Abstract
Upward-propagating solar tides are responsible for a large part of atmospheric variability in the mesosphere and lower thermosphere (MLT) region, and they are also an important source of ionospheric variability. Tides can be divided into the parts that are symmetric and antisymmetric about the equator. Their distinction is important, as the electrodynamic responses of the ionosphere to symmetric and antisymmetric tides are different. This study examines symmetric and antisymmetric tides using 21 years of temperature measurements by the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry. The main focus is on the solar migrating semidiurnal tide (SW2), which is one of the dominant tides in the MLT region. It is shown that symmetric and antisymmetric parts of SW2 are comparable in amplitude. However, their spatiotemporal characteristics are different. That is, the symmetric part is strongest during March–June at 30–35° latitude, while the antisymmetric part is most prominent during May–September with the largest amplitude at 15–20° latitude. The symmetric and antisymmetric parts can be well described by the first two symmetric and antisymmetric Hough modes, respectively. Amplification is observed in the antisymmetric part during the major sudden stratospheric warmings (SSWs) in January 2006, 2009, 2013 and 2019. Atmospheric model simulations for the 2009 and 2019 SSWs confirm the amplification in the antisymmetric part of SW2. The enhanced antisymmetric tidal forcing explains the previously-reported asymmetric response of the ionospheric solar-quiet current system to SSWs.
