Microwave Bursts of Solar Radio Emission

Abstract Wide-band compound dynamic spectra of solar radio outbursts have been studied in the frequency range from 9500 Mc to 67 Mc. The range of 9500–1000 Mc has been covered by the records at several fixed frequencies, and both the swept frequency records and fixed frequency records have been used on the frequencies below 1000 Mc. A typical outstanding outburst may be as follows. A short-lived burst whose duration is several minutes begins almost simultaneously at the frequencies above 1000 Mc. The flux density of the burst is maximum at about 9000 Mc-3000 Mc, decreases with decreasing frequency and is generally very weak on the frequencies below 1000 Mc. A few minutes later, a Type II burst begins in the meter wave region with a sharp high frequency cut-off which is usually below 350 Mc for the fundamental wave (and below 700 Mc for the second harmonic). Following the Type II burst, a broad-band Type IV burst begins and lasts one hour or two. The frequencies for the maximum flux density of the Type-IV bursts distribute depending on the case from 200 Mc to 10,000 Mc. Above mentioned short-lived burst at the frequencies above 1000 Mc is neither an extension of the Type II burst nor the same as the Type IV burst. It is a distinctive type of bursts and may be called “Microwave burst” or “M-type” burst. In comparison with the current classification of bursts at the microwaves, short-lived SD (Simple and Distinct) and CD (Complex and Distinct) bursts whose durations are less than about 10 minutes seem to be the “M-type” bursts, while long-lived CD bursts might be Type IV bursts.