WAVEGUIDE THEORY FOR IONOSPHERIC PROPAGATION OF HYDROMAGNETIC EMISSIONS

Results of earlier experimental and theoretical work (reviewed by Tepley [1965a, b] and Wentworth [1965]) suggest that hydromagnetic (hm) emissions are generated by a plasma instability at 4–8 earth radii in the vicinity of the equatorial plane. The emissions then propagate in the ion-cyclotron (Alfven) mode along high-latitude geomagnetic field lines toward the earth. Part of the signal energy penetrates the ionosphere in or near the auroral zones, and the remainder is reflected back along geomagnetic field lines and is successively amplified upon passing through the equatorial interaction region. In this note we suggest that part of the signal energy that penetrates the ionosphere is transformed (possibly through collisional processes) into the isotropic (fast) hm wave mode and then propagates horizontally in an ionospheric waveguide around the Alfven (or phase) velocity minimum near the F2 peak. The complete propagation path is illustrated schematically in Figure 1. The suggestion of waveguide propagation is based upon the experimental results presented in the accompanying letter [Wentworth et al., 1966], which indicate hm emission propagation times on the order of seconds between widely separated stations in the same hemisphere.