Type-III solar radio bursts with spike-like toppings

TL;DR

This study presents the largest statistical analysis of spike-type III solar radio bursts, revealing their temporal, spectral, and polarization characteristics, and providing new insights into their origin in dynamic electron-acceleration regions.

Contribution

It offers the first large-scale statistical characterization of spike-type III bursts, highlighting their diverse morphologies, polarization, and temporal offsets, thus advancing understanding of their emission mechanisms.

Findings

Spike-like clusters precede type III bursts by 0.5-3 s and 3-30 MHz.

Over 64% of clusters show strong circular polarization (>0.6).

Bi-directional drifting structures indicate source motion toward and away from the Sun.

Abstract

Spike-type III burst pairs represent a distinct class of solar radio emissions in which clusters of spike-like bursts appear atop the highfrequency onset of type III bursts. Using high time-frequency resolution data from the Chashan Broadband Solar radio spectrometer at meter wavelengths (CBSm), we present the largest statistical study to date of such events, comprising 502 spike-type III pairs from 35 events recorded between November 2023 and October 2025. We find that spike-like clusters systematically precede their associated type III bursts by 0.5-3 s in time (~87% of pairs) and by 3-30 MHz in frequency (~80%), a temporal and spectral offset that differs from earlier reports. The spike-like clusters exhibit diverse morphologies, including point-like, blob-like, drifting, and diffuse structures, with durations of ~0.5-5 s and bandwidths of 15-150 MHz. Bi-directional drifting structures with rates of ~20-100 MHz s-1 are observed, consistent with source motion both toward and away from the Sun. Furthermore, spike emission is predominantly strongly circularly polarized, with more than 64% of clusters showing maximum polarization exceeding 0.6, in stark contrast to the generally weak polarization of type III bursts. These findings point to an origin of the spike radiation in a multiscale, inhomogeneous, and highly dynamic electron-acceleration region, providing novel observational constraints on the mechanisms underlying coherent solar radio bursts.