The geomagnetic quiet nighttime thermospheric wind pattern over Fritz Peak Observatory during Solar Cycle Minimum and Maximum

Abstract

Nighttime thermospheric winds and temperatures at F region heights have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer from about November 1972 to the present. The winds and temperatures are obtained from the Doppler shifts and line profiles of the (O I) 15,867 K (630 nm) line emission. The data obtained during geomagnetic quiet conditions (Ap ≤ 20) have been separated into two groups representing solar cycle minimum conditions (1972–1977) and solar cycle maximum conditions (1978–1979). The monthly variation of the nighttime zonal and meridional winds are presented for both groups. The results show that during solar cycle minimum the zonal winds are predominantly eastward during the winter months at speeds of 50–75 m s⁻¹ and westward during the summer months at speeds of 50–100 m s⁻¹. The meridional winds measured to the north and south of Fritz Peak Observatory are equatorward throughout most of the night at nearly the same speed. The equatorward winds in the summer peak near midnight at about 100–150 m s⁻¹. The equatorward winds in the winter also peak near midnight but with smaller speeds, 50–75 m s⁻¹. During solar cycle maximum the zonal winds are again predominantly eastward during the winter months, reaching peak speeds of 60–75 m s⁻¹ in the early evening hours and decreasing afterward. In the summer the winds are eastward during the early evening hours, shifting to westward near midnight and increasing to a maximum speed of 75–100 m s⁻¹ in the early morning hours. The meridional winds are poleward at ∼20 m s⁻¹ in the early evening hours, shifting to equatorward near 2100 LT and then increasing in magnitude with maximum speeds near or after midnight. There is a considerable difference in the speed of the winds measured to the north and south of Fritz Peak Observatory. During the summer the equatorward winds measured to the north are 150–175 m s⁻¹, whereas the equatorward winds measured to the south are 50–75 m s⁻¹. During the winter the speeds are smaller, 50–75 m s⁻¹ and 25–30 m s⁻¹ to the north and south of the station, respectively. Calculations made with the National Center for Atmospheric Research thermospheric general circulation model for different seasons are in reasonable agreement with the observations during both solar minimum and solar maximum. During solar cycle maximum the observed and calculated meridional wind differences to the north and south of Fritz Peak Observatory are attributed to enhanced auroral heating and ion drag associated with magnetospheric convection north of the observatory. The equatorward driven winds slow south of Fritz Peak Observatory owing to the diminishing influence of high‐latitude forcings.