Abstract
Abstract Solar radio type III bursts are believed to be the most sensitive signatures of near-relativistic electron beam propagation in the corona. A solar radio type IIIb-III pair burst with fine frequency structures, observed by the Low Frequency Array (LOFAR) with high temporal (∼10 ms) and spectral (12.5 kHz) resolutions at 30–80 MHz, is presented. The observations show that the type III burst consists of many striae, which have a frequency scale of about 0.1 MHz in both the fundamental (plasma) and the harmonic (double plasma) emission. We investigate the effects of background density fluctuations based on the observation of striae structure to estimate the density perturbation in the solar corona. It is found that the spectral index of the density fluctuation spectrum is about −1.7, and the characteristic spatial scale of the density perturbation is around 700 km. This spectral index is very close to a Kolmogorov turbulence spectral index of −5/3, consistent with a turbulent cascade. This fact indicates that the coronal turbulence may play the important role of modulating the time structures of solar radio type III bursts, and the fine structure of radio type III bursts could provide a useful and unique tool to diagnose the turbulence in the solar corona.
Highlights
Solar radio type III bursts are a common signature of nearrelativistic electrons streaming through plasma in the solar corona and interplanetary space, and they provide a useful way to remotely trace these electrons (e.g., Wild 1950; Pick & Vilmer 2008)
Solar radio type III bursts are produced by near-relativistic electrons streaming through the corona and such bursts can provide important information about the background plasma properties
The background plasma density corresponding to a frequency of several tens of megahertz in the high corona is nonuniform, and type III bursts should have flux variations associated with density fluctuations, as is evident from numerical simulations
Summary
Solar radio type III bursts are a common signature of nearrelativistic electrons streaming through plasma in the solar corona and interplanetary space, and they provide a useful way to remotely trace these electrons (e.g., Wild 1950; Pick & Vilmer 2008). Li et al (2012), using numerical methods, have further confirmed that enhanced density structures along the beam path and either electron or ion temperature enhancements of unknown origin can reproduce striae-like features They find that the second harmonic emission should be stronger than the fundamental emission, which is inconsistent with the observations. Our observations of a type IIIb radio burst support the result that density turbulence in the corona is likely to cause the striae structure, and that the escaping plasma radio emission fluctuates along the direction that the beam propagates with a power-law flux fluctuation spectrum.
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