Recent Investigations of Hydromagnetic Emissions Part II. Theoretical Interpretation

Recent experimental and theoretical work suggests the hypothesis that hydromagnetic (hm) emissions are hydromagnetic wave packets guided by field lines as they bounce back and forth between hemispheres of the earth. As such they are analogous to whistlers, and one test of the hypothesis is the quantitative calculation of plasma densities in the outer magnetosphere and their comparison with whistler densities closer to the earth.Many hm emission exhibit measurable dispersion when displayed in the form of sonagrams (frequency vs time displays). In such cases, the measurement of group bounce times at two different emission frequencies can be combined with an accurate model of the magnetosphere to determine the zero-frequency (Alfvén) bounce period and the equatorial cyclotron frequency above which the wave cannot propagate. These two quantities uniquely define the field line along which the hm emission propagated and the integrated plasma density along that field line.Analysis of 9 events showed they occurred on field lines crossing the equatorial plane between 4 and 10 earth radii, and showed equatorial plasma densities in agreement with extrapolated whistler “knee” densities.