Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport

Reid, H. A.S. and Kontar, E. P. (2018) Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport. Astronomy and Astrophysics, 614, A69. (doi:10.1051/0004-6361/201732298)

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Context. Solar type III radio bursts contain a wealth of information about the dynamics of electron beams in the solar corona and the inner heliosphere; this information is currently unobtainable through other means. However, the motion of different regions of an electron beam (front, middle, and back) have never been systematically analysed before. Aims. We characterise the type III burst frequency-time evolution using the enhanced resolution of LOFAR (Low Frequency Array) in the frequency range 30–70 MHz and use this to probe electron beam dynamics. Methods. The rise, peak, and decay times with a ∼ 0.2 MHz spectral resolution were defined for a collection of 31 type III bursts. The frequency evolution was used to ascertain the apparent velocities of the front, middle, and back of the type III sources, and the trends were interpreted using theoretical and numerical treatments. Results. The type III time profile was better approximated by an asymmetric Gaussian profile and not an exponential, as was used previously. Rise and decay times increased with decreasing frequency and showed a strong correlation. Durations were shorter than previously observed. Drift rates from the rise times were faster than from the decay times, corresponding to inferred mean electron beam speeds for the front, middle, and back of 0.2, 0.17, 0.15 c, respectively. Faster beam speeds correlate with shorter type III durations. We also find that the type III frequency bandwidth decreases as frequency decreases. Conclusions. The different speeds naturally explain the elongation of an electron beam in space as it propagates through the heliosphere. The expansion rate is proportional to the mean speed of the exciter; faster beams expand faster. Beam speeds are attributed to varying ensembles of electron energies at the front, middle, and back of the beam.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Kontar, Professor Eduard and Reid, Dr Hamish
Authors: Reid, H. A.S., and Kontar, E. P.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Astronomy and Astrophysics
Publisher:EDP Sciences
ISSN (Online):1432-0746
Published Online:19 February 2018
Copyright Holders:Copyright © 2018 ESO
First Published:First published in Astronomy and Astrophysics 614: A69
Publisher Policy:Reproduced in accordance with the publisher copyright policy
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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
652881Simulating Sun-Earth Solar Radio BurstsHamish ReidThe Royal Society (ROYSOC)RG130642S&E P&A - PHYSICS & ASTRONOMY