{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T00:35:25Z","timestamp":1760142925202,"version":"build-2065373602"},"reference-count":16,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2024,1,9]],"date-time":"2024-01-09T00:00:00Z","timestamp":1704758400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"European Union","award":["H2020-INFRAIA-2018-2020101007599"],"award-info":[{"award-number":["H2020-INFRAIA-2018-2020101007599"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>The time evolution of the total number of free electrons in the Earth\u2019s ionosphere, i.e., the Global Electron Content (GEC), during more than two solar cycles is analyzed in this work. The GEC time series has been extracted from the Global Ionospheric Maps (GIMs) of Vertical Total Electron Content (VTEC) estimated by UPC-IonSAT with TOMION-v1 software from global GPS measurements since the end of 1996. A dual-layer voxel-based tomographic model solved with a forward Kalman scalar filter, from dual-frequency carrier GPS data only, provides the so-called UQRG GIM after VTEC kriging interpolation, with a resolution of 15 min in time, 5\u00b0 in longitude and 2.5\u00b0 in latitude. UQRG is one of the best behaving GIMs in the International GNSS Service (IGS).In this context, the potential application of the GEC spectrum evolution as a potential space weather index is discussed and demonstrated.<\/jats:p>","DOI":"10.3390\/s24020393","type":"journal-article","created":{"date-parts":[[2024,1,9]],"date-time":"2024-01-09T03:36:58Z","timestamp":1704771418000},"page":"393","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["The Spectrum of Global Electron Content: A New Potential Indicator of Space Weather Activity"],"prefix":"10.3390","volume":"24","author":[{"given":"Josep Maria","family":"Aroca-Farrerons","sequence":"first","affiliation":[{"name":"UPC-IonSAT, 08034 Barcelona, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9687-5850","authenticated-orcid":false,"given":"Manuel","family":"Hern\u00e1ndez-Pajares","sequence":"additional","affiliation":[{"name":"UPC-IonSAT, 08034 Barcelona, Spain"},{"name":"IEEC, 08034 Barcelona, Spain"}]},{"given":"Haixia","family":"Lyu","sequence":"additional","affiliation":[{"name":"UPC-IonSAT, 08034 Barcelona, Spain"},{"name":"GNSS Research Center, Wuhan University, Wuhan 430079, China"},{"name":"Department of Physics and Mathematics, University of Alcal\u00e1, 28801 Alcal\u00e1 de Henares, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4450-6073","authenticated-orcid":false,"given":"David","family":"Roma-Dollase","sequence":"additional","affiliation":[{"name":"IEEC, 08034 Barcelona, Spain"},{"name":"Instituto de Ciencias del Espacio (ICE-CSIC, IEEC), Campus Universitat d\u2019Autonoma de Barcelona, Carrer de Can Magrans, 08193 Cerdanyola del Vall\u00e8s, Spain"}]},{"given":"Raul","family":"Orus-Perez","sequence":"additional","affiliation":[{"name":"ESA, ESTEC, 2201 AZ Noordwijk, The Netherlands"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6226-4851","authenticated-orcid":false,"given":"Alberto","family":"Garc\u00eda-Rigo","sequence":"additional","affiliation":[{"name":"UPC-IonSAT, 08034 Barcelona, Spain"},{"name":"IEEC, 08034 Barcelona, Spain"}]},{"given":"Victoria","family":"Graffigna","sequence":"additional","affiliation":[{"name":"UPC-IonSAT, 08034 Barcelona, Spain"},{"name":"IEEC, 08034 Barcelona, Spain"}]},{"given":"Germ\u00e1n","family":"Olivares-Pulido","sequence":"additional","affiliation":[{"name":"UPC-IonSAT, 08034 Barcelona, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4907-0494","authenticated-orcid":false,"given":"Enric","family":"Monte-Moreno","sequence":"additional","affiliation":[{"name":"UPC-TALP, 08034 Barcelona, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1583-2307","authenticated-orcid":false,"given":"Heng","family":"Yang","sequence":"additional","affiliation":[{"name":"UPC-IonSAT, 08034 Barcelona, Spain"},{"name":"UPC-TALP, 08034 Barcelona, Spain"},{"name":"School of Electronic Information and Engineering, Yangtze Normal University, Chongqing 408100, China"}]},{"given":"Qi","family":"Liu","sequence":"additional","affiliation":[{"name":"UPC-IonSAT, 08034 Barcelona, Spain"},{"name":"College of Geography and Environmental Science, Henan University, Kaifeng 475004, China"}]}],"member":"1968","published-online":{"date-parts":[[2024,1,9]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"335","DOI":"10.5194\/angeo-26-335-2008","article-title":"Global electron content: A new conception to track solar activity","volume":"26","author":"Afraimovich","year":"2008","journal-title":"Ann. 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