Intense electrostatic waves near the upper hybrid resonance frequency

Abstract

Plasma wave measurements using instruments on the Imp 6 and Hawkeye 1 satellites are utilized in a study of very intense electrostatic waves near the upper hybrid resonance frequency in the region just outside the plasmapause. These intense plasma wave events have electric field strengths of ∼1–20 mV m−1 and are among the most intense waves seen in the earth's magnetosphere. Detailed studies of more than 140 of these intense electrostatic disturbances reveal that the events occur at all local times and at magnetic latitudes varying from the equator to as high as 50°. The polarization of these waves is such that the wave electric field vector is oriented perpendicular to the geomagnetic field. In most cases the center frequency of the intense waves appears to correspond to an (n + ½)fg− harmonic near the upper hybrid resonance frequency. A survey of plasma measurements made simultaneously using the Hawkeye 1 Lepedea shows that the occurrence of the intense electrostatic waves is not strongly controlled by the intensities of ∼1‐ to 20‐keV electrons but that specific details of the hot electron distribution function are directly related to the wave turbulence. All events at magnetic latitudes less than about 10° show strong pitch angle anisotropy with the greatest intensities at α ≈ 90°. The hot distribution function f(υ⊥, υ∥) is described for a few events showing two sources of free energy; a temperature anisotropy and a loss cone distribution. One event shown suggests that a bump on tail in υ⊥ may also contribute free energy in some cases. A possible mechanism for producing intense waves near the upper hybrid resonance frequency is suggested which draws upon current theories applied to the generation of (n + ½)fg− bands. Evidence is given which suggests that the intense electrostatic waves may be a source of nonthermal continuum radiation.