Upper-thermospheric Observations And Neutral-gas Dynamics At High Latitudes During Solar Maximum.

The primary objective of this work was to understand the time-dependent dynamics of high-latitude thermospheric neutral winds. To do this, a unique 70-orbit December-solstice dataset of Dynamics Explorer 2 (DE-2) satellite data was established, with coverage of both polar caps during the same orbit. Analysis of this data led to the characterization of four basic high-latitude neutral-wind signature categories for each hemisphere under various interplanetary-magnetic-field (IMF) configurations. In addition, neutral-gas forcing simulations for the same solar-maximum December-solstice conditions using the NCAR thermospheric general circulation model (TGCM) showed that the orientation of the pressure-gradient force in the winter northern hemisphere was different from the summer southern hemisphere, explaining the differences in neutral-wind flow patterns systematically observed by DE-2 in opposite hemispheres. The variations in the pressure-gradient forces in opposite hemispheres were ascribed to density variations and temperature gradients, resulting from a superposition of solar-EUV, Joule and cusp heating. The DE-2 dataset was then used to conduct two different time-dependent tests of the NCAR-TGCM. The first test compared TGCM-simulated neutral winds to DE-2 measured neutral winds. The results indicated that the NCAR-TGCM provides good first-order simulated neutral winds. However, the TGCM time-dependent neutral winds differed from the measured neutral wind velocities by an average of 10-40%, primarily due to the inability to accurately model the ion-drag neutral-gas momentum source. The second test used DE-2 ion-drift data as a means of providing a realistic time-dependent prescription of ion-convection parameters. Results showed that under selected geophysical conditions, use of satellite data to constrain model boundary conditions brought simulated neutral winds closer to observed values. Finally, a technique for short-range forecasting of the high-latitude ion-convection parameters was noted. While techniques of this type show promise, more combined ion drift and neutral wind data is needed for a comprehensive evaluation.Ph.D.Physics, Atmospheric SciencePure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128130/2/8801433.pd