We present the results of the remaining ten soundings of Titan's atmosphere by radio occultations using the Cassini spacecraft that have not been previously reported. Three were in 2008 and 2009, and used the Ultra Stable Oscillator onboard the spacecraft. The rest were in 2014 and 2016 after the USO had failed and were executed in two-way mode. Comparison of the later soundings with those earlier in the Cassini mission provide an indication of the seasonal change at mid latitudes. The southern hemisphere has shown a dramatic cooling during the onset of autumn; change in the northern hemisphere is noticeable, but more subdued. Changes at low latitudes are even smaller. The additional retrievals from 2008 to 2009 add a better constraint on the latitude structure of the previously reported destabilization of the temperature profiles at northern latitudes during winter and early spring. They show that the most pronounced effect is at polar latitudes (>70 N), suggesting that they are associated with the descending circulation over the winter pole that has been proposed by several authors. The full set of occultations indicates minimal variation with latitude and time in the troposphere, likely attributable to the large atmospheric radiative damping time and efficient meridional heat transports, but somewhat more variation near the surface, which has a smaller thermal inertia.

This publisher's note renumbers the reference list in Appl. Opt.56, 5274 (2017)APOPAI0003-693510.1364/AO.56.005274.

The Cassini spacecraft orbiting Saturn carries the composite infrared spectrometer (CIRS) designed to study thermal emission from Saturn and its rings and moons. CIRS, a Fourier transform spectrometer, is an indispensable part of the payload providing unique measurements and important synergies with the other instruments. It takes full advantage of Cassini's 13-year-long mission and surpasses the capabilities of previous spectrometers on Voyager 1 and 2. The instrument, consisting of two interferometers sharing a telescope and a scan mechanism, covers over a factor of 100 in wavelength in the mid and far infrared. It is used to study temperature, composition, structure, and dynamics of the atmospheres of Jupiter, Saturn, and Titan, the rings of Saturn, and surfaces of the icy moons. CIRS has returned a large volume of scientific results, the culmination of over 30 years of instrument development, operation, data calibration, and analysis. As Cassini and CIRS reach the end of their mission in 2017, we expect that archived spectra will be used by scientists for many years to come.

The Ultra Stable Oscillator aboard the Cassini spacecraft failed in late 2011, which means that all radio occultations after that date have to be done in two‐way mode, using a ground‐based signal transmitted to the spacecraft as the frequency reference. Here we present the numerical technique we use to analyze the data from the two‐way atmospheric radio occultations of both Saturn and Titan that have occurred since the Ultra Stable Oscillator (USO) failure, along with the theoretical reasons behind this technique. Since our two‐way technique is based upon our earlier one‐way technique which used the USO as the frequency reference, we also present our one‐way technique which we used for Saturn occultations prior to the loss of the USO.

Titan, after Venus, is the second example in the solar system of an atmosphere with a global cyclostrophic circulation. The origin and maintenance of these superrotating atmospheres is not well understood, but Titan has a strong seasonal modulation in the middle atmosphere, and the seasonal changes in the winds may offer clues. The pole in winter and early spring is characterized by temperatures 20-30 K cooler at 140-170 km than those at low latitudes, and strong circumpolar winds as high as 190 m/s at 200- 250 km. At these levels the polar region is characterized by enhanced concentrations of several organic gases, and also detectable condensates. All this suggests that the polar vortex provides a mixing barrier between winter polar and lower-latitude air masses, analogous to the polar ozone holes on Earth. Because the concentrations of organic gases increase with altitude in the middle atmosphere, the observed enhancements suggest subsidence over the winter pole. Consistent with this are the observed temperatures approximately 200 K at the winter-polar stratopause (280 km), making it the warmest part of the atmosphere. The warm stratopause likely results from adiabatic heating associated with the subsidence. Recent observations in late northern winter and early spring indicate that the warm anomaly at the winter-polar stratopause is weakening;. In contrast to the middle atmosphere, latitude contrasts in tropospheric temperatures are muted. During the northern winter season, they were approximately 5 K at the tropopause and 3 K or less near the surface, being coldest at high northern latitudes. This is understandable in terms of the long radiative relaxation times in the troposphere, compared to times that are much shorter than a season in the upper stratosphere and higher. Curiously, the transition between the small meridional contrast (and presumably seasonal variations) in temperatures observed in the troposphere and the large variations observed at higher altitudes occurs abruptly above 80 km. Here the temperatures in the lower stratosphere, generally increasing with altitude, exhibit a sudden drop with increasing altitude at high northern latitudes, producing the contrast between low and high northern winter latitudes in the upper stratosphere described above. While the radiative relaxation time associated with infrared gaseous coolants decreases with altitude in the stratosphere, the abrupt transition suggests the presence of an optically thick condensate at thermal-infrared wavelengths. Near the surface, temperature lapse rates are adiabatic over the lowest 2 km, with the suggestion of a nocturnal stable inversion over the lowest 200 m in radio-occultation soundings near the morning terminator. At mid and high latitudes in both winter and summer hemispheres, the profiles are more statically stable (i.e., subadiabatic). This is most pronounced in the winter hemisphere.

We present the results of six soundings of the atmosphere of Titan by the radio occultation technique using the Cassini spacecraft currently in orbit around Saturn. These occultations occurred during four separate targeted Titan encounters in both the Prime and Equinox missions of Cassini over 3 years. They cover a wide range of latitude from 75 degrees S to 79 degrees N, split so that three soundings are in the northern hemisphere and three are in the southern hemisphere. Techniques and error analysis are similar to Schinder et al. (2011). The six temperature-altitude profiles presented here are compared to those earlier results. Of special interest is the sudden cooling observed at altitudes of similar to 80-100 km in the two high northern (winter) soundings at 74 degrees N and 80 degrees N, where the temperature drops by about 10 K over the course of 20 km. The northern profiles also exhibit a transition between the troposphere and stratosphere that is much more abrupt than in the south, and the northern tropopause temperatures are much cooler. (C) 2012 Elsevier Inc. All rights reserved.

We use the existing thirteen Cassini radio'occultation soundings to construct a meridional cross section of geopotential height vs. pressure and latitude. The assumption of balanced flow permits the construction of a similar cross section of zonal winds, from near the surface to the 0.1'mbar level. In the lower troposphere, the winds are approx.10 m/s, except within 20deg of the equator, where they are much smaller. The winds increase higher up in the troposphere to nearly 40 m/s in the tropopause region, but then decay rapidly in the lower stratosphere to near'zero values at 20 mbar (approx.80 km), reminiscent of the Huygens Doppler Wind Experiment result. This null zone extends over most latitudes, except for limited bands at mid'latitudes. Higher up in the stratosphere, the winds become larger. They are highest in the northern (winter) hemisphere. We compare the occultation results with the DWE and CIRS retrievals and discuss the similarities and differences among the data sets.