Investigation on the Anomalous Weakening of Diurnal Tides in the Mesosphere‐Lower Thermosphere

• Published in: Journal of geophysical research. Space physics 2023-01, Vol.128 (1), p.n/a
• Main Authors: Prijith, S. S., Kumar, K. Kishore
• Format: Article
• Language: English
• Subjects: Amplitudes; Atmosphere; Atmospheric tides; Atmospheric waves; Coupling; Diurnal tides; Equatorial regions; Excitation; Gravity waves; Ionosphere; Ionospheric composition; Latitude; Lower thermosphere; Mesosphere; mesosphere‐lower thermosphere; meteor wind radar; nonlinear wave‐wave interaction; parametric excitation; Planetary waves; quasi‐two‐day wave; Thermosphere; Tidal amplitude; Tides; Wind measurement
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2022JA030725

Variations in tidal characteristics assume importance as far as the atmosphere‐ionosphere coupling is concerned. The present study examines short‐term variabilities in diurnal tidal characteristics, using one decade (2006–2015) of wind measurements in the mesosphere‐lower thermosphere (MLT) region over a near equatorial location, Thumba (8.5°N, 77°E). The study presents two distinct events of anomalous tidal weakening, with decrease in monthly mean tidal amplitude by −38 ms−1 (−26 ms−1) in February 2010 and −17 ms−1 (−8 ms−1) in January 2012 at 98 km (91 km). These events are observed to be coinciding with the occurrence of strong quasi‐two‐day wave (QTDW). Compared to pre‐event conditions, decrease (increase) in tidal (QTDW) amplitudes at 91 km is −13.03 ms−1 (34.97 ms−1) and −15.65 ms−1 (52.81 ms−1) during the first and second event, respectively. However, causative mechanisms are found to be different for both the events. While the former event is primarily due to parametric excitation of phase locked 2‐day waves by diurnal tide, the latter one is mainly due to non‐linear interaction between tide and QTDW and subsequent generation of secondary waves. While QTDW phase is relatively stable during the former event, it shows steep temporal variations during the latter one. Changes in background wind conditions are also found to be more conducive for weakening of diurnal tide during the former event. These anomalous changes in tidal characteristics would have significant impacts on ionospheric processes. The significance of this study lies in providing observational evidence for the existence of different pathways through which QTDW affects tidal amplitudes over the low‐latitude MLT region. Plain Language Summary Structure and dynamics of Earth's atmosphere in the 60–100 km region, which is known as mesosphere lower thermosphere (MLT), are dominated by spectrum of atmospheric waves comprising of tides, planetary waves and gravity waves. Among these, diurnal tide has largest amplitude in the MLT region over low latitudes. Changes in atmospheric tides affect vertical transport of energy and momentum and hence the atmosphere‐ionosphere coupling. Tidal characteristics are modified in short time scales by several processes. The present study reports relatively rare events of anomalous tidal weakening in the MLT region, over Thumba (8.5°N, 77°E), in boreal winter. The weakening of diurnal tide during these events is very prominent in a decade of observations. Thes