Heating of ionospheric O+ ions by shear Alfvén waves

Ionospheric ions, in particular O⁺ ions, which have been transversely heated, are often observed flowing upward along auroral field lines. A new mechanism, heating by current-driven shear (or kinetic) Alfvén waves (SAW), is proposed. An electron current drives oblique SAWs unstable near ω≃Ω₀, and these waves are in turn gyroresonantly absorbed by the ions. The mechanism is similar to ion heating by current-driven electrostatic ion cyclotron waves (EICW). However, the SAW differs from the EICW in that as the perpendicular temperature of the ions increases, growth of the SAW can still occur, whereas growth of the EICW becomes suppressed. As a consequence the SAW is able to provide sustained perpendicular heating of ions with smaller currents being required for the heating than for heating via EICWs.