Electron hole structure and its stability depending on plasma magnetization

Abstract

Observations from Polar have revealed that electron hole (e-hole) structure critically depends on the plasma magnetization determined by the ratio Ω̄=Ωe / ωpe, where Ωe and ωpe are the electron cyclotron and plasma frequencies, respectively. Using three-dimensional parallel particle-in-cell) simulations, we have studied the formation and structure of e-holes by varying Ω̄ showing that (1) for Ω̄<1 e-holes are highly transitory while for Ω̄ > 1 long-lasting e-holes form, especially when Ω̄ ≥ 2, (2) in the transitory e-holes for Ω̄ < 1, the e-holes are essentially planar with parallel electric fields E∥ ≫ E⊥, the perpendicular field, (3) when Ω̄ ≥ 2 a variety of structures are possible ranging from spheroidal structures with Ex ∼ Ey ∼ E∥ to a planar one with Ex ∼ Ey ≪ E∥, where Ex and Ey are the x and y components of the perpendicular field E⊥. It is also possible that Ex ≪ Ey ∼ E∥ or Ey ≪ Ex ∼ E∥. We find that e-holes with long transverse structures undergo a perpendicular modulation, which eventually breaks the long structures into fragments of e-holes. The fragments are seen to further decay by emitting lower hybrid (LH) waves. A linear model of these instabilities is developed showing an e-hole is an effective radiator of plasma waves. An e-hole with long transverse structure is effective in radiating high-frequency whistler waves which have long perpendicular wavelengths. When such waves are radiated out, the remnant structure is transversely modulated and has relatively short scale length. On the other hand, the fragments with the short perpendicular scale lengths are effective in radiating the LH waves. These findings from the simulations are compared with findings from the data from Polar [Franz et al., 2000]. Copyright 2001 by the American Geophysical Union.