On the high percentage of occurrence of type-B 150-km echoes during the year 2019 and its relationship with mesospheric semi-diurnal tide and stratospheric ozone. Issue 1 (1st January 2022) - Dutta, Reetambhara
Highlights: Anomalously high occurrence of 150-km radar echoes over Kototabang during September 2019. In the years 2017 and 2019, SW2 dominates DW1 due to the larger stratospheric ozone, when compared to other years. Dominant SW2 along with solar minimum conditions may lead to large echo occurrence through interchange instability. Abstract: The Kototabang (0.20°S, 100.3°E, 10.36°S dip latitude) Equatorial Atmosphere Radar (EAR) observations of valley region field-aligned irregularities (FAI) reveal maximum percentage of occurrence (PO) of high signal to noise ratio (greater than 3 dB) type-B 150-km echoes during boreal summer (June-August) and winter (December-January) of solar moderate to minimum years 2016–2019. The PO is anomalously high in the solar minimum year 2019 and particularly during the September equinox when a major austral sudden stratospheric warming (SSW) event has occurred. Its possible relation with mesospheric tides and stratospheric ozone is investigated. The space–time spectral analysis of upper mesospheric temperature information obtained from the spaceborne radiometer instrument Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) reveals that migrating semi-diurnal tide (SW2) is dominant during June-August, when the PO of the 150-km echoes maximizes, whereas migrating diurnal tide (DW1) is dominant during equinox months. It also reveals quasi-biennial variability of the DW1 tide probably due to similar variability of the stratospheric ozone. It is suggested that the DW1 tides generated due to solar radiation absorption by stratospheric ozone and tropospheric water vapour can have phases opposite to each other, leading to the suppression of DW1 tide during September 2017 and 2019 resulting in the relative dominance of SW2 tide over DW1 tide. The meridional wind shear associated with the relatively dominant SW2 tide results in an interchange instability developed on the gradient of daytime descending ion layer along with solar minimum conditions leads to plasma irregularities responsible for these echoes. The large eastward propagating diurnal tide with zonal wavenumber 3 (DE3) observed during June-September 2019 is unlikely to cause the instability, due to its weak meridional component.