If the current sheet between adjacent twisted magnetic flux tubes is sufficiently thin, the electron flow velocity becomes comparable to the Alfvén velocity and can destabilize collisionless Alfvén waves. The threshold for instability in force-free plasmas is calculated for both inertial and kinetic Alfvén-wave regimes. When there is strong magnetic shear, unstable kinetic Alfvén waves can resonantly accelerate ions to energies much higher than the electron temperature.

• Received 24 May 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.4768