**1. Introduction**

With the development and advancement of space exploration technology, researchers have paid increasing attention to seismic ionosphere detection and lithosphere–ionosphere coupling. However, earthquake prediction has been in the exploratory stage because of the complexities behind the causes of earthquakes. There are many reports regarding pre-earthquake geological structural changes and other pre-earthquake anomalies related to earthquakes; among these, the pre-earthquake ionospheric anomaly is a popular topic of contemporary research [1–3].

Davies (1964) first detected abnormal ionospheric disturbances above the epicenter of a 9.2 magnitude Alaskan earthquake [4]. Pulinets (2004) observed anomalies appearing in electron densities of the ionospheric F region a few days before some strong earthquakes through the analysis of GPS TEC and other ionospheric parameters from different dedicated satellites [5]. The preliminary results from the above research show that the energy released during seismogenesis can propagate upward into the Earth's atmosphere, resulting in ionospheric disturbances. The identification of seismo-ionospheric anomalies was made from

**Citation:** Wei, L.; Li, J.; Liu, L.; Huang, L.; Zheng, D.; Tian, X.; Huang, L.; Zhou, L.; Ren, C.; He, H. Lithosphere Ionosphere Coupling Associated with Seismic Swarm in the Balkan Peninsula from ROB-TEC and GPS. *Remote Sens.* **2022**, *14*, 4759. https://doi.org/10.3390/ rs14194759

Academic Editors: José Fernández, Juan F. Prieto and Serdjo Kos

Received: 11 August 2022 Accepted: 20 September 2022 Published: 23 September 2022

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the analysis of different GNSS satellite measurements based on different statistical methods. For example, by using GPS TEC, Sanchez (2022) found TEC in the form of traveling ionospheric disturbances (TIDs) within 1 h after the mainshock onset during an earthquake in California [6]. Liu (2001) detected TEC 15 days before an earthquake (Mw 7.7) in Taiwan and found three anomalies that provided empirical evidence of earthquake-executed ionosphere anomalies within five days before the mainshock [7]. Chen (2004) found a significant correlation between anomalies and earthquakes in Taiwan and the anomalous depression of the maximum plasma frequency in the ionospheric F2 layer by analyzing these TEC anomalies [8]. Hobara (2005) found from a dedicated satellite's mission over a seismogenic zone that an abnormal ionospheric disturbance appeared during an 8.3 magnitude earthquake in Hachiko [9]. Ryu (2014) discovered earthquake–ionospheric coupling by detecting ionospheric electron density (IED) data in seismic regions accompanied by PEIA; this report suggested the possible coupling of the lithosphere and ionosphere by the integration of TEC [10]. Similarly, Yao (2016) used a singular spectrum analysis of the TEC time series around the epicenter of the Nepal earthquake and showed positive ionospheric anomalies in the epicenter region before the earthquake [11]. Shah (2020) used global TEC observation data to study ionospheric anomalies before and after earthquakes at different latitudes, and found there were pre-earthquake disturbances and coseismic responses in the temporal and spatial distribution of the TEC data [12]. Tariq (2021) studied the ionospheric anomalies before earthquakes in Pakistan and Islamabad, and the results showed positive anomalies in the ionosphere ten days before the two earthquakes [13]. In general, seismo-ionospheric anomalies appear either a few days to two weeks before large earthquakes or around the earthquake time [14]. Various kinds of great earthquake precursors have been reported so far, but there are few studies on the impact of the seismic swarm on the ionosphere. Therefore, this paper comprehensively describes the ionospheric disturbances of the Balkan–Greece seismic swarm with a case study of both the co- and pre-seismic ionospheric disturbances of the seismic swarm.

#### **2. Experimental Data**

To avoid the interference of magnetic storms, earthquakes that occurred under geomagnetic conditions during a quiet period were selected (Kp < 4, F10.7 < 100 and Dst > −30 nT). The Kp and Dst indices were usually lower than 4 and greater than −30 nT, respectively, which indicate that the geomagnetic activity is quiet [15,16]. The seismic data were obtained from the United States Geological Survey (available online: https://earthquake.usgs.gov/earthquakes/search/ (accessed on 10 July 2021)), which provided the reference for calculating the radius of the seismogenic area [11,17]. We also analyzed TEC data from GPS stations operating within the seismogenic zone of the seismic swarm in the Balkan Peninsula (Figure 1). Further details are presented in Table 1.

We retrieved high-quality TEC data from the ROB by using GPS observations, and the products consist of ionospheric vertical TEC maps over Europe, which were estimated in near real-time every 15 min with 0.5◦ × 0.5◦ grids. The maps are available online with a latency of ~3 min in IONEX format at: ftp://gnss.oma.be (accessed on 11 July 2021) and as interactive web pages at: www.gnss.be (accessed on 11 July 2021) [18]. In addition, we calculated the VTEC at seven GPS stations operating within the seismogenic zone of the seismic swarm in the Balkan Peninsula (available online: https://www.epncb.oma.be/ (accessed on 12 July 2021)). The correlation index for the solar activity and geomagnetic activity was obtained from the Goddard Space Flight Center of NASA (available online: https://omniweb.gsfc.nasa.gov/ (accessed on 12 July 2021)).

**Figure 1.** Geographic map of the locations of seismic swarm in the Balkan Peninsula in 2019. Seismogenic zones are shown by circles. Earthquake information is shown with every epicenter (circular). GPS stations are shown by filled hexagons.

**Table 1.** Details of the earthquakes that occurred in the Balkan Peninsula.

