Observation of the neutral-ion coupling through 6-day planetary wave

A westward 6 day oscillation with zonal wave number 1 is identified in the global maps of the ionospheric total electron content (TEC) during May 2003. This signature coincides with the wave activities in the mesosphere and lower thermosphere (MLT) region with a similar zonal structure and period, deduced from the temperature and wind observations from the Thermosphere Ionosphere and Mesosphere Electric Dynamics (TIMED) satellite. The vertical wavelength of this 6 day wave is estimated to be ~65 km, which enables it to propagate up into the lower thermosphere and therefore modulate the ionosphere through E region wind dynamo. The zonal wind perturbations of the 6 day wave maximize in the equatorial region with an amplitude of ~30 m/s and the meridional wind perturbations peak at middle latitudes with an amplitude of 15–20 m/s. The 6 day oscillation in TEC peaks at the geomagnetic equatorial ionization anomaly (EIA) crests in both hemispheres with a deep minimum at the equator. The absolute TEC perturbations maximize at ~1400-1800 LT with an amplitude of ~9 (~7) total electron content unit (TECU; 1 TECU = ¹01⁶ m⁻²) in the southern (northern) hemisphere, which account for ~16% (~10%) of the background TEC. Larger relative TEC perturbations of ~20% are found at 0200-0400 LT. The similar wave number-period spectra and the consistent temporal variations of the 6 day periodical signatures in both neutral atmosphere and ionosphere suggest a strong neutral-ion coupling through planetary wave.