Microbursts at meter-decameter wavelengths

We study the characteristics of microbursts using a large data base obtained with the multifrequency radioheliograph of the Clark Lake Radio Observatory. Most of the new observations were made during July 29, 1985 to August 2, 1985; we also include for statistical studies the microburst data used in our earlier studies. We perform a statistical analysis of many characteristics such as frequency drift, source size and brightness temperature and compare them with the properties of normal type III bursts. We investigate the coronal structures and surface activities associated with some of the events. We find that (i) the brightness temperature is in the range 6 × 10⁵ K to 6 × 10⁷ K; (ii) the drift rate of the microbursts is slightly smaller than that of normal type III bursts, implying electron beams with speeds 0.2c.We explore various theoretical interpretations of the observed low brightness temperatures. We show that the microbursts can be explained as due to spontaneously emitted Langmuir waves by electron beams whereas normal type III bursts are due to coherently emitted plasma waves in a two-stream instability. We estimate the range of number densities for electron beams responsible for microbursts.On leave from Indian Institute of Astrophysics, Bangalore, India.