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Solar radio emission features a large number of fine structures demonstratinggreat variability in frequency and time. We present spatially resolved spectralradio observations of type IIIb bursts in the $30-80$ MHz range made by the LowFrequency Array (LOFAR). The bursts show well-defined fine frequencystructuring called "stria" bursts. The spatial characteristics of the striasources are determined by the propagation effects of radio waves; theirmovement and expansion speeds are in the range of 0.1-0.6c. Analysis of thedynamic spectra reveals that both the spectral bandwidth and the frequencydrift 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 driftrates vary from zero to ~0.3 MHz s^-1. The observed spectral characteristics ofthe stria bursts are consistent with the model involving modulation of the typeIII burst emission mechanism by small-amplitude fluctuations of the plasmadensity along the electron beam path. We estimate that the relative amplitudeof the density fluctuations is of dn/n~10^-3, their characteristic length scaleis less than 1000 km, and the characteristic propagation speed is in the rangeof 400-800 km/s. These parameters indicate that the observed fine spectralstructures could be produced by propagating magnetohydrodynamic waves.

LOFAR observations of fine spectral structure dynamics in type IIIb radio bursts