Optimal Reconstruction of Atmospheric Gravity Waves from Traveling Ionospheric Disturbances
Sebastijan Mrak, University of Colorado Boulder; Joe Huba, Syntek Technologies; Erich Becker, Sharon Vadas, North West Research Associates
Global Navigation Satellite System (GNSS) receivers are routinely used to estimate the ionospheric Total Electron Content (TEC) using phase delay measurements at two frequencies. The GNSS-TEC estimates have been a crucial space weather diagnostic used to identify salient large-scale, large-amplitude spatial features for decades. Recently, the GNSS-TEC observations are being used to identify transient traveling ionospheric disturbances (TIDs) at spatial scales ranging between 100s (medium-scale - MSTIDs) to 1000s of km (large-scale - LSTIDs). These features have significantly smaller amplitudes, usually in the order of 1% of the background TEC, yet they provide a crisp spatial depiction of (space) weather phenomena, leveraging dense spatial observations from numerous receivers with ionospheric pierce points (IPP) at baseline distances in the order of 10s of km. Most of the MSTIDs are a manifestation of Gravity Wave (GW) forcing, pushing plasma up and down the magnetic field lines.
The TID-inferred horizontal wave parameters, spatial figure, and spatiotemporal evolution are used to establish links with the underlying physics and source region. However, these estimated parameters are sensitive to the TID image creation, where line-of-sight TEC observations are projected to a fixed altitude (i.e., IPP) using some physical assumption. Normally, this assumption is blindly set to an average height of the ionospheric F-peak (HmF ~300 km), or it is refined based on the F-peak height informed by an empirical model such as the IRI. While this assumption is satisfactory for some circumstances, it is erroneous for the majority of GWs originating in the lower atmosphere/thermosphere, with only a few surviving altitudes >250 km. For these cases, horizontal TID parameters extracted by traditional assumptions are likely wrong yielding erroneous scientific conclusions related to the GWs propagation and their sources. We will devise an optimal GNSS-TID imaging algorithm using synthetic TEC data from coupled HI Altitude Mechanics General Circulation Model (HIAMCM) and NRL’s SAMI3 model of the ionosphere-plasmasphere. The derived TID parameters will be validated against the true GWs (i.e., HIAMCM), and the reconstruction of the GW field and the underlying electron density perturbations will be demonstrated using inverse ray tracing.
In this paper, we will present a reconstruction from a case study where we identified wintertime concentric TIDs over the region of the Continental United States (CONUS). We devised an autofocusing algorithm that finds an optimal altitude for the TID projection height which peaks at around 220 km altitude. We use numerical simulations of the neutral atmosphere using the HIAMCM, which is then coupled to the SAMI3 model computing the resulting plasma density perturbations. We compute synthetic TEC profiles using the same GPS receiver locations as for the case study and compute the corresponding synthetic TID map using the autofocusing algorithm. We analyze the differences between observations and model reconstruction and compare them to the underlying GW parameters from the HIAMCM model. The findings are fundamental for defining the GW observability constraints using GNSS-TID maps.