March 19, 2015

Scientific Results obtained with MAPS experiments set

MAPS work group

Saturn's magnetosphere is a complex, multiphasic environment, in which the components (solid surfaces, neutral gas, plasma) are closely linked. This characteristic makes it necessary to carry out the combined analysis of different magnetospheric parameters measured individually by certain of Cassini's instruments. The MAPS (Magnetosphere and Plasma Science) interdisciplinary study group of the Cassini-Huygens mission was therefore set up with the aim of highlighting a multi-instrumental view of the whole of the magnetospheric observations and of thereby maximising the scientific benefits of the mission. The principal responsibility for the MAPS consortium was shared between Tamas Gombosi (University of Michigan) and Michel Blanc (IRAP, Toulouse and Ecole Polytechnique, Palaiseau). This activity is supported by CNES.

The instruments participating in this consortium are the instruments for measuring the populations of plasma, neutral gases, and dusts, magnetic fields, and the following waves:

  • CAssini Plasma Spectrometer (CAPS) measures low energy plasma, ions and electrons using an ion mass spectrometer (IMS), an ion beam spectrometer (IBS), an electron spectrometer (ELS) and a motorised actuator scanning the angular space.
  • Magnetospheric IMaging Instrument (MIMI) measures high energy plasma. The instrument comprises 3 detectors: the sensor measuring the distributions of charged particles (LEMMS, Low Energy Magnetospheric Measurement System), the charge-energy-mass spectrometer (CHEMS, charge-energy-mass-spectrometer) and the Ion and Neutral CAmera (INCA).
  • MAGnetometer (MAG) studies the magnetic field of Saturn and the interaction between the magnetosphere and the solar wind.
  • Radio and Plasma Wave Science (RPWS) measures the plasma waves and radio emissions: electrical (from 1 Hz to 16 MHz) and magnetic fields (from 1 Hz to 12.6 kHz).
  • Ion and Neutral Mass Spectrometer (INMS) measures the ions and neutral particles in proximity to Saturn, Titan and the moons in order to study their atmosphere and ionosphere.
  • Cosmic Dust Analyser (CDA) measures grains of ice and dust in Saturn's environment and analyses their composition.
  • UltraViolet Imaging Spectrograph (UVIS) measures the ultraviolet emissions of the neutral gas populations present in Saturn's environment, which makes it possible to determine and characterise the plasma sources in the magnetosphere.
  • Radio Science Subsystem (RSS) measures the ionospheric density of Saturn.

MAPSKP tool and summarised database

The dynamic of the multi-instrumental MAPS group gave rise to a tool for simultaneous visualisation of the data from the MAPS instruments: the MAPSKP tool (MAPS Key Parameters) developed at IRAP by Etienne Pallier in collaboration with the University of Michigan and CNES. The website provides access to the summarised 'key parameters' plasma data, at low resolution (1 min), allowing for a quick glimpse of the events to be studied within a multi-instrumental framework. Its usage is restricted and it offers three functionalities to the researchers in the MAPS study group:

  1. A function for visualising predefined data sets, which makes it possible to visualise a data set according to its type (trajectory, magnetic field, electronic and ionic plasma parameters, etc.);
  2. An interactive trace function for the various data sets in existence, which makes it possible to choose and combine a group of data to be displayed for a given period of time in order to create multi-instrumental visualisations. The time and intensity scales are modulable in order to target the observation windows around events of interest;
  3. A function for downloading the summarised data.

The MAPSKP tool, through the sharing and instant accessibility of a large amount of data, thereby makes it possible to revitalise collaboration between scientific groups and increase the scientific benefits of the mission.

Multi-instrumental view of the summarised data
Multi-instrumental view of the summarised data (top to bottom) magnetic field via MAG, energetic particles (electrons) from MIMI LEMMS and CAPS ELS, energetic particles (ions) from MIMI LEMMS and CAPS IMS, and radio and wave data from RPWS, over time. Figure obtained using the MAPSKP tool.

Cartography of the whole structure of Saturn's magnetosphere

Saturn's magnetosphere is now being studied in detail - from the microscopic to the macroscopic, and vice versa - by the Cassini-Huygens mission.

At the beginning of 2004, during the probe's planetary approach phase, the distant observations of Saturn's magnetosphere made it possible to observe and study its auroral, radio and ultraviolet emissions, as well as the dust escaping from the environment into the interplanetary medium.

Then, on July 1st, 2004, the orbit insertion around Saturn made it possible to carry out the first mapping of the whole of the environment since the Voyager probes in the early 1980s. Cassini's orbit insertion around Saturn was the most detailed orbit by each of the instruments in the MAPS working group, individually. The totality of the observations from these instruments during this orbit were combined in order to provide a unified description of the large-scale structure of Saturn's magnetosphere, through the identification of the regions that characterise it, as well as their frontiers.

Identification of the magnetospheric regions through which Cassini passed during its orbit insertion around Saturn on June 30th and July 1st, 2004
Identification of the magnetospheric regions through which Cassini passed during its orbit insertion around Saturn on June 30th and July 1st, 2004, a work recently featured on the cover of volume 46 of Reviews of Geophysics. From top: time-energy spectrograph by MIMI LEMMS (electrons and ions), components and intensity of magnetic field by MAG, time-energy spectrograph by CAPS (electrons and ions), time-frequency spectrograph by RPWS, according to time and distance from the planet. © André et al. (2008).

The plasma regions identified during this orbit consist of the ionosphere of Saturn's rings (region 1, within 3 planetary radii), a cold plasma torus (region 2, up to 5-6 planetary radii) in the inner magnetosphere, which extends in a long plasma sheet (region 3) and an outer magnetosphere, explored at high latitude (region 4, beyond 12-14 planetary radii). Each of these regions had been identified during previous missions, but in far less detail.

The physico-chemical and dynamic processes operating in each of these regions were also demonstrated. The inner magnetosphere is characterised by the presence of a strong magnetic field and contains the major sources of plasma and neutral gas in the planetary environment, giving rise to a magnetosphere dominated by the by-products of the water molecule. The extended plasma sheet, where strong electrical currents were observed, is a dynamic region with highly elongated magnetic field lines due to the presence of plasma. It contains a mixture of populations of cold and hot plasma, this mixture resulting from the radial transporting of plasma from the inner magnetosphere outwards. Finally, the outer magnetosphere at high latitude is characterised by the absence of plasma, this being concentrated by the centrifugal force in the equatorial regions of the magnetosphere.