Nocturnal thermospheric neutral wind and temperature measurement using a Fabry-Perot Interferometer: First results from an equatorial Indian station

Abstract

Results on first systematic measurements of nocturnal thermospheric neutral winds and temperature using a Fabry-Perot Interferometer (FPI) from Trivandrum (8.5°N, 77°E, 0.5° dip latitude), an Indian station close to the geomagnetic equator, are presented. These measurements were made during the low solar activity period of June 2017 to April 2018. The OI 6300 Å thermospheric nightglow emission has been used as the tracer. Using these measurements, nocturnal variability of monthly mean temperature and wind have been obtained. These mean temperature and wind are compared with the predictions of the neutral atmospheric models, NRL (USA)’s Mass Spectrometer and Incoherent Scatter Radar 2000 (MSISE-00) and Horizontal Wind Model 2014 (HWM14). The FPI temperatures reveal an overall decreasing trend beginning post-sunset till the end of observation for all months. The lowest monthly mean temperature was estimated to be 601°K in the pre-morning hours in March 2018, which is about 45°K less than the corresponding model value. There are overall agreements between the FPI and the MSIS model temperatures. Although, in some months,measurements exhibit large deviations from the model predictions indicating its limitations as applied to the equatorial thermosphere. The mean meridional wind is found to agree with the predictions of HWM14 model remarkably well and also corroborates with the wind retrieved from the collocated ionosonde measurements reported earlier from Trivandrum. Meridional wind during solsticial months clearly exhibits signatures of inter-hemispheric flow. The mean zonal wind, however, deviates significantly from the model predictions on most of the occasions. The mean zonal wind is found to be less compared to that reported elsewhere in the African and Brazillian longitude sectors. It is ascribed to be due to the relatively less pre-reversal enhancement (PRE) and the consequent strong ion-drag over Trivandrum. Further, it appears that PRE is relatively stronger in winter solstice than in the equinoxes in agreement with an earlier investigation carried out from Trivandrum.