Geomagnetism 6 Ionosphere 6 Vertical total electron content Electrons 4 GPS 4 GPS ionospheric model 4 Geomagnetic fields 4 Geomagnetic storm 4 Geomagnetic substorm 4 Geostationary satellites 4 Global positioning system 4 Ionospheric perturbation 4 Observatories 4 Average growth rate 2 Buildings 2 Computer simulation 2 Geographical distribution 2 Geomagnetic disturbances 2 Geomagnetic field 2 Geomagnetic storms 2 Geomagnetic variation 2 Geostationary operational environmental satellites 2 Hard X ray 2 Ionization 2 Ionization of gases 2 Ionized gas 2 Ionospheric solar flare effects 2 Magnetic fields 2 Magnetosphere 2 Near-Earth 2