Role of electron temperature anisotropy in the onset of magnetic reconnection

Predictions of tearing saturation in a neutral sheet range from minute amplitude to explosive growth. It is shown that in 2D, single island tearing saturates at very small amplitudes due to preferential electron heating in the parallel direction. However, the presence of multiple unstable modes allows the system to get past the stabilization and grow to ion scales. In 3D there are two current aligned instabilities that can affect the tearing mode. One is the Weibel instability driven by electron anisotropy T-perpendicular toe/T-parallel toe < 1 and the other is the lower hybrid drift instability (LHDI). The Weibel instability is shown to at most increase the single island tearing saturation amplitude up to the singular layer thickness. On the other hand, recent studies suggest that LHDI can affect tearing through the generation of T-perpendicular to e/T-parallel to e > 1 within the current layer. In this limit, the parallel Weibel instability merges with tearing to give rise to anisotropic tearing, which becomes the dominant electron anisotropy driven mode with a broad angular spectrum. The level of electron anisotropy generated by the LHDI can significantly enhance the tearing growth rate and extend the spectrum to very short wavelength which expedites the transition to large scale reconnection. Thus LHDI's main contribution to reconnection onset is not through anomalous resistivity but through its effect on tearing mode.