CAPS: Cassini Plasma Spectrometer


Photograph from D. T. Young et al. of the CAPS flight model prior to delivery to the Cassini spacecraft.



About CAPS

The Cassini Plasma Spectrometer (CAPS) onboard the Cassini spacecraft was one of multiple fields and particles instruments tasked with exploring Saturn's magnetosphere. CAPS was comprised of a triumvirate of distinct sensors intended to study the full variety of plasma phenomena in the Saturn system, producing comprehensive all-sky mass-resolved measurements. The need for three separate sensors was driven by the dynamic nature of Saturn's magnetosphere and the wide range of measurement objectives laid out for CAPS. The CAPS instrument itself represents a collaboration among several institutions. The institutions who contributed to the construction of the instruments included the Southwest Research Institute, Los Alamos National Laboratory, the Centre d'Étude des Environnements Terrestre et Planetaires, the Goddard Space Flight Center, Mullard Space Science Laboratory of University College London, the University of Oulu and the University of Virginia.
Scientific objectives for CAPS include the following:
  • Characterize the multiple sources and sinks of plasma in the Saturnian system
  • Investigate plasma transport within and the dynamics of Saturn's magnetosphere
  • Determine the mechanisms leading to the generation of aurorae at Saturn
  • Investigate the composition of Saturn's rings and icy satellites by measuring the sputtering-generated plasmas coming from these bodies
  • Characterize the ionosphere of Titan and the detailed interactions between Titan and the Saturnian magnetosphere

CAPS detectors:

  • Ion Mass Spectrometer (IMS): Measures Low-concentration ion species and hot, diffuse magnetospheric plasmas
  • Electron Spectrometer (ELS): Measures velocity distributions of electrons and ions
  • Ion Beam Spectrometer (IBS): Measures Well-defined ion beams in the solar wind out near Saturn, highly directional ion fluxes in Titan's ionosphere and field-aligned auroral fluxes

CAPS instrument characteristics *

             IMS ELS IBS
Energy/charge Response      
        range (eV/e-) 1–50280 0.6–28750 1–49800
        resolution(ΔE/E)FWHM 0.17 0.17 0.014
Angular Response      
        # of elevation sectors 8 8 3
        instantaneous FOV (AZ X EL)FWHM 8.3º x 160º 5.2º x 160º 1.4º x 150º
        angular resolution (AZ X EL)FWHM 8.3º x 20º 5.2º x 20º 1.4º x 1.5º
Temporal Response      
        per sample (s) 6.25 x 10-2 3.125 x 10-2 7.813 x 10-3
        energy-elevation (s) 4.0 2.0 2.0
        energy-elevation-azimuth (s)   180  
Mass/charge Response Medium Res.   High Res.    
        range (amu/e-) 1 ~ 400           1 ~ 100 - -
        resolution(M/ΔM)FWHM 8                     60 - -
        energy-geometric factora (cm2 sr eV/eV) 5 x 10-3           5 X 10-4 1.4 x 10-2 4.7 x 10-5
aapplies to total field-of-view and includes efficiency factors
*values taken from Table I of Young et al. (2004) in Space Science Reviews

CAPS featured the Electron Spectrometer (ELS) (see Linder et al. 1998), the Ion Mass Spectrometer (IMS) and the Ion Beam Spectrometer (IBS), each of which were used to make in-situ measurements of various portions of the plasma population at Saturn. The ELS was used to measure the velocity distribution of electrons with energies between ~1 eV and 28.25 keV, covering the range expected for thermal electrons from Titan and near the ring plane. The IMS was designed to measure low-concentration ion species and hot, diffuse magnetospheric plasmas. It was sensitive to ions at energies between 1 eV and 50.28 keV and with an atomic resolution M/ΔM ~70. The IBS was intended to measure well-defined ion beams in the solar wind out near Saturn, highly directional ion fluxes in Titan's ionosphere and field-aligned auroral fluxes. It measured ion velocity distributions at high angular and energy resolution from 1 eV to 49.8 keV.
The engineering details of the CAPS instrument and the science objectives it was built to address are described in further detail in the Space Science Reviews paper by Young et al. (2004).

CAPS Data

Data Search Tools

  • Preliminary Fields and Particles and Auroral Schedules to help find data at the planned observation dates
  • The Event Calendar is an interactive event finding-tool that can be used to search for data associated with particular events.
  • The Master Schedule is a time-ordered listing of observations by all instruments. This can be used to find data based on particular events.

Browse Uncalibrated Data Products on PDS

How uncalibrated data is organized:

  • Split by mission phase (Cruise, Earth, Jupiter, Saturn), then
  • By year, date range, year+day-of-year
Then, for each day, the first few characters of the file name will indicate the contents:
  • ACT: actuator angle, measured every 1 or 8 s
  • ANC: ancillary files providing position, velocity, and orientation information
  • ELS: electron spectrometer data (counts per accumulation)
  • IBS: ion beam spectrometer (counts per accumulation for each of 3 anodes)
  • LOG: rarely used.
  • IMS data is in three different file types:
    • ION: several datasets for different ion species—where ion counts are partitioned by species
    • SNG: ion counting data, in counts/accumulation for each of 8 IMS anodes
    • TOF: time-of-flight ion data, gathered over longer intervals on 1 anode

Browse Calibrated Data Products on PDS

How calibrated data is organized:

  • Split by year, day-of-year
Then, for each day, the first few characters of the file name will indicate the contents:
  • ELS: electron spectrometer data (counts per accumulation)
  • IBS: ion beam spectrometer (counts per accumulation for each of 3 anodes)
  • IMS data is in four different file types:
    • ION: several datasets for different ion species—where ion counts are partitioned by species
    • SNG: ion counting data, in counts/accumulation for each of 8 IMS anodes
    • TOFLEF: Ion Mass Spectrometer 'time of flight' linear electric field data
    • TOFST: Ion Mass Spectrometer 'time of flight' straight through data product

Derived Data Products

Selecting Data Products

  • List of Times associated with Titan and Icy Satellites encounters may help to find a time frame of interest.
  • ELS/IBS/IMS browse plots (under the time on each plot), or MPQRG (mission planning quick reference guide) will provide basic geometric information. The directory containing ELS, IBS and IMS browse plots is sorted by target, then year and day.
  • List of Times contains information about when CAPS was operating at a high data rate.
  • Users can find specific observations using the Events Calendar

Analyzing CAPS Data

  • Calibration Documentation and Data describes how to calibrate CAPS data and provides examples. This will be needed for users who download the uncalibrated CAPS data. Also see CAPS CALIB and CAPS_USER_GUIDE
  • CAPS Time of Flight Data Analysis Procedures [PDF] The purpose of this document is to provide a guide to the reduction and analysis of time-of-flight (TOF) data generated by the Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer (IMS) which is one of three CAPS sensors
  • SPICE WebGeocalc allows users to search for observation computations by geometry. Be aware that since CAPS was able to articulate its pointing relative to the spacecraft, SPICE will display this information as well.




For questions and comments, visit the PDS Cassini Contact Page