Abstract
Abstract Low-frequency radio observations make it possible to study the solar corona at distances up to 2–3 R ☉. Frequency of plasma emission is a proxy for electron density of the emitting plasma and, therefore, observations of solar radio bursts can be used to probe the density structure of the outer corona. In this study, positions of solar radio sources are investigated using the Low-Frequency Array (LOFAR) spectral imaging in the frequency range 30–50 MHz. We show that there are events where apparent positions of the radio sources cannot be explained using the standard coronal density models. Namely, the apparent heliocentric positions of the sources are 0.1–0.7 R ☉ further from the Sun compared with the positions predicted by the Newkirk model, and these shifts are frequency-dependent. We discuss several possible explanations for this effect, including enhanced plasma density in the flaring corona, as well as scattering and refraction of the radio waves.
Highlights
Most observations of the solar atmosphere are limited to the lower corona, where the ambient plasma density and magnetic field are sufficiently high to produce substantial fluxes of X-ray, extreme ultra-violet, and microwave emissions, which can be used for diagnostics of thermal and nonthermal plasma components
If the emission is produced at the local fundamental plasma frequency, its frequency depends on local electron density as f [MHz] = 8.93 ́ 10-3 n[cm-3]
The average height of the source observed on 2015 June 25 (Section 3.3) requires densities from the Newkirk model multiplied by a factor of 2.5–4.5 with the projection angle of 90°; the shape of FD function of this source corresponds to the hydrodynamic scale length of about 1 Re
Summary
Most observations of the solar atmosphere are limited to the lower corona, where the ambient plasma density and magnetic field are sufficiently high to produce substantial fluxes of X-ray, extreme ultra-violet, and microwave emissions, which can be used for diagnostics of thermal and nonthermal plasma components. Energetic electrons propagating through the solar corona and heliosphere result in the generation of plasma waves, which, in turn, produce coherent radio emission at and above the local plasma frequency, from ∼1 GHz in the lower corona to few megahertz in the outer corona (see, e.g., Ginzburg & Zhelezniakov 1958; Kundu 1965; Takakura 1967; Melrose 1980, for review). Solar radio observations with both spatial and spectral resolution substantially enhance diagnostics opportunities, making possible simultaneous diagnostics of the nonthermal and thermal plasma components in the coronal plasma (e.g., Bastian et al 2001; Pick & Vilmer 2008; Kontar et al 2017b)
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