Origin of the Modulation of the Radio Emission from the Solar Corona by a Fast Magnetoacoustic Wave

Abstract

Observational detection of quasi-periodic drifting fine structures in a type III radio burst associated with a solar flare SOL2015-04-16T11:22, with Low Frequency Array, is presented. Although similar modulations of the type III emission have been observed before and were associated with the plasma density fluctuations, the origin of those fluctuations was unknown. Analysis of the striae of the intensity variation in the dynamic spectrum allowed us to reveal two quasi-oscillatory components. The shorter component has the apparent wavelength of ∼ 2 Mm, phase speed of ∼ 657 km s −1 , which gives the oscillation period of ∼ 3 s, and the relative amplitude of ∼ 0.35%. The longer component has the wavelength of ∼ 12 Mm, and relative amplitude of ∼ 5.1%. The short frequency range of the detection does not allow us to estimate its phase speed. However, the properties of the shorter oscillatory component allowed us to interpret it as a fast magnetoacoustic wave guided by a plasma non-uniformity along the magnetic field outwards from the Sun. The assumption that the intensity of the radio emission is proportional to the amount of plasma in the emitting volume allowed us to show that the superposition of the plasma density modulation by a fast wave and a longer-wavelength oscillation of an unspecified nature could readily reproduce the fine structure of the observed dynamic spectrum. The observed parameters of the fast wave give the absolute value of the magnetic field in the emitting plasma of ∼ 1.1 G which is consistent with the radial magnetic field model.

Quasi-periodic Fast Propagating Magnetoacoustic Waves during the Magnetic Reconnection Between Solar Coronal Loops

“…This speculation is consistent with the observations that less heating and acceleration of plasma are detected during the reconnection. Moreover, different from Goddard et al (2016) and Kolotkov et al (2018), neither the QFPM wave radio signature, nor the reconnection radio signature is detected from Nobeyama observations. This indicates that there is no significant (detectable) non-thermal emission during the reconnection.…”

A Review of Recent Solar Type III Imaging Spectroscopy

“…With previous imaging, the spectral resolution was too sporadic to have multiple images of one stria. A number of studies using LOFAR imaging spectroscopy (Kontar et al, 2017;Chen X. et al, 2018;Kolotkov et al, 2018;Sharykin et al, 2018) have analyzed striae bursts, concentrating on one specific event on the 16 April 2015.…”

“…The mean striae speed from the drift rate was around 600 km s −1 , larger than the typical sound speed of 200 km s −1 and smaller than the type III burst speed of 0.2c. Kolotkov et al (2018) analyzed the same type III event, finding quasi-periodicity in the signal. The authors explained the periodicity by a propagating fast wave train that modulated the radio emission produced by the electron beam.…”

The Frequency Drift and Fine Structures of Solar S-bursts in the High Frequency Band of LOFAR

“…The density fluctuations of the the corona plasma can result in the fine-structured solar radio bursts, and these fine structures can be used to study the density fluctuations of the corona. Kolotkov et al (2018) used the quasi-periodic drifting fine structures of type III radio bursts to study the density variation caused by fast magnetoacoustic waves. Loi et al (2014) used simulations to show that the fine structure in type III radio bursts could be due to the background density fluctuations.…”