Figure 8. Latitudinal distribution of ACS NIR occultation with ranges selected for search of local time variations (black vertical lines). Bottom panels: zoom at local time inside the indicated bins with red curves for morning terminator and blue curves for evening terminator.
Figure 9 presents a latitude-altitude cross-section of temperature and water vapor at morning and evening terminators, with the difference between them. A typical signature of thermal tides is prominent in the difference, with negative and positive temperature differences alternating along the vertical and stretching horizontally along latitudes. Two bins around aphelion, one in MY35 after solstice Ls=115°-135° and one in MY36 before solstice, Ls=50°-70°, reveals positive difference at 40 and 80 km and negative difference at 60 km in low latitudes where sampled local times correspond to 06:00AM and 06:00PM (Fig. 8). These patterns are reminiscent of MCS/MRO observations (Kleinböhl et al., 2013) and ACS TIRVIM/TGO (Guerlet et al., 2022). At 45°S and 45°N, there is a flip of sign in the temperature difference. There, the temperature difference exceeds 20 K. During these seasonal bins, water vapor mixing ratios were below 3-5 ppmv between 40-60 km, and from 10 to 200 ppmv below 40 km at low-to-mid latitudes. Despite the warm temperature anomaly observed at the morning terminator at 60 km in the equatorial region, water is located mostly below. At 40 km, the temperature was warmer in the evening by 20-25 K , and we observed more saturation in the morning. In the lower atmosphere more water was observed in the evening below 30 km in the middle northern latitude where 5-10K warmer temperatures were measured.
During the dusty season (at Ls=200°-220° and 270°-290° of MY35) the evening temperature was warmer by 20-30 K from 30°S to 30°N at altitudes ranging from 40 to 65 km. This maximum is correlated with low saturation ratio in the evening (~1-2) compared to strong supersaturation in the morning. Water distribution also varies with local time. At Ls=200°-220°, water abundance increases by 10-40 ppmv in the evening at all altitudes. From Ls 270° to 290°, more water is observed in the evening between the equator and 45°S. This morning-to-evening increase might be attributed to an intensification of the mean upward motion during daytime. A similar configuration has also been seen during Ls 230° to 250° of MY35 before solstice.
Close to the northern vernal equinox (330°-350°) of MY35 the warmer temperature prevailing from 40 to 60 km in the low-to-mid southern latitudes in the evening also raises the saturation altitude by 20 km compared to the morning, with water vapor mixing ratio being higher by 10-30 ppmv compared to the morning.