remote sensing Article Long-Term Variations of Plasmaspheric Total Electron Content from Topside GPS Observations on LEO Satellites Shuanggen Jin 1,2, * , Chao Gao 1,3 , Liangliang Yuan 1 , Peng Guo 1 , Andres Calabia 2 , Haibing Ruan 2 and Peng Luo 1,4   Citation: Jin, S.; Gao, C.; Yuan, L.; Guo, P.; Calabia, A.; Ruan, H.; Luo, P. Long-Term Variations of Plasmaspheric Total Electron Content from Topside GPS Observations on LEO Satellites. Remote Sens. 2021, 13, 545. https://doi.org/10.3390/ rs13040545 Academic Editor: Roberta Giuliani Received: 7 January 2021 Accepted: 29 January 2021 Published: 3 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China; cgao@shao.ac.cn (C.G.); llyuan@shao.ac.cn (L.Y.); gp@shao.ac.cn (P.G.); luopeng@shao.ac.cn (P.L.) 2 School of Remote Sensing and Geomatics Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China; andres@calabia.com (A.C.); rhb@nuist.edu.cn (H.R.) 3 School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China 4 School of Communication and Information Engineering, Shanghai University, Shanghai 200444, China * Correspondence: sgjin@shao.ac.cn or sg.jin@yahoo.com; Tel.: +86-021-34775292 Abstract: The plasmasphere is located above the ionosphere with low-energy plasma, which is an important component of the solar-terrestrial space environment. As the link between the ionosphere and the magnetosphere, the plasmasphere plays an important role in the coupling process. Therefore, it is of great significance to study the electron content variation of the plasmasphere for the solar- terrestrial space environment. Nowadays, the topside global positioning system (GPS) observations on Low Earth Orbit (LEO) satellites provide a unique opportunity to estimate and study variations in the plasmasphere. In this paper, the plasmaspheric total electron content (PTEC) is estimated, and its long-term variations are studied from topside GPS observations onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). The PTEC in the daytime is higher than that in the nighttime, with the peak between 14:00 and 17:00 in the magnetic local time, while the minimum value of PTEC in the belt appears between 3:00 and 6:00 in the magnetic local time before sunrise. For seasonal variations, the PTEC is the highest in spring of the northern hemisphere and the lowest in summer of the northern hemisphere regardless of the state of the solar activity. The long-term variation in PTEC is further analyzed using 11-year COSMIC GPS observation data from 2007 to 2017. A high correlation between PTEC and the F10.7 indices is found. Particularly in the geomagnetic high-latitude region during the daytime, the correlation coefficient reaches 0.93. The worst case occurs during the nighttime in the geomagnetic middle-latitude region, but the correlation coefficient is still higher than 0.88. The long-term variations of plasmaspheric TEC are mainly related to the solar activity. Keywords: plasmasphere; PTEC; GPS; GCPM; F10.7 index 1. Introduction With the continuous exploration into deep space and the increasing variety of elec- tromagnetic applications, such as communication and navigation, monitoring and under- standing of the solar-terrestrial space environment have become a hot field, including the Earth’s neutral atmosphere, ionosphere, plasmasphere, magnetosphere, and so on [1]. The plasmasphere is a part of magnetosphere, also called the inner magnetosphere [2], which starts from the top of the ionosphere and ends at the plasmapause. The plasmas- phere is a donut-shaped region surrounding the Earth, containing the coldest plasma of the magnetosphere [3]. It is currently believed that the charged particles in the plasmas- phere mainly come from escape of the ionosphere and capture from the solar wind [4,5]. Richards et al. [6] examined the relative importance of ionospheric and thermospheric densities and temperatures in producing the annual variation of the plasmaspheric electron density. Lee et al. [7] compared the global plasmaspheric total electron content (TEC) with Remote Sens. 2021, 13, 545. https://doi.org/10.3390/rs13040545 https://www.mdpi.com/journal/remotesensing