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.
