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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A8, 1213, doi:10.1029/2001JA000315, 2002

Interpreting observations of MF/HF radio emissions: Unstable wave modes and possibilities to passively diagnose ionospheric densities

A. T. Weatherwax

Institute for Physical Science and Technology, University of Maryland, College Park, Maryland, USA


Peter H. Yoon

Institute for Physical Science and Technology, University of Maryland, College Park, Maryland, USA


J. LaBelle

Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire, USA


Abstract

Recent satellite observations by the Wind spacecraft, as well as earlier observations by the ISIS 1 and 2 satellites, of MF/HF radio emissions have been attributed to upper hybrid/Z mode waves generated in the topside ionosphere at two and three times the local electron gyrofrequency fce. We calculate the growth rates of such emissions for various wave modes as a function of altitude in the topside ionosphere and for a variety of electron density profiles, employing a modified loss-cone/horseshoe electron distribution function as the free energy source. These calculations represent the first application of this instability model to the topside ionosphere, in parallel to earlier efforts involving wave excitation in the bottomside of the F region. In addition to the Z mode, which is known to be unstable at points where the upper hybrid frequency matches cyclotron harmonics, the X mode can be directly generated near the first harmonic of the electron cyclotron frequency for certain density profiles. We further put forth how these emissions might be used as a remote diagnostic of ionospheric plasma properties.

Published 28 August 2002.

Index Terms: 2407 Ionosphere: Auroral ionosphere (2704); 2471 Ionosphere: Plasma waves and instabilities; 7847 Space Plasma Physics: Radiation processes; 7867 Space Plasma Physics: Wave/particle interactions.

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Citation: Weatherwax, A. T., P. H. Yoon, and J. LaBelle (2002), Interpreting observations of MF/HF radio emissions: Unstable wave modes and possibilities to passively diagnose ionospheric densities, J. Geophys. Res., 107(A8), 1213, doi:10.1029/2001JA000315.