Convective instabilities of electromagnetic ion cyclotron waves in the outer magnetosphere

Journal Article · Thu Sep 01 04:00:00 EDT 1994 · Journal of Geophysical Research

DOI:https://doi.org/10.1029/94JA01259· OSTI ID:168130

Horne, R B ^[1]; Thorne, R M ^[2]

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1. Natural Environmental Research Council, Cambridge (United Kingdom)
2. Univ. of California, Los Angeles, CA (United States)

The path-integrated linear growth of electromagnetic ion cyclotron waves in the outer (L {le} 7) magnetosphere is investigated using a realistic thermal plasma distribution with an additional anisotropic energetic ring current H{sup +} to provide free energy for instability. For conditions typical of the dayside magnetosphere, high plasma beta effects reduce the group velocity and significantly increase the spatial growth rates for left-hand polarized instabilities just below the helium gyrofrequency {Omega}{sub He{sup +}}, and on the unguided mode above {Omega}{sub He{sup +}} but below the cross over frequency {omega}{sub cr}. Relatively high densities, typical of the afternoon local time sector, favor these low group velocity effects for predominantly field-aligned waves. These waves are reflected in the magnetosphere and can exist for several equatorial transits without significant damping. They are left-hand polarized only on the first equatorial crossing and become linearly polarized for the remainder of the ray path. Consequently, these waves should be observed with basically linear polarization at all frequencies and all latitudes in the early morning local time sector. Waves below {Omega}{sub He{sup +}} are not subject to reflection when the O{sup +} concentration is small and therefore should be observed with left-hand polarization near the equator and essentially linear polarization at higher latitudes. Since the He{sup +} concentration is usually large in the afternoon sector, guided mode waves above {Omega}{sub He{sup +}} reflect to form a background distribution with basically linear polarization. The authors suggest that strong left-hand polarized emissions observed by AMPTE in the afternoon sector near the equator are probably due to strongly growing low group velocity waves at frequencies just below {Omega}{sub He{sup +}} and on the unguided mode above {Omega}{sub He{sup +}}. 50 refs., 13 figs.

• OSTI ID: 168130
• Journal Information: Journal of Geophysical Research, Journal Name: Journal of Geophysical Research Journal Issue: A9 Vol. 99; ISSN JGREA2; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
CONVECTIVE INSTABILITIES
EARTH MAGNETOSPHERE
ELECTROMAGNETIC RADIATION
INSTABILITY GROWTH RATES
ION CYCLOTRON-RESONANCE
POLARIZATION
WAVE PROPAGATION