Thermospheric response observed over Fritz Peak, Colorado, during two large geomagnetic storms near Solar Cycle Maximum

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.

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

“…Combined with the north looking measurement (which is looking at the bright aurora and is minimally contaminated), this error manifests as an apparent convergence in the horizontal wind field. This likely explains the horizontal convergence that is commonly reported at midlatitudes during storms [e.g., Hernandez and Roble, 1976;Hernandez et al, 1982;Biondi, 1984;Makela et al, 2014] and perhaps explains some convergences reported near the aurora [e.g., Anderson et al, 2011]. Of course, there are many other effects which can cause apparent convergences, such as inaccurate Doppler referencing, hydroxyl contamination, instrumental or laser calibration fluctuations, and actual vertical winds that are not properly handled in the analysis.…”

Thermospheric dynamics during the March 22, 1979, magnetic storm: 1. Model simulations

“…The NCAR TGCM has been used to calculate timedependent circulation, temperature, and compositional structure of the thermosphere for the geophysical conditions appropriate for March 22, 1979. The basic model structure without coupling between dynamics and composition has been described in detail by Dickinson et al [1981] and was used by Roble et al [1982 ., 1984]. A diagnostic package for the TGCM has also been designed to analyze some of the physical processes that are operating within the model .…”

“…Both the Joule heating and the ion drag momentum source due to the relative drift between the ions and neutrals are updated at each time step in the model to determine the time-dependent Joule heating rate and momentum forcing. Other details of the model parameterizations and operational characteristics have been described in the papers byDickinson et al [1981,Roble et al [-1982, and Roble and the NCAR TGCM during magnetically disturbed periods is the spatial distribution and temporal evolution of the high-latitude electric field. In practice this takes the form of appropriately specifying the empirical convection model ofHeelis et al [1982], which involves choosing values for 11 independent (adjustable) variables.…”

The Midlatitude Thermospheric Dynamics From an Interhemispheric Perspective

“…While the midlatitude thermosphere is the region most observed and investigated for insight into upper atmospheric dynamics (e.g., Hernandez et al, ; Hernandez & Roble, , , ; Huang et al, ; Wu et al, ; Wu et al, ), it remains least understood in many respects. Midlatitude thermospheric winds respond to lower thermospheric waves, geomagnetic activity, solar EUV conditions, and Sub‐Auroral Polarization Streams (SAPS), and vary with latitude, longitude/UT, and season (e.g.,Wang et al, ; Wang et al, ; Wang et al, ; Wang & Lühr, ; Wu, Noto, et al, ; Wu, Yuan, et al, ).…”