Coordinated measurements of ELF transmission anomalies and the precipitation of energetic particles into the ionosphere

Abstract

Stimulated by previous findings of a positive qualitative correlation between anomalous ELF signal levels and the occurrence of significant fluxes of precipitating energetic particles, coordinated exercises were conducted between satellite measurements of the precipitating particles and ELF transmissions between the US Navy Wisconsin Test Facility and receiving stations in Maryland, Greenland, Norway, and Italy. Significant fluxes of precipitating electrons were often measured with the Lockheed pay load on the polar‐orbiting satellite 1972–076B. In March‐April 1976 the received signal strengths were found generally to increase during periods when the geomagnetic K indices were higher and when the fluxes of precipitating electrons >160 keV were greater on the average. During other coordinated exercises in March‐April 1975 the general correlations were less evident. Based on the fluxes, energy spectra, and pitch‐angle distributions of the particles, ion‐pair production profiles have been calculated and electron density profiles subsequently obtained with application of known effective electron loss rates. From the coordinated data, correlative studies between signal strengths and electron density profiles have been performed with use of an ELF waveguide mode computer program developed at the Naval Ocean Systems Center. The propagation path to each station of interest is segmented into different regions with an averaging perpendicular to the direction of propagation of the waveguide parameters. With the inclusion of data from several stations, the transmission signal strengths could be studied simultaneously under a variety of ionospheric conditions. The waveguide computer program calculations indicated that energetic electron precipitation events may cause a signal enhancement or degradation, depending upon where the disturbance is occurring along the path. The consistent occurrence during electron precipitation events of signal strength anomalies in either direction is supported by the coordinated satellite/ELF station measurements.