Direct measurements of nighttime thermospheric winds and temperatures, 2. Geomagnetic storms

Abstract

A high-resolution Fabry-Perot spectrometer has been used at Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), to determine the nighttime thermospheric temperatures and winds during geomagnetic storm periods from the line profiles and Doppler shifts of the (O I) 15867-K (6300 A) line emission. Data obtained during four geomagnetic storm periods in 1974 are presented: March 20–22, October 15–20, June 29 to July 8, and April 16–27. The data are compared to temperatures determined from the Ogo 6 global empirical model of neutral composition and temperature and to winds calculated from a semiempirical dynamic model of the neutral thermosphere. The results show considerable variations in the dynamic character of the thermosphere which are also related to the intensity of geomagnetic activity. During geomagnetic storms the nighttime equatorward winds are greatly enhanced from their quiet time values with a maximum measured velocity of 640 m s−1 during a Kp = 9 storm. The zonal winds generally develop a westward component relative to the geomagnetic quiet zonal winds. During intense geomagnetic activity the zonal winds are westward at 100–200 m s−1 in the early evening hours, flowing in the direction of magnetospheric convection and opposite of model predictions. The meridional winds measured south of the site are generally smaller than those measured to the north. Sometimes the meridional winds measured north and south of the station flow in opposite directions. The nighttime neutral gas temperatures are observed to increase from their geomagnetic quiet values during the storm and are in general concurrence with the Ogo 6 model predictions.