Low frequency radio observations of bi-directional electron beams in the solar corona

Abstract

The radio signature of a shock travelling through the solar corona is known as a type II solar radio burst. In rare cases these bursts can exhibit a fine structure known as `herringbones', which are a direct indicator of particle acceleration occurring at the shock front. However, few studies have been performed on herringbones and the details of the underlying particle acceleration processes are unknown. Here, we use an image processing technique known as the Hough transform to statistically analyse the herringbone fine structure in a radio burst at $\sim$20-90 MHz observed from the Rosse Solar-Terrestrial Observatory on 2011 September 22. We identify 188 individual bursts which are signatures of bi-directional electron beams continuously accelerated to speeds of 0.16${}_{-0.10}^{+0.11}c$. This occurs at a shock acceleration site initially at a constant altitude of $\sim$0.6 R${}_{\odot }$ in the corona, followed by a shift to $\sim$0.5 R${}_{\odot }$. The anti-sunward beams travel a distance of 170${}_{-97}^{+174}$ Mm (and possibly further) away from the acceleration site, while those travelling toward the sun come to a stop sooner, reaching a smaller distance of 112${}_{-76}^{+84}$ Mm. We show that the stopping distance for the sunward beams may depend on the total number density and the velocity of the beam. Our study concludes that a detailed statistical analysis of herringbone fine structure can provide information on the physical properties of the corona which lead to these relatively rare radio bursts.

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• LOFAR: The LOw-Frequency ARray
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• Dynamics of electron beams in the inhomogeneous solar corona plasma
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Cited by in corpus (8)

• Multiple Regions of Shock-accelerated Particles during a Solar Coronal Mass Ejection
• Type II radio bursts and their association with coronal mass ejections in solar cycles 23 and 24
• Shock-accelerated electrons during the fast expansion of a coronal mass ejection
• Localised acceleration of energetic particles by a weak shock in the solar corona
• First Results from the REAL-time Transient Acquisition backend (REALTA) at the Irish LOFAR station
• Radio evidence for a shock wave reflected by a coronal hole
• Detection of spike-like structures near the front of type-II bursts
• Observations of shock propagation through turbulent plasma in the solar corona