Abstract:                                       

Global Navigation Satellite Systems (GNSS) have been demonstrated in numerous studies as a practical technique for monitoring ionospheric responses induced by seismic activity, such as earthquakes, tsunamis, and volcanic eruptions. One of the key parameters frequently used for monitoring Coseismic Ionospheric Disturbances (CID) is Total Electron Content (TEC), which can be calculated from dual-frequency GNSS observations. However, the overlap of ionospheric TEC anomalies caused by both seismic and non-seismic factors remains an uncertainty in CID interpretation. Therefore, this study utilizes high-rate GNSS data to determine whether statistically significant differences in ionospheric anomalies can be identified, which may help to distinguish CID from other influencing factors. For a comprehensive assessment, we examine GNSS data across a range of sampling rates, from a standard rate (one observation every 30 seconds) up to a high rate of 50 Hz (50 observations per second). We then apply a combination of sine wave analyses to quantify the parameters of GNSS-TEC variations. Our preliminary results reveal that GNSS-TEC signals fluctuate at an average frequency of ~0.12 Hz, which falls within the infrasound band of atmospheric acoustic waves, a range proven to be generated by seismic events. These ionospheric TEC disturbances typically occur around ten minutes after the mainshock of large seismic events, consistent with the speed of acoustic wave propagation from the Earth's surface to the ionosphere. Additionally, using high-rate Global Positioning System (GPS) data, we detect ionospheric TEC disturbances in the Extremely Low Frequency (ELF) band, ranging from ~80 Hz to ~200 Hz. These frequencies are indicative of plasmaspheric hiss waves, which are often generated in the plasmasphere, a region of plasma extending from the ionized upper atmosphere to ~2 to ~6 Earth radii. This distance aligns well with the GPS satellite altitude, which orbits at ~20200 km above the Earth's surface, enabling it to explain why GNSS signals can capture this wave band. Although further work is needed to fully address the challenges associated with wave resonances and their influence on signal interpretation, our findings have provided convincing evidence to enhance understanding of the causal mechanisms behind ionospheric anomalies in general.

Keywords: GNSS, GNSS-TEC disturbances, Seismic activity, Infrasound waves, Plasmaspheric hiss waves

Authors: Nhung Le^1,2*, Yuri Shprits^1,3, Xingzhi Lyu¹, Naofumi Takamatsu^1,4, Kłos Anna⁵, Harald Schuh^1,6

¹GFZ Helmholtz Centre for Geosciences, Potsdam, Germany
²Hanoi University of Natural Resources and Environment, Vietnam
³Universität Potsdam, Germany 4Geospatial Information Authority of Japan
⁵Military University of Technology, Poland
⁶Technische Universität Berlin

Corresponding: nhung@gfz.de