Storm time response of the midlatitude thermosphere: Observations from a network of Fabry‐Perot interferometers

Abstract

Observations of thermospheric neutral winds and temperatures obtained during a geomagnetic storm on 2 October 2013 from a network of six Fabry‐Perot interferometers (FPIs) deployed in the Midwest United States are presented. Coincident with the commencement of the storm, the apparent horizontal wind is observed to surge westward and southward (toward the equator). Simultaneous to this surge in the apparent horizontal winds, an apparent downward wind of approximately 100 m/s lasting for 6 h is observed. The apparent neutral temperature is observed to increase by approximately 400 K over all of the sites. Observations from an all‐sky imaging system operated at the Millstone Hill observatory indicate the presence of a stable auroral red (SAR) arc and diffuse red aurora during this time. We suggest that the large sustained apparent downward winds arise from contamination of the spectral profile of the nominal thermospheric 630.0 nm emission by 630.0 nm emission from a different (nonthermospheric) source. Modeling demonstrates that the effect of an additional population of 630.0 nm photons, with a distinct velocity and temperature distribution, introduces an apparent Doppler shift when the combined emissions from the two sources are analyzed as a single population. Thus, the apparent Doppler shifts should not be interpreted as the bulk motion of the thermosphere, calling into question results from previous FPI studies of midlatitude storm time thermospheric winds. One possible source of contamination could be fast O related to the infusion of low‐energy O+ ions from the magnetosphere. The presence of low‐energy O+ is supported by observations made by the Helium, Oxygen, Proton, and Electron spectrometer instruments on the twin Van Allen Probes spacecraft, which show an influx of low‐energy ions during this period. These results emphasize the importance of distributed networks of instruments in understanding the complex dynamics that occur in the upper atmosphere during disturbed conditions.

Observations of storm time midlatitude ion‐neutral coupling using SuperDARN radars and NATION Fabry‐Perot interferometers

“…For the event presented in this study, the FPIs made cardinal mode measurements, allowing estimates of the thermospheric zonal, meridional, and vertical winds. However, it should be noted that in a previous study of the NATION FPI observations from this storm period, Makela et al [] concluded that observations made to the zenith by each NATION FPI indicated the presence of an anomalously large and persistent downward Doppler shift of the thermospheric redline emission resulting in downward apparent vertical winds of the order of 100–140 m/s (Figure f). That study hypothesized that this was evidence for a contamination of this emission due to downward moving hot O atoms, and therefore, the inferred velocities are not completely indicative of the background thermospheric motion.…”

Atmospheric scattering effects on ground‐based measurements of thermospheric vertical wind, horizontal wind, and temperature

“…However, deviations from hydrostatic balance are short‐lived and cannot explain the large winds lasting more than an hour reported in some studies. Recently, Makela et al [] observed 100 m/s apparent downward winds sustained for several hours during the 2 October 2013 geomagnetic storm at midlatitudes. This downward wind was observed by six independent FPIs spanning 10° in latitude and longitude.…”

“…This downward wind was observed by six independent FPIs spanning 10° in latitude and longitude. Makela et al [] concluded that these large apparent vertical winds must be an artifact of some type of contamination and hypothesized that the source of contamination is energetic precipitating oxygen ions.…”

Estimation of mesoscale thermospheric wind structure using a network of interferometers

“…Recently, networks of spatially separated FPIs with overlapping fields of view have been developed, which can provide information regarding the spatial distribution of the wind, as well as its temporal evolution [ Meriwether , ]. Examples of such networks include an SDI and FPI in Antarctica [ Anderson et al , ], two SDIs in Alaska [ Anderson et al , ], three FPIs in Peru, one of which is located in Arequipa and described by Meriwether et al [], three FPIs in Scandinavia [ Aruliah et al , ], two FPIs in Brazil [ Makela et al , ], and five FPIs in the eastern United States [ Makela et al , ]. The recent growth of these networks has motivated a need to develop a technique that combines measurements from multiple FPIs in order to estimate the regional thermospheric wind field, i.e., the latitudinal and longitudinal distribution of the three‐dimensional wind vector, which has zonal, meridional, and vertical components, and which is assumed to be representative of the wind at an altitude of 250 km.…”