Anomalous Increase in the Occurrence of Post‐Midnight FAI Radar Echoes in September 2019 and Its Relation with the Austral Sudden Stratospheric Warming

The F-region observations of field aligned irregularities (FAI) by the Equatorial Atmosphere Radar (EAR) at Kototabang (0.2 degrees S, 100.3 degrees E, 10.36 degrees S dip latitude) show maximum percentage of occurrence (PO) of post-midnight echoes due to spread F during boreal summer of solar minimum years. However, the PO is observed to be anomalously high during September 2019, when compared to the similar solar minimum year 2018 even after removing geomagnetically disturbed days indicating the lower atmospheric dynamical forcing. Coincidently, an austral polar sudden stratospheric warming (SSW) event has occurred during September 2019, which precedes large planetary wave activity. Due to the circulation changes associated with the SSW, though the accumulation of ozone over the equatorial stratosphere is inferred from the space-borne Sounding of the Atmosphere by Broadband Emission Radiometry (SABER) measurements, the expected enhancement in the amplitude of the upper mesospheric migrating semi-diurnal tide (SW2) is not observed in SABER temperature. But there is a drastic decrease in the amplitude of the migrating diurnal tide (DW1). The enhancement of stationary diurnal tide (DS0) suggests that the DW1 may have interacted with the planetary wave of zonal wavenumber 1 to generate DS0 at the expense of the DW1 tide. It is suggested that the relative increase of SW2 over DW1 leads to the dominant semi-diurnal variation of the zonal electric field, which can become eastward at a later time as observed in the ionosonde measurements and lift the F-layer to higher heights creating favorable conditions for the growth of Rayleigh-Taylor instability to generate spread F around midnight.