Sources of Variability in the Topside Ionosphere
Hawkins, Jessica M.
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The topside ionosphere is a dynamic region of the Earth’s upper atmosphere that couples with solar and terrestrial regions and affects communication and navigation systems. The solar extreme ultraviolet (EUV) irradiance is a major driver of variability in the topside, and further work is needed to understand how EUV drives variability, particularly in light of the recent availability of spectrally-separated EUV measurements. Variability in the topside is also driven from below, as there is increasing evidence that atmospheric effects are propagated across a broad range of altitudes. The WN4 pattern has been recognized as a signature of non-migrating tides originating in the troposphere, but its variations in the topside have yet to be comprehensively studied. Ionospheric models have struggled to accurately represent the shape and variability of topside ion density profiles, in part due to the relative scarcity of in situ measurements. Nevertheless, empirical models such as the internationally-recognized standard International Reference Ionosphere (IRI) provide a crucial resource to scientists, and improvements to IRI are ongoing. This dissertation addresses these research needs in three studies. First, we investigated the response of the topside ionosphere to EUV variability using thermal plasma measurements from the Defense Meteorological Satellite Program (DMSP) over 23 years, a period of time never considered before. The response was found to be closely related to the composition due to the effect of this parameter on the scale height. The composition varies dramatically with the solar cycle and the SZA, and these changes in composition have a compounding effect on the response of the topside ionosphere to changes in solar EUV irradiance. We investigated and quantified the signature of WN4 in the topside ionosphere across local time, season, and varying levels of solar activity using monthly averages of ion densities from DMSP. This study provides compelling evidence of the effects of tropospheric weather on space weather even at DMSP altitudes, which had not been comprehensively presented before. Tidal patterns are strongly present along the magnetic equator throughout the year, and WN4 is clearest at September equinox. During solstice months the WN4 signature is modified by zonal and meridional winds. WN4 moved eastward with increasing F10.7 and drifted eastward at about 2o GLON per hour MLT. To address the scarcity of in situ topside measurements needed to improve IRI, we presented a new data set of over 11,000 magnetic conjunctions between DMSP and C/NOFS in 2009-2015. We investigated the error of the most recent version of IRI in reproducing the topside profile shape under different conditions. The model was found to underestimate the plasma scale height in the morning but matched the profile shape well in the afternoon. The percent error showed patterns consistent with a diffusive equilibrium condition in the afternoon, whereas strong ion flows in the morning produced a more complex relationship between the Ni profile shape and the plasma temperatures.