Symmetric and Antisymmetric Solar Migrating Semidiurnal Tides in the Mesosphere and Lower Thermosphere

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 degrees latitude, while the antisymmetric part is most prominent during May-September with the largest amplitude at 15-20 degrees 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. Upward-propagating tides play an important role in vertical atmospheric coupling, as they transfer energy and momentum from the lower atmosphere to the upper atmosphere. Tides can be divided into two parts; one is symmetric and the other is antisymmetric about the equator. Their effects on the upper atmosphere are different. We examine symmetric and antisymmetric parts of tides separately, using 21 years of satellite-based temperature observations. We focus on the solar semidiurnal tide, which is known to have a significant impact on the upper atmosphere. We describe how the symmetric and antisymmetric parts of the solar semidiurnal tide depend on latitude and height, and how they change from month to month and from year to year. These results help us understand some aspects of spatiotemporal variability in the upper atmosphere. Symmetric and antisymmetric parts of the solar migrating semidiurnal tide are examined using satellite observations of air temperature The two parts are comparable in amplitude in the mesosphere and lower thermosphere but their spatiotemporal features are different Amplification occurs in the antisymmetric part during major sudden stratospheric warmings