Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan

The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager l spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling’’ and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R _S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge–energy–mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ∼ 2.4 cm² sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5^∘ full width half maximum) over the range 7 keV/nuc < E < 3 MeV/nuc. CHEMS uses electrostatic deflection, TOF, and energy measurement to determine ion energy, charge state, mass, and 3-D anisotropy in the range 3 ≤ E ≤ 220 keV/e with good (∼0.05 cm² sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E≤ 0.884 MeV in the forward direction (G ∼ 0.02 cm² sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ∼ 0.4 cm² sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-α measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ∼60 R _S every 2–3 h (every ∼10 min from ∼20 R _S). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be < 0.1 s, which is more than adequate for microsignature studies. Data obtained during Venus-2 flyby and Earth swingby in June and August 1999, respectively, and Jupiter flyby in December 2000 to January 2001 show that the instrument is performing well, has made important and heretofore unobtainable measurements in interplanetary space at Jupiter, and will likely obtain high-quality data throughout each orbit of the Cassini mission at Saturn. Sample data from each of the three sensors during the August 18 Earth swingby are shown, including the first ENA image of part of the ring current obtained by an instrument specifically designed for this purpose. Similarily, measurements in cis-Jovian space include the first detailed charge state determination of Iogenic ions and several ENA images of that planet’s magnetosphere.This revised version was published online in July 2005 with a corrected cover date.     

A Revisit of the Phobos Events

The magnetic field disturbances detected by the Phobos-2 spacecraft in 1989 have been suggested to be caused by a ring of dust and/or gas emitted from the Martian moon, Phobos. The physical nature of these ``Phobos events' is examined using results from related investigations over the last twenty years. It is concluded that there is no clear evidence at present to support the association of magnetic field disturbances in the solar wind with Phobos. The situation will be further clarified taking advantage of the multi-spacecraft observations of the Yinghuo-1(YH-1), Mars Express and MAVEN missions beginning in 2012. It is expected that many novel features of solar wind interaction with Phobos (and possibly also Deimos) itself will also be revealed.      

The Cassini Ion and Neutral Mass Spectrometer (INMS) Investigation

The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation will determine the mass composition and number densities of neutral species and low-energy ions in key regions of the Saturn system. The primary focus of the INMS investigation is on the composition and structure of Titan’s upper atmosphere and its interaction with Saturn’s magnetospheric plasma. Of particular interest is the high-altitude region, between 900 and 1000 km, where the methane and nitrogen photochemistry is initiated that leads to the creation of complex hydrocarbons and nitriles that may eventually precipitate onto the moon’s surface to form hydrocarbon–nitrile lakes or oceans. The investigation is also focused on the neutral and plasma environments of Saturn’s ring system and icy moons and on the identification of positive ions and neutral species in Saturn’s inner magnetosphere. Measurement of material sputtered from the satellites and the rings by magnetospheric charged particle and micrometeorite bombardment is expected to provide information about the formation of the giant neutral cloud of water molecules and water products that surrounds Saturn out to a distance of ∼12 planetary radii and about the genesis and evolution of the rings.The INMS instrument consists of a closed ion source and an open ion source, various focusing lenses, an electrostatic quadrupole switching lens, a radio frequency quadrupole mass analyzer, two secondary electron multiplier detectors, and the associated supporting electronics and power supply systems. The INMS will be operated in three different modes: a closed source neutral mode, for the measurement of non-reactive neutrals such as N₂ and CH₄; an open source neutral mode, for reactive neutrals such as atomic nitrogen; and an open source ion mode, for positive ions with energies less than 100 eV. Instrument sensitivity is greatest in the first mode, because the ram pressure of the inflowing gas can be used to enhance the density of the sampled non-reactive neutrals in the closed source antechamber. In this mode, neutral species with concentrations on the order of ≥10⁴ cm⁻³ will be detected (compared with ≥10⁵ cm⁻³ in the open source neutral mode). For ions the detection threshold is on the order of 10⁻² cm⁻³ at Titan relative velocity (6 km sec⁻¹). The INMS instrument has a mass range of 1–99 Daltons and a mass resolutionM/ΔM of 100 at 10% of the mass peak height, which will allow detection of heavier hydrocarbon species and of possible cyclic hydrocarbons such as C₆H₆.The INMS instrument was built by a team of engineers and scientists working at NASA’s Goddard Space Flight Center (Planetary Atmospheres Laboratory) and the University of Michigan (Space Physics Research Laboratory). INMS development and fabrication were directed by Dr. Hasso B. Niemann (Goddard Space Flight Center). The instrument is operated by a Science Team, which is also responsible for data analysis and distribution. The INMS Science Team is led by Dr. J. Hunter Waite, Jr. (University of Michigan).This revised version was published online in July 2005 with a corrected cover date.     

The ionospheres and plasma tails of comets

The present understanding of cometary ionospheres and plasma tails is critically evaluated. Following a brief introduction of the significance of the study of cometary ionospheres and tails (Section 1), the observational statistics and spectroscopic observations are summarized in Sections 2 and 3.The complicated and time varying morphology of the plasma tail and the ionosphere as revealed both by photographs as well as visual drawings is discussed in Section 4.The evidence for a strong comet-solar wind interaction, the possible nature of this interaction and also the use of comets as probes of the solar wind are considered in the next 3 sections (5, 6, 7). This is followed by a discussion of the various processes so far proposed for the ionization of cometary gases and their limitations (Section 8).Hydrodynamic models of the solar wind-comet interaction, which refers essentially to the region outside the tangential discontinuity, are presented and evaluated in Section 9. A discussion of the ion chemistry and structure of the region inside the tangential discontinuity (which is here referred to as the cometary ionosphere) follows in Section 10.The largely indirect evidence for the existence of substantial magnetic fields in cometary ionospheres and type 1 tails is evaluated and their likely origin is considered in Section 11. The associated electric currents; their size and closure as well as their importance as sources of ionization in the inner coma are also discussed.Finally in Section 12, some of the directions in which future research should progress, in order to provide a more complete and secure knowledge of cometary ionospheres and plasma tails, are stressed.     

Physical studies of the planetary rings

In this review paper, the physical properties of the Saturnian and Uranian rings as derived from ground-based observations are first discussed. Focus is then shifted to the study of the orbital dynamics of the ring particles. Numerical simulations of the evolutionary history of a system of colliding particles in differential rotation together with theoretical modelling of the inelastic collision processes are surveyed. In anticipation of the information returned from in situ measurements by space probes, interactions of the planetary rings with the interplanetary meteoroids and planetary magnetospheres are briefly considered. Finally, models of planetary ring origin are examined. In this connection, some recent work on the satellite resonant perturbation effects on the ring structure are also touched upon.     

Seasonal patterns of condensation and sublimation cycles in the cryptic and non-cryptic regions of the South Pole

The South Pole of Mars is characterized by an asymmetric residual ice cap composed of water ice and CO₂ ice. On the opposite side of the residual cap, there exists an area called cryptic region which is relatively free of ice during summer time. Many fan-shaped km-scale structures apparently caused by a wind-blown system of dust-laden gas jets occurred dozens degrees of Ls before the complete sublimation of the CO₂ frost layer. We have examined the seasonal cycles of condensation and sublimation in the cryptic and non-cryptic regions by using the topographic data from the MOLA/MGS measurements. Using the MOLA topography data collected over one Martian year (1999–2001), we have studied the temporal elevation change and the seasonal cycle of the carbon dioxide frost on the southern polar caps. We have produced mapping of the seasonal CO₂ frost thickness variation for seven Ls (30°, 60°, 90°, 120°, 150°, 180°, 210°, 240°, 270° and 330°). It is found that the time variations of the CO₂ frost thickness in these two regions are quite similar. The greatest thickness of the CO₂ frost layer is about 0.76–0.78 m in both places occurs at Ls = 150°.     

Threshold Control for Automatic Detection in Radar Systems

In automatic detection in radar systems an estimate of background clutter power is used to set the detection threshold. Usually detection cells surrounding the cell under test for the presence of a target are used to estimate the clutter power. In the research reported herein, the target location is taken to be uncertain and thus returns from a target could corrupt this clutter power estimate. It is shown how the threshold should be varied to compensate for the resulting degradation in detection performance. The threshold control procedure is based on a priori information about target location that could be supplied by the radar's tracking system. In addition, a simple procedure for calculating detection and false alarm probabilities for Swerling II target models is presented.     

A surface thermal model and exospheric ballistic transport code of planet Mercury

The atmosphere of Mercury is of an exospheric nature. Its formation is due to several physical mechanisms including meteoroid impact, surface sputtering by solar wind ions and photon sputtering by solar UV radiation. The molecules and atoms emitted from the surface materials of Mercury include H, He, O, Ar, and S, etc. It is important to study their spatial distributions across the planetary surface via ballistic random walk. We have developed a surface thermal model coupled with Hodges-type Monte Carlo calculations to simulate the exosphere of Mercury, which will be a major scientific target of the BepiColombo mission of ESA and JAXA.