spike emission region on the order of ${\sim}\:10^8$ cm, with brightness temperature as high as $10^{13}$ K. The observed spatial, spectral, and temporal properties of the individual spike bursts are also suggesting the radiation responsible for spikes escapes through anisotropic density turbulence in closed loop structures with scattering preferentially along the guiding magnetic field oriented parallel to the limb in the scattering region. The dominance of scattering on the observed time profile suggests the energy release time is likely to be shorter than what is often assumed. The observations also imply that the density turbulence anisotropy along closed magnetic field lines is higher than along open field lines.