Statistics of Titan's South Polar Tropospheric Clouds

We present the first long-term study of the behavior of the sporadically observed tropospheric clouds recently discovered near Titan's south pole. We find that one or more small individual cloud systems is present in the 70°-80° south region during every night of observation. These clouds account for 0.5%-1% of Titan's 2.0 μm flux, consistent with a global cloud cover fraction of 0.2%-0.6%. Clouds observed over multiple-night observing periods remained nearly fixed in brightness and position with respect to Titan's surface. The continual presence of south polar clouds is consistent with the hypothesis that surface heating during the long period of continuous polar sunlight at the time of Titan's southern summer solstice drives seasonal convection and cloud formation at the pole

Modeling low order aberrations in laser guide star adaptive optics systems

When using a laser guide star (LGS) adaptive optics (AO) system, quasi-static aberrations are observed between the measured wavefronts from the LGS wavefront sensor (WFS) and the natural guide star (NGS) WFS. These LGS aberrations, which can be as much as 1200 nm RMS on the Keck II LGS AO system, arise due to the finite height and structure of the sodium layer. The LGS aberrations vary significantly between nights due to the difference in sodium structure. In this paper, we successfully model these LGS aberrations for the Keck II LGS AO system. We use this model to characterize the LGS aberrations as a function of pupil angle, elevation, sodium structure, uplink tip/tilt error, detector field of view, the number of detector pixels, and seeing. We also employ the model to estimate the LGS aberrations for the Palomar LGS AO system, the planned Keck I and the Thirty Meter Telescope (TMT) LGS AO systems. The LGS aberrations increase with increasing telescope diameter, but are reduced by central projection of the laser compared to side projection

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan's atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titan's atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission

Comet Shoemaker‐Levy 9: No effect on the Io plasma torus

Observations of the Io plasma torus made before, during, and after the impact of Comet Shoemaker‐Levy 9 with Jupiter reveal no comet‐induced changes. Three weeks of high spectral‐resolution ground‐based visible spectroscopy show no changes larger than typical day‐to‐day variations in the torus densities, ion temperatures, or rotation velocities. Comparison with six months of identically obtained data from 1991 and 1992 also shows no differences

Quasi-static aberrations induced by laser guide stars in adaptive optics

Laser Guide Star Adaptive Optics (LGS AO) systems use the return from an artificial guide star to measure the wavefront aberrations in the direction of the science object. We observe quasi-static differences between the measured wavefront and the wavefront aberration of the science object. This paper quantifies and explains the source of the difference between the wavefronts measured using an LGS and a natural guide star at the W. M. Keck Observatory, which can be as high as 1000 nm RMS

Jupiter's visible aurora and Io footprint

Images obtained by the Galileo spacecraft's solid-state imaging (SSI) system represent the first survey of Jupiter's northern auroral emissions at visible wavelengths and on the nightside of the planet. These images captured the emissions with unprecedented spatial resolutions down to ∼26 km pixel^(−1). Four classes of emission were observed: (1) a continuous, primary arc associated with the middle/outer magnetosphere, (2) a variable secondary arc associated with the region just beyond Io's torus, (3) diffuse “polar cap” emission, and (4) a patch and tail associated with the magnetic footprint of Io. The primary arc emission occurs at an altitude 245±30 km above the 1-bar pressure level. Its horizontal width is typically a few hundred kilometers, and its total optical power output varied between ∼10^(10) and ∼10^(11) W in observations taken months apart. The location of the primary arc in planetary coordinates is similar to that on dayside images at other wavelengths and does not vary with local time. The morphology of the primary arc is not constant, changing from a multiply branched, latitudinally distributed pattern after dusk to a single, narrow arc before dawn. Emission from Io's ionospheric footprint is distinct from both the primary and secondary arcs. Measurements of its optical power output ranged from 2 to 7×10^8 W

Geographic Control of Titan's Mid-Latitude Clouds

Observations of Titan's mid-latitude clouds from the W. M. Keck and Gemini Observatories show that they cluster near 350°W longitude, 40°S latitude. These clouds cannot be explained by a seasonal shift in global circulation and thus presumably reflect a mechanism on Titan such as geysering or cryovolcanism in this region. The rate of volatile release necessary to trigger cloud formation could easily supply enough methane to balance the loss to photolysis in the upper atmosphere

Real-time wavefront control for the PALM-3000 high order adaptive optics system

We present a cost-effective scalable real-time wavefront control architecture based on off-the-shelf graphics processing units hosted in an ultra-low latency, high-bandwidth interconnect PC cluster environment composed of modules written in the component-oriented language of nesC. We demonstrate the architecture is capable of supporting the most computation and memory intensive wavefront reconstruction method (vector-matrix-multiply) at frame rates up to 2 KHz with latency under 250 &mgr;s for the PALM-3000 adaptive optics systems, a state-of-the-art upgrade on the 5.1 meter Hale Telescope that consists of a 64x64 subaperture Shack-Hartmann wavefront sensor and a 3368 active actuator high order deformable mirror in series with a 349 actuator "woofer" DM. This architecture can easily scale up to support larger AO systems at higher rates and lower latency