ULF wave electromagnetic energy flux into the ionosphere: Joule heating implications

Abstract

Ultralow-frequency (ULF) waves-in particular, Alfvén waves-transfer energy into the Earth's ionosphere via Joule heating, but it is unclear how much they contribute to global and local heating rates relative to other energy sources. In this study we use Time History of Events and Macroscale Interactions during Substorms satellite data to investigate the spatial, frequency, and geomagnetic activity dependence of the ULF wave Poynting vector (electromagnetic energy flux) mapped to the ionosphere. We use these measurements to estimate Joule heating rates, covering latitudes at or below the nominal auroral oval and below the open/closed field line boundary. We find ULF wave Joule heating rates (integrated over 3-30 mHz frequency band) typically range from 0.001 to 1 mW/m 2 . We compare these rates to empirical models of Joule heating associated with large-scale, static (on ULF wave timescales) current systems, finding that ULF waves nominally contribute little to the global, integrated Joule heating rate. However, there are extreme cases with ULF wave Joule heating rates of ≥10 mW/m 2 -in these cases, which are more likely to occur when Kp ≥ 3, ULF waves make significant contributions to the global Joule heating rate. We also find ULF waves routinely make significant contributions to local Joule heating rates near the noon and midnight local time sectors, where static current systems nominally contribute less to Joule heating; the most important contributions come from lower frequency (<7 mHz) waves.

Global Alfvén Wave Power in the Auroral Zone in Relation to the AE Index

“…In comparison, others have suggested using event case studies that the Alfvénic contribution to Joule heating in the auroral zone should be less than 30% of the Alfvén wave power entering the AAR (Dombeck et al, ). In a statistical study, Hartinger et al () also investigated the Joule heating due to ultralow‐frequency waves generated in the equatorial region, but they did not differentiate (i.e., lumping them together) between Joule heating and the conversion of energy from Alfvén waves to particle kinetic energy; thus, direct comparisons with their results are not useful. In contrast, the energy flux of upward traveling ions on the nightside is much smaller (several percent) than that of precipitating electrons (discussed in Wygant et al, ).…”

Frequency variability of standing Alfvén waves excited by fast mode resonances in the outer magnetosphere

“…Since FLRs transfer energy to radiation belt electrons [ Mann et al , ] and the ionosphere [ Hartinger et al , ], predicting when, where, and why these occur is important. While direct solar wind driving may account for ∼32% of events [ Viall et al , ], such an assessment for these coupled fast mode resonances (cFMRs) has not yet been possible since observational evidence or lack thereof for cFMRs has often involved searching for the (still heavily disputed) CMS frequencies.…”

Correspondence between the ULF wave power spatial distribution and auroral oval boundaries

“…For high but reasonable values of the mirror resistance this mechanism is likely to dominate over ionospheric damping [Fedorov et al, 2001]. Nonetheless, in studies of Pc5 wave energy dissipation in the ionosphere only the Joule heating was considered so far [Rae et al, 2007;Hartinger et al, 2015];…”