Investigation of The Main Ionospheric Trough with GNSS and DMSP Data

Abstract:
This study presents a visual and statistical analysis of the main ionospheric trough (MIT) during geomagnetically active dates in 2013 and 2014. The MIT is visualized using total electron content (TEC) values from the CEDAR Madrigal database, which provides a variety of data from ground based GNSS receivers, and data from the Defense Meteorological Satellite Program (DMSP). The MIT is a region of plasma depletion occurring at sub-auroral latitudes in the night-time F-region (~200-500 kilometers in altitude) ionosphere [1]. The MIT can be calculated as regions of low TEC, typically less than 50% below the background electron content. Observations thus far have shown a hemispherical asymmetry in trough occurrence near solstices and magnetospheric data alignment in the Northern hemisphere primarily on the dusk side. Based on our visualizations of the MIT, we observe gaps typically occurring near dusk. We hypothesize that there may exist a connection between this structure and sub-auroral polarization streams (SAPS).
SAPS are associated with field-aligned currents (FACs), which require closure in the conducting ionosphere. SAPS tend to occur just after sunset and before midnight due to convection patterns across the polar cap, but new research shows that there may be a similar phenomenon happening on the dawn side of the Earth-Sun system. These newly hypothesized energy flows are referred to as dawn auroral polarization streams (DAPS) [2], which raise new questions about how SAPS-like flows can occur on the dawn side of the Earth. Our goal is to visualize and analyze behavior of the ionospheric trough and verify our results using other studies and data. Our analysis is compared with previous work done by Liu et al. (2020) [2] and Liu and Xiong (2020) [3] and offers reasonable and similar results.
The location of the MIT is calculated from TEC using a Python algorithm developed by Greg Starr [4] (based on the work of Aa et al. (2020 [1]) which allows us to both visualize and analyze depleted regions of the ionosphere. TEC values are averaged over an hour and interpolated using nearest neighbors in order to improve data coverage. Along with the MIT itself, we plot Poynting flux (PF), ion drift velocity (VF), and electric field (EF) vectors from DMSP over these maps. Since PF is derived from magnetic field perturbation, electric field, and ion drift observations, strong flows are a good indication of SAPS. The EF and VF vectors offer verification that the ions are flowing in the expected direction for SAPS (along a poleward EF).
We find correlation between trough gaps and PF on the dusk side with similar behavior on the dawn side of the Earth (typically in the Southern hemisphere), however it is unclear if these may be due to poor data coverage. Statistical analysis on both trough characteristics and the observed data alignment will be presented to demonstrate how this behavior depends on latitude, MLT, and its potential connection to SAPS. This analysis gives us insight to the precise latitudes and MLT occurrences of both trough gaps and Poynting flux to see how closely these values are aligned. This work opens the door to further investigation of dawnside and Southern hemisphere ionospheric behavior.

References
[1] Aa, E., Zou, S., Erickson, P. J., Zhang, S.-R., & Liu, S. (2020). Statistical analysis of the main ionospheric trough using Swarm in situ measurements. Journal of Geophysical Research: Space Physics, 125.
[2] Liu, J., Lyons, L. R., Wang, C.-P., Hairston, M. R., Zhang, Y., & Zou, Y. (2020). Dawnside auroral polarization streams. Journal of Geophysical Research: Space Physics, 125.
[3] Liu, Y., & Xiong, C. (2020). Morphology evolution of the midlatitude ionospheric trough in nighttime under geomagnetic quiet conditions. Journal of Geophysical Research: Space Physics, 125, 2019JA027361. https://doi.org/10.1029/2019JA027361
[4] Starr, G., Mrak, S., Nishimura, Y., Hirsch, M., Ishwar, P., & Semeter, J. (2022). Automatic identification of the main ionospheric trough in Total Electron Content images. Space Weather, 20, e2021SW002994. https://doi.org/10.1029/2021SW002994