Hydromagnetic emissions, X-ray bursts, and electron bunches: 1. Experimental results

Abstract

Recent experimental results are presented concerning the occurrence, structure, and frequency-latitude dependence of hydromagnetic emissions (regular oscillatory micropulsations in the frequency range 0.4–7 cps). Results are also presented for the occurrence of noise bursts (irregular micropulsations in the same frequency range). Evidence is considered suggesting that both hm emissions and noise bursts are generated by related mechanisms. The experimental results are summarized as follows: (1) Occurrence. First, short hm emission bursts sometimes occur about l min after the sudden commencement of magnetic storms. Second, simultaneous occurrence with X-ray bursts and increased riometer absorption has been noted. (2) Structure. Hydromagnetic emissions tend to occur in distinct frequency bands. The bands are usually characterized by a fine structure consisting of a superposition of repetitive wave trains of a few minutes' duration and rapidly increasing frequency. (3) Frequency--latitude dependence. An inverse relationship has been found between the highest observed hm emission frequency and the geomagnetic latitude at which the signal is observed.