Stratospheric aftermath of the 2010 Storm on Saturn as observed by the TEXES instrument. I. Temperature structure

Abstract: We report on spectroscopic observations of Saturn's stratosphere in July 2011 with the Texas Echelon Cross Echelle Spectrograph (TEXES) mounted on the NASA InfraRed Telescope Facility (IRTF). The observations, targeting several lines of the CH$_4$ $\nu_4$ band and the H$_2$ S(1) quadrupolar line, were designed to determine how Saturn's stratospheric thermal structure was disturbed by the 2010 Great White Spot. A study of Cassini Composite Infrared Spectrometer (CIRS) spectra had already shown the presence of a large stratospheric disturbance centered at a pressure of 2~hPa, nicknamed the beacon B0, and a tail of warm air at lower pressures (Fletcher et al. 2012. Icarus 221, 560--586). Our observations confirm that the beacon B0 vertical structure determined by CIRS, with a maximum temperature of $180\pm1$K at 2~hPa, is overlain by a temperature decrease up to the 0.2-hPa pressure level. Our retrieved maximum temperature of $180\pm1$K is colder than that derived by CIRS ($200\pm1$K), a difference that may be quantitatively explained by terrestrial atmospheric smearing. We propose a scenario for the formation of the beacon based on the saturation of gravity waves emitted by the GWS. Our observations also reveal that the tail is a planet-encircling disturbance in Saturn's upper stratosphere, oscillating between 0.2 and 0.02~hPa, showing a distinct wavenumber-2 pattern. We propose that this pattern in the upper stratosphere is either the signature of thermal tides generated by the presence of the warm beacon in the mid-stratosphere, or the signature of Rossby wave activity.