132,659 research outputs found
Cassini detection of Enceladus' cold water-group plume ionosphere
No full textThis study reports direct detection by the Cassini plasma spectrometer of freshly-produced water-group ions (O+, OH+, H2O+, H3O+) and heavier water dimer ions (HxO(2))(+) very close to Enceladus where the plasma begins to emerge from the plume. The data were obtained during two close ( 52 and 25 km) flybys of Enceladus in 2008 and are similar to ion data in cometary comas. The ions are observed in detectors looking in the Cassini ram direction exhibiting energies consistent with the Cassini speed, indicative of a nearly stagnant plasma flow in the plume. North of Enceladus the plasma slowing commences about 4 to 6 Enceladus radii away, while south of Enceladus signatures of the plasma interaction with the plume are detected 22 Enceladus radii away. Citation: Tokar, R. L., R. E. Johnson, M. F. Thomsen, R. J. Wilson, D. T. Young, F. J. Crary, A. J. Coates, G. H. Jones, and C. S. Paty ( 2009), Cassini detection of Enceladus' cold water-group plume ionosphere, Geophys. Res. Lett., 36, L13203, doi:10.1029/2009GL038923
Preliminary interpretation of Titan plasma interaction as observed by the Cassini Plasma Spectrometer: Comparisons with Voyager 1
No full textThe Cassini Plasma Spectrometer (CAPS) instrument observed the plasma environment at Titan during the Cassini orbiter's TA encounter on October 26, 2004. Titan was in Saturn's magnetosphere during the Voyager 1 flyby and also during the TA encounter. CAPS measurements from this encounter are compared with measurements made by the Voyager 1 Plasma Science Instrument (PLS). The comparisons focus on the composition and nature of ambient and pickup ions. They lead to: A) the major ion components of Saturn's magnetosphere in the vicinity of Titan are H+, H-2(+) and O+/CH4+ ions; B) finite gyroradius effects are apparent in ambient O+ ions as the result of their absorption by Titan's extended atmosphere; C) the principal pickup ions are composed of H+, H-2(+), N+/CH2+, CH4+, and N-2(+); D) the pickup ions are in narrow energy ranges; and E) there is clear evidence of the slowing down of background ions due to pickup ion mass loading
Cassini observations of ion and electron beams at Saturn and their relationship to infrared auroral arcs
No full text[1] We present Cassini Visual and Infrared Mapping Spectrometer observations of infrared auroral emissions from the noon sector of Saturn's ionosphere revealing multiple intense auroral arcs separated by dark regions poleward of the main oval. The arcs are interpreted as the ionospheric signatures of bursts of reconnection occurring at the dayside magnetopause. The auroral arcs were associated with upward field-aligned currents, the magnetic signatures of which were detected by Cassini at high planetary latitudes. Magnetic field and particle observations in the adjacent downward current regions showed upward bursts of 100–360 keV light ions in addition to energetic (hundreds of keV) electrons, which may have been scattered from upward accelerated beams carrying the downward currents. Broadband, upward propagating whistler waves were detected simultaneously with the ion beams. The acceleration of the light ions from low altitudes is attributed to wave-particle interactions in the downward current regions. Energetic (600 keV) oxygen ions were also detected, suggesting the presence of ambient oxygen at altitudes within the acceleration region. These simultaneous in situ and remote observations reveal the highly energetic magnetospheric dynamics driving some of Saturn's unusual auroral features. This is the first in situ identification of transient reconnection events at regions magnetically conjugate to Saturn's magnetopause
ALTIMETRY BY CASSINI RADAR: PROCESSING AND SIMULATION
No full textThe Cassini mission, that is a joint NASA/ESA/ASI effort, has recently offered the unique possibility of exploring Titan, the largest moon of Saturn, that is the only satellite in the solar system to host an appreciable atmosphere, which unfortunately made the surface below very difficult to be observed from the Earth with optical instruments. In fact, a smoggy haze, mostly composed of aerosols resulting from photochemistry between methane and hydrogen and other hydrocarbons, completely envelops the satellite. The Radar Altimeter of the Cassini Mission to Titan operates in a transition region between pulse and beam-limited condition. Due to the specific observation geometry, low values of mispointing angle have been found to significantly affect altimeter impulse response. This involves a non-conventional formulation of the system response which has been the main goal of this research doctorate. An analytical model of the average return power waveform, valid for near-nadir altimetry measurements, has been developed in order to cope with the particular operating conditions of Cassini Mission. The model used to approximate the altimeter waveform is based on the same general assumptions of the classical Brown’s model (1977), but exploits a flat surface response approximation by Prony’s methods. Both theoretical considerations and simulated data have been taken into account to support the accuracy of the proposed model. To infer the main geophysical parameters describing surface topography from altimetry data, a parametric estimation procedure has been used. The Maximum Likelihood Estimator (MLE) procedure has been chosen since in principle it can assure optimal performance as consequence of the analytical model we used to describe the system impulse response. Performances of the implemented method have been numerically evaluated through simulation of data received by CASSINI in high-resolution altimeter mode. The algorithms have been implemented in specific software tools for processing, managing, visualizing and archiving scientific output products containing all the retrieved information about the Titan surface topography, starting from the raw data as delivered by JPL/NASA. The developed processing system is currently in charge of producing standard altimetric Cassini products to be archived in the Planetary Data System (PDS) format. The retrieved topography of the fly-bys performed up to now are shown and shortly commented. Further activities have been dedicated to altimetry echo waveform simulation. The main reason for this effort is to better understand the Titan’s surface characteristics by analyzing the signals received by CASSINI radar in altimetric mode. The approach followed is based on a fractal characterization of Titan’s surface that enables a closed form for the scattering coefficient. A preliminary analysis has been performed on actual data (T30 fly-by) for estimating fractal parameters of Titan’s surface
Location of Saturn's northern infrared aurora determined from Cassini VIMS images
No full textThe location of Saturn's northern infrared aurora is described in detail using 12 selected images acquired by Cassini VIMS during 2006–2008. Bright main oval arcs and prevalent polar features are displayed, which do not exhibit a preferred local time or spatial extent. The average equatorward limit of the aurora was determined using a best fit circle method. The average circle has a radius of 16.4 ± 0.2° latitude, centred 1.6° anti-sunward of Saturn's pole. The low standard deviation of the fitted circle radii (<1°) indicates the relative stability of the equatorward auroral boundary. For the first time we show that the average location of the infrared oval is similar to that of the ultraviolet oval, suggesting that the auroral emissions at different wavelengths are very likely produced by a common driver despite the distinct emission mechanisms
Simultaneous Cassini VIMS and UVIS observations of Saturn's southern aurora: Comparing emissions from H, H-2 and H-3(+) at a high spatial resolution
No full textHere, for the first time, temporally coincident and spatially overlapping Cassini VIMS and UVIS observations of Saturn's southern aurora are presented. Ultraviolet auroral H and H-2 emissions from UVIS are compared to infrared H-3(+) emission from VIMS. The auroral emission is structured into three arcs - H, H-2 and H-3(+) are morphologically identical in the bright main auroral oval (similar to 73 degrees S), but there is an equatorward arc that is seen predominantly in H (similar to 70 degrees S), and a poleward arc (similar to 74 degrees S) that is seen mainly in H-2 and H-3(+). These observations indicate that, for the main auroral oval, UV emission is a good proxy for the infrared H-3(+) morphology (and vice versa), but for emission either poleward or equatorward this is no longer true. Hence, simultaneous UV/IR observations are crucial for completing the picture of how the atmosphere interacts with the magnetosphere
The influence of the secondary electrons induced by energetic electrons impacting the Cassini Langmuir probe at Saturn
No full textThe Cassini Langmuir Probe (LP) onboard the Radio and Plasma Wave Science experiment has provided much information about the Saturnian cold plasma environment since the Saturn Orbit Insertion in 2004. A recent analysis revealed that the LP is also sensitive to the energetic electrons (250–450 eV) for negative potentials. These electrons impact the surface of the probe and generate a current of secondary electrons, inducing an energetic contribution to the DC level of the current-voltage (I-V) curve measured by the LP. In this paper, we further investigated this influence of the energetic electrons and (1) showed how the secondary electrons impact not only the DC level but also the slope of the (I-V) curve with unexpected positive values of the slope, (2) explained how the slope of the (I-V) curve can be used to identify where the influence of the energetic electrons is strong, (3) showed that this influence may be interpreted in terms of the critical and anticritical temperatures concept detailed by Lai and Tautz (2008), thus providing the first observational evidence for the existence of the anticritical temperature, (4) derived estimations of the maximum secondary yield value for the LP surface without using laboratory measurements, and (5) showed how to model the energetic contributions to the DC level and slope of the (I-V) curve via several methods (empirically and theoretically). This work will allow, for the whole Cassini mission, to clean the measurements influenced by such electrons. Furthermore, the understanding of this influence may be used for other missions using Langmuir probes, such as the future missions Jupiter Icy Moons Explorer at Jupiter, BepiColombo at Mercury, Rosetta at the comet Churyumov-Gerasimenko, and even the probes onboard spacecrafts in the Earth magnetosphere
Structure of Titan's mid-range magnetic tail: Cassini magnetometer observations during the T9 flyby
No full textWe analyze the magnetic structure of Titan's mid-range magnetic tail (5-6 Titan radii downstream from the moon) during Cassini's T9 flyby. Cassini magnetometer (MAG) measurements reveal a well-defined, induced magnetic tail consisting of two lobes and a distinct central current sheet. MAG observations also indicate that Saturn's background magnetic field is close to the moon's orbital plane and that the magnetospheric flow has a significant component in the Saturn-Titan direction. The analysis of MAG data in a coordinate system based on the orientation of the background magnetic field and an estimation of the incoming flow direction suggests that Titan's magnetic tail is extremely asymmetric. An important source of these asymmetries is the connection of the inbound tail lobe and the outbound tail lobe to the dayside and nightside hemispheres of Titan, respectively. Another source could be the perturbations generated by changes in the upstream conditions
Supercorotating return flow from reconnection in Saturn's magnetotail
No full textDetecting plasma dynamics in Saturn's magnetosphere is essential for understanding energy flow through the system. It has been proposed that both the Dungey and Vasyliunas cycles operate at Saturn, and the competition between these cycles has been debated. We examine data taken by the Cassini spacecraft in Saturn's post-dawn magnetosphere, similar to 17.5 Saturn radii from the planet, and identify an example of return flow from magnetotail reconnection. The flow included water group ions and had elevated ion temperatures (of order 1 keV), consistent with Vasyliunas cycle return flow. The flow was also supercorotating (similar to 200 km s(-1), similar to 120% of corotation), which is highly atypical of Saturn's outer magnetosphere. Our results suggest that return flows are time-variable, and our results concerning Dungey cycle return flows are inconclusive. We propose that supercorotating flows in Saturn's dawn magnetosphere strongly influence the current system that is responsible for the planet's main auroral emission. Citation: Masters, A., M. F. Thomsen, S. V. Badman, C. S. Arridge, D. T. Young, A. J. Coates, and M. K. Dougherty (2011), Supercorotating return flow from reconnection in Saturn's magnetotail, Geophys. Res. Lett., 38, L03103, doi: 10.1029/2010GL046149
Global MHD simulations of Saturns's magnetosphere at the time of Cassini approach
No full textWe present the results of a 3D global magnetohydrodynamic simulation of the magnetosphere of Saturn for the period of Cassini's initial approach and entry into the magnetosphere. We compare calculated bow shock and magnetopause locations with the Cassini measurements. In order to match the measured locations we use a substantial mass source due to the icy satellites (\sim1 x 10^{28} s^{-1} of water product ions). We find that the location of bow shock and magnetopause crossings are consistent with previous spacecraft measurements, although Cassini encountered the surfaces further from Saturn than the previously determined average location. In addition, we find that the shape of the model bow shock and magnetopause have smaller flaring angles than previous models and are asymmetric dawn-to-dusk. Finally, we find that tilt of Saturn's dipole and rotation axes results in asymmetries in the bow shock and magnetopause and in the magnetotail being hinged near Titan's orbit (\sim20 R _S)
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