Alfvénic heating in the cusp ionosphere‐thermosphere

The effect of electromagnetic variability on cusp-region ionosphere-thermosphere heating is examined. The study is motivated by observed correlations between anomalous thermospheric density enhancements at F region altitudes and small-scale field-aligned currents, previously interpreted as evidence of ionospheric Alfven resonator modes. Height-integrated and height-dependent heating rates for Alfven waves incident from the magnetosphere at frequencies from 0.05 to 2 Hz and perpendicular wavelengths from 0.5 to 20 km have been calculated. The velocity well in Alfven speed surrounding the F region plasma density maximum facilitates energy deposition by slowing, trapping, and intensifying resonant waves. The Alfvenic Joule heating rate maximizes at the resulting resonances. F region Joule heating resulting from quasistatic and Alfvenic variability with the same root-mean-square amplitude in the F region are shown to be comparable. At the same time, Alfvenic variability deposits little electromagnetic power in the E region, whereas quasistatic variability greatly enhances E region heating. When measured electric and magnetic fields are used to constrain the amplitude and spectral content of superposed Alfven waves incident from the magnetosphere, the calculated F region heating rate ranges from 5 to 10 nW/m(2).