Quasi-parallel whistler mode waves observed by THEMIS during near-earth dipolarizations

We report on quasi-parallel whistler emissions detected by the near-earth satellites of the THEMIS mission before, during, and after local dipolarization. These emissions are associated with an electron temperature anisotropy alpha = T-perpendicular to e/T-parallel to e > 1 consistent with the linear theory of whistler mode anisotropy instability. When the whistler mode emissions are observed the measured electron anisotropy varies inversely with beta(parallel to e) (the ratio of the electron parallel pressure to the magnetic pressure) as predicted by Gary and Wang (1996). Narrow band whistler emissions correspond to the small alpha existing before dipolarization whereas the broad band emissions correspond to large alpha observed during and after dipolarization. The energy in the whistler mode is leaving the current sheet and is propagating along the background magnetic field, towards the Earth. A simple time-independent description based on the Liouville's theorem indicates that the electron temperature anisotropy decreases with the distance along the magnetic field from the equator. Once this variation of alpha is taken into account, the linear theory predicts an equatorial origin for the whistler mode. The linear theory is also consistent with the observed bandwidth of wave emissions. Yet, the anisotropy required to be fully consistent with the observations is somewhat larger than the measured one. Although the discrepancy remains within the instrumental error bars, this could be due to time-dependent effects which have been neglected. The possible role of the whistler waves in the substorm process is discussed.