Magnetospheric Physics at Saturn: Is It Earth-Like, Jupiter-Like, or Even Like It Was 25 Years Ago? III Posters
Presiding: D G Mitchell, Applied Physics Laboratory, Johns Hopkins University; J T Steinberg, Los Alamos National Laboratory
SM13A-01 1330h
Using Pickup Ions to Determine the Solar Wind Speed at the Cassini Spacecraft
We calculate solar wind speeds at the Cassini spacecraft when it's in the solar wind, as it was during the cruise to Saturn and as it is for a substantial portion of its orbits around Saturn. This benefits magnetospheric studies by providing a key measure of the upstream conditions. Similarly, a solar wind speed monitor at 9 AU (and during the 2000--2004 cruise phase between 1 and 9 AU) facilitates interplanetary studies. Since pickup ions have an intensity peak at twice the solar wind speed, we use the MIMI/CHEMS measurement of 3--220 keV/e singly charged He to determine the pickup He spectra and calculate speeds down to one-hour resolution. These speeds agree well with solar wind speeds directly measured by Cassini's CAPS instrument during a month long period in 2004, during which speeds varied from 450 to 600 km/s. Aided by further calibration with CAPS, CHEMS can provide solar wind speeds during the frequent periods when the spacecraft orientation prevents CAPS from making the measurement. We calculate solar wind speeds upstream of Saturn during 2004 and 2005 and compare with global variations of the magnetosphere.
SM13A-02 1330h
The Compton Getting effect in ENA images of energetic ion injections at Saturn
The Cassini Ion and Neutral Camera (INCA) images local energetic ion populations and energetic neutral atoms (ENAs). During the final months of approach to Saturn, INCA frequently imaged the whole Saturnian magnetosphere. Data from that period reveal an intermittent signal in total ENA flux, sometimes near our detector background. After closest approach, INCA images from orbit frequently show intense brightenings confined in local time. We have interpreted these signals as localized bursts of injected ions which then corotate with the planet. Often the injection related component of the ENAs is more intense than all the ENA emission from the rest of the magnetosphere. We suspect that the strongest ENA signals detected on approach were likely due to similar injection events. We have further observed that the ENAs created by the injected population seem to be brightest in the midnight sector. We have demonstrated that this is due primarily to the Compton-Getting effect, which for the detection of energetic ions in a flowing plasma gives higher counts in the sectors viewing into the oncoming flow. The relative distance between the spacecraft and the injected population will also affect the total ENA counts in an INCA image because of the R-2 dependence from the source distribution. Unlike ENA imaging at Earth (where corotation velocities are about 5% of those at the same L value in Saturn's magnetosphere), the Compton-Getting effect is essential to the interpretation of ENA images of the rapidly rotating magnetospheres of the giant planets.
SM13A-03 1330h
High resolution measurements of Langmuir waves upstream of the Saturnian bow shock
The Cassini spacecraft is currently in the fourth orbit of its four-year prime mission to study the Saturnian system. The Radio and Plasma Wave Science (RPWS) investigation is designed to study the radio emissions and plasma waves in the vicinity of Saturn. The RPWS instrument includes a wideband receiver (WBR) that provides high time and frequency resolution over a range of frequencies, allowing the fine structure of various radio and plasma waves to be examined. Langmuir waves produced upstream of the Saturnian bowshock were detected by Cassini during the approach to Saturn, and also during each of the first four orbits of Cassini The structure of the Langmuir waves detected by Cassini spans a broad range of time scales, from tens of minutes to structures with time scales of milliseconds. The large scale structure includes upshifts and downshifts in the frequency of the Langmuir waves, probably due to the location of Cassini in the Saturnian foreshock, and also variation in the frequency of the Langmuir wave emission related to density variations in the solar wind. The small scale structure includes beat-like waveforms suggestive of a non-linear process. The observed structures, both the large and the fine time scales, are very similar to the structure observed in Langmuir waves at Venus, Earth, and Jupiter. The characteristics of these waves will be examined and the distribution of their electric field amplitude will be determined. These results will be compared to the various theories of Langmuir wave production and propagation.
SM13A-04 1330h
Modeling Saturn's Inner Plasmasphere: Cassini's Closest Approach
Ion densities from the three-dimensional Saturn-Thermosphere-Ionosphere-Model (STIM, Moore et al., 2004) are extended above the plasma exobase using the formalism of Pierrard and Lemaire (1996, 1998), which evaluates the balance of gravitational, centrifugal and electric forces on the plasma. The parameter space of low-energy ionospheric contributions to Saturn's plasmasphere is explored by comparing results that span the observed extremes of plasma temperature, 650 K to 1700 K, and a range of velocity distributions, Lorentzian (or Kappa) to Maxwellian. Calculations are made for plasma densities along the path of the Cassini spacecraft's orbital insertion on 1 July 2004. These calculations neglect any ring or satellite sources of plasma, which are most likely minor contributors at 1.3 Saturn radii. Modeled densities will be compared with Cassini measurements as they become available. Moore, L.E., M. Mendillo, I.C.F. Mueller-Wodarg, and D.L. Murr, Icarus, 172, 503-520, 2004. Pierrard, V. and J. Lemaire, J. Geophys. Res., 101, 7923-7934, 1996. Pierrard, V. and J. Lemaire, J. Geophys. Res., 103, 4117, 1998.
SM13A-05 1330h
Cassini Measurements of Solar Wind Near Saturn
In situ studies of Saturn's magnetosphere from Cassini do not enjoy the benefit of simultaneous measurements of the solar wind immediately upstream. In lieu of such measurements, it will be highly valuable to quantify the typical range and the variability of solar wind conditions near Saturn. The Cassini Plasma Spectrometer (CAPS) includes a detector called the Ion Beam Spectrometer (IBS), which is optimized for measuring solar wind protons and alpha particles. Solar wind measurements have been obtained with IBS during some extended intervals, including much of January 2004 during the approach to Saturn, as well as in late July through late September 2004, during the first apoapsis. The solar wind observed thus far is characterized by corotating interaction regions (CIRs) separating fast and slow streams. For example a forward shock on July 25, 2004 heralded the arrival of a CIR that lasted until August 1, 2004. It has been reported elsewhere that the solar wind ram pressure is a primary controlling parameter for Saturn's aurora. A preliminary look suggests that the peak ram pressures seen so far are found in association with the compressed solar wind inside CIRs. We will present the Cassini solar wind measurements made thus far, and discuss the general character of the solar wind near Saturn.
SM13A-06 1330h
Warm flux tubes in Saturn's cool E-ring plasma torus
As Cassini passed through Saturn's E ring, the magnetometer onboard observed a number of diamagnetic cavities isolated in time from any other disturbance. The decrease in magnetic pressure within these flux tubes implies the presence of an additional plasma energy density of up to 1 keV/cm3. The magnetic fields within these cavities are more dipolar and hence less stressed than their neighbors, suggesting that they may contain less mass or are traveling at a lower azimuthal velocity relative to their surroundings. The magnetometer also observed at about 6 Saturn radii, outward from these tubes, an irregular transition from predominantly cool to predominantly warm flux tubes. In the jovian magnetosphere, at the outer edge of the Io torus, a similar boundary has been found. Well inside the location of the jovian boundary, inward moving flux tubes have been reported in the Galileo magnetometer data. These tubes have enhanced field strength and have been interpreted as depleted flux tubes. The saturnian flux tubes exhibit magnetic strength decreases, but they still may contain less plasma than their surroundings. In short, the two differently appearing phenomena may both be different expressions of the same process.
SM13A-07 1330h
Are Identical Twins, Truly Identical? A Comparison of Mass Spectra Recorded by "Identical" Mass Spectrometers.
When the time of flight (TOF) Ion Mass Spectrometer (IMS) for the Cassini Plasma Spectrometer (CAPS) instrument was launched, the prototype model was upgraded to be identical to the flight model electro-optically and electronically. The primary goal of the upgrade was to have an instrument which could be tested in the laboratory to help analyze and interpret the data being returned from Cassini IMS measurements at Saturn for its four year tour and after a seven year journey. Since launch the prototype model has been used to test the instruments response to a wide variety of ion beam masses, energies, fluxes and compositions, many of which could not be covered during ground calibration of the flight instrument. This testing broadened our understanding of how IMS works and responds beyond what was accomplished before launch. However, when making comparisons between calibration data taken with the flight and the prototype units using beams with the same composition and energy, some systematic differences have been observed. In particular the mass spectra generated by the prototype appear to have a shift in the TOF from that of the flight instrument, the size of which depends on both the energy and mass of the incident ion. The differences will be discussed with particular reference to potential causes. This information is important for understanding the complications in calibrating and comparing the results from instruments that are assumed to function identically, in particular as applied to future constellation missions with instrument of identical design. It may also be useful to others designing similar TOF instruments.
SM13A-08 1330h
Hybrid Simulations of Ion Cyclotron Waves at Io
We study the evolution of ions picked up into a plasma flowing perpendicular to a background magnetic field using a 1D electromagnetic hybrid simulation code. Such a pickup geometry occurs in the Io plasma torus, where ions originating from volcanic activity on Io are mass loaded into the corotating plasma torus in the presence of a predominantly southward magnetic field. Free energy associated with this mass loading process leads to the generation of low amplitude ion cyclotron waves. Previously, dispersion analyses of the wave generation process at Io employed dispersion solvers which approximated the mass loaded ring distribution of ions with gaussian distributions. In this simulation ring beam ions are realistically simulated. We compare simulations of various ion species and loading rates with linear theory.
SM13A-09 1330h
Spacecraft Observations of Satellite Control of Jupiter's Radio Emission
Observations from both the Galileo and Voyager spacecraft show the influence of the four Jovian satellites on the radio emission generated in the Jovian magnetosphere. In an attempt to quantify the significance of these correlations, we have completed a statistical analysis of Jupiter's emission intensity and occurrence probability with all four Galilean satellites. We analyzed the peak correlations of satellite phase versus Jovian longitude, and present the significance as standard deviations (σ) above the non-Io-A background. Our analysis shows peaks of significance of at least 2σ for all four Galilean satellites. We hope to present new data from the Cassini spacecraft flyby at Jupiter to confirm these results in different frequency bands.
SM13A-10 1330h
Plasma Sheet Thickness at Jupiter From Galileo Measurements of Electron Density and a New Model of Jupiter's Magnetic Field
Electron density has been determined throughout much of Galileo's primary mission at Jupiter (December 7, 1995 to November 6, 1997) by observing plasma waves measured by the plasma wave instrument on board the spacecraft. The density data set is used here to identify spacecraft encounters with Jupiter's magnetotail plasma sheet during the primary mission by assuming that electron density is highest at the center of the plasma sheet. As Jupiter rotates, the spacecraft encounters one pair of plasma sheet crossings during each ten-hour rotation period. Electron density is usually seen to increase as Galileo enters the plasma sheet, reach a maximum value near the center of the plasma sheet, and then decrease as the spacecraft exits the plasma sheet. This signature is clearest in the data at radial distances between 20 RJ, and 50 RJ from Jupiter. Plasma sheet thickness is determined by identifying the z-coordinate of the spacecraft as it enters and exits the plasma sheet. The z-position is measured with respect to a newer magnetic field model by Khurana and Kivelson. This work seeks to determine the plasma sheet thickness in Jupiter's magnetotail, where other instruments observe a thicker plasma sheet in the midnight and dusk sectors and a thinner, more distinct sheet in Jupiter's dawn sector.
SM13A-11 1330h
Equatorial electron beams at Jupiter; Signatures of Earth-like spatial structuring within Jupiters main auroral oval?
The regions of intense discrete aurora at Earth show high degrees of spatial structuring down to the less than 1 km scale. Specifically, intense and narrow auroral arcs associated with strong upward (with respect to Earth) magnetic field-aligned electric currents are often accompanied with adjacent regions of downward currents which constitute part of the return current of the auroral magnetosphere-ionosphere current system. Within Earth's near-equatorial magnetosphere, a distinct signature of these downward electric current regions has been discovered; intense, magnetic field-aligned, bi-directional electron beams with broad energy distributions extending at times to 10s of keV (Klumpar et al., 1988; interpreted by Carlson et al., 1998 with low altitude measurements). Such electron beams were observed by Galileo within Jupiter's middle, near equatorial magnetosphere between about 15 to greater than 30 RJ (Bhattacharya et al, 2001; Tomas et al., 2004; Frank and Paterson, 2004), and are undoubtedly related to beams observed at high latitudes by Ulysses (Lanzerotti et al, 1993). The equatorial beams occur within the region of space identified as mapping to Jupiter's main region of strong auroral emissions where, on average, it is thought that the global electric currents are upward with respect to Jupiter. We show here that the observed electron beams are highly transient or structured when observed in high time resolution. We argue that the beams represent evidence of Earth-like structuring within Jupiter's main aurora, with narrow regions of downward current imbedded with and accompanying regions of upward current, all within the broad region where, on average, the electric currents are upward with respect to Jupiter.