Simultaneous FPI and TMA Measurements of the Lower Thermospheric Wind in the Vicinity of the Poleward Expanding Aurora After Substorm Onset

Abstract

Lower thermospheric wind fluctuations in the vicinity of an auroral arc immediately before and after a substorm onset were examined by analyzing data from a ground‐based green line Fabry‐Perot interferometer (FPI; optical wavelength of 557.7 nm) at Tromsø, Norway, and in situ measurements from a trimethyl aluminum (TMA) trail released from a sounding rocket launched during the Dynamics and Energetics of the Lower Thermosphere in Aurora 2 (DELTA‐2) campaign on 26 January 2009. Soon after the rocket launch but before disappearance of the TMA trail, a substorm onset occurred. The DELTA‐2 TMA experiment appears to be the first case in which the substorm onset occurred during the TMA wind measurement. It is known that energy dissipation induced by the ionospheric closure current is compacted at the poleward side of the discrete arc in the ionospheric morning cell. Both FPI and TMA measurements were made at the poleward side, but the FPI measured winds nearer to the poleward edge of the arc than the TMA by 110–130 km. The FPI winds at distance of 53–74 km relative to the arc edge showed clear fluctuations immediately after the substorm onset, but there was no obvious similar fluctuation in the TMA‐measured winds. The difference in the response at the two locations suggests that energy dissipation sufficient to be detected as the FPI/TMA wind perturbations was confined to the area from the poleward edge of the arc to a relative distance shorter than 163–203 km but longer than 53–74 km in this event.

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

“…This indicates that the zonal wind acceleration is also affected by the local ion drift due to the auroral arc electrodynamics (see, e.g., Aikio et al, ; Conde et al, ), and strong meridional gradient of the zonal wind may exist in the vicinity of the arc (within a latitudinal range of 1°). This spatial scale is close to the observation presented by Oyama et al (), who showed that clear fluctuations in lower thermospheric wind were confined within a region from the edge of an arc, which had a width between 53 and 203 km.…”

“…Observations by the Fabry‐Perot interferometer (FPI) and its all‐sky type, scanning Doppler imager, reveal that thermospheric winds are more dynamic and responsive to the magnetospheric forcing in both vertical and horizontal components than modeling or statistical studies typically show (Anderson et al, , , ; Aruliah et al, ; Griffin et al, ). Mesoscale and small‐scale winds have been found in association with auroral structures, such as auroral arcs (Conde et al, ; Kosch et al, ), poleward expanding aurora after substorm onset (Oyama et al, ), and auroral patches inside pulsating aurora (Oyama et al, , ). Those findings also suggest that the thermospheric wind has more rapid responses than theories have predicted.…”

Interhemispheric conjugate effect in longitude variations of mid-latitude ion density

“…In Figure 1a, northern midlatitude N i distribution shows a dependence on geomagnetic field configuration. In general, N i tends to be lower at the longitudes with larger inclinations, such as around 270°E where neutral wind induced transport is smaller since inclinations are larger (>45°) and the vertical neutral wind is much smaller than the horizontal neutral winds under geomagnetic quiet condition (e.g., Fisher et al, 2015;Oyama et al, 2017), especially at higher latitudes (such as 45°-60°N) where the F 2 peak region is still irradiated. Although geomagnetic declination is important for neutral wind induced vertical transport at mid-latitudes, inclination effects seem to be more important than declination effects in Figure 1a.…”