Migrating and non-migrating tidal influences on the high occurrence of post-midnight spread F over Ascension Island during solar minimum

The digisonde observations over Ascension Island (8 degrees S, 14 degrees W, dip latitude: 16 degrees S) reveal that the occurrence of post-midnight spread F (or more appropriately delayed spread F) is more when compared to that of the post-sunset spread F during solar minimum years. While the monthly variations of the percentage of occurrence (PO) of post-sunset spread F (18:00-22:00) do not now show any consistent seasonal variation, the PO of the post-midnight spread F reveals a seasonal variation with a high PO during austral summer (November-February) and minimum PO during austral winter (May-August) and it shows a large interannual variability. The high PO during September 2019 may be associated with the austral sudden stratospheric warming (SSW). In the year 2020, the PO is higher (>60%) in November-February. When the PO of the post-midnight spread F is larger, there is a strong semi-diurnal variation of peak electron density height of F-layer (hmF2) with a maximum height at noon and midnight hours, suggesting a possible semi-diurnal tidal influence. It is also found that the occurrence of spread F just after sunset hours is less over Ascension Island and it does not show any clear seasonal variation. The space-time spectral analysis of SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) of TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) satellite reveals the dominant presence of migrating semidiurnal tide (SW2) at 10 degrees N during January-February 2020. During November-December 2020, though the SW2 tidal amplitudes are weaker, there is a dominant presence of the eastward propagating non-migrating diurnal (DE3) tide. The thermospheric winds measured by MIGHTI (Michelson Interferometer for Global High-resolution Thermospheric Imaging) instrument of ICON (Ionospheric Connection Explorer) mission show equatorward winds in the late evening hours during November-February 2020. The migrating diurnal tide (DW1) dominates in MIGHTI zonal winds at F-region heights, whereas SW2, as well as the eastward propagating semi-diurnal tide with zonal wavenumber 2 (SE2), tidal amplitudes are comparable to DW1 in the meridional wind. It is suggested that the semi-diurnal variation in the meridional winds turning the winds equatorward during late evening hours can cause eastward electric field that can lift the F layer to higher heights and thereby favouring the Rayleigh-Taylor Instability (RTI) to act, which can cause the post-midnight spread F. It is also shown that the growth rate can be sufficiently positive around midnight hours to cause the RTI to act. (C) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

Automated summary

Digisonde observations over Ascension Island show that post-midnight spread F is more common during solar minimum years, with a pronounced seasonal variation. The study also reveals a connection between the sudden stratospheric warming in the Southern Hemisphere and higher occurrence of spread F.