Preprint: Alfv¶en Wave Instability of Current Sheets in Force-Free Collisionless Plasmas

If the current sheet between adjacent twisted magnetic °ux tubes is suf̄ciently thin, the electron °ow velocity becomes comparable to the Alfv¶en velocity and can destabilize collisionless Alfv¶en waves. The threshold for instability in force-free plasmas is calculated for both inertial and kinetic Alfv¶en wave regimes. When there is strong magnetic shear, unstable kinetic Alfv¶en waves can resonantly accelerate ions to energies much higher than the electron temperature. The direction of the magnetic ̄eld rotates abruptly at the interface between two adjacent twisted °ux tubes [1] corresponding to a thin current sheet °owing in this interface. The character of the current sheet depends [2] on the ratio of plasma thermal to magnetic energy ̄ = 210n·T=B : When ̄ » 1 (denoted high ̄), current sheets are associated with a magnetic ̄eld null and a localized region of increased plasma pressure [3,4]. When ̄ << 1 (denoted low ̄), the magnetic ̄eld magnitude remains approximately constant throughout the current sheet, there is no associated change in plasma pressure, and the plasma is nearly force-free. Low ̄ plasmas constitute an important class of plasmas and include the solar corona, the near-Earth magnetosphere, spheromaks, reversed̄eld pinches, and tokamaks. Magnetic reconnection is experimentally observed in all these plasmas and involves the magnetic ̄eld becoming locally unfrozen from the plasma. This has been traditionally modeled by invoking ̄nite Ohmic resistivity to allow magnetic ̄eld di®usion and dissipation at the current sheet with consequent modi ̄cation of the current sheet. However, for many important situations the plasma is essentially collisionless so that resistive models cannot explain magnetic reconnection. There is consequently great interest in ̄nding collisionless mechanisms which can modify current sheets and thereby enable magnetic reconnection. Recently this author [5] proposed that the radiation resistance associated with Alfv¶en wave emission by a thin current sheet could act as an e®ective collisionless resistivity, but