The effect of non-migrating tides on the morphology of the equatorial ionospheric anomaly: seasonal variability

Recent observations of the low-latitude F-region ionosphere at times near equinox have shown that it varies with a predominant zonal wavenumber-four pattern in a fixed local-time frame. It has been shown that this pattern corresponds well to the non-migrating diurnal eastward wavenumber-three atmospheric tide (DE3) at E-region altitudes simulated by the Global Scale Wave Model (GSWM). Here we present details of the morphology of the F-region ionosphere from TIMED GUVI with simultaneous observations of the non-migrating diurnal tides at E-region altitudes from TIMED SABER. For the case of equinox (March 2002), the correspondence of the SABER and GUVI observations confirms the relationship previously established using the GSWM simulations. There is also a wavenumber-one signature that is present which may be related to the semi-diurnal westward wavenumber-three, possibly in conjunction with changes in the magnetic field with longitude. During July 2002, when the amplitude of the DE3 maximizes, the amplitude of the wavenumber-four pattern in the F-region ionosphere intensifies. There is also evidence of a strong wavenumber-three pattern in the F-region ionosphere. which call be attributed to the strong diurnal eastward wavenumber-two tide during this period. During January 2003, the amplitude of all non-migrating components observed by SABER are either small or asymmetric and the ionosphere does not display either a wavenumber-three or -four pattern. During both solstice periods, a strong wavenumber-one is seen that is attributed to the offset of the subsolar point and the geomagnetic equator that maximizes at solstice, possibly in conjunction with other geomagnetic effects. During all seasons, significant hemispheric asymmetries in the airglow wavenumber spectra are seen. The combined GUVI and SABER observations presented here demonstrate that the large-scale periodic longitudinal structure of the F-region ionosphere responds significantly to changes in the forcing by non-migrating diurnal tides at E-region altitudes.