Abstract
The study investigates ionospheric electric field responses to the earthquake (EQ) of magnitude 8.3, and the related seismic activity and tsunami triggered by the mainshock in Chile-Illapel region, at 22:54 UTC, in the evening of September 16, 2015. The work is a wider review of available ground and satellite data and techniques available to detect seismically induced traveling ionospheric disturbances (TID) and irregularities of smaller scale. The data used in the experiment includes several types of ground and satellite observations from low-Earth orbit (LEO) satellites. The number of techniques applied here is also extended and includes spectral analysis of LEO along-track data and composed analysis of ground GNSS data. The timeframe of the analyses is focused on September 16 and 17, 2015 but also extended to several adjacent days, where an enhanced seismic activity has been recorded. Several examples of seismically triggered TIDs are shown, as detected by combined observations from more than one source and applying different methods, including spectral analysis. These disturbances occur before the mainshock, just after, or in time following this large EQ, and can be found in close neighborhood of Chile-Illapel or far away from the epicenter. The objective of the work was to demonstrate an increasing number of available data and techniques, which can be limited when applied alone, but their combination can provide many advantages in the analysis of seismically disturbed ionosphere. The combination of LEO satellite data reaching all regions of the globe with local but dense ground-based GNSS data and ionospheric HF sounders looks promising, especially in view of the nearby availability of CubeSat constellations equipped with instruments for ionosphere sounding. An important conclusion coming from the study is a need for spectral analysis techniques in the processing of LEO along-track data and the requirement of the validation of LEO observations with separate LEO data or ground-based data. A general but key finding refers to the complementarities of different observations of the ionospheric electric field, which is critically important in the case of analyzing ionospheric irregularities in the extended and composed ionosphere, especially if not every sounding direction can successfully find it.
Highlights
The study investigates ionospheric electric field responses to the earthquake (EQ) of magnitude 8.3, and the related seismic activity and tsunami triggered by the mainshock in Chile-Illapel region, at 22:54 UTC, in the evening of September 16, 2015
The extremely dense ground total electron content (TEC) observations performed by the GEONET network enabled clear distinguishing of acoustic-gravity waves generated at the epicenter, acoustic waves coupled with the Rayleigh wave, and gravity waves coupled with the following tsunami (Occhipinti et al, 2013; Jin et al, 2014; Chou et al, 2020)
The objective of this work was to demonstrate the synergistic effect of a wider scope of data sources and techniques worth considering together, as well as a wider spatiotemporal window analyzing seismically triggered changes in the ionosphere and changes correlated with tsunami
Summary
The composed process for characterizing the TIDs associated with EQ and tsunami is done in two stages (see details in Yang et al, 2019). A preliminary coincidence of Swarm B disturbance and TIDs in-ground GNSS data (Fig. 9) must be inspected by the scatter plot because the keogram indicates only a common distance of the observations from the epicenter and not necessarily a common location. An extension of the applicability of the TOMION model (multiTOMION) has been done by combining GNSS-POD LEOs-, GNSS ground- and vessel-, and DORISbased measurements with the corresponding improvement of VTEC Global Ionospheric Maps (Hernández-Pajares et al, 2020) This extension can provide a direct estimation of 4D ED gradients as a good tool for future tsunami detection and characterization, augmented with the use of LEO constellations much larger than the Swarm one, such as e.g., Spire CubeSats.
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