Enhancement of ultra-low frequency wave amplitudes at the plasmapause

Abstract:

We present measurements of ultra-low frequency (ULF) wave amplitudes measured by the THEMIS probes while crossing the plasmapause. During one crossing on 24 June 2007 which we study in detail, all three probes of which data was obtained show an increase in the ULF wave amplitude between 10 and 50 mHz by about 30\%. These results are confirmed by a statistical study that examines the ULF wave amplitude in the same frequency range averaged over 132 plasmapause crossings between June 2007 and December 2007 made by THEMIS C, D, and E. We find that, assuming the plasmapause can be approximated by a tangential MHD discontinuity, a ULF wave amplitude enhancement of 30\% is in agreement with theoretical transmission coefficient calculations if the plasma density increases by a factor of about 130 while simultaneously the plasma temperature decreases by a factor of about 73. While the plasma density estimate is confirmed by observations derived from spacecraft potential measurements, the temperature decrease cannot be confirmed because the cold plasmaspheric particle populations are not detected by the THEMIS particle instruments; however, the value is in reasonable agreement with previous measurements of magnetospheric and plasmaspheric plasma temperatures. These results might have important implications for the detection of global fast modes by satellites as their amplitude is hence expected to by higher inside the plasmasphere than outside.