Abstract
Nighttime thermospheric winds and temperatures have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high resolution Fabry‐Perot spectrometer. The winds and temperatures are obtained from the Doppler shifts and line profiles of the (O I) 15,867K (630 nm) line emission. Measurements made during two large geomagnetic storm periods near solar cycle maximum reveal a thermospheric response to the heat and momentum sources associated with these storms that is more complex than the ones measured near solar cycle minimum. In the earlier measurements made during solar cycle minimum, the winds to the north of Fritz Peak Observatory had an enhanced equatorward component and the winds to the south were also equatorward, usually with smaller velocities. The winds measured to the east and west of the observatory both had an enhanced westward wind component. For the two large storms near the present solar cycle maximum period converging winds are observed in each of the cardinal directions from Fritz Peak Observatory. These converging winds with speeds of hundreds of meters per second last for several hours. The measured neutral gas temperature in each of the directions also increases several hundred degrees Kelvin. Numerical experiments done with the NCAR thermospheric general circulation model (TGCM) suggest that the winds to the east and north of the station are driven by high‐latitude heating and enhanced westward ion drag associated with magnetospheric convection. The cause of the enhanced poleward and eastward winds measured to the south and west of Fritz Peak Observatory, respectively, is not known. During geomagnetic quiet conditions the circulation is typically from the southwest toward the northeast in the evening hours. We speculate that the enhanced flow to the south and west of Fritz Peak Observatory may be due to either (1) a hemispheric difference in the thermospheric energy input rate, (2) equatorial heating by neutral hydrogen precipitation, or (3) an increased midnight equatorial temperature bulge or some combination of these sources. Since the measurements indicate that the winds in the 630‐nm airglow layer converge from all directions at the observatory, we speculate that Fritz Peak is at the boundary of two circulation patterns during part of the storm.
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