On the energy principle and ion tearing in the magnetotail

We re‐examine the use of the energy principle as applied to the tearing instability in the magnetotail. We demonstrate that when a magnetic field component normal to the current sheet is present, electron pitch‐angle diffusion (PAD) either by micro‐turbulence or by chaotic orbits cannot remove the strong stabilization of the tearing mode caused by electron compressibility. We find that our conclusions are in agreement with those of Pellat et al. [1991], who argued on the basis of canonical P y conservation that the stabilization of the ion tearing mode cannot be removed by the introduction of PAD. Our results are at variance with those of Kuznetsova and Zelenyi [1991], who argued that the application of the energy principle used by Pellat et al. [1991] is incorrect, and that tearing is in fact unstable in the limit of strong PAD. We show that the disagreement between these two studies can be traced to an incorrect orbit evaluation first introduced by Coroniti [1980] and subsequently used by Kuznetsova and Zelenyi [1991].