Alfvénic Heating in the Cusp Ionosphere‐Thermosphere

Abstract

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 Alfvén resonator modes. 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 are shown to be comparable. At the same time, Alfvénic 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 Alfvén waves incident from the magnetosphere, the calculated F region heating rate ranges from 5 to 10 nW/m3.

A New Framework to Incorporate High‐Latitude Input for Mesoscale Electrodynamics

“…The Joule heating rate, which is treated as the sum of ion to neutral heat transfer and frictional heating rates, is shown in Figure 18c. It can be seen that the GITM-HIME Lotko and Zhang (2018) found the heating due to variability (66 mV/m) around 10 nW/m −3 and due to a quasi-static field (similar to our GITM-HIME[B] approach) around 38 nW/m −3 higher than the simulated response with a Weimer type electric field at the same altitude. Overall, the GITM-HIME[V] and GITM-HIME[T] rates produce three peaks at 0632, 0707, and 0742 UT.…”

“…Figure 19 illustrates the altitude profile of the total ion to neutral volumetric energy transfer (Joule heating) rates obtained from the simulations. It can be seen that the GITM-HIME Lotko and Zhang (2018) found the heating due to variability (66 mV/m) around 10 nW/m −3 and due to a quasi-static field (similar to our GITM-HIME[B] approach) around 38 nW/m −3 higher than the simulated response with a Weimer type electric field at the same altitude. The simulated values at F region altitudes follow a similar trend; however, the magnitudes are very low due to nighttime electron densities to be compared directly.…”

Challenges to Understanding the Earth's Ionosphere and Thermosphere

“…This energy input provides structured electric fields that can be of similar magnitude to those electrostatic features described earlier (Keiling et al, 2019). They do not affect the bulk circulation of the plasma in the ionosphere but selectively heat the I-T in the upper region due to the resonant trapping of Alfvén waves by the plasma density gradient in the topside ionosphere (e.g., Lotko & Zhang, 2018;Verkhoglyadova et al, 2018). Alfvénic energy sources need to be identified in a systematic way to be incorporated into models of the I-T behavior since they represent heat sources that redistribute the plasma and the neutral gas horizontally and vertically.…”

“…There exist some observational evidence, to suggest that some neutral density enhancements are associated with small-scale field-aligned current and Alfvénic waves causing ohmic heat in the plasma that is subsequently transferred to the neutral gas (Liu et al, 2010;. Lotko and Zhang (2018) examined Alfvénic heating effects in the cusp finding an increase in the F region Joule heating rate, which can be effective in heating the F region neutrals. Cusp neutral density enhancements associated with increased electron temperature, small-scale FAC, and vertical plasma motion are consistent with increased electron precipitation (e.g., Kervalishvili & Lühr, 2013;Lühr & Marker, 2013).…”

Time-dependent responses of the neutral mass density to magnetospheric energy inputs into the cusp region in the thermosphere: A high-resolution two-dimensional local modeling

“…Since alternating electric fields keep velocity differences between neutrals and ions large, the mass density's time evolution may differ from what we presented above. Lotko and Zhang (2018) have shown that Joule heating rates generated by Alfvén waves are maximized at F layer altitudes, with the altitude profile depending on wavelength and frequency. In contrast, quasi-static electric fields maximize Joule heating rates at E layer altitudes.…”