Ultrafast Kelvin wave variations in the surface magnetic field

A suite of general circulation models is used to investigate the surface magnetic perturbations due to the ionospheric currents driven by an eastward-propagating ultrafast Kelvin wave (UFKW) packet with periods between 2 and 4 days and zonal wave number s = -1. The simulated daytime UFKW-driven meridional magnetic perturbations dBn (similar to +/- 5 nT) (or zonal currents) between about 5 degrees and 20 degrees magnetic latitude in each hemisphere are opposite in sign to those equatorward of +/- 5 degrees and produced by the equatorial electrojet (EEJ), with the directions on any given day determined by the phase of the UFKW as it propagates eastward with respect to the sunlit ionosphere. Since the nominal daytime Sq zonal current between similar to +/- 30 degrees is uniformly eastward flowing, the present results are consistent with the hypothesis that the EEJ is part of a local current vortex with oppositely directed currents near the equator versus those between 5 degrees and 20 degrees at low latitudes. UFKWs are a special wave type wherein meridional winds are relatively small, which leads to our finding that the EEJ dBn constitutes a simple quantitative proxy for E-region UFKW neutral winds near the 107-km peak height of the Hall conductivity, including the variable wave period of the UFKW packet. Numerical experiments are also performed to understand the longitude distribution of actual ground magnetometer measurements that are needed to reliably extract the UFKW dBn signal and hence the neutral winds, both of which are closely linked to plasma drifts and electron densities in the equatorial F region. Using actual magnetometer data it is moreover shown that the UFKW dBn signal is easily measurable. Therefore measurements of EEJ dBn can potentially be used to infer UFKW activity for scientific investigations focusing on coupling between the tropical troposphere and the ionosphere-thermosphere.