Multiinstrument Studies of Thermospheric Weather Above Alaska

Abstract

We describe how space weather impacts thermospheric winds above Alaska, using an array of ground and space-based instrumentation. Forcing of the thermosphere is observed using coherent (Super Dual Auroral Radar Network) and incoherent (Poker Flat Incoherent Scatter Radar) radars to measure ion convection, plus ground and space-based imagers to map auroral precipitation. The primary emphasis in this work is to determine the neutral thermospheric wind response at F region heights to these drivers, using an array of ground-based all-sky imaging Fabry-Perot spectrometers that record Doppler spectra of optical emissions at 630-nm wavelength, originating from a height of around 240 km. Line-of-sight winds are derived from Doppler shifts and, because each instrument only directly measures this one component, considerable postprocessing must be applied to infer the full three-component vector velocity field. Several techniques for doing so are identified, and a detailed description is presented of the hybrid algorithm used here. Results show that thermospheric winds at these latitudes respond strongly to magnetospheric drivers associated with the aurora over length scales down 100 km or less, and on time scales as short as 15 min. These scales are considerably smaller than those resolved by previous observations, or those captured in semiempirical models such as the Horizontal Wind Model or global-scale first principle models such as the Thermospheric Global Circulation Model and its relatives. Divergence and vorticity commonly occur in the observed horizontal wind fields, over regions spanning hundred of kilometers across and with magnitudes exceeding 0.5 × 10 − 3 s −1 . Plain Language Summary This article combines data from multiple instruments to show howEarth's aurora and flows of ionosphere plasma perturb winds in the neutral atmosphere at 240-km altitude. It shows how atmospheric weather at this height is driven by space weather.

Challenges to Understanding the Earth's Ionosphere and Thermosphere

“…However, the large-scale driver of the momentum forcing, the plasma drift, can change rapidly and be variable over a wide range of spatial scales. The momentum exchange between neutral gas and plasma can be surprisingly fast, ranging from a few minutes to more than an hour (e.g., Conde et al, 2018;Kosch et al, 2010). At lower altitudes the time scales are much longer, but in all cases it should be recognized that the neutral dynamics reacts generally on time scales longer than those associated with the reconfiguration of the global plasma convection pattern, on the order of 13 min (Ridley et al, 1998).…”

“…Recently a comprehensive observational network over Alaska has allowed the study of winds in the F region in connection with ion flow and particle precipitation with unprecedented spatial and temporal resolution (Conde et al, 2018). These observations indicate that the F region wind (approximately at 240 km altitude) responds to changes in the drivers within 15 min on spatial scales of 100 km or smaller.…”

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

“…As studied by Conde et al (2018) and Zou et al (2018), the drastic increase in ion density introduced by precipitation will enhance the neutral-ion collision frequency, which in turn strengthens ion drag. However, the potential variability of lags with local time, and thus universal time, is not well understood and could affect the results shown here.…”

“…Both of these studies exploited results from a Scanning Doppler Imager (SDI), a type of FPI which can measure more than one single point neutral wind measurement (Conde & Smith, 1997) Shepherd, 2014Shepherd, , as of 2018. Thus, we would usually expect the neutral velocities observed by SCANDI to have longer lag times than those found by Conde et al (2018) and Zou et al (2018) due to less ionization from increased particle precipitation, resulting in fewer collisions between the plasma and neutrals. SCANDI allows a neutral wind delay to changes in the plasma to be resolved for each neutral wind vector determined, at a spatial resolution of approximately 100 km at 250-km altitude, thus allowing the examination of mesoscale changes in ion drag.…”

High-latitude thermospheric wind study using a Fabry–Perot interferometer at Tromsø in Norway: averages and variations during quiet times

“…Various important mechanisms can drive high-latitude winds, such as tides, plasma convections, and Joule heating. Both ground-based and in situ wind measurements are used widely to study how the neutral wind in the thermosphere responds to geomagnetic activities such as auroras and/or different substorm phases (e.g., Price and Jacka 1991;Price et al 1995;Conde and Smith 1998;Conde et al 2018;Kosch et al 2010;Xu et al 2019;Ritter et al 2010;Oyama et al 2016). Besides these studies focused on winds during disturbed times, others have investigated the high-latitude wind pattern and how it depends on various factors such as solar and geomagnetic conditions (e.g., Aruliah et al 1991;Lathuillère et al 1997).…”