Mars Climate Sounder observations of the semi-diurnal tide in the Martian atmosphere

The Mars Climate Sounder (MCS) is a mid- and far-infrared thermal emission radiometer on board the Mars Reconnaissance Orbiter (MRO). From the measured radiances, vertical profiles of atmospheric temperature, dust and water ice opacity are retrieved up to altitudes of about 80 km. MRO is in a sun-synchronous orbit with equator crossings around 3AM and 3PM Martian local time. As MCS routinely views the limb in the forward direction along the orbit track, it obtains its measurements at these local times. Over the last Mars year, MCS has used its azimuth actuator to view the limb cross-track as well as along-track. These measurement sequences were performed in 11 observation campaigns, each of about 4 weeks duration. The cross-track geometry yields measurements that are offset by about +/-1.5 hours in local time in the equatorial region. Coverage of local time increases towards the poles. Here we present analyses of the temperature fields from these cross-track campaigns. The additional local time coverage from the cross-track measurements allow the identification of tidal modes that would be aliased in data with only two local times available. We show that in addition to the migrating diurnal tide, a significant semi-diurnal component is observed at low latitudes. This semi-diurnal tide is present throughout the Martian year, with significant amplitudes in both the clear and the dusty season. At high latitudes, the semi-diurnal tide is the dominant tidal contribution in the upper middle atmosphere. It is present both in northern and southern high latitudes in the winter season of each hemisphere. Simulations using a Mars General Circulation Model suggest that the tide is likely forced by elevated layers of radiatively active water ice clouds.