Linear theory of anisotropy driven modes in a Harris neutral sheet

There are several sources of electron and ion anisotropies that can have a profound effect on the stability of the current sheets in the magnetosphere. A new semianalytical approach is introduced and utilized to develop the linear theory of anisotropy driven modes in a neutral sheet. This technique is intermediary between analytical models and those that solve the exact linear Vlasov equation. Its advantage is in its accuracy and speed. Both the parallel and perpendicular limits are considered, and improved stability criteria and growth rates for Weibel, anisotropic tearing, and Fried-Weibel modes are obtained. For the same anisotropy levels, electron anisotropy is much more effective in modifying the stability of the modes, but the presence of large ion anisotropy in the magnetosheath can still have a significant effect on the growth of the tearing mode. Effects of ion anisotropy are more pronounced for thicker sheets, whereas the electron anisotropy is weakly dependent on the current sheet thickness. Although the current expectation is that magnetic reconnection in the magnetosphere is associated with the formation of thin current sheets, our results suggest an interesting possibility of fast growth rates for thick sheets in the presence of sufficient electron and/or ion anisotropies.