Auroral hiss, Z mode radiation, and auroral kilometric radiation in the polar magnetosphere: DE 1 observations

Abstract

The polar‐orbiting DE 1 spacecraft is now providing the first measurements of high‐latitude auroral phenomena in the radial distance range between about 2 and 5 RE, where several important types of auroral plasma wave emissions are believed to be generated. This paper describes the initial observations from the DE 1 plasma wave instrument in this interesting region. Three principal types of plasma wave emissions are discussed: auroral hiss, Z mode radiation, and auroral kilometric radiation. Whistler mode auroral hiss emissions are observed on essentially every pass over the auroral zone. The auroral hiss usually has a characteristic ‘funnel‐shaped’ frequency‐time spectrum which can be explained by a simple whistler mode propagation effect if the radiation is emitted from a spatially localized source below the spacecraft. Ray path studies show that the auroral hiss is propagating upward from a source, the lower boundary of which is an altitude of about 0.7–0.9 RE. The DE 1 observations have identified broadband Z mode emissions in the low‐density region over the auroral zone and polar cap. This radiation usually has a sharply defined upper cutoff near the electron gyrofrequency and extends downward in frequency to a cutoff called the fL = 0 cutoff. The Z mode radiation is sometimes difficult to distinguish from the auroral hiss, which occurs in the same general frequency range with about the same intensity. The Z mode radiation probably corresponds to the noise previously identified as ‘continuum radiation’ in the Hawkeye polar region data. The auroral hiss can usually be distinguished from the Z mode radiation by the sharp upper cutoff of the whistler mode at the local electron plasma frequency. Broadband emissions identified as auroral kilometric radiation are frequently observed over the evening auroral regions at frequencies between about 100 kHz and 400 kHz. These emissions are very intense, usually 30–50 dB above the intensity of the auroral hiss and Z mode radiation, and are highly variable, sometimes disappearing completely. The auroral kilometric radiation usually occurs at frequencies above the electron gyrofrequency, consistent with earlier measurements which indicate that this radiation is propagating in the free‐space R‐X mode.