Alfven wave instability of current sheets in force-free plasmas: comparison to ion acoustic instability

Abstract

Current sheets necessarily form at the interface between adjacent twisted flux tubes and so are ubiquitous to magnetic configurations having non-trivial field topology. In an earlier publication (P. M. Bellan, Phys. Rev. Letters, 83,4768, 1999) the author showed that current sheets in a low β plasma will be kinetically unstable with respect to Alfvén wave emission if the current sheet becomes sufficiently thin for the field-aligned electron flow to become super-Alfvenic. At first sight it might appear that before the current sheet becomes thin enough to develop a super-Alfvenic flow, ion acoustic instabilities will develop when the sheet becomes thin enough for the electron flow to exceed the ion acoustic velocity. However, it is shown here that because of strong ion Landau damping on ion acoustic waves for plasmas with comparable electron and ion temperatures, ion acoustic waves have a much higher instability threshold than the Alfvén instability. Thus, Alfvén instability should dominate ion acoustic instability in thin current sheets.