28 research outputs found
Saturn's aurora observed by the Cassini camera at visible wavelengths
The first observations of Saturn's visible-wavelength aurora were made by the
Cassini camera. The aurora was observed between 2006 and 2013 in the northern
and southern hemispheres. The color of the aurora changes from pink at a few
hundred km above the horizon to purple at 1000-1500 km above the horizon. The
spectrum observed in 9 filters spanning wavelengths from 250 nm to 1000 nm has
a prominent H-alpha line and roughly agrees with laboratory simulated auroras.
Auroras in both hemispheres vary dramatically with longitude. Auroras form
bright arcs between 70 and 80 degree latitude north and between 65 and 80
degree latitude south, which sometimes spiral around the pole, and sometimes
form double arcs. A large 10,000-km-scale longitudinal brightness structure
persists for more than 100 hours. This structure rotates approximately together
with Saturn. On top of the large steady structure, the auroras brighten
suddenly on the timescales of a few minutes. These brightenings repeat with a
period of about 1 hour. Smaller, 1000-km-scale structures may move faster or
lag behind Saturn's rotation on timescales of tens of minutes. The persistence
of nearly-corotating large bright longitudinal structure in the auroral oval
seen in two movies spanning 8 and 11 rotations gives an estimate on the period
of 10.65 0.15 h for 2009 in the northern oval and 10.8 0.1 h for 2012
in the southern oval. The 2009 north aurora period is close to the north branch
of Saturn Kilometric Radiation (SKR) detected at that time.Comment: 39 pages, 8 figures, 1 table, 6 supplementary movies, accepted to
Icaru
Cassini ISS Observation of Saturnâs North Polar Vortex and Comparison to the South Polar Vortex
We present analyses of Saturnâs north pole using high-resolution images captured in late 2012 by the Cassini spacecraftâs Imaging Science Subsystem (ISS) camera. The images reveal the presence of an intense cyclonic vortex centered at the north pole. In the red and green visible continuum wavelengths, the north polar region exhibits a cyclonically spiraling cloud morphology extending from the pole to 85°N planetocentric latitude, with a 4700 km radius. Images captured in the methane bands, which sense upper tropospheric haze, show an approximately circular hole in the haze extending up to 1.5° latitude away from the pole. The spiraling morphology and the âeyeâ-like hole at the center are reminiscent of a terrestrial tropical cyclone. In the System III reference frame (rotation period of 10h39m22.4s, Seidelmann et al. 2007; Archinal et al. 2011), the eastward wind speed increases to about 140 m s^(â1) at 89°N planetocentric latitude. The vorticity is (6.5± 1.5)Ă10^(â4)s^(â1) at the pole, and decreases to (1.3± 1.2)Ă10^(â4)s^(â1) at 89°N. In addition, we present an analysis of Saturnâs south polar vortex using images captured in January 2007 to compare its cloud morphology to the north pole. The set of images captured in 2007 includes filters that have not been analyzed before. Images captured in the violet filter (400 nm) also reveal a bright polar cloud. The south polar morphology in 2007 was more smooth and lacked the small clouds apparent around the north pole in 2012. Saturn underwent equinox in August 2009. The 2007 observation captured the pre-equinox south pole, and the 2012 observation captured the post-equinox north pole. Thus, the observed differences between the poles are likely due to seasonal effects. If these differences indeed are caused by seasonal effects, continuing observations of the summer north pole by the Cassini mission should show a formation of a polar cloud that appears bright in short-wavelength filters
Phase light curves for extrasolar Jupiters and Saturns
We predict how a remote observer would see the brightness variations of giant
planets similar to Jupiter and Saturn as they orbit their central stars. We
model the geometry of Jupiter, Saturn and Saturn's rings for varying orbital
and viewing parameters. Scattering properties for the planets and rings at
wavelenghts 0.6-0.7 microns follow Pioneer and Voyager observations, namely,
planets are forward scattering and rings are backward scattering. Images of the
planet with or without rings are simulated and used to calculate the
disk-averaged luminosity varying along the orbit, that is, a light curve is
generated. We find that the different scattering properties of Jupiter and
Saturn (without rings) make a substantial difference in the shape of their
light curves. Saturn-size rings increase the apparent luminosity of the planet
by a factor of 2-3 for a wide range of geometries. Rings produce asymmetric
light curves that are distinct from the light curve of the planet without
rings. If radial velocity data are available for the planet, the effect of the
ring on the light curve can be distinguished from effects due to orbital
eccentricity. Non-ringed planets on eccentric orbits produce light curves with
maxima shifted relative to the position of the maximum planet's phase. Given
radial velocity data, the amount of the shift restricts the planet's unknown
orbital inclination and therefore its mass. Combination of radial velocity data
and a light curve for a non-ringed planet on an eccentric orbit can also be
used to constrain the surface scattering properties of the planet. To summarize
our results for the detectability of exoplanets in reflected light, we present
a chart of light curve amplitudes of non-ringed planets for different
eccentricities, inclinations, and the viewing azimuthal angles of the observer.Comment: 40 pages, 13 figures, submitted to Ap.
Cassini ISS Observation of Saturnâs North Polar Vortex and Comparison to the South Polar Vortex
We present analyses of Saturnâs north pole using high-resolution images captured in late 2012 by the Cassini spacecraftâs Imaging Science Subsystem (ISS) camera. The images reveal the presence of an intense cyclonic vortex centered at the north pole. In the red and green visible continuum wavelengths, the north polar region exhibits a cyclonically spiraling cloud morphology extending from the pole to 85°N planetocentric latitude, with a 4700 km radius. Images captured in the methane bands, which sense upper tropospheric haze, show an approximately circular hole in the haze extending up to 1.5° latitude away from the pole. The spiraling morphology and the âeyeâ-like hole at the center are reminiscent of a terrestrial tropical cyclone. In the System III reference frame (rotation period of 10h39m22.4s, Seidelmann et al. 2007; Archinal et al. 2011), the eastward wind speed increases to about 140 m s^(â1) at 89°N planetocentric latitude. The vorticity is (6.5± 1.5)Ă10^(â4)s^(â1) at the pole, and decreases to (1.3± 1.2)Ă10^(â4)s^(â1) at 89°N. In addition, we present an analysis of Saturnâs south polar vortex using images captured in January 2007 to compare its cloud morphology to the north pole. The set of images captured in 2007 includes filters that have not been analyzed before. Images captured in the violet filter (400 nm) also reveal a bright polar cloud. The south polar morphology in 2007 was more smooth and lacked the small clouds apparent around the north pole in 2012. Saturn underwent equinox in August 2009. The 2007 observation captured the pre-equinox south pole, and the 2012 observation captured the post-equinox north pole. Thus, the observed differences between the poles are likely due to seasonal effects. If these differences indeed are caused by seasonal effects, continuing observations of the summer north pole by the Cassini mission should show a formation of a polar cloud that appears bright in short-wavelength filters
Saturnâs visible lightning, its radio emissions, and the structure of the 2009â2011 lightning storms
Visible lightning on Saturn was first detected by the Cassini camera in 2009 at âŒ35° South latitude. We report more lightning observations at âŒ35° South later in 2009, and lightning in the 2010â2011 giant lightning storm at âŒ35° North. The 2009 lightning is detected on the night side of Saturn in a broadband clear filter. The 2011 lightning is detected on the day side in blue wavelengths only. In other wavelengths the 2011 images lacked sensitivity to detect lightning, which leaves the lightning spectrum unknown.
The prominent clouds at the west edge, or the âheadâ of the 2010â2011 storm periodically spawn large anticyclones, which drift off to the east with a longitude spacing of 10â15° (âŒ10,000 km). The wavy boundary of the stormâs envelope drifts with the anticyclones. The relative vorticity of the anticyclones ranges up to âf/3, where f is the planetary vorticity. The lightning occurs in the diagonal gaps between the large anticyclones. The vorticity of the gaps is cyclonic, and the atmosphere there is clear down to level of the deep clouds. In these respects, the diagonal gaps resemble the jovian belts, which are the principal sites of jovian lightning.
The size of the flash-illuminated cloud tops is similar to previous detections, with diameter âŒ200 km. This suggests that all lightning on Saturn is generated at similar depths, âŒ125â250 km below the cloud tops, probably in the water clouds. Optical energies of individual flashes for both southern storms and the giant storm range up to 8 Ă 10^9 J, which is larger than the previous 2009 equinox estimate of 1.7 Ă 10^9 J. Cassini radio measurements at 1â16 MHz suggest that, assuming lightning radio emissions range up to 10 GHz, lightning radio energies are of the same order of magnitude as the optical energies.
Southern storms flash at a rate âŒ1â2 per minute. The 2011 storm flashes hundreds of times more often, âŒ5 times per second, and produces âŒ10^(10) W of optical power. Based on this power, the stormâs total convective power is of the order 10^(17) W, which is uncertain by at least an order of magnitude, and probably is underestimated. This power is similar to Saturnâs global internal power radiated to space. It suggests that storms like the 2010â2011 giant storm are important players in Saturnâs cooling and thermal evolution
Saturn's north polar vortex structure extracted from cloud images by the optical flow method
The paper presents velocity fields with ~3âkm spatial resolution of Saturn's north polar vortex (NPV) retrieved using the optical flow method from a sequence of polarâprojected cloud images captured by the Imaging Science Subsystem camera on board NASA's Cassini spacecraft. The fields of the velocity magnitude, velocity variation, relative vorticity, divergence, and second invariant are determined to characterize the flow structures of the inner core of the NPV. The mean zonal and mean meridional velocity profiles of the NPV are compared with previous measurements. We also describe the relevant details of application of the optical flow method to planetary cloudâtracking wind measurements. The mean zonal velocity profile is consistent with the previous measurements using correlation image velocimetry methods. The small but significant meridional velocity corresponds to outwardly spiraling streams observed in the region near the north pole (NP). The concentrated vorticity and second invariant within 1° planetographic latitude of the NP indicate strong rotational motion of the fluid. An analysis is presented to explore a possible physical origin of the observed spiraling node at the NP
Dynamics of Saturn's South Polar Vortex
The camera onboard the Cassini spacecraft has allowed us to observe many of Saturn's cloud features. We present observations of Saturn's south polar vortex (SPV) showing that it shares some properties with terrestrial hurricanes: cyclonic circulation, warm central region (the eye) surrounded by a ring of high clouds (the eye wall), and convective clouds outside the eye. The polar location and the absence of an ocean are major differences. It also shares properties with the polar vortices on Venus, such as polar location, cyclonic circulation, warm center, and long lifetime, but the Venus vortices have cold collars and are not associated with convective clouds. The SPV's combination of properties is unique among vortices in the solar system
Peak electron densities in Saturn's ionosphere derived from the low-frequency cutoff of Saturn lightning
International audienceRadio bursts from Saturn lightning have been observed by the Cassini Radio and Plasma Wave Science instrument at frequencies of a few megahertz during several month-long storms since 2004. As the radio waves traverse Saturn's ionosphere on their way to the spacecraft, one can determine the peak electron density from the measurement of the low-frequency cutoff below which the radio bursts are not detected. In this way we obtained 231 profiles of peak electron densities that cover all Saturnian local times at a kronocentric latitude of 35°S, where the storms were spotted by the Cassini camera. Peak electron densities show a large variation at dawn and dusk and are around 5 à 104 cm-3, in fair agreement with radio occultation measurements at midlatitudes. At noon and midnight, the densities are typically somewhat above 105 cm-3 and around 104 cm-3, respectively. The diurnal variation is about 1 to 2 orders of magnitude for averaged profiles over one storm at 35°S. This is somewhat less compared to previous Voyager measurements which showed more than 2 orders of magnitude variation. The diurnal variation as well as the peak electron densities of Saturn's ionosphere tend to decrease with the decreasing solar EUV flux from 2004 until the end of 2009
Cassini ISS observation of Saturnâs String of Pearls
We present the dynamics of the String of Pearls (SoPs) feature observed by the Cassini spacecraftâs Imaging Science Subsystem (ISS) camera between 2007 and 2010. The SoPs was originally discovered in the 5 ÎŒm images captured by Cassini VIMS instrument, where it appeared as a chain of infrared-bright spots (Momary, T.W., et al. [2006]. The Zoology of Saturn: The Bizarre Features Unveiled by the 5 Micron Eyes of Cassini/VIMS. AAS/Division for Planetary Sciences Meeting Abstracts 38, 499). Using ISS images of Saturn, we found a chain of 23â26 dark spots at 33.2°N planetocentric latitude with characteristics that are consistent with those of SoPs. Our measurements imply that the feature propagated at â2.26 ± 0.02° day^â1 in longitude (â22.27 ± 0.2 m s^â1, negative values denote westward) during the observed period that spans three Earth years. Our measurements imply that the SoPs is a chain of cyclones, which we infer from the motion of clouds on the periphery of the individual pearls. We tracked the motion of 26 pearls for 6 months in 2008 and noted a few pearls appearing and disappearing, all near the eastâwest termini of the SoPs feature. During this period, a few of the pearls, varying between 6 and 10, harbored a small circular cloud at the center, which we call the central peaks. In general, a group of vortices with the same sign of vorticity tend to merge; however, our measurements did not detect merger of pearls. The interest in the feature was heightened when the latest planet-encircling storm erupted from the SoPs on December 5, 2010 (Sayanagi, K.M., Dyudina, U.A., Ewald, S.P., Fischer, G., Ingersoll, A.P., Kurth, W.S., Muro, G.D., Porco, C.C., West, R.A. [2013]. Icarus 223, 460â478). The storm severely disrupted the region; the SoPs was last seen on December 24, 2010 in the turbulent wake of the storm, and has not reappeared as of August 2013