Over the last fifty years, a multitude of spacecraft and rocket experiments have studied plasma wave emissions from Earth's auroral regions. One such emission is auroral hiss, a low-frequency whistler-mode wave that is produced in the auroral zone. Observations from Earth-orbiting spacecraft show that auroral hiss is generated by field-aligned electron beams, with the resulting plasma wave emission propagating along the resonance cone. This propagation results in auroral hiss appearing as a V-shaped funnel when observed on a frequency-time spectrogram. This thesis presents the first comprehensive study of auroral hiss at a planet other than Earth, using the Cassini spacecraft to study auroral hiss at Saturn. NASA's Cassini spacecraft, currently in orbit around Saturn, has allowed for the first opportunity to study this emission in detail at another planet. Since 2006, the Cassini spacecraft has twice been in a series of high inclination orbits, allowing investigation and measurements of Saturnian auroral phenomena. During this time, the Radio and Plasma Wave Science (RPWS) Investigation on Cassini detected low frequency whistler mode emissions propagating upward along the auroral field lines, much like terrestrial auroral hiss. Comparisons of RPWS data with observations from several other Cassini instruments, including the Dual-Technique Magnetometer (MAG), Magnetospheric Imaging Instrument (MIMI), and the Cassini Plasma Spectrometer (CAPS), have revealed a complete picture of this emission at Saturn. Observations from these instruments have been used to make a variety of determinations about auroral hiss at Saturn. RPWS has only observed this emission when Cassini was at high-latitudes, although these observations have shown no preference for local time. Tracking the times this emission has been observed revealed a clear periodicity in the emission. Further study later revealed not one but two rotational modulations, one in each hemisphere, rotating at rates of 813.9 and 800.7 degrees per day in the northern and southern hemispheres, respectively. These rates match with observations of the clock-like Saturn Kilometric Radiation. Study of the field-aligned current structures in the auroral regions revealed a strong upward-directed current in both hemispheres on the lower-latitude side of the auroral hiss emission. Along with correlating particle densities, these observations were used to infer the presence of a high-density plasmasphere at low latitudes, with the series of field-aligned current structures lining up with the outer boundary at L-shell values of around 12-15. Analysis of electron beams observed in conjunction with auroral hiss shows that these beams produce large growth rates for whistler-mode waves propagating along the resonance cone, similar to terrestrial auroral hiss. Analytical calculation of the normalized growth rates of ten electron beam events on Day 291, 2008, yielded a wide range of growth rates, from 0.004 to over 6.85 times the real frequency. The latter, a non-physical result, came from a violation of the weak growth approximation, suggesting there was so much growth that the analytical calculation was not valid in this instance. Numerical calculation using a plasma dispersion-solving code called WHAMP produced a growth rate of about 0.3, a still very large number, suggesting the detected beams may be the source of the observed auroral hiss plasma wave emission.

<p>This report presents a model to determine the solar Extreme UltraViolet (EUV) intensity and the ion flux in the vicinity of Saturn, by using measurements from the Langmuir probe, a plasma investigation instrument, of the Cassini satellite. The model is based on in situ measurements and does therefore provide an improved estimation of the wanted parameters compared to previously used calculations based only on the EUV flux measured near Earth. The solar EUV and ion flux were determined by analysing and processing the current measurements from the Cassini Langmuir probe in several steps. Initially the time intervals where the measured current were expected to be due only to the photoelectron current was extracted. The photoelectron current is the part of the measured probe current that is only due to electrons ejected from the probe by photons coming from the Sun. The measurements showed a periodic behaviour which was concluded to be due to the attitude of the satellite. This interfering effect was corrected for and the data was then plotted against an EUV index, estimated from a traditionally used proxy of the EUV flux near Earth; the F10.7 solar radio flux index. In agreement with the theory of the photoelectric effect a linear relationship between the EUV flux and the photoelectron current <em>m_ph</em> was expected. A least square linear fit to the extracted photoelectron current data provided the relation, for the Langmuir probe on Cassini, in the form of the equation <em>m_ph</em>=0.1842<em>EUV</em>+0.2405, where <em>m_ph</em> is the photoelectron current in nA and <em>EUV</em> is the EUV index in W/Hzm^2. The derived equation is the result of the study, showing how to estimate the solar EUV flux using the Langmuir probe current measurements. This result was used to derive the other wanted parameter, the ion flux. The derivation was done by calculating the photoelectron current <em>m_ph </em>at all time and subtracting the result from the total current. The retrieved difference gives the magnitude of the ion current for every measurement.</p>

Les ceintures de radiations provoquent des dégâts irréversibles sur les satellites les traversant, détériorant ainsi les instruments de mesures embarqués. Les étudier est utile au développement de matériaux adaptés et résistants. Depuis les années 90, l'ONERA-DESP étudie les ceintures de radiations des planètes magnétisées, telle que la Terre ou Jupiter, grâce au modèle Salammbô. Salammbô prend en compte les processus physiques de l'environnement radiatif pour recréer les populations d'électrons peuplant les ceintures. Dans cette étude, il s'agit de développer un modèle des ceintures de radiations internes de Saturne, basé sur les travaux précédents. Avant les années 2000, Pioneer 11 et Voyager 2 ne permettaient pas un développement suffisamment avancé d'un modèle de ceintures de radiations de Saturne. La mission Cassini apporte ensuite quantités d'observations et de données pour mener une étude plus approfondie de ces ceintures. Cette thèse débute par l'analyse de la magnétosphère interne de Saturne : anneaux, satellites, nuage de neutres... L'interaction des particules des ceintures de radiations avec ces différents paramètres se traduit par le calcul de coefficients de diffusion. Ces coefficients sont intégrés à l'équation de transport et permettent de comprendre les mécanismes perturbant la distribution des électrons au sein des ceintures de radiation. Les résultats obtenus sont comparés aux mesures faites par les sondes Pioneer 11, Voyager 2 et Cassini.

This report presents a model to determine the solar Extreme UltraViolet (EUV) intensity and the ion flux in the vicinity of Saturn, by using measurements from the Langmuir probe, a plasma investigation instrument, of the Cassini satellite. The model is based on in situ measurements and does therefore provide an improved estimation of the wanted parameters compared to previously used calculations based only on the EUV flux measured near Earth. The solar EUV and ion flux were determined by analysing and processing the current measurements from the Cassini Langmuir probe in several steps. Initially the time intervals where the measured current were expected to be due only to the photoelectron current was extracted. The photoelectron current is the part of the measured probe current that is only due to electrons ejected from the probe by photons coming from the Sun. The measurements showed a periodic behaviour which was concluded to be due to the attitude of the satellite. This interfering effect was corrected for and the data was then plotted against an EUV index, estimated from a traditionally used proxy of the EUV flux near Earth; the F10.7 solar radio flux index. In agreement with the theory of the photoelectric effect a linear relationship between the EUV flux and the photoelectron current mph was expected. A least square linear fit to the extracted photoelectron current data provided the relation, for the Langmuir probe on Cassini, in the form of the equation mph=0.1842EUV+0.2405, where mph is the photoelectron current in nA and EUV is the EUV index in W/Hzm^2. The derived equation is the result of the study, showing how to estimate the solar EUV flux using the Langmuir probe current measurements. This result was used to derive the other wanted parameter, the ion flux. The derivation was done by calculating the photoelectron current mph at all time and subtracting the result from the total current. The retrieved difference gives the magnitude of the ion current for every measurement.

Thèse de doctorat : Sciences de l'univers et de l'environnement : Paris 12 : 2007. / Titre provenant de l'écran-titre. Bibliogr. : 188 réf.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1999. / Cover title. Computer printout. Abstract. Includes bibliographical references (p. [103]-112). Available electronically via Internet.

We use particle and magnetic field data from the Ion and Neutral Camera (INCA) and the magnetometer (MAG) onboard Cassini to detect and examine an energetic particle event that occurred upstream from the Saturnian bow shock during DOY 229/2007. The energetic (&gt;100 keV) O+ ions are observed only when the Interplanetary Magnetic Field (IMF) connects the spacecraft with the planetary bow shock. We provide strong evidence showing the magnetospheric origin of the observed ions: (1) We detect singly ionized oxygen (O+) which is not resident of the solar wind, (2) the particle pitch angle distribution indicates that the ions travel along the field line connecting the spacecraft to the bow shock and (3) the ion intensity increases are observed only during the periods of magnetic connection to the bow shock. Our results show that the Saturnian dayside magnetosphere is not as sealed as thought to be, but can -under certain circumstances- allow high energy magnetospheric plasma to leak into the nearby solar wind and further in space.

University, Diss., 2002--Bonn.

Despite atmospheric and compositional differences on Titan and Earth, the similarity in the shape and spacing of linear dunes of the Belet Sand Sea of Titan and the Namib Sand Sea of Earth suggests that comparisons will yield a better understanding of the dictating factors of duneforming processes. We present a methodology for the collection of dune width and spacing measurements representative of the Namib and Belet sand seas. 94,304 locations in Belet from Cassini SAR images and 5,563 locations in the Namib from IKONOS images are used for measurements. The average width and spacing of linear dunes in Belet are 1,235 m and 2,776 m, respectively, with a standard deviation of 422 and 859 respectively. In the Namib, the average linear dune width and spacing is 736 m and 2,203 m, with a standard deviation of 204 and 592. We also analyze these morphometrics according to potential dictating factors such as elevation and distance to sand sea margins. We establish significant trends according to distance to margin, which confirms that the largest and most widely spaced dunes are generally found in the center of the sand sea. We also observe increasing dune width with increasing elevation. The strongest trend we observe is distance to the western margin in the Namib Sand Sea. In Belet, none of these trends were found to be significant. Analysis of width vs. spacing is significant in both sand seas. The disparity in results of the two sand seas suggests factors such as age, sand sea size, or proximity to source may influence linear dune morphometrics.

Analyzing dune parameters such as dune width and spacing can be useful in determining the reaction of dunes to changes in atmospheric and sedimentary conditions currently and in the recent geologic past. Dune parameters, dune width and spacing, were measured for linear dunes in regions across Saturn's moon Titan from images T21, 23, 28, 44 and 48 collected by Synthetic Aperture RADAR aboard the Cassini spacecraft in order to reconstruct the surface history of Titan. Dunes in the five study swaths are all linear in form, but lack superimposed or flanking dunes. They have a mean width of 1.3 km and mean crest spacing of 2.7 km, wider and farther apart on average than similar terrestrial dunes in the Namib and Agneitir Sand Seas though larger linear dunes exist on Earth. Because of the lack of superimposed and flanking dunes and their size, Titan's dunes are classified as very large simple linear dunes. The large size, spacing and uniform morphology are all indicators that Titan's dunes are very mature and long-lived features. The ratio of dune width to spacing for Titan's dunes is similar to that found in terrestrial dunes in that dune spacing tends to be twice dune width. In addition to being similar in size, this is further evidence that terrestrial dunes can be used as analogues for Titan's dunes and vice versa and that the essential dune-forming processes are the same on both bodies. Dune width and spacing decrease northward, which is attributed to, but not limited to, increased maturity of dune fields to the south or increased sediment stabilization to the north. Sediment stabilization may be caused by Titan's asymmetric seasons and a net transport of moisture from south to north. The majority of dunes have spacings consistent with an upper limit of 2 to 4 km established by the atmospheric boundary layer, further evidence they are mature. Dunes are more widely spaced in the south are evidence they have been growing toward a steady state for a longer period of time than those in the north. Titan's large linear dunes have long reconstitution times. This is in part due to the fact that winds sufficient for saltation are reached only near the Titan equinox every 14 Earth years. Based on rates for similar terrestrial dunes the reconstitution time for Titan's dune is 600,000 Earth years or more, and therefore substantial changes in dune form should not be observable over Cassini's lifetime. Cumulative probability plots of dune parameters measured at different locations across Titan indicate there is a single population of dunes on Titan. This suggests that, unlike analogous dunes in the Namib and Agneitir Sand Seas, dune-forming conditions that currently exist on Titan are either the only dune-friendly conditions in the moon's history, or the current conditions have been stable and active long enough to erase any evidence of past conditions.