Prompt GPS TEC response to magnetospheric compression

A new type of total electron content (TEC) variation was observed when an interplanetary shock impacted on the Earth's magnetosphere on 17 March 2015. With hundreds of ground-based Global Positioning System (GPS) receivers, instantaneous TEC impulses were detected in the signals from four GPS satellites cruising in the dayside equatorial magnetosphere. Despite the small amplitude (0.3 TECU), the impulses were synchronously registered by receivers spreading from low to middle latitudes on the ground; they lasted for several minutes, during which period the interplanetary magnetic field (IMF) kept northward. The TEC impulses therefore discriminate themselves from usual traveling ionospheric disturbances and exclude possible effects from the southward IMF-driven magnetic storm. We suggest that the TEC variation is caused by shock-induced magnetospheric compression, which moves plasma earthward in the dayside plasmasphere. As a result, some plasma outside of GPS orbit (4.2 R_E) is moved to the inside and contributes to the plasma content traversed by GPS raypath. The GPS TEC technique thus exhibits an unprecedented capability to capture small tremor of the magnetosphere, and with the dense receiver network on the ground it can be a feasible tool for remote sensing of the plasma dynamics around 4.2 R_E.