Hydromagnetic emissions, X-ray bursts, and electron bunches: 2. Theoretical interpretation

The existence of bunches of monoenergetic electrons oscillating back and forth along field lines in the magnetosphere has been suggested by a number of workers to explain various geophysical phenomena. The simultaneous occurrence of X-ray and hydromagnetic emission bursts (regular oscillatory micropulsations at frequencies in the range 1–2 cps) immediately after sudden commencements and during magnetically disturbed periods, as discussed in part 1, raises the possibility that hm emissions could be generated by such electron bunches. A model is presented that considers certain aspects of the motion of geomagnetically trapped electrons relevant to the production of hm emissions by electron bunches. A new result is the calculation of electron motion in a dipole field confined to the interior of a sphere (as a first approximation to the solar wind cavity). The model calculations are applied to experimental results discussed in part 1, and it is shown that the electrons responsible for the X-ray burst after the sudden commencement of June 27, 1960, could have produced the hm emission burst at that time, and that the maximum observed hm emission frequency as a function of geomagnetic latitude is consistent with the calculated value. Hydromagnetic emissions during magnetically quiet times are characterized by a fine structure containing overlapping bands of rapidly increasing frequency. This is tentatively interpreted as indicating rapid acceleration of electron bunches; hence, if the model is correct, hm emissions will provide a powerful tool for studying the acceleration mechanism for electrons in the magnetosphere.