The Discovery of Large Amplitude Solitary Waves at High Altitudes in the Cusp

C. A. Cattell

Observations of plasmas at low altitudes in the earth's auroral zones have provided evidence of the existence of small amplitude solitary waves and double layers (which have a net potential drop and accelerate auroral electrons) at altitudes of a few thousand to ten thousand kilometers. Data from the electric field instrument on the Polar satellite have recently shown that large amplitude solitary waves also occur in the low altitude auroral zone (see Nugget 1). At higher altitudes (30,0000 km to 50,000 km), there are many different regions which contain boundaries between a plasma of high density and one of low density. Strong currents, which have been observed by the Polar magnetometer, usually flow on the boundaries, providing a source of energy which can cause waves to grow. Two types of waves (unipolar and bipolar parallel pulses) occurring on one such boundary are described in other nuggets.

The cusp is a region on magnetic field lines which have recently been reconnected where magnetosheath plasma has direct access to the magnetosphere. Polar is the first satellite instrumented to make high time resolution measurements of the three-dimensional electric field and spacecraft potential at high altitudes in the polar cusp and polar cap. Using this capability, Wygant et al. [Fall AGU 1996] have shown that one to two orders of magnitude enhancements in the thermal plasma density with very rapid rise times (~30s) and slow decay times (1-2 hours) are often observed. These enhancements are observed to have a repetition rate of a few hours, with the peak densities decreasing as the satellite moves towards the nightside. On some days, the pattern of enhancements is less regular. We have examined the electric field waveform at frequencies up to 800 Hz in order to understand the type of plasma waves and time domain structures which are excited in association with the thermal density enhancements in the polar cusp and polar cap. These measurements show, for the first time, that solitary waves and double layers, similar to those observed at low altitudes, occur in association with field-aligned currents at the edges of the regions of enhanced plasma density.

Previous satellite observations of the electric fields in the cusp, which were made in the frequency domain, described the waves which occur there as being broadband. The Polar observations of the electric fields in the time domain reveal that, in fact, the waves are nonlinear solitary waves, a very different physical phenomena than linear broadband waves with different implications for the excitation of the waves and their energization of the plasma particles. This study reinforces the discoveries made elsewhere in the magnetosphere by time domain electric field instruments that the physics of boundary regions is dominated by nonlinear, rather than linear, electric fields.

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