Heavy ion mass loading of the geomagnetic field near the plasmapause and ULF wave implications

Abstract

[1] The structure of the density discontinuity across the plasmapause is often based on electron and H + density profiles with the contribution of heavy ions (He + , O + etc) neglected. Electron and ion density measurements in this region may differ significantly due to the presence of heavy ions and it is important for the intercomparison of different datasets to understand these differences. Dynamics Explorer (DE-1) magnetic field and plasma composition data have been used to compare heavy ion responses across the plasmapause and to calculate the mass loaded ion density (r) profiles. To illustrate this we investigate mass loading through radial profile variations in the Alfven velocity (V A ). Results show that the gradient in r and V A across the plasmapause is modified when mass loading due to multiple heavy ion species is included, particularly in the presence of the O + torus. Application to ultra-low frequency (ULF) field line resonance is used as an example where the contribution from heavy ions smoothes out the expected ULF wave resonant frequency discontinuity at the plasmapause.

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

“…The plasma morphology of the event summarized by Figs. 8 and 9 is in agreement with the presence of a heavy ion ''torus'' or ''shell'' in the vicinity of the plasmapause (Horwitz et al 1984;Singh et al 1992;Fraser et al 2005) as the oxygen ion density significantly exceeds the proton density. Exactly how this affects ULF wave generation is not yet fully understood (Fraser et al 2005) but it is suggested to be possible at locations where the drift-bounce resonance frequency matches the eigenfrequency of local standing Alfvén waves.…”

“…8 and 9 is in agreement with the presence of a heavy ion ''torus'' or ''shell'' in the vicinity of the plasmapause (Horwitz et al 1984;Singh et al 1992;Fraser et al 2005) as the oxygen ion density significantly exceeds the proton density. Exactly how this affects ULF wave generation is not yet fully understood (Fraser et al 2005) but it is suggested to be possible at locations where the drift-bounce resonance frequency matches the eigenfrequency of local standing Alfvén waves. The presence of cold high-density plasma may, therefore, be a controlling factor for ULF wave generation in this event as it determines the L value of the field line on which ULF waves are excited, i.e., where the field line eigenfrequency and drift-bounce Observations by THEMIS-D on 15 September 2011: a electron density derived from the spacecraft potential measurement; b-f wavelet power spectra of magnetic field (b radial, c azimuthal, d parallel) and electric field (e radial, f azimuthal) components in the MFA coordinate system; g spectra of ion energy flux between 1 and 30 keV Adapted from resonance frequency match.…”

“…It is unclear how plasma mass density structure near the plasmapause and plasmasphere boundary layer (PBL) affect other properties of ULF waves and field line resonances (Fraser et al 2005). Recent exploration of the role Fig.…”

“…2). There is already evidence of enhanced oxygen ion densities in the inner magnetosphere (e.g., Yau et al 1985;Hamilton et al 1988;) and significantly decreased eigenfrequencies of magnetic field lines (Fraser et al 2005). The source of this enhanced mass density may be drift-bounce resonant oxygen ions that mass load magnetic field lines, allowing Pc5 waves to penetrate to significantly lower L than normal (e.g., Lee et al 2007).…”

Pi2 pulsations in a small and strongly asymmetric plasmasphere

“…[20] Recently Fraser et al [2005] reported a heavy ion (O + ) torus near the plasmapause and showed that the FLR frequency discontinuity at the plasmapause is smoothed out. If we consider such a heavy ion effect near the plasmapause in our study, the fundamental FLR frequencies near the plasmapause will shift to a lower value.…”