Quiet-time ionospheric density variations observed by the Radio Receiver Instrument on e-POP/Swarm-E

Quiet geomagnetic conditions provide a unique window into ionospheric dynamics driven by lower-atmospheric forcing and weak background magnetospheric coupling. By applying Faraday rotation rate–based methods to transionospheric HF radio-wave polarization measurements from the Radio Receiver Instrument (RRI) on Swarm-E/e-POP, differential total electron content (dTEC) in the ionosphere can be derived at substantially higher along-track resolution than provided by GPS. In this work, dTEC observations from GPS and RRI during two geomagnetically quiet days in December 2017 are examined in order to characterize background ionospheric dynamics under weak magnetospheric forcing. Similar large-scale wavelike structures observed on consecutive days by both RRI and GPS indicate persistent regional density perturbations. Additionally, RRI resolves small-scale (7 to 50 km) dTEC variations with amplitudes of ±1 to 2 TECU that are not captured by GPS. Repetitive enhancements and depletions confined to narrow latitudinal bands of about 0.25°, corresponding to roughly 25 km, indicate a quiet-time ionosphere structured by continuous mesoscale and small-scale forcing. This is consistent with upward-propagating disturbances from the lower atmosphere that are associated vertical coupling with the ionosphere.