Abstract
AbstractIt 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.
Highlights
The mechanisms by which the high-latitude thermosphere and ionosphere are linked are not completely understood
We have identified an event that shows clear forcing of the thermospheric neutral wind from ionospheric convection above Svalbard
A cross-correlation analysis was performed using spatially resolved SuperDARN and Scanning Doppler Imager (SCANDI) data to quantitatively determine the timescale upon which ion drag fully accelerates the neutrals into the direction of plasma motion and if there was any variation of this lag over a range of approximately 1,000 km
Summary
The mechanisms by which the high-latitude thermosphere and ionosphere are linked are not completely understood. Joshi et al (2015) used midlatitude Super Dual Auroral Radar Network (SuperDARN) radars and FPI instruments to calculate τ with values falling in the range of 10 to 360 min, varying rapidly from measurement to measurement This makes clear an issue when trying to determine a neutral wind lag time from plasma or neutrals with high temporal variability; sudden increases or decreases in plasma velocity which are not sustained will mean that the neutrals do not have enough time to respond and never fully accelerate or decelerate to some equilibrium state. Shown are the fields of view of the two overlapping SuperDARN radars and the geomagnetic north pole as of 2018
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
