Space weather phenomena such as scintillations of Global Navigation Satellite Systems (GNSS) signals are of increasing importance for aviation, the maritime, and civil engineering industries. The ionospheric plasma irregularities causing scintillations are associated with strong gradients in ionospheric plasma density. To provide nowcasts and forecasts of space weather effects, it is vital to monitor the ionosphere and detect strong density variations. To reconstruct plasma density variations in the polar cap ionosphere, we use total electron content (TEC) estimates from the Swarm satellites' GPS receivers. By considering events where the Swarm satellites are in close proximity, we obtain plasma density variations by inverting TEC measurements on a two-dimensional grid. We first demonstrate the method using synthetic test data, before applying it to real data. The method is validated using in situ Langmuir probe measurements and ground-based TEC observations. We find that the new method can reproduce density variations, although it is sensitive to the geometry of the Swarm satellite constellation and to the calculated plasma temperature. Our proposed method opens new possibilities for ionospheric plasma monitoring that uses GPS receivers aboard low Earth orbit (LEO) satellites.

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GPS signals are subject to disturbances in the polar regions, caused in part by clouds of high density plasma (polar cap patches). For the Swarm satellites, this noise results in positioning errors in the order of centimeters. In this study, we use data from the GPS receiver and compare it to in situ density gradient data from the Langmuir probes. 65 We find that when GPS satellites are in front (azimuth φ=0±20°) of the Swarm, there is a direct proportionality between the density gradient dn/ds and the GPS observable dL4/dt. The same is true for when GPS satellites are positioned behind the Swarm satellite (azimuth φ=180±20°).Our results suggest that dL4/dt measurements from any satellite can be used to extrapolate the distribution of the plasma density gradient in the surrounding volume of the satellite, so that plasma density gradients can be detected from GPS receiver data alone. @en