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

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 $(0.1\,\mbox{--}\,0.6)c$ . 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 ${\approx}\,(20\,\mbox{--}\,100)$  kHz and the striae drift rates vary from zero to ${\approx}\,0.3~\mbox{MHz}\,\mbox{s}^{-1}$ . 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 $\Delta n/n\sim10^{-3}$ , their characteristic length scale is less than 1000 km, and the characteristic propagation speed is in the range of $400\,\mbox{--}\,800~\mbox{km}\,\mbox{s}^{-1}$ . These parameters indicate that the observed fine spectral structures could be produced by propagating magnetohydrodynamic waves.