S. Livi

The Saturnian plasma sheet as revealed by energetic particle measurements

[1]xa0Since July 2004 Cassini is in orbit around Saturn providing in-situ measurements of the Saturnian magnetosphere. One of the three sensors of the Magnetospheric Imaging Instrument (MIMI) is the Low Energy Magnetospheric Measurement System (LEMMS) that responds to energetic particles which can serve as indicators of key regions (Krimigis et al., 2005) and ongoing plasma processes in the magnetosphere. In this paper we identify and characterize, based on energetic particle and magnetic field measurements, the radiation belts, the plasma sheet, and the lobe region. The transition between plasma sheet and lobe region sometimes occurs very rapidly, and sometimes occurs with the period of Saturns rotation. We explain the highly variable nature of the Saturnian plasma sheet as a combination of the geometry of the Cassini trajectory, together with the variable location of the boundary between open and closed field lines caused by a strong localized magnetic anomaly in the Saturnian field.

Heliospheric energetic particle observations during the October--November 2003 events

[1]xa0The intense level of solar activity recorded from 19 October to 12 November 2003 led to unusually high energetic particle intensities observed throughout the heliosphere. The fleet of spacecraft distributed in the inner and outer heliosphere offers us the opportunity to study both the effects of these events in different regions of the heliosphere and the evolution of the energetic particle intensities measured at different heliocentric radial distances. Observations at 1 AU by the ACE and GOES-11 spacecraft show multiple particle intensity enhancements associated with individual injections of solar energetic particles (SEPs) and with the arrival of shocks driven by coronal mass ejections (CMEs), resulting in a long time interval (∼40 days) of elevated low-energy (<1 MeV) ion and near-relativistic (<315 keV) electron intensities. Observations from the Ulysses spacecraft at 5.2 AU, 6° north of the ecliptic, and 120–90° west of the Earth also showed elevated low-energy ion and near-relativistic electron intensities for more than ∼40 days that were modulated by the effects of recurrent corotating interaction regions (CIRs) and the passage of a fast interplanetary coronal mass ejection (ICME). The Cassini spacecraft at 8.7 AU, 3° south of the ecliptic, and 75–35° west of the Earth saw an intense low-energy ion and near-relativistic electron event in association with the passage of an enhanced magnetic field structure formed by the compression of transient solar wind flows and CIRs. The prompt component of the SEP event at Cassini was largely reduced due to the modulating effect of intervening transient flows propagating between the Sun and the spacecraft. The Voyager-2 spacecraft at 73 AU and 25° south of the ecliptic did not observe these events until April 2004. The arrival of a merged interaction region (MIR) at Voyager-2 produced a ∼70-day period with elevated <17 MeV proton and <60 keV electron intensities. Particle fluences computed over the duration of the events at each spacecraft show a radial dependence that decays more slowly than that expected from a simple model assuming adiabatic cooling of an isotropic particle population uniformly distributed in a shell symmetrically expanding at the solar wind speed. Although the SEP events were observed throughout the heliosphere, both (1) the solar particle injections occurring at different times and longitudes, and (2) the marked differences in the interplanetary stream structures propagating toward different longitudes resulted in distinct time-intensity histories at each spacecraft, and therefore periods with equal particle intensities were not observed by this fleet of spacecraft.

Evidence of Enceladus and Tethys microsignatures

We present evidence of two icy satellite microsignatures in the Cassini LEMMS data. Just upstream of Enceladus, a deep and narrow decrease in the flux of several MeV electrons is consistent with a recent absorption by that moon. This microsignature is collocated with a deep depletion in the MeV proton flux. The proton feature is much wider than the satellite diameter, suggesting multiple interactions and/or losses to the E Ring and neutral gas. An observed increase in proton flux toward the planet suggests a possible inner magnetospheric source. A decrease in the low energy electron intensity downstream of Tethys is also consistent with a microsignature approximately the size of the satellite that has drifted slightly toward the planet near midnight. Modeling suggests that microsignatures near Tethys orbit would persist for more than a complete rotation of Saturn and the radial diffusion coefficient is about 10 -9 R 2 S /s.

Leakage of energetic particles from Jupiter's dusk magnetosphere: Dual spacecraft observations

[1]xa0For the first time, two spacecraft, Galileo and Cassini, observed Jupiters magnetosphere simultaneously for nearly half a year between October 2000 and March 2001. This provided an unprecedented opportunity to disentangle spatial and temporal aspects of the dynamics of the Jovian magnetosphere. In this paper we report new results on the source of the leakage of energetic particles (electrons with energy 15 keV to several MeV and ions with energy > 30 keV) from the dusk side of the magnetosphere. The dual spacecraft measurements show clearly that magnetospheric particles leak directly into the interplanetary medium from the closed magnetosphere, and are the source for the “upstream” particle events [Baker et al., 1996; Zwickl et al., 1981; Krimigis, 1992; Haggerty and Armstrong, 1999; Anagnostopoulos et al., 1998] that have been reported from instruments during prior single spacecraft encounters with the planet. These events, consisting of high-energy particles of Jovian origin, have been observed throughout the heliosphere [Baker and Van Allen, 1976] and their propagation has recently been modelled [Fichtner et al., 2000; Ferreira et al., 2001]. Jupiter then is an important contributor to the interplanetary charged particle fluxes, especially within an astronomical unit of the planet.

An Interstellar Neutral Atom Detector (INAD)

Direct detection of interstellar neutrals is a powerful technique for enlarging our knowledge about the media surrounding our solar system. We present in this paper a combination of two telescopes and a pointing device that would enable precise and detailed measurements of the density, velocity, temperature, and composition of the neutral particles that penetrate through the heliospheric bow shock and the heliopause.

Miniaturized electron magnetic spectrometer

Abstract Traditionally magnetic electron spectrometers provide the most reliable electron measurements in space at energies above ∼10 keV. However, the inclusion of powerful magnets presents problems for spacecraft with stringent magnetic cleanliness requirements, and the magnetic yoke required to close the instrument is bulky and heavy. To mediate the aforementioned shortcomings, we report a preliminary conceptual design on a new miniature magnetic electron spectrometer that measures energetic electrons from 50 keV to 1.5 MeV. The new detector covers close to 360° in the azimuthal direction and ± 3° off the plane, and if mounted on a spinning spacecraft, will measure the full spherical angular distribution twice per spin. This represents a very large advance over previous designs, which typically measure a very limited angular cone at any one time. As a result of the placement of the magnets in this design, the magnetic flux closes intrinsically, hence no bulky flux-containing magnetic yoke, typical in previous magnetic electron spectrometer designs, is required. We are funded by a NASA instrument development grant to develop a prototype of this sensor over the next 2 years.