Tidal influence on the longitudinal variabilities of the post-midnight spread F during September 2019

Abstract The analysis of digisonde ionograms of Ascension Island (7.9°S, 14.4°W geographic; dip latitude: 16°S) for the geomagnetically quiet days shows a high percentage of occurrence (PO) of post-midnight spread F (PMSF) in September 2019. However, Jicamarca (11.95°S, 76.87°W, dip latitude: 1.10°N) digisonde observations do not show any significant increase in the PO. Besides, the wavelet spectrum of the peak height of the F-layer (hmF2) shows a strong semi-diurnal (12 hours) variation over Ascension Island, whereas diurnal (24 hours) variation is dominant in Jicamarca. The high PO of PMSF and the semi-diurnal variation indicates the lower atmospheric tidal forcing. The two-dimensional spectral analysis of the space-borne SABER (Sounding of the Atmosphere by Broadband Emission Radiometry) temperature measurements shows that the migrating semi-diurnal (SW2) tidal amplitude exceeds the migrating diurnal (DW1) tidal amplitude at 10°N, which is close to the dip equator in the Ascension Island longitude, whereas the DW1 dominates at 10°S over the dip equatorial station, Jicamarca. The SW2 dominated tidal spectrum can drive semi-diurnal variation in the zonal electric field. Though the presence of the F-layer at lower heights and the westward electric field might have prevented the growth of the Rayleigh-Taylor Instability (RTI) after sunset, the dominant semi-diurnal variation of the zonal electric field can become eastward at a later time and lift the F-layer to higher heights creating favourable conditions for the growth of the RTI to generate the spread F around midnight.