Confinement of nonthermal continuum radiation to low latitudes

Abstract

Applying a wave intensity map technique to 4 years of plasma wave observations from the VLF electric field instrument on the Hawkeye spacecraft reveals the latitudinal distribution of the Earth's nonthermal continuum radiation. The wave intensity maps at frequencies below (23.7 kHz), near (31.1 kHz), and above (56 kHz) the average magnetopause plasma frequency clearly identified a source region and propagation extent of the nonthermal continuum. The Hawkeye observations show that the nonthermal continuum radiation is primarily observed at relatively low latitudes, is rarely observed over the high-altitude polar cap, and is consistent with previous observations as being beamed outward from the dawn source region. The observed angular distribution of the nonthermal continuum radiation is modeled by ray-tracing calculations which show that the trapped and escaping components of the nonthermal continuum can exist simultaneously at the same frequency but with very different spatial distributions in the magnetosphere. In addition, the observations and the ray-tracing calculations show that the trapped continuum radiation component does not uniformly illuminate the magnetospheric cavity as some have previously believed but is confined to mostly low latitudes.