The Earth as a radio source: The nonthermal continuum

In addition to the intense and highly variable auroral kilometric radiation the earth also radiates a weak nonthermal continuum from energetic electrons in the outer radiation zone. The intensity of this continuum radiation decreases with increasing frequency and is usually below the cosmic noise level at frequencies above 100 kHz. In this paper we show that the frequency spectrum of the continuum radiation consists of two components, a trapped component, which is permanently trapped within the magnetosphere at frequencies below the solar wind plasma frequency, and an escaping component, which propagates freely away from the earth at frequencies above the solar wind plasma frequency. The low‐frequency cutoff of the continuum radiation spectrum is at the local electron plasma frequency, which can be as low as 500 Hz in the low‐density regions of the distant magnetotail. Direction‐finding measurements and measurements of the spatial distribution of intensity for both the trapped and the freely escaping components are used to determine the region in which the continuum radiation is generated. These measurements all indicate that the continuum radiation is generated in a broad region which extends through the morning and early afternoon from about 4.0 to 14.0 hours local time immediately beyond the plasmapause boundary. In contrast to the auroral kilometric radiation, which is generated in the high‐latitude auroral zone regions, the continuum radiation appears to be generated over a broad range of latitudes, including the magnetic equator. In some cases the continuum radiation appears to be closely associated with intense bands of electrostatic noise which are observed near the electron plasma frequency at the plasmapause. Possible mechanisms by which this radiation could be generated, including gryosynchrotron radiation from energetic electrons in the outer radiation zone, are discussed.