Observational Evidence for the Role of Hall Conductance in Alfvén Wave Reflection

Abstract

Electromagnetic energy carried by magnetohydrodynamic modes is an important mechanism in the energy transfer between the magnetosphere and the ionosphere. Alfvén waves are known to carry field-aligned currents and thus play an important role in the dynamics of the ionosphere-magnetosphere coupling. The role of Hall conductance in this interplay has been explored in magnetohydrodynamic models of the ionosphere but has hitherto not been observed in situ. We use 5 years of observations from the Swarm mission to shed light on this interplay. We present a high-latitude climatology of both the measured Poynting flux and the measured Alfvén wave reflection coefficient. Our results indicate that high-energy precipitation, which penetrates deep into the ionosphere and directly leads to strongly enhanced Hall conductance, is an important cause of positively interfering Alfvén wave reflection. We present such observational evidence and, with that, suggest that Hall conductance is substantially more important in the ionospheric wave reflection climatology than hitherto believed.

High‐Resolution Poynting Flux Statistics From the Swarm Mission: How Much Is Being Underestimated at Larger Scales?

“…For the unfiltered 16 Hz and first few SGLP filtered datasets, the average downward integrated Poynting flux is roughly 2.5 times larger than the equivalent upward for all IMF orientations. These results are consistent with the dominance of magnetospheric forcing to the ionosphere rather than vice versa (as a potential result of the neutral wind dynamo; Kelley et al, 1991), and with previous studies who observed lower magnitude upward Poynting fluxes statistically (e.g., Gary et al, 1995;Ivarsen et al, 2020;Keiling et al, 2003). At large SGLP filters (e.g., 40+ s/600+ km), the downward integrated Poynting flux is closer to 3 times larger than the upward for all IMF orientations.…”

“…Statistical patterns of Swarm derived upward Poynting flux, not sorted by the IMF orientation, can be found in Ivarsen et al. (2020).…”

“…To calculate the Poynting flux using Equation 1, the large-scale convection electric field and terrestrial magnetic field must be removed from the Swarm E and B data. We apply the same method of determining the "background" fields as that used in Ivarsen et al (2020), that is, by using a second-order Savitsky-Golay low-pass (SGLP) filter of 225 s in width. Using a long SGLP filter such as this has the added benefit of extracting large-scale offsets present in the data that is not of geophysical origin (Koustov et al, 2019).…”

“…We do however, toward the end of the paper, show hemispherically integrated values of upward Poynting flux in a different format, as it is significantly less "noisy" and less affected by one-off events. Statistical patterns of Swarm derived upward Poynting flux, not sorted by the IMF orientation, can be found in Ivarsen et al (2020).…”

Effect of Interplanetary Magnetic Field on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents During Different Seasons

“…Using Swarm A magnetic and electric field measurements, Ivarsen et al. (2020) found a larger Alfvén wave reflection coefficient in the NH than in the SH. They interpreted this observation as a consequence of hemispheric asymmetry in the precipitation induced conductance, with the largest hemispheric asymmetry seen during local winter.…”