Characterizing Quiet-Time IT Variability Using Data from GOLD and ICON

Solar geomagnetic activity and lower-atmosphere waves are two important forcings for the short-term variability of the Ionosphere-Thermosphere (IT) system. Here, we use data from the Global-scale Observations of the Limb and Disk (GOLD) and the Ionospheric Connection Explorer (ICON) satellites to characterize the quiet-time (Kp < 3) variability of the IT environment. We analyze Oxygen to Nitrogen-2 (O/N2) ratios and neutral temperatures (from GOLD) as well vertical ion drifts and mean ion densities and temperatures (from ICON) over a two-year period from 2019 to 2020. The full two-year period is used to study long term, quiet-time variability and seasonal variations in O/N2 ratios and ion temperatures and densities. Then, we focus in on the one-year period of 2020 to study short term (<11 day) variability in the IT environment. To quantify short term variability, we calculated relative standard deviations by first subtracting an 11-day sliding window mean from each data set then calculating the standard deviations of the residuals. Both mean ion density and standard deviation show prominent wave-4 structure, which are driven by the nonmigrating tides from the lower atmosphere. The absolute standard deviation of mean ion density at an altitude of ~575 km is ~6.3 · 104 N/cm3 and occurs at ~80°W and local solar time (LT) 21h. The absolute standard deviation of mean ion temperature is ~570 K and occurs at 80°W and LT 6h. Both maxima occur at the equator (± 2.5° lat.) In terms of the relative ion density variability, there are two strong peaks near the magnetic equator. One is ~73% appearing at LT 5h and the other is ~75% at 100°W and LT 21h. There is only one peak (~43%) for ion temperature occurring at 7h. Such a LT dependence may be related to the large conductivity gradient near dawn and dusk, which deserve a further modeling study.