Semidiurnal Tidal Influence on the Occurrence of Postmidnight F Region FAI Radar Echoes

Abstract

The Equatorial Atmosphere Radar observations at Kototabang (0.2°S, 100.3°E) are used to study the possible semidiurnal tidal influence on the occurrence of postmidnight echoes from the field‐aligned irregularities (FAIs) due to spread F. It is found that the postmidnight FAI echoes show high percentage of occurrence (PO) during June–July and low PO in December–January of low solar activity years. As solar activity approaches minimum, the PO increase is extended to May, August, and September. The space‐time Fourier analysis on the temperature information obtained from the Sounding of Atmosphere by Broadband Emission Radiometry instrument onboard Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite reveals that at the low‐latitude upper mesospheric heights, though the migrating diurnal tide propagating westward with zonal wavenumber (k) 1 (DW1) dominates the migrating semidiurnal tide propagating westward with k = 2 (SW2) particularly during the equinox months, the SW2 tidal amplitudes are larger than DW1 during June–July. The postmidnight FAI echoes appear to be more frequent during the sudden stratospheric warming of the Solar Minimum Years 2018–2019 and 2008–2009 but not in the Solar Maximum Year 2013. The dominance of SW2 over DW1 is also noted during these years. This indicates that the SW2 plays a major role in the occurrence of postmidnight spread F. It is suggested that the dominant semidiurnal variation of zonal electric field can become eastward and lift the F layer to higher heights during midnight hours, which can favor the growth of the Rayleigh‐Taylor instability to cause delayed spread F around midnight.

On the solar activity dependence of midnight equatorial plasma bubbles during June solstice periods

“…There are several studies that report the effect of semi-diurnal tides as a probable reason for the enhanced zonal electric field during June soltice midnight hours (Stoneback et al, 2011;Sridharan and Meenakshi, 2020). From simultaneous observations using Ionosonde, MF radar, and C/NOFS over the Indian sector, Chakrabarty et al (2014) showed that the mean amplitude of the meridional component of the semi diurnal tide during 2000-2011 increased with decreasing solar activity and was maximized during summer months of 2008.…”

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

“…Chakrabarty et al (2014) observed the shift of the F 2 layer around midnight and attributed it to be due to the influence of the thermospheric semi-diurnal tide. Sridharan and Meenakshi (2020) noted that the hmF2 variations show semiannual variation with higher MEENAKSHI ET AL.…”

“…(2014) observed the shift of the F 2 layer around midnight and attributed it to be due to the influence of the thermospheric semi‐diurnal tide. Sridharan and Meenakshi (2020) noted that the hmF2 variations show semiannual variation with higher heights during equinox months at post‐sunset hours and they show annual variation with higher heights during June solstice. They attributed it to the enhancement of mesospheric SW2 during June solstice.…”

Destabilizing Influence of the Neutral Winds for the Midnight and Post‐Midnight Ionospheric Irregularities in Brazil Sector

“…The LT variations of the irregularity occurrence, as mentioned above, are merely noticed and need to be explained. The pre-midnight irregularities are well known to rely on the PRE of the eastward electric field (C.-S. Huang, 2018), while the midnight or post-midnight irregularities are usually suggested to be related to the decreasing western electric field and ion recombination at lower altitudes that destabilizes the ionosphere (Ajith et al, 2016;Nicolls et al, 2006;Otsuka, 2018;Sridharan & Meenakshi, 2020;Zhan & Rodrigues, 2018). But, an inconvenient fact is the strongest trans-equatorial winds in Brazil sector, which have been found to significantly redistribute the equatorial ionospheric plasma (Tian et al, 2021).…”

“…The occurrence is usually observed around June solstice and under low solar activity, when the PRE is weak and the electric field is on average westward throughout the night (e.g., Fejer et al., 2008; C. Y. Huang et al., 2012). The post‐midnight irregularities are then found to be well related to the uplift of the F region (Nishioka et al., 2012), which could be caused by a decreasing westward electric field during the midnight hours in conjunction with the chemical recombination in the bottomside F region (Ajith et al., 2016; Nicolls et al., 2006; Otsuka, 2018; Sridharan & Meenakshi, 2020; Zhan & Rodrigues, 2018).…”