This manuscript presents the analysis of data from multiple ground- and space-based sensors in the North American region before, during, and after the 12 Oct. 2021 geomagnetic storm. The data show the formation and equatorward propagation of a density trough, which manifested within bottom-side and top-side electron density data as well as within maps of total electron content (TEC). During the recovery phase on the 13th, the equatorward edge of the trough settled at around 30° latitude and exhibited a steep density gradient. By the 14th, this sharp boundary had disappeared. Near this edge on the 13th, small-scale irregularities formed. The impact of these was observed within Global Positioning System (GPS) data as elevated rate of TEC index (ROTI) and presented as strong 35 MHz scintillations of cosmic radio sources as well as spread-F within ionograms from multiple digisonde systems. GPS and 35-MHz data demonstrated that the irregularity region was narrowly confined (≤5° wide) near the trough edge. The 35-MHz scintillation data also showed that the irregularities were moving relatively slowly at ~7 m s-1, likely toward the southeast. Density and velocity measurements demonstrate that the conditions near the trough boundary we highly favorable for the gradient drift instability (GDI) with the one-dimensional growth rate estimated to be ~0.01 s-1. Since these conditions persisted for many hours, this growth rate was more than sufficient for the GDI to be considered the primary driver of irregularity formation in this case.

The results of a study aimed at assessing the utility of transionospheric 35 MHz scintillation measurements toward cosmic radio sources for estimating the level of spatial coherence in high frequency (HF) skywave systems are presented. This was done using an array of four antennas in southern Maryland called the Deployable Low-band Ionosphere and Transient Experiment (DLITE). Two of the antennas within a ~350-m north/south baseline were used to monitor 35-MHz intensity variations of two bright cosmic sources, Cynus A and Cassiopeia A. The other two antennas, which were within a ~420-m east/west baseline, recorded the 7.85 MHz skywave from the CHU radio station near Ottawa, Ontario. These HF measurements were used to quantify the level of spatial coherence by measuring the amplitudes of the cross correlation of the two antennas’ recorded voltages relative to the received power, which were typically ~0.5-0.9, but occasionally near zero. A method was developed to estimate the expected cross-correlation amplitude based on the 35-MHz scintillations. This method assumes the case of weak scattering, which is generally appropriate for mid-latitudes, and that the irregularity distribution follows that of the background electron density. These calculations typically captured the day-to-day variations in spatial coherence quite well (correlation coefficient r≈0.6) while only marginally reproducing hour-to-hour variations (r≈0.2). Thus, this method holds promise as an economical and passive means to assess the spatial coherence expected for skywave propagation within a given mid-latitude region.

Magnetohydrodynamic (MHD) models have been used for nearly four decades to study the dynamics of magnetospheric substorms. However, until recently no demonstration has been made that MHD models can consistently reproduce substorm onset times in a statistical sense. To test whether MHD can reproduce observed substorm onset times, we developed a procedure for identifying substorm onsets that can be applied both to observational data and to MHD output. Our substorm identification procedure aims to improve upon existing methods of substorm identification by using multiple types of observations to corroborate each identified substorm. Using this procedure, we identified over 100 substorms from the period 1-31 January 2005. Using this list of substorm onset times, we show that the MHD model has weak, but statistically significant skill in predicting substorm onset times. We explore paths to improving the ability of the MHD model to predict substorm dynamics by testing different configurations of the MHD model.

Total electron content (TEC) observations can provide insights into electron density variations in the ionosphere. Such variations are associated with many aspects of ionospheric dynamics, including traveling ionospheric disturbances. In the present work we assimilate observations of TEC and horizontal TEC gradients into the SAMI3 (SAMI3 is Another Model of the Ionosphere 3D) ionosphere model. Assimilation into SAMI3 is accomplished using an ensemble Kalman filter implemented within LightDA, an extensible data assimilation library. Our TEC gradient observations are obtained from the Very Large Array Low-band Ionosphere and Transient Experiment (VLITE) and the TEC measurements are derived from GNSS receiver data. VLITE provides high precision and high spatial resolution TEC gradient observations over a small area, while GNSS observations supplement these with global coverage. By leveraging TEC observations in a physics-based model through data assimilation, we aim to improve our understanding of ionospheric processes and develop tools for improved ionospheric forecasting capabilities.