Polar observations of solitary waves at high and low altitudes and comparison to theory

Solitary waves with large electric fields (up to 100 mV/m) are often observed by Polar in the low altitude auroral zone and, at high altitudes(~ 4-8 R _E ), during crossings of the plasma sheet boundary and cusp. Electron solitary waves are ubiquitous, and are observed for wide range of f _ce /f _pe . In contrast, to date, ion solitary waves have only been observed in the auroral zone at low altitudes in the region where f _ce /f _pe ≥1. We describe some results of statistical studies of ion solitary waves at low altitudes and electron solitary waves at high altitudes. Ion solitary waves, observed in regions of upward field-aligned currents and ion beams, are negative potential structures and have velocities between the speeds of the associated O ⁺ and H ⁺ beams, scale sizes of approximately 10λ _D , and normalized amplitudes, eφ/kT _e , of order 0.1 because the electron temperatures are large (plasma sheet values). In addition, the amplitude increases with both the velocity and the scale size which is inconsistent with the predictions of ion acoustic soliton theory. The observations are well modeled by the simulations of Crumley et al. (2001) which include only the plasma sheet electrons and the beam ions. Both observations and the simulations are consistent with an ion hole mode associated with the ion two stream instability. The high altitude electron solitary waves have velocities from ~1000km/s to >2500 km/s. Observed scale sizes are on the order of 1-10λ _D with eφ/kT _e up to O(1). For these solitary waves also, the amplitude increases with both the velocity and the scale size, consistent with electron hole modes as was observed at low altitudes. Even the very large amplitude solitary waves are stable based on the criterion developed by Muschietti et al.(1999).