We present the dynamics of the String of Pearls (SoPs) feature observed by the Cassini spacecraft... more We present the dynamics of the String of Pearls (SoPs) feature observed by the Cassini spacecraft's Imaging Science Subsystem (ISS) camera between 2007 -2010. The SoPs was originally discovered in the 5-micron images captured by Cassini VIMS instrument, where it appeared as a chain of infrared-bright spots (Momary et al., 2004). Using ISS images of Saturn, we found a chain of 23-26 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 −2.26±0.02 • day −1 in longitude (−22.27±0.2 m s −1 , 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 east-west terminuses 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 et al., 2013). 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. 3
Three-dimensional numerical simulations of freely evolving stratified geostrophic turbulence on t... more Three-dimensional numerical simulations of freely evolving stratified geostrophic turbulence on the  plane are presented as a simplified model of zonal jet formation on Jupiter. This study samples the parameter space that covers the low, middle, and high latitudes of Jupiter by varying the central latitude of the  plane. The results show that robust zonal jets can emerge from initial small-scale random turbulence through the upscale redistribution of the kinetic energy in the spectral space. The resulting flow's sensitivities to the flow's deformation radius L D and the two-dimensional Rhines length L  ϭ ͌ U/ (U is the characteristic turbulence velocity and  is the meridional gradient of the planetary vorticity) are tested, revealing that whether the outcome of the upscale energy transfer becomes dominated by jets or vortices depends on the relative values of L D and L  . The values of L  and L D are varied by tuning the -plane parameters, and it is found that the flow transitions from a jet-dominated regime in L  Շ L D to a vortical flow in L  տ L D . A height-to-width ratio equal to f /N, the Coriolis parameter divided by the Brunt-Väisälä frequency, has previously been established for stable vortices, and this paper shows that this aspect ratio also applies to the zonal jets that emerge in these simulations.
The exploration of Jupiter has played a pivotal role in the development of our understanding of t... more The exploration of Jupiter has played a pivotal role in the development of our understanding of the history of our solar system; it has served as a paradigm for the interpretation of exoplanetary systems around other stars, and as a fundamental laboratory for the myriad of physiochemical phenomena evident on the gas giants. Yet, despite great successes in the studies of Jupiter over four centuries of research, our characterisation of Jupiter remains incomplete. Jupiter's atmosphere is distinguished from Saturn's and the Ice Giants by its larger mass, dynamic "weather layer", multiple long-lived vortices and the smaller significance of seasonal variability. We review the scientific goals for Jovian exploration in the coming decade: 1. The bulk composition (e.g. heavy elements, isotopes), cooling history and internal structure (the existence of a core) of Jupiter as signatures of planetary formation and evolutionary models, along with comparisons to the other gas giants. 2. The development of a global three-dimensional understanding of the structure, meteorology and chemistry of the troposphere, stratosphere and mesosphere; the mechanisms for transport of energy, momentum and chemical species (tracers) vertically and horizontally, and the role of moist convection. 3. The coupling of the deep motions within the interior to the dynamical manifestations observed in the visible cloud layers. 4. Interactions between the lower neutral atmosphere and the upper atmosphere (thermosphere, ionosphere, magnetosphere), along with energy sources and redistribution responsible for aurora, radiolytic chemistry and high thermospheric temperatures. 5. Time-variable phenomena over a range of timescales to determine the underlying mechanisms and significance of the evolution of discrete atmospheric features, quasi-periodic global upheavals, energetic particle precipitation, asteroidal/cometary impacts and wave activity. These themes for Jupiter science will reveal the connections between deep interior, atmospheric weather layer and charged upper atmosphere; and constrain the origin and subsequent evolution of the gas giant.
Observations of Saturn between 1994-2004 revealed that the equatorial cloud-top wind blows at ~27... more Observations of Saturn between 1994-2004 revealed that the equatorial cloud-top wind blows at ~275~ms-1, approximately half the speed of the Voyager-observed speed, ~470~ms-1, in 1981. It has been hypothesized that the equatorial wind has been slowed by a large equatorial disturbance called the Great White Spot (GWS) in 1990. However, the fact that the clouds are estimated to be higher today than in the Voyager era makes it difficult to observationally isolate the vertical shear effects on the cloud-tracking speed measurements from a true wind speed change. We perform numerical simulations, using the Explicit Planetary Isentropic Coordinate (EPIC) model, to determine whether a GWS-like storm can decelerate the equatorial jet. Possible deceleration mechanisms the storm may trigger are potential vorticity (PV) homogenization, which accelerates the equatorial wind westward, and atmospheric waves transporting momentum away from the jet. Our simulations show that the storm does slow the equatorial wind in the stratosphere, but much less than what has been observed. When our model is initialized with no wind, the storm caused a westward acceleration of as much as 60~ms-1. When initialized with the Voyager wind profile, the largest slowdown we obtain is ~30~ms-1. This is consistent with our order of magnitude calculation, which shows that it is difficult for an atmospheric wave that could plausibly be generated by a GWS to change the tropospheric wind by the observed magnitude. Our result also implies that the storm's PV homogenization effect is weak. Our simulations hint that smaller scale effects such as gravity waves may contribute in slowing the equatorial wind although further study is necessary to estimate their strengths. We also present effects of the storm on the equatorial cloud morphology and compare our simulation results to observations of the GWS evolution.
ABSTRACT We present our analysis of Saturn’s northern hemisphere using images captured by Cassini... more ABSTRACT We present our analysis of Saturn’s northern hemisphere using images captured by Cassini ISS camera. Saturn’s north pole emerged from winter darkness in August 2009; however, the spacecraft stayed in the equatorial plane and the polar regions remained out of view. The spacecraft went into a high-inclination orbit in late 2012 and started returning visible-light images of the north pole. Our images revealed a large cyclonic vortex centered on the north pole. In the System III reference frame (rotation period of 10h39m19s, Seidelmann et al 2007), the wind speed peaks at about 150 m s-1 at 89 degree N planetocentric latitude. The vorticity peaks at the pole at 8.9×10-4 s-1. The zonal wind speed drops to 10±10 m s-1 at around 80 degree N, and peaks again at 100±10 m s-1 at 76 degree N. The 76 degree N zonal jet follows the outline of the Hexagon originally discovered during the Voyager flybys (Godfrey, 1988). We detect a non-zero westward propagation speed of the Hexagon with respect to the System III reference frame. We also present an update on the aftermath of the 2010-2011 Great Storm on Saturn as the region continues to evolve. The anticyclonic vortex that was spawned by the storm continues to drift eastward. The 42 degree N eastward jet has developed a dark streak in a bright band, reminiscent of its appearance during the Voyager flybys when the wave was called the Ribbon. We also continue to monitor the zonal wind speed. Our study is supported by the Cassini Project, NASA Outer Planet Research Program grant NNX12AR38G, and NSF Astronomy and Astrophysics grant 1212216.
We examine new images returned from Cassini spacecraft's ISS camera to analyze tropospheric cloud... more We examine new images returned from Cassini spacecraft's ISS camera to analyze tropospheric cloud morphology of Saturn. We compare our findings to Voyager 2 observations to search for changes in global cloud morphology. Images were acquired around the equinox and our datasets provide near-global coverage in multiple wavelength bands. We find that the clouds exhibit the highest contrasts in infrared continuum centered at 752 & 939 nm (CB2 & CB3 filters, respectively) and 727 & 890 nm methane bands (MT2 & MT3). We compare the present day location of Saturn's bands to those of the Voyager era. We reconfirm multiple features that were previously found in Voyager's visible and Cassini infrared images in the northern hemisphere. First, we examine the Ribbon wave found by the Voyager missions (Sromovsky et al. 1983; Godfrey and Moore, 1986) at multiple wavelength bands. Next, we examine the behavior of a dark vortex that exhibits many similarities to the vortex labeled by Sromovsky et al. as Brown Spot 1. Our data also shows a visible-light counterpart to the String of Pearls feature, which appeared as a series of planet encircling bright spots in 5-micron VIMS images (Momary et al. 2006), suggesting that they are cloud clearings. Our images confirm that there are indeed a string of dark cloud-free spots in the region, which enable us to study their dynamics and compare our results to the VIMS measurements by Choi et al (2009). We also compare the appearance of the north-polar hexagon at multiple wavelengths. Our data also provides good coverage in the southern hemisphere, and we compare our results to Voyager images in 1980-81 and images acquired during the early phase of Cassini mission. Supported by the Cassini Project and a Summer Undergraduate Research Fellowship from Caltech.
Jupiter's cloud-level zonal jets are remarkably steady in time despite their sharp curvat... more Jupiter's cloud-level zonal jets are remarkably steady in time despite their sharp curvature (i.e., second latitudinal derivative of the zonal wind profile). The stable jets must be supported by a proper sub-cloud wind and thermal structure; however, the large-scale deep structure of the zonal jets and temperature remain a major unknown in the gas-giant planet atmospheres. Past studies suggest two
We present new increased-resolution Venus spin-up experiments with topography using the latest hy... more We present new increased-resolution Venus spin-up experiments with topography using the latest hybrid vertical coordinate EPIC atmosphere model (Dowling et. al., Icarus, 182, 2006). We previously reported lower-resolution simulations and showed that the presence of topography substantially accelerates the spin-up of Venusian superrotating winds (Herrnstein and Dowling, JGR, 112, 2007). Our experiments also showed that, with 5- degree resolution, a
We present high-resolution modeling results of two iconic features of planetary atmospheres, Satu... more We present high-resolution modeling results of two iconic features of planetary atmospheres, Saturn's Polar Hexagon and Jupiter's Great Red Spot. In 1988, Voyager images revealed the presence of a circumpolar wave at 76 degrees planetocentric latitude in the northern hemisphere of Saturn. It was characterized for having a zonal planetary wavenumber of six, appearing like a hexagon in polar projected
In 1980, Voyager images revealed the presence of a circumpolar wave at 78 degrees planetographic ... more In 1980, Voyager images revealed the presence of a circumpolar wave at 78 degrees planetographic latitude in the northern hemisphere of Saturn. It was notable for having a dominant planetary wavenumber-six zonal mode, and for being stationary with respect to Saturn's Kilometric Radiation rotation rate measured by Voyager. The center of this hexagonal feature was coincident with the center of
The jetstreams on Saturn at 47N and 77N planetographic latitude are notable in exhibiting wavy cl... more The jetstreams on Saturn at 47N and 77N planetographic latitude are notable in exhibiting wavy cloud morphologies that have been called the Ribbon and the Hexagon, respectively. Recent laboratory and numerical experiments identified several distinct scenarios that can lead to meandering zonal jets. For the Ribbon, the 3D numerical model of Sayanagi et al. (in print, JAS) predicts that the
We present numerical simulations of the wave-like feature in Saturn's northern mid-latitu... more We present numerical simulations of the wave-like feature in Saturn's northern mid-latitude known as the Ribbon wave. It was discovered in analyzing the images returned by the Voyager spacecraft in 1980-81 (Sromovsky et al. 1983, JGR). In the visible wavelengths, the Ribbon appears as a dark line which meanders around 48 degree north planetographic latitude and completely engirdles the planet.
We present the dynamics of the String of Pearls (SoPs) feature observed by the Cassini spacecraft... more We present the dynamics of the String of Pearls (SoPs) feature observed by the Cassini spacecraft's Imaging Science Subsystem (ISS) camera between 2007 -2010. The SoPs was originally discovered in the 5-micron images captured by Cassini VIMS instrument, where it appeared as a chain of infrared-bright spots (Momary et al., 2004). Using ISS images of Saturn, we found a chain of 23-26 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 −2.26±0.02 • day −1 in longitude (−22.27±0.2 m s −1 , 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 east-west terminuses 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 et al., 2013). 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. 3
Three-dimensional numerical simulations of freely evolving stratified geostrophic turbulence on t... more Three-dimensional numerical simulations of freely evolving stratified geostrophic turbulence on the  plane are presented as a simplified model of zonal jet formation on Jupiter. This study samples the parameter space that covers the low, middle, and high latitudes of Jupiter by varying the central latitude of the  plane. The results show that robust zonal jets can emerge from initial small-scale random turbulence through the upscale redistribution of the kinetic energy in the spectral space. The resulting flow's sensitivities to the flow's deformation radius L D and the two-dimensional Rhines length L  ϭ ͌ U/ (U is the characteristic turbulence velocity and  is the meridional gradient of the planetary vorticity) are tested, revealing that whether the outcome of the upscale energy transfer becomes dominated by jets or vortices depends on the relative values of L D and L  . The values of L  and L D are varied by tuning the -plane parameters, and it is found that the flow transitions from a jet-dominated regime in L  Շ L D to a vortical flow in L  տ L D . A height-to-width ratio equal to f /N, the Coriolis parameter divided by the Brunt-Väisälä frequency, has previously been established for stable vortices, and this paper shows that this aspect ratio also applies to the zonal jets that emerge in these simulations.
The exploration of Jupiter has played a pivotal role in the development of our understanding of t... more The exploration of Jupiter has played a pivotal role in the development of our understanding of the history of our solar system; it has served as a paradigm for the interpretation of exoplanetary systems around other stars, and as a fundamental laboratory for the myriad of physiochemical phenomena evident on the gas giants. Yet, despite great successes in the studies of Jupiter over four centuries of research, our characterisation of Jupiter remains incomplete. Jupiter's atmosphere is distinguished from Saturn's and the Ice Giants by its larger mass, dynamic "weather layer", multiple long-lived vortices and the smaller significance of seasonal variability. We review the scientific goals for Jovian exploration in the coming decade: 1. The bulk composition (e.g. heavy elements, isotopes), cooling history and internal structure (the existence of a core) of Jupiter as signatures of planetary formation and evolutionary models, along with comparisons to the other gas giants. 2. The development of a global three-dimensional understanding of the structure, meteorology and chemistry of the troposphere, stratosphere and mesosphere; the mechanisms for transport of energy, momentum and chemical species (tracers) vertically and horizontally, and the role of moist convection. 3. The coupling of the deep motions within the interior to the dynamical manifestations observed in the visible cloud layers. 4. Interactions between the lower neutral atmosphere and the upper atmosphere (thermosphere, ionosphere, magnetosphere), along with energy sources and redistribution responsible for aurora, radiolytic chemistry and high thermospheric temperatures. 5. Time-variable phenomena over a range of timescales to determine the underlying mechanisms and significance of the evolution of discrete atmospheric features, quasi-periodic global upheavals, energetic particle precipitation, asteroidal/cometary impacts and wave activity. These themes for Jupiter science will reveal the connections between deep interior, atmospheric weather layer and charged upper atmosphere; and constrain the origin and subsequent evolution of the gas giant.
Observations of Saturn between 1994-2004 revealed that the equatorial cloud-top wind blows at ~27... more Observations of Saturn between 1994-2004 revealed that the equatorial cloud-top wind blows at ~275~ms-1, approximately half the speed of the Voyager-observed speed, ~470~ms-1, in 1981. It has been hypothesized that the equatorial wind has been slowed by a large equatorial disturbance called the Great White Spot (GWS) in 1990. However, the fact that the clouds are estimated to be higher today than in the Voyager era makes it difficult to observationally isolate the vertical shear effects on the cloud-tracking speed measurements from a true wind speed change. We perform numerical simulations, using the Explicit Planetary Isentropic Coordinate (EPIC) model, to determine whether a GWS-like storm can decelerate the equatorial jet. Possible deceleration mechanisms the storm may trigger are potential vorticity (PV) homogenization, which accelerates the equatorial wind westward, and atmospheric waves transporting momentum away from the jet. Our simulations show that the storm does slow the equatorial wind in the stratosphere, but much less than what has been observed. When our model is initialized with no wind, the storm caused a westward acceleration of as much as 60~ms-1. When initialized with the Voyager wind profile, the largest slowdown we obtain is ~30~ms-1. This is consistent with our order of magnitude calculation, which shows that it is difficult for an atmospheric wave that could plausibly be generated by a GWS to change the tropospheric wind by the observed magnitude. Our result also implies that the storm's PV homogenization effect is weak. Our simulations hint that smaller scale effects such as gravity waves may contribute in slowing the equatorial wind although further study is necessary to estimate their strengths. We also present effects of the storm on the equatorial cloud morphology and compare our simulation results to observations of the GWS evolution.
ABSTRACT We present our analysis of Saturn’s northern hemisphere using images captured by Cassini... more ABSTRACT We present our analysis of Saturn’s northern hemisphere using images captured by Cassini ISS camera. Saturn’s north pole emerged from winter darkness in August 2009; however, the spacecraft stayed in the equatorial plane and the polar regions remained out of view. The spacecraft went into a high-inclination orbit in late 2012 and started returning visible-light images of the north pole. Our images revealed a large cyclonic vortex centered on the north pole. In the System III reference frame (rotation period of 10h39m19s, Seidelmann et al 2007), the wind speed peaks at about 150 m s-1 at 89 degree N planetocentric latitude. The vorticity peaks at the pole at 8.9×10-4 s-1. The zonal wind speed drops to 10±10 m s-1 at around 80 degree N, and peaks again at 100±10 m s-1 at 76 degree N. The 76 degree N zonal jet follows the outline of the Hexagon originally discovered during the Voyager flybys (Godfrey, 1988). We detect a non-zero westward propagation speed of the Hexagon with respect to the System III reference frame. We also present an update on the aftermath of the 2010-2011 Great Storm on Saturn as the region continues to evolve. The anticyclonic vortex that was spawned by the storm continues to drift eastward. The 42 degree N eastward jet has developed a dark streak in a bright band, reminiscent of its appearance during the Voyager flybys when the wave was called the Ribbon. We also continue to monitor the zonal wind speed. Our study is supported by the Cassini Project, NASA Outer Planet Research Program grant NNX12AR38G, and NSF Astronomy and Astrophysics grant 1212216.
We examine new images returned from Cassini spacecraft's ISS camera to analyze tropospheric cloud... more We examine new images returned from Cassini spacecraft's ISS camera to analyze tropospheric cloud morphology of Saturn. We compare our findings to Voyager 2 observations to search for changes in global cloud morphology. Images were acquired around the equinox and our datasets provide near-global coverage in multiple wavelength bands. We find that the clouds exhibit the highest contrasts in infrared continuum centered at 752 & 939 nm (CB2 & CB3 filters, respectively) and 727 & 890 nm methane bands (MT2 & MT3). We compare the present day location of Saturn's bands to those of the Voyager era. We reconfirm multiple features that were previously found in Voyager's visible and Cassini infrared images in the northern hemisphere. First, we examine the Ribbon wave found by the Voyager missions (Sromovsky et al. 1983; Godfrey and Moore, 1986) at multiple wavelength bands. Next, we examine the behavior of a dark vortex that exhibits many similarities to the vortex labeled by Sromovsky et al. as Brown Spot 1. Our data also shows a visible-light counterpart to the String of Pearls feature, which appeared as a series of planet encircling bright spots in 5-micron VIMS images (Momary et al. 2006), suggesting that they are cloud clearings. Our images confirm that there are indeed a string of dark cloud-free spots in the region, which enable us to study their dynamics and compare our results to the VIMS measurements by Choi et al (2009). We also compare the appearance of the north-polar hexagon at multiple wavelengths. Our data also provides good coverage in the southern hemisphere, and we compare our results to Voyager images in 1980-81 and images acquired during the early phase of Cassini mission. Supported by the Cassini Project and a Summer Undergraduate Research Fellowship from Caltech.
Jupiter's cloud-level zonal jets are remarkably steady in time despite their sharp curvat... more Jupiter's cloud-level zonal jets are remarkably steady in time despite their sharp curvature (i.e., second latitudinal derivative of the zonal wind profile). The stable jets must be supported by a proper sub-cloud wind and thermal structure; however, the large-scale deep structure of the zonal jets and temperature remain a major unknown in the gas-giant planet atmospheres. Past studies suggest two
We present new increased-resolution Venus spin-up experiments with topography using the latest hy... more We present new increased-resolution Venus spin-up experiments with topography using the latest hybrid vertical coordinate EPIC atmosphere model (Dowling et. al., Icarus, 182, 2006). We previously reported lower-resolution simulations and showed that the presence of topography substantially accelerates the spin-up of Venusian superrotating winds (Herrnstein and Dowling, JGR, 112, 2007). Our experiments also showed that, with 5- degree resolution, a
We present high-resolution modeling results of two iconic features of planetary atmospheres, Satu... more We present high-resolution modeling results of two iconic features of planetary atmospheres, Saturn's Polar Hexagon and Jupiter's Great Red Spot. In 1988, Voyager images revealed the presence of a circumpolar wave at 76 degrees planetocentric latitude in the northern hemisphere of Saturn. It was characterized for having a zonal planetary wavenumber of six, appearing like a hexagon in polar projected
In 1980, Voyager images revealed the presence of a circumpolar wave at 78 degrees planetographic ... more In 1980, Voyager images revealed the presence of a circumpolar wave at 78 degrees planetographic latitude in the northern hemisphere of Saturn. It was notable for having a dominant planetary wavenumber-six zonal mode, and for being stationary with respect to Saturn's Kilometric Radiation rotation rate measured by Voyager. The center of this hexagonal feature was coincident with the center of
The jetstreams on Saturn at 47N and 77N planetographic latitude are notable in exhibiting wavy cl... more The jetstreams on Saturn at 47N and 77N planetographic latitude are notable in exhibiting wavy cloud morphologies that have been called the Ribbon and the Hexagon, respectively. Recent laboratory and numerical experiments identified several distinct scenarios that can lead to meandering zonal jets. For the Ribbon, the 3D numerical model of Sayanagi et al. (in print, JAS) predicts that the
We present numerical simulations of the wave-like feature in Saturn's northern mid-latitu... more We present numerical simulations of the wave-like feature in Saturn's northern mid-latitude known as the Ribbon wave. It was discovered in analyzing the images returned by the Voyager spacecraft in 1980-81 (Sromovsky et al. 1983, JGR). In the visible wavelengths, the Ribbon appears as a dark line which meanders around 48 degree north planetographic latitude and completely engirdles the planet.
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Papers by Kunio Sayanagi