Anomalous Increase in the Occurrence of Post‐Midnight FAI Radar Echoes in September 2019 and Its Relation With the Austral Sudden Stratospheric Warming. Issue 6 (21st June 2021) - Meenakshi, S.
Abstract: The F‐region observations of field aligned irregularities (FAI) by the Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S, 100.3°E, 10.36°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. Plain Language Summary: The post‐sunset radar echoes from F‐region irregularities normally occur largely during equinox months (March and September) of solar maximum years. Besides, radar echoes are observed recently during post midnight hours also and their characteristics are different from those of post‐sunset echoes. These post‐midnight echoes occur during June solstice months of solar minimum years. However, during September 2019, an unusually high occurrence of post‐midnight radar echoes from F‐region irregularities has been observed. It is investigated what might have caused the unusual high occurrence during September. During September 2019, an austral sudden stratospheric warming (SSW) event has occurred. Accumulation of stratospheric ozone molecules over equatorial latitudes has been observed. The dominance of migrating semi‐diurnal tide over migrating diurnal tide has been noted. It is suggested that the dominant semi‐diurnal character in the zonal electric field might have caused lifting of the F‐region to higher heights where it is more susceptible to Rayleigh‐Taylor instability leading to the delayed occurrence of F‐region irregularities. Key Points: Large increase in the occurrence of post‐midnight radar echoes in September 2019, when there is an austral polar stratospheric warming event Relative dominance of semi‐diurnal (SW2) tidal amplitudes over diurnal (DW1) tidal amplitudes due to accumulation of ozone in the equatorial stratosphere Suppression of diurnal tide (DW1) is due to its interaction with planetary waves and semi‐diurnal character in zonal electric field can cause radar echoes