# Oscillation of solar radio emission at coronal acoustic cut-off frequency

## TL;DR

This study links observed 80-minute periodic radio emissions in the solar corona to acoustic cut-off oscillations, suggesting that coronal perturbations generate recurrent shocks and magnetic reconnection events.

## Contribution

It provides new evidence connecting coronal acoustic oscillations with periodic radio emissions and the formation of density structures in the solar wind.

## Key findings

- Detected 80-minute periodicity in radio emissions across multiple frequencies.
- Linked the periodicity to the coronal acoustic cut-off frequency at 1 MK.
- Proposed a mechanism involving jets, shocks, and magnetic reconnection.

## Abstract

Recent SECCHI COR2 observations on board STEREO-A spacecraft have detected density structures at a distance of 2.5--15~R propagating with periodicity of about 90~minutes. The observations show that the density structures probably formed in the lower corona. We used the large Ukrainian radio telescope URAN-2 to observe type IV radio bursts in the frequency range of 8--32~MHz during the time interval of 08:15--11:00~UT on August 1, 2011. Radio emission in this frequency range originated at the distance of 1.5--2.5 R according to the Baumbach-Allen density model of the solar corona. Morlet wavelet analysis showed the periodicity of 80~min in radio emission intensity at all frequencies, which demonstrates that there are quasi-periodic variations of coronal density at all heights. The observed periodicity corresponds to the acoustic cut-off frequency of stratified corona at a temperature of 1~MK. We suggest that continuous perturbations of the coronal base in the form of jets/explosive events generate acoustic pulses, which propagate upwards and leave the wake behind oscillating at the coronal cut-off frequency. This wake may transform into recurrent shocks due to the density decrease with height, which leads to the observed periodicity in the radio emission. The recurrent shocks may trigger quasi-periodic magnetic reconnection in helmet streamers, where the opposite field lines merge and consequently may generate periodic density structures observed in the solar wind.