2‐day ionospheric oscillations at low latitudes due to an ultra‐fast Kelvin wave

This study establishes an observational connection between an ultra‐fast Kelvin wave (UFKW) and the occurrence of counter electrojet (CEJ) in the equatorial E region and planetary wave‐like oscillations in low‐latitude total electron content (TEC), based on a case study of CEJ events that occurred in December 2017. During this period, noontime CEJ was observed by ground‐based magnetometers with a periodicity of ∼2 days. The analysis of hourly wind data from the JAGUAR‐DAS Whole neutral Atmosphere Reanalysis reveals the presence of an eastward‐propagating UFKW with a zonal wavenumber of 1 (E1) and a period of ∼2 days along with an enhancement of eastward wind in the equatorial dynamo region which coincided with occurrence of CEJ. Observations from the Swarm satellite mission confirm the presence of equatorial electrojet (EEJ) oscillations associated with the UFKW. This is the first time that an UFKW has been identified as a source of day‐to‐day variability in CEJ. As CEJ is linked to a reduced daytime upward plasma drift over the magnetic equator, TEC at low latitudes also varies at a period of ∼2 days. The analysis of global TEC maps reveals that the 2‐day TEC variations consist not only of the eastward propagating wavenumber 1 component but also of the westward propagating wavenumber 2 (W2) component. The latter arises from the interaction of the E1 UFKW with the diurnally varying ionosphere. The 2‐day TEC variations exhibit significant longitudinal dependence due to the interplay of constructive and destructive interferences between the E1 and W2 2‐day oscillations.