M-I Coupling via Alfvén Waves and Suprathermal Particles: Perspectives from Swarm and ePOP

Abstract:

The three Swarm satellites A, B, and C, and the Enhanced Polar Outflow Probe "ePOP" (also known as Swarm Echo) have been in operation since late 2013 in nearly-polar orbits. Swarm A-C feature precise calibrated and validated measurements of magnetic fields and ion drift velocities at approximately 500 km altitude which have been used in numerous published studies of low-frequency electrodynamics at all latitudes. In this talk we feature recent results demonstrating the wide-spread nature of M-I coupling via Alfvén waves [Pakhotin et al., JGR, 2019; Wu et al., JGR, 2020], but no clear, one-to-one relation between regions of intense Alfvénic fluctuations on one hand, and large-scale FACs (upward or downward) or auroral arcs on the other, indicating that the ultimate source of these intense wave regions remains an open question.

ePOP is notable for its high time resolution. Shen et al. [JGR, 2018] used the ePOP Suprathermal Electron/Ion Imager (SEI) to identify narrow (< 1 km) regions of intense ion heating by plasma waves to energies of tens of eV at altitudes as low as 350 km. Wave-particle heating was previously thought not to be feasible at such altitudes due to collisional damping, however Shen and Knudsen [JGR, 2020a] show through simulations that the observed temperatures are consistent with a balance between heating by observed waves on one hand and collisional cooling on the other. Using the SEI in electron mode in the topside ionosphere (~1000 km altitude), Shen and Knudsen [JGR, 2020b] report the first direct measurements of suprathermal electron acceleration transverse to B, to energies reaching 300 eV and likely higher, also in narrow regions less than 1 km across. By resolving these localized heating regions, these measurements show the ionosphere to be a source of both ion and electron heating to much higher energies than previously appreciated.