Microscopic spectral features in solar decametric bursts and coronal irregularities

Abstract

A crossed Yagi antenna array at 35 MHz was employed in conjunction with a polarization switch so as to enable spectral observations of solar noise storm activity in R and L polarizations. Intense decametric solar noise storms were recorded during the third week of November 1975 and fourth week of March 1976 with the help of a high resolution spectroscope operating near 35 MHz. The paper describes some of the new microscopic spectral features observed during these two noise storms. Three sets of high resolution dynamic spectra of decametric solar bursts, two of which are explained in terms of induced scattering of Langmuir waves by thermal ions and the third in terms of additional propagation effects through dense coronal irregularities, are presented. The microscopic bursts, classified as 'inverted U' 'U and dots', represent small-scale (~10⁴ km) phenomena with durations of less than a second. Some burst spectra appear as chain of dots with individual bandwidths ~40 kHz and durations ~0.3 sec. It is suggested that the bandwidth of such dot emissions (~40 kHz) provides an evidence that they might indeed be generated by the process of induced scattering of plasma waves which predicts emission bandwidth ~f × 10^-3, where f is the center frequency. Some bursts are observed as a chain of striations showing curvature along the frequency axis which is attributed to dispersion in propagation delays through the dense coronal irregularities.