2025 THEMIS SCIENCE NUGGETS
Statistical Analysis of Ultra-Low-Frequency Total Electron Content Disturbances: Relationship to Magnetospheric Waves
Michael Hartiinger
UCLA Earth, Planetary, and Space Sciences, Los Angeles, California
Introduction
The ionosphere, a region of Earth’s upper atmosphere that includes ions and electrons, is highly dynamic. Large, wave-like changes in electron density can lead to a range of space weather impacts, including disruptions of radio signals that affect GPS positioning. Since multiple phenomena below (earthquakes, explosions, thunderstorms…) and above (e.g., electrical currents and wave-like variations in magnetic field and plasma) the ionosphere affect electron dynamics, it’s challenging to track and predict the source of these changes. To address this, we combine THEMIS satellite measurements above the ionosphere with GPS receiver and magnetometer measurements below the ionosphere (Figure 1a), along with lists of earthquakes and other surface disturbances, to isolate and study conditions when plasma waves above the ionosphere affect electron dynamics in the ionosphere. We focus on the Total Electron Content (TEC), a frequently measured and widely used parameter corresponding to the total number of electrons measured in a line of sight between a GPS satellite and ground-based receiver (Figure 1b).
|
| Figure 1. (Figure and caption taken from Shen et al., 2024 Figure 1): Schematic diagram showing coordinated observations from THEMIS and ground-based GPS receiver (FAIR) of (a) modulation of whistler-mode waves near the magnetic equator by ULF waves, electron pitch-angle scattering into the loss cone, and precipitation into the ionosphere (red arrows) induced by modulated whistler-mode waves; and (b) the modulated electron precipitation with energies of 0.1–30 keV deposits their energies at altitudes between 100–400 km and induces modulated impact ionization and dTEC having amplitudes as large as 0.5 TECU and spanning scales of 5–100 km. This dTEC modulation was captured by the signal from GPS43, which has a high elevation, but was overlooked by the signal from GPS40, which has a relatively lower elevation. |
Results
Through many THEMIS satellite-GPS receiver-ground magnetometer conjunctions (i.e., Figure 1), we found that magnetospheric Ultra Low Frequency (ULF) waves frequently drive auroral zone Total Electron Content (TEC) variations with frequencies of ~2-50 mHz. We also found that the TEC variation amplitude correlates with ground magnetic variation amplitude, and the correlation depends on magnetic field component. Finally, we found that TEC variation amplitude correlates with Kp, AE, and solar wind speed, with the largest amplitudes usually occurring near local midnight.
|
| Figure 2. (Figure and caption taken from Hartinger et al., 2025): (a) TEC wave power versus ground-based magnetic field wave power, where color indicates the component of the magnetic field where x is towards magnetic north, z is directed down into the Earth, and y completes the right‐hand orthogonal set (roughly eastward). (b) The same as panel (a), but for satellite magnetic field wave power in a magnetic field-aligned coordinate system where z is parallel to the mean B, y is in the local azimuthal direction for SM cylindrical coordinates, and x completes the right‐hand orthogonal set and is directed radially outward. Additionally, only events where the observing satellite is located close to the magnetic equatorial plane (BzGSM |B| >0.95) are shown to remove effects related to field‐aligned mode structure. In both panels, lines are for least squares fits where color again indicates component of the magnetic field; corresponding slopes and R‐values are shown in the insets. |
One of the conjunction events in the database was studied by Shen et al., [2024], who found that the TEC variations were linked to whistler waves modulated by ULF waves, both of which were also observed by THEMIS and led to electron precipitation that drove the TEC variations (illustrated in Figure 1). This is one example of the many ways ULF waves can modulate TEC. The published data and survey plots [Hartinger et al., 2025b,c] likely show a wide range of other modulation mechanisms; for more details see Hartinger et al., [2025a].
Conclusion
The results in this study have implications for both remote sensing and future magnetosphere-ionosphere coupling studies. In particular, the results shown in Figure 2 indicate that TEC wave power can be used as a proxy for ULF wave power; this finding has the potential to substantially increase spatial coverage of ULF wave observations from sparse satellites and in regions that currently lack ground magnetometers but have GPS receivers (e.g., West Antarctica). More broadly, the fact that so many events in the conjunction database were driven from the magnetosphere suggests that more diagnostics are needed to separate events “driven from above” and those “driven from below” the ionosphere when studying TEC variations. Additionally, the fact that ULF waves drive TEC disturbances with amplitudes comparable to background TEC levels (30-40%) indicates that future studies are needed to explore whether linear theory is adequate to capture the spatial and temporal evolution of these wave events. Future studies are also needed to explore whether these TEC disturbances lead to space weather impacts such as scintillation.
Biographical Note
Michael Hartiinger is a Research Scientist at Space Science Institute and Associate Researcher at UCLA. He earned his Ph.D. from the UCLA Earth, Planetary, and Space Sciences Department in 2012. His research focuses on plasma waves and space weather impacts such as geomagnetically induced currents, as well as north-south hemisphere asymmetries.
References
Hartinger, M. D., Shi, X., Verkhoglyadova, O., Meng, X., Ozturk, D. Su., Moore, A., et al. (2025a). Statistical analysis of ultra-low-frequency total electron content disturbances: Relationship to magnetospheric waves. Journal of Geophysical Research: Space Physics, 130, e2024JA033456. https://doi.org/10.1029/2024JA033456Hartinger, M. (2024b). Total electron content and magnetic field time series survey plots during THEMIS/CMO/FAIR magnetic conjunctions [Dataset]. Zenodo. https://doi.org/10.5281/zenodo.13770417
Hartinger, M. (2024c). ULF wave events during THEMIS‐CMO‐FAIR magnetic conjunctions [Dataset]. Zenodo. https://doi.org/10.5281/zenodo.11536584
Shen, Y., Verkhoglyadova, O. P., Artemyev, A., Hartinger, M. D., Angelopoulos, V., Shi, X., and Zou, Y. (2024). Magnetospheric control of ionospheric TEC perturbations via whistler-mode and ULF waves. AGU Advances, 5, e2024AV001302. https://doi.org/10.1029/2024AV001302
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu