IMF Control of Alfvénic Energy Transport and Deposition at High Latitudes

Abstract

We investigate the influence of the interplanetary magnetic field (IMF) clock angle ϕIMF on high‐latitude inertial Alfvén wave (IAW) activity in the magnetosphere‐ionosphere transition region using Fast Auroral SnapshoT (FAST) satellite observations. We find evidence that negative IMF Bz coincides with nightside IAW power generation and enhanced rates of IAW‐associated electron energy deposition, while positive IMF Bz coincides with enhanced dayside wave and electron energy deposition. Large ( ≳0.3em0.3em0.3em5 nT) negative IMF By coincides with enhanced postnoon IAW power, while large positive IMF By coincides with enhanced but relatively weaker prenoon IAW power. For each ϕIMF orientation we compare IAW Poynting flux and IAW‐associated electron energy flux distributions with previously published distributions of Alfvénic Poynting flux over ∼2–22 mHz, as well as corresponding wave‐driven electron energy deposition derived from Lyon‐Fedder‐Mobarry global MHD simulations. We also compare IAW Poynting flux distributions with distributions of broad and diffuse electron number flux, categorized using an adaptation of the Newell et al. (2009) precipitation scheme for FAST. Under negative IMF Bz in the vicinity of the cusp (9.5–14.5 magnetic local time), regions of intense dayside IAW power correspond to enhanced diffuse electron number flux but relatively weaker broadband electron precipitation. Differences between cusp region IAW activity and broadband precipitation illustrate the need for additional information, such as fields or pitch angle measurements, to identify the physical mechanisms associated with electron precipitation in the vicinity of the cusp.

Identification of Auroral Electron Precipitation Mechanism Combinations and Their Relationships to Net Downgoing Energy and Number Flux

“…Their results, as well as previous results (e.g., Keiling et al, ) indicate that Alfvénic Poynting flux is more evenly distributed around local noon. But both our results and those of Hatch et al (, ) indicate that intense Alfvénic precipitation has a drop out near noon. Exclusive classification of the downgoing spectra method (e.g., Newell et al, ) gives the illusion of being able to assign electron energy and number flux to particular mechanisms.…”

“…We do not present net electron energy and number flux by location and mechanism as our method is not mutually exclusive by mechanism. It should be noted that the energy and number fluxes we used are different than both Newell et al () and Hatch et al (, ) who used downgoing fluxes rather than net downward fluxes. It should also be noted that the methods utilized in both papers for determining precipitating (downward) flux likely overestimate the contribution from events they classified as broadband.…”

“…Hatch et al (, ) have recently published results from FAST utilizing a modified Newell et al () classification methodology along with electric and magnetic field measurements to investigate Alfvénic (and to a lesser degree diffuse) precipitation. In particular, they determined kinetic (inertial) Alfvén waves (from FAST measured wave fields) occurrence and Poynting flux and compared them to electron precipitation associated with broadband characteristics.…”

“…In particular, they determined kinetic (inertial) Alfvén waves (from FAST measured wave fields) occurrence and Poynting flux and compared them to electron precipitation associated with broadband characteristics. In Hatch et al () they also compared these to Lyon‐Fedder‐Mobarry simulation expectations of these wave characteristics from Zhang et al (). Since our results indicate that the combination of Alfvénic and QSPS acceleration occurs frequently (and sometimes with significant NAIP) within the same spin all of the energy and number precipitation they associated with broadband does not necessarily come from Alfvénic acceleration.…”

Alfvénic Heating in the Cusp Ionosphere‐Thermosphere

“…Observations from the Polar (Keiling et al, ) and FAST (Chaston et al, , ; Hatch et al, ) satellites show that magnetic variability associated with intense Alfvén‐wave Poynting fluxes flowing into the ionosphere are commonly observed in the statistical locations of the thermospheric density anomalies and associated bursts of small‐scale field‐aligned current. In fact, the previously mentioned soft electron precipitation induced by wave‐particle interactions is expected in regions of Alfvénic activity (Zhang, Lotko, et al, ).…”