Cassini MAG: Magnetometer


PIA06430 shows the dynamic atmosphere of Saturn's icy moon Enceladus that was detected by the magnetometer.



About MAG

The Cassini Magnetometer (MAG) measured the three vector components and magnitude of the magnetic field in Saturn's magnetosphere and interplanetary space. MAG consists of two separate sensors, a Fluxgate Magnetometer (FGM) and a Vector/Scalar Helium Magnetometer (V/SHM). The V/SHM was built at the Jet Propulsion Laboratory, Pasadena, CA, while the FGM was built and managed at Imperial College, London, UK.
Scientific objectives for the MAG include the following:
  • Measure the magnetic field generated in Saturn's interior
  • Characterize Saturn's global magnetospheric magnetic field
  • Understand the coupling between the magnetosphere and ionosphere
  • Characterize the interaction between the icy moons and Saturn's magnetosphere
  • Determine the magnetic state of Titan and characterize its magnetospheric interaction

MAG detectors:

  • Fluxgate Magnetometer (FGM): measured magnetic field direction and magnitude. This is the primary magnetometer used by the mission because it gathered data during the entire Saturn tour.
  • Vector Helium Magnetometer (VHM): measured magnetic field direction and magnitude. This detector failed in November, 2005.
  • VHM Scalar Mode (SHM): high-accuracy measurements of magnetic field magnitude while FGM was measuring vector magnetic field. This detector failed in November, 2005.
MAG Instrument Characteristics *
Sensor FGM VHMa SHMa
  Fluxgate
Magnetometer
Vector Helium
Magnetometer
VHM Scalar Mode
Downlink Data Rate 32 vectors/s 2 vectors/s 1 (values/s)
Dynamic Range Bins R0 R1 R2 R3 R0 R1
Dynamic Range (nT) ±40 ±400 ±10000 ±44000 ±32 ±256 256–16384
Resolution (nT) 4.9 48.8 1200 5400 0.0039 0.0312 0.0360
a the helium magnetometer (VHM and SHM) failed in November 2005
* includes values taken from Tables III, IV and V of Dougherty et al. (2004) in Space Science Reviews

The instrument returned nearly continuous magnetic field measurements throughout the entire Cassini mission and part of interplanetary cruise on the way to the Saturn system. The magnetometers were mounted on an 11 m boom to minimize interference from spacecraft generated fields. The V/SHM was mounted at the end of the boom and the FGM at the mid-point. The goal of two separate sensors was to provide two independent measurements of the local field for cross-calibration.
The helium magnetometer has two modes of operation, a vector mode (VHM) that measures the magnetic field direction and magnitude and a scalar mode (SHM) that measures the magnetic field magnitude only. The scalar mode of operation was intended to provide highly accurate measurements of the magnetic field magnitude while the FGM simultaneously measured the vector magnetic field. It was intended to be used in strong magnetic fields close to Saturn to make an accurate determination of Saturn's internal planetary field. (The helium magnetometer failed on November 17, 2005 thus data from this sensor are not available after this time.)
The magnetometer acquired data almost continuously throughout the mission, although the data rate varied based on available resources (solid state recorder space and downlink capability). The maximum data rates for each of the modes are as follows: FGM (32 vectors/s), VHM (2 vector/s) and SHM (1 value/sec).
The User's Guide includes detailed information on instrument characteristics, how to search for, calibrate and transform MAG data into various coordinate systems of interest. It also includes step-by-step instructions of how to replicate examples from the published literature. The engineering details of the MAG instrument and the science objectives it was built to address are described in further detail in the Space Science Reviews paper by Dougherty et al. (2004).

MAG 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.

Calibrated Data

Calibrated magnetometer data that has been transformed into several different geophysical coordinate systems is available on PDS. Data is available in RTN (Radial Tangential Normal) coordinates for interplanetary cruise data and KRTP (Kronocentric body-fixed, J2000 spherical Coordinates), KSM (Kronocentric Solar Magnetospheric Coordinates), KSO (Kronocentric Solar Orbital Coordinates) for the Saturn orbits. For more on coordinate systems, see the MAG data-set information page, or download the MAG User's Guide [PDF].
These data files include the magnetic field vector components and magnitude as well as basic trajectory information appropriate for the particular coordinate system.
Calibrated magnetometer data is available on PDS at the full time resolution as well as 1-minute and 1-second averages:
  • Full Resolution, organized by year and by a set of days, with the indicated coordinate data for that set of days—for example, for 2003 days 91–120 in RTN coordinates, DATA/2003/091_120_APR/03110_FGM_RTN/
  • One-Second Averages, organized by year and then by days, with multiple coordinate systems available—for example, for 2006 days 36–68, DATA/2006/06036_06068_21_FGM_RTN_1S
  • One-Minute Averages, organized by year, with multiple coordinate systems available—for example, DATA/2005/2005_FGM_KSM_1M/
Calibrations required by the magnetometer are described in detail in the MAG User's Guide [PDF]. Calibrations for the V/SHM consist of subtracting zero levels, multiplying by an orthogonality-sensitivity matrix, rotating from the sensor coordinates to the spacecraft coordinates, and subtracting spacecraft generated fields. A precise determination of the alignment of the magnetometer sensors with respect to the spacecraft body is accomplished by use of the Science Calibration Subsystem (SCAS) calibration coils. The SCAS consists of two coils rigidly mounted on the spacecraft body with a known alignment to the spacecraft axes. (SCAS calibration activities are described here)

Uncalibrated MAG Data

  • Uncalibrated Magnetometer Data is available on the PDS Planetary Plasma Interactions node. The MAG User Guide also provides a detailed description of the calibration procedure. Calibration files and software tools to perform the calibration are also described in the User Guide and provided on PDS.

Derived Data Products

Data in moon-centered coordinates for all targeted moon flybys, including the magnetic field components and magnitude, spacecraft trajectory vector, and distance.
  • Titan (Temporarily unavailable)
  • Enceladus (Temporarily unavailable)
  • Dione (Temporarily unavailable)
  • Rhea (Temporarily unavailable)
  • Tethys (Temporarily unavailable)
  • Iapetus (Temporarily unavailable)
Complete set of Conjugate Maps (Temporarily unavailable) of Cassini spacecraft in the Saturnian magnetosphere, based on an empirical magnetosphere model.

Selecting Data Products

Comprehensive lists of events may be useful in selecting a timeframe to study. These events are available within the Event Calendar, or directly from the files below.
Both the calibrated and uncalibrated magnetometer data are archived in the PDS. For most researchers, unless a specific reason is identified to recalibrate the data, the calibrated data will be most useful.

Analyzing MAG Data

The calibrated magnetometer data in units of nanoteslas is available on the PDS in text format. Data files contain the 3 vector components, the magnitude of the magnetic field and basic trajectory information (range, latitude, longitude, local time) in the relevant coordinate system.
Once data products have been selected and retrieved, users will need appropriate software to read, manipulate and display that data. Software packages that will enable users to make use of MAG data are listed below.


For questions and comments, visit the PDS Cassini Contact Page