Fabry‐Perot Interferometer Observations of Thermospheric Horizontal Winds During Magnetospheric Substorms

Abstract

The high‐latitude ionosphere‐thermosphere system is strongly affected by the magnetospheric energy input during magnetospheric substorms. In this study, we investigate the response of the upper thermospheric winds to four substorm events by using the Fabry‐Perot interferometer at Tromsø, Norway, the International Monitor for Auroral Geomagnetic Effects magnetometers, the EISCAT radar, and an all‐sky camera. The upper thermospheric winds had distinct responses to substorm phases. During the growth phase, westward acceleration of the wind was observed in the premidnight sector within the eastward electrojet region. We suggest that the westward acceleration of the neutral wind is caused by the ion drag force associated with the large‐scale westward plasma convection within the eastward electrojet. During the expansion phase, the zonal wind had a prompt response to the intensification of the westward electrojet (WEJ) overhead Tromsø. The zonal wind was accelerated eastward, which is likely to be associated with the eastward plasma convection within the substorm current wedge. During the expansion and recovery phases, the meridional wind was frequently accelerated to the southward direction, when the majority of the substorm WEJ current was located on the poleward side of Tromsø. We suggest that this meridional wind acceleration is related to a pressure gradient produced by Joule heating within the substorm WEJ region. In addition, strong atmospheric gravity waves during the expansion and the recovery phases were observed.

Thermospheric wind variations observed by a Fabry–Perot interferometer at Tromsø, Norway, at substorm onsets

“…If we assume that the observed wind variations occurred during 30 min, then the amplitudes of acceleration rates of zonal winds were 0.0097-0.0272 m/s 2 (+ 17.5 m/s of R1, + 48.9 m/s of R2). These values are comparable with the acceleration rates of zonal winds (0.0189-0.1 m/s 2 ) during/near the expansion phase as summarized in Table 2 of Cai et al (2019).…”

“…It seems that there were no strong prehistory plasma convection activities that can contribute to the observed wind variations at local substorm onsets. Cai et al (2019) used the same FPI (630.0 nm) at Tromsø to study thermospheric wind variations during different substorm phases. The four substorm events in their study are different from our four events and have stronger magnetic field variations.…”

Colocated Observations of the E and F Region Thermosphere During a Substorm

“…The E region winds retaining an eastward component (aligned with E × B drifting plasma) could in part be due to the long E region ion‐drag timescales discussed earlier but also reflect that the ion‐neutral collision frequency at lower altitudes is much higher when compared to F region altitudes. Thus, E region winds better represent the strengthening westward electrojet current during substorm expansion, in agreement with Cai et al (2019).…”

“…Large increases in Joule heating due to the sudden injection of auroral energy trigger atmospheric gravity waves (AGWs) (Hines, 1960), which cause neutral wind disturbances to propagate to lower latitudes, and potentially even into the opposite hemisphere, as observed in simulations (Fuller‐Rowell et al, 1994; Richmond & Matsushita, 1975). Other model simulations (e.g., Fuller‐Rowell & Rees, 1984, and numerous since) found that ion drag was enhanced throughout the entire polar region during an isolated substorm, and Cai et al (2019) noted similar strengthening of ion drag during substorm expansion on the mesoscale, using ground‐based FPI observations. So far, however, observations of the effect of substorms on neutral dynamics at both E and F region altitudes, with regards to ion drag from plasma convection, have not been examined.…”

“…At auroral latitudes and altitudes, particle precipitation can also enhance collisions due to an increased plasma conductivity, resulting in thermospheric winds that align very quickly with the flow of plasma on the order of minutes (e.g., Billett et al, 2020; Conde et al, 2018; Kiene et al, 2018; Zou et al, 2018). During “quiet” periods, that is, times when the ionospheric conductivity is minimal or plasma convection velocity is slow, localized (mesoscale) neutral wind reconfiguration timescales are on the order of tens of minutes to hours at F region altitudes (∼150–300 km; Cai et al, 2019; Xu et al, 2019, and references therein). In the lower E region, the ion drift is impeded by neutral collisions and can no longer be considered as E × B drifting (Sangalli et al, 2009).…”