Geotail observation of low frequency wave activity in a 3D structure of magnetotail reconnection site
Abstract:We have statistically examined plasma wave activity observed in the near Earth magnetotail reconnection events. Geotail encountered about 40 clear reconnection sites during 20 years observation where the enhanced cross-tail electron current layer was detected in association with the simultaneous plasma flow and magnetic field reversals. The intense plasma wave activity in wide frequency range is observed in the electron-ion decoupling region around the X-line. However, surprisingly, wave intensity right at the center of the electron current layers, those are possible X-lines, is much weaker than that in its surrounding region. Zenitani et al. (2012) successfully estimated the energy dissipation rate of the best reconnection event (May 15, 2013 event), and their result is also consistent with our interpretation that the observed wave activity cannot be a major player of the reconnection dissipation. These Geotail observed events suggest that the magnetic diffusion region of the near Earth magnetotail reconnection site is mainly controlled by the physics of the collisionless reconnection process, rather than the anomalous resistivity.
Despite this conclusion, the wave activity observed in the electron-ion decoupling region and the plasma sheet boundary region has still unignorable wave power to the electron heating. In order to address the role of the wave activity in the lower-hybrid frequency range around the X-lines, we have done a statistical survey of the wave activity and its relation to the structure of the magnetotail reconnection site. Based on the reconnection events shown in Nagai et al. (2013), we have examined the excitation process of the wave activity in the context of the 3-dimensional structure of the magnetotail reconnection. We found that wave activity in the dusk-side reconnection site (MHD like dawnward flow region) is much weaker than that in the ion-electron decoupling region of the post-midnight reconnection sites while similar activity can be seen in the dawn-side reconnection sites. These new findings will be a hint to understand the dawn-dusk evolution of the magentaotail diffusion region.