ALBERT

Description: Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) observations of the total electron content (TEC) above and below 800 km are used to study the local time and seasonal variation of longitude structures in both the F region ionosphere as well as the topside ionosphere and plasmasphere. The COSMIC observations reveal the presence of distinct longitude variations in the topside ionosphere-plasmasphere TEC, and these further exhibit a seasonal and local time dependence. The predominant feature observed at all local times in the topside ionosphere-plasmasphere TEC is a substantial maximum (minimum) during Northern Hemisphere winter (summer) around 300°–360° geographic longitude. Around equinox, at a fixed local time, a wave 4 variation in longitude prevails in the daytime F region TEC as well as the topside ionosphere-plasmasphere TEC. The wave 4 variation in longitude persists into the nighttime in the F region; however, the nighttime topside ionosphere-plasmasphere TEC exhibits two maxima in longitude. The COSMIC observations clearly reveal the presence of substantial longitude variations in the F region and topside ionosphere-plasmasphere, and to elucidate the source of the longitude variations, results are presented based on the coupling between the Global Ionosphere Plasmasphere model and the Thermosphere Ionosphere Electrodynamics General Circulation Model. The model simulations demonstrate that the orientation of the geomagnetic field plays a fundamental role in generating significant longitude variations in the topside ionosphere-plasmasphere but does not considerably influence longitude variations in the F region ionosphere. The model results further confirm that nonmigrating tides are the primary mechanism for generating longitude variations in the F region ionosphere. The coupled model additionally demonstrates that nonmigrating tides are also of considerable importance for the generation of longitude variations in the topside ionosphere-plasmasphere TEC.

Description: The complete mechanism of how upward propagating tropospheric tides connect to the upper atmosphere is not yet fully understood. One proposed mechanism is via ionospheric wind dynamo. However, other sources can potentially alter the vertical E × B drift: gravity and plasma pressure gradient driven current, the geomagnetic main field, and longitudinal variation in the conductivities. In this study we examine the contribution to the vertical drift from these sources, and compare them. We use March equinox results from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model. We found that the gravity and plasma pressure gradient driven current and the longitudinal variation of the conductivities excluding the variation due to the geomagnetic main field do not change the longitudinal variation of the vertical drift significantly. Modifying the geomagnetic main field will change the vertical drift at 5–6 LT, 18–19 LT and 23–24 LT at almost all longitudes. In general the influence of the geomagnetic main field on the vertical drift is larger, with respect to the maximum difference, at 18–19 LT and 23–24 LT, equal at 5–6 LT, and smaller at 14–15 LT than the influence due to nonmigrating tidal components in the neutral winds. Examination of the contribution from E- and F-region neutral winds to the vertical drift shows that their importance depends on the local time and the solar activity. This implies that the vertical drift has to be analyzed at specific local times to examine the relation between the wave number in the vertical drift and in the neutral winds.

Description: Accurate magnetic field measurements at ground and Low-Earth Orbit (LEO) are crucial to describe Earth's magnetic field. One of the challenges with processing LEO magnetic field measurements to study Earthâ's magnetic field is that the satellite flies in regions of highly varying ionospheric currents which needs to be characterized accurately. The present study focuses on ionospheric current systems due to gravity and plasma pressure gradient forcing, and aims to provide guidance on the estimation of their magnetic effect at LEO altitudes with the help of numerical modeling. We assess the diamagnetic approximation which estimates the magnetic signal of the plasma pressure gradient current. The simulations indicate that the diamagnetic effect should not be removed from LEO magnetic observations without considering the gravity current effect, as this will lead to an error larger than the magnetic signal of these currents. We introduce and evaluate a method to capture the magnetic effect of the gravity driven current. The diamagnetic and gravity current approximations ignore the magnetic effect from currents set up by the induced electric field. The combined gravity and plasma pressure gradient magnetic effect tends to cancel above the F-region peak, however between approximately 300 km and the peak it exhibits a significant height and latitudinal variation with magnitudes up to 8nT. During solar minimum the combined magnetic signal is less than 1nT above 300 km. In addition to the solar cycle dependence, the magnetic signal strength varies with longitude (approximately by 50%) and season (up to 80%) at solar maximum.

Description: Ionospheric currents are driven by several different physical processes and exhibit complex spatial and temporal structure. Magnetic field measurements of ionospheric sources are often spatially sparse, causing significant challenges in visualizing current flow at a specific time. Standard methods of fitting equivalent current models to magnetic observations, such as line currents, spherical harmonic analysis, spherical cap harmonic analysis, and spherical elementary current systems (SECS), are often unable to capture the full spatial complexity of the currents, or require a large number of parameters which cannot be fully determined by the available data coverage. These methods rely on a set of generic basis functions which contain limited information about the geometries of the various ionospheric sources. In this study, we develop new basis functions for fitting ground and satellite measurements, which are derived from physics-based ionospheric modeling combined with principal component analysis (PCA). The physics-based modeling provides realistic current flow patterns for all of the primary ionospheric sources, including their daily and seasonal variability. The PCA technique extracts the most relevant spatial geometries of the currents from the model run into a small set of equivalent current modes. We fit these modes to magnetic measurements of the Swarm satellite mission at low and mid-latitudes and compare the resulting model with independent measurements and with the SECS approach. We find that our PCA method accurately reproduces features of the equatorial electrojet and Sq current systems with only 10 modes, and can predict ionospheric fields far from the data region.