LOFAR Observations of Fine Spectral Structure Dynamics in Type IIIb Radio Bursts

Abstract

Solar radio emission features a large number of fine structures demonstrating great variability in frequency and time. We present spatially resolved spectral radio observations of type IIIb bursts in the 30 − 80 MHz range made by the Low Frequency Array (LOFAR). The bursts show well-defined fine frequency structuring called "stria" bursts. The spatial characteristics of the stria sources are determined by the propagation effects of radio waves; their movement and expansion speeds are in the range of (0.1−0.6)c. Analysis of the dynamic spectra reveals that both the spectral bandwidth and the frequency drift rate of the striae increase with an increase of their central frequency; the striae bandwidths are in the range of ∼ (20 − 100) kHz and the striae drift rates vary from zero to ∼ 0.3 MHz s −1 . The observed spectral characteristics of the stria bursts are consistent with the model involving modulation of the type III burst emission mechanism by small-amplitude fluctuations of the plasma density along the electron beam path. We estimate that the relative amplitude of the density fluctuations is of ∆n/n ∼ 10 −3 , their characteristic length scale is less than 1000 km, and the characteristic propagation speed is in the range of 400 − 800 km s −1 . These parameters indicate that the observed fine spectral structures could be produced by propagating magnetohydrodynamic waves.

Anisotropic Radio-wave Scattering and the Interpretation of Solar Radio Emission Observations

“…Turbulent density fluctuations which have a power that is weaker in the parallel direction compared to the perpendicular to radial direction result in a reduced time-broadening effect (i.e. radio-wave cloud broadening along the z direction); consequently the results with anisotropy factor α = 0.3 give a characteristic decay time ∼ 0.6 s, exactly as observed (see Figure 4 in Sharykin et al 2018).…”

“…Sub-arcminute imaging observations of Type III solar radio bursts have shown that intrinsic sources with sizes ∼ < 0 ′ .1 result in observed sources as large as ∼ 20 ′ at 30 MHz (Kontar et al 2017;Sharykin et al 2018), demonstrating that scattering dominates the properties of observed source sizes. Moreover, the locations of the upper and lower sub-band sources of Type II solar radio bursts are observed to be spatially separated (e.g., Zimovets et al 2012;Chrysaphi et al 2018), with the amount of separation being consistent with radio-wave scattering of plasma radio emission from a single region (Chrysaphi et al 2018).…”

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.…”

“…A characteristic spatial scale of striae was estimated around 700 km, using the Newkirk density model (Newkirk, 1961) rather than spatial positions because the source was located over the solar disc. Sharykin et al (2018) extended this study to analyse the spatial motion of the individual striae. These structures have an instantaneous bandwidth around 20-100 kHz that increases with increasing central frequency.…”

First imaging spectroscopy observations of solar drift pair bursts

“…Such high speeds are likely to be associated with apparent motion of the scattered image rather than the actual motion of the emitter (cf. Kontar et al 2017;Sharykin et al 2018).…”