Alfvénic Heating in the Cusp Ionosphere-Thermosphere

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Wednesday, December 19, 2018

The effect of electromagnetic variability on ionosphere-thermosphere heating in the low-altitude geomagnetic cusp is examined. The study is motivated by observed correlations between anomalous thermospheric density enhancements at F region altitudes (200-500 km) and small-scale field-aligned currents, previously interpreted as evidence of ionospheric Alfvén resonator modes.

Graph showing volumetric Joule heating rates
Volumetric Joule heating rates in the low-altitude geomagnetic cusp vs. altitude for an intense burst of Alfvénic power (W1), 26-s average Alfvénic power (W26), typical cusp-region dc power (Wdc), and "quasistatic" (noninductive) dissipation with the same rms electric field as the 26-s average Alfvénic value at 400 km altitude.

Height-integrated and height-dependent heating rates for Alfvén 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 Alfvén speed surrounding the F region plasma density maximum facilitates energy deposition by slowing, trapping, and intensifying resonant waves. The Alfvénic Joule heating rate maximizes at the resulting resonances. F region Joule heating resulting from quasistatic and Alfvénic variability with the same root-mean-square amplitude in the F region is shown to be comparable. At the same time, Alfvénic variability deposits little electromagnetic power in the E region (90-150 km altitude), 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 Alfvén waves incident from the magnetosphere, the calculated F region heating rate ranges from 5 to 10 nW/m2.

Publication Name: Journal of Geophysical Research: Space Physics

First HAO Author's Name: William Lotko