Cassini MIMI<br>Magnetospheric IMaging

Historical Background Information

NASA's Cassini Mission, managed by the Jet Propulsion Laboratory in Pasadena, California, will perform a comprehensive scientific investigation of all aspects of the planet Saturn. Included among the focuses of this mission are Saturn's interior, its atmosphere, its satellites, its rings, its near planet space environment (called the magnetosphere), and the detailed interactions that occur between all of these elements. The satellite Titan is a particularly key focus, and the Cassini spacecraft will launch a separate probe that will ultimately land of the surface of Titan. More general information about the Cassini mission as a whole, and its overall science goals, can be found at the Jet Propulsion Laboratory.

The Cassini Magnetospheric Imaging Instrument (MIMI) will be one of 12 science instruments on the main (non-Probe) spacecraft and one of 6 instruments designed primarily to investigate the space environments around Saturn and around its satellites. The space environment consists of electromagnetic fields, plasmas (ionized gases), energetic charged particle (ions and electrons), neutral gases, and dust particulates. More general information about the space environment of Saturn and the science investigations designed to probe these environments can be found at the Cassini MAPS: Magnetospere & Plasma Science Working Group.

As described more fully below, the magnetospheric or space environment of a planet interacts in important and interesting ways with the other constituents of a planetary system. Interest in the study of planetary magnetospheres is broadly based. In particular, planetary magnetospheres constitute the few astrophysical plasma environments that are accessible to direct, in- situ measurement. Most astrophysical plasmas (e. g. pulsar environments, astrophysical jets, other x-ray emitting regions such as the crab nebula, stellar atmospheres, etc.) are accessible only to remote sensing, and the detailed physical processes that give rise to the global configurations and behaviors must be guessed. Planetary magnetospheres offer astrophysical plasma environments whereby the global configurations can be imaged with new technologies while at the same time the detailed physical processes can be measured directly in-situ. The study of planetary magnetospheres therefore has broad implications for the study of the behaviors of astrophysical plasmas throughout the Universe. A schematic of a sample planetary magnetosphere is provided here, including references to some of the terminology and regions of space referred to in the discussions given below.

An apparently fundamental characteristic of planetary magnetospheres is that they are prodigious accelerators of charged particles. Within the Earth's magnetosphere the most well-known consequences of this process are the beautiful optical displays of the polar regions called auroras (Northern and Southern lights) and the Van Allen radiation belts that surround the Earth, posing radiation hazards to both astronauts and sensitive equipment. Because of the acceleration properties of magnetospheres, the ions and electrons (charged particles) measured by the MIMI instrument are always significant, and sometimes dominant, components of the particle populations and plasmas that fill planetary magnetospheres. The characterization of the processes that generate aurorae and the radiation belts at Saturn are among the objectives of the MIMI instrument.

Saturn, its atmosphere, rings, moons, and plasma envelope (magnetosphere) are all closely coupled, and consequently interact through the exchange of matter and energy. Planetary magnetospheres are populated primarily by the exchange of matter with the planetary ionospherex, satellites, and rings. Energetic particles and plasmas modify the upper atmospheres (yielding ionization, heating, bulk motions or winds, and chemical modification), satellite surfaces (removing material via, e. g., sputtering and modifying the nature of the surface materials), and rings (again modifying the surfaces and participating in the transport of particulate matter via surface and deep penetration charging effects). Planetary magnetospheres inject fast neutral atoms, plasmas and energetic particles into the interplanetary medium. In turn, the interplanetary medium, via the supersonic solar wind pressures, magnetic fields, etc. help establish the magnetospheric configuration including the bowshock, magnetopause, magnetotail; and it apparently energizes such dynamical events as magnetospheric substorms (global magnetospheric reconfigurations that result from the conversion of electromagnetic energy into charged particle heating and acceleration). At Saturn, Titan presents a dense atmosphere within Saturn's magnetosphere where escape of atmospheric constituents contributes to a neutral gas cloud that is an important, if not dominant, source of plasma for the magnetosphere.

As described elsewhere, the MIMI instrument will measure the energetic charged particle and hot plasma environment of Saturn by means of remote imaging of the hot ions populations and with localized, in-situ measurements. In particular, MIMI will utilize the new technique of global magnetospheric imaging to help the MIMI team achieve its science goals (see Energetic Neutral Atom Imaging for sample images of Saturn's magnetosphere). The MIMI science goals are listed under science objectives.

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