Spatially Resolved Neutral Wind Response Times During High Geomagnetic Activity Above Svalbard

Abstract

It has previously been shown that in the high‐latitude thermosphere, sudden changes in plasma velocity (such as those due to changes in interplanetary magnetic field) are not immediately propagated into the neutral gas via the ion‐drag force. This is due to the neutral particles (O, O2, and N2) constituting the bulk mass of the thermospheric altitude range and thus holding on to residual inertia from a previous level of geomagnetic forcing. This means that consistent forcing (or dragging) from the ionospheric plasma is required, over a period of time, long enough for the neutrals to reach an equilibrium with regard to ion drag. Furthermore, mesoscale variations in the plasma convection morphology, solar pressure gradients, and other forces indicate that the thermosphere‐ionosphere coupling mechanism will also vary in strength across small spatial scales. Using data from the Super Dual Auroral Radar Network and a Scanning Doppler Imager, a geomagnetically active event was identified, which showed plasma flows clearly imparting momentum to the neutrals. A cross‐correlation analysis determined that the average time for the neutral winds to accelerate fully into the direction of ion drag was 75 min, but crucially, this time varied by up to 30 min (between 67 and 97 min) within a 1,000‐km field of view at an altitude of around 250 km. It is clear from this that the mesoscale structure of both the plasma and neutrals has a significant effect on ion‐neutral coupling strength and thus energy transfer in the thermosphere.

Multi-instrument Observations of Ion-Neutral Coupling in the Dayside Cusp

“…In fact, recent studies (Conde et al, 2018;Zou et al, 2018) have certainly shown this to be the case, observing a rapid (<20 minutes) neutral wind response during auroral activity. This is much shorter than previous estimations (on the order of a few hours) during periods of little or no precipitation (Kosch et al, 2010;Joshi et al, 2015;Billett et al, 2019).…”

“…Thus, there are often large delays (on the order of hours) before velocity changes in the high latitude plasma translate fully into the neutral wind (e.g. Billett et al, 2019). Traditionally, a quantitative time delay is determined by calculating the time for the neutral wind to accelerate to 1/e of the plasma velocity (e.g.…”

Ion Heating in the Polar Cap Under Northwards IMF Bz

“…Billett et al. (2019) estimated the neutral wind response time to a change in plasma convection at high‐latitudes to be roughly 75–90 min, depending on the strength of the event. Consequently, ion heating events may be more common when the IMF is highly variable and the convection pattern is frequently reconfigured.…”

Statistical Analysis of Joule Heating and Thermosphere Response During Geomagnetic Storms of Different Magnitudes

“…Besides, the response time of thermospheric density to energy injection during geomagnetic storms, which contributes to study for how long Joule heating might be enhanced (Billett et al, 2019), is also an important topic in magnetosphere‐ionosphere/thermosphere coupling studies. To quantify this response time, we take the time lag between the peak of thermospheric density and the peak of Joule heating (“time lag” hereafter) as the probe for each geomagnetic storm case.…”