CAPS Science Observations for tour
Tour 18-5
The following is a brief statement of CAPS science observations by region for the purpose of Cassini science planning discussions. The items apply in general to times when CAPS is designated as the primary instrument and are not exclusive of other (ride along) measurements with the same objectives. No account has been taken of CAPS coordinated measurements with MAPS or ORS.
  1. SOI - 2 year
  2. Solar wind measurements during this interval require orienting s/c Z-axis roughly orthogonal to s/c-Sun line with the Sun within 45° of the -Y axis. Do not roll the s/c during this orientation.
    CAPS will measure interstellar pickup ions when solar wind is not in view.

  3. SOI
  4. CAPS will be on and operating in a ride-along mode during Saturn insertion. Plan to obtain pitch angle distributions using ACT.

  6. The sub-solar magnetopause may be the best candidate area because the garden-hose angle is near 90 deg. Using the CAPS/MAPS analysis tools (S. Maurice), the regions are roughly and arbitrarily defined as:

    Solar wind: 122 hr. in LT; > 40 Rs, 30 deg lat.
      s/c: z-axis perp. sun-Sat. line in any direction.
    Bowshock: 121 hr. in LT; 25-40 Rs, 15deg latitude.
      s/c orientation: z-axis perp. sun-Saturn line.
    Magnetopause: 121 hr. in LT; 20-30 Rs, 15deg latitude.
      s/c orientation: transition to z-axis // sun-Saturn (DFPW mode ok).

    NOTE: Additional observations will be made of these regions at any time the tour permits.

  8. The survey has to supply a sufficiently dense sample of all the regions of magnetosphere. (Give a quantitative estimate.) It also has to contain a good sample of the phase space distributions in each region, including the rest frame pitch angle (pa) distribution and local flow directions. In general this requires nearly 4 pi coverage of the sky obtained with the CAPS FOV (whether articulated or not, the requirement is the same). Two survey categories are envisioned:

    Standard: Much of the spatial and ~ of the phase space survey can be achieved as ride-along during DFPW or ORS modes.


    1. Some survey objectives will require spacecraft pointing control: Co-rotationally aligned flows near dawn and dusk cannot be seen in DFPW mode and require s/c s-axis perp. to Saturn-sun line.
    2. It may be worthwhile to try to get complete survey coverage on a few orbits at a sampling of LT's. This would imply getting more than the "average" amount of pointing control.

  10. Current definition is OK (> 60 deg lat., < 8 Rs, all LT)

    Spacecraft orientation is problematic outside of the DFPW mode because of the need to see both directions along B-field. CAPS can barely pick up both hemispheres using the ACT provided the s/c y-axis is perp. to the local B-direction. Will need control of pointing for at least 3 auroral passes to obtain pitch angles.

  11. SKR
  12. Current definition OK (> 60 deg lat.,< 8 Rs, 08-14 LT)
    Spacecraft orientation same as for auroral zone.

  14. Suggest using < 8 Rs and 1.5 Rs above/below equatorial plane (rather than 2.5°).

    As mentioned for survey, we need to get good phase space coverage (approx. 4π sr) throughout this region. The difference between this and survey is that higher density of high telemetry rate coverage is needed to study plasma sources and sinks in details.

  16. These would be high inclination orbits (say I > 45 deg), < 8 Rs, all lat. along the orbit, all LT.

    Objective is to map bulk plasma and current flows away from and into the equatorial source region. Spacecraft trajectories that roughly parallel B-field lines are ideal. Control of s/c pointing needed to get full pitch angle distribution.

  17. FLYBY's
  18. Based on GLL experience with Jovian satellites and on the possible large extent of interactions between Titan and the external environment (pick-up ions, wakes), it would be desirable to observe for 2 hours (clock time) on either side of any satellite c/a. It is highly desirable to design some flybys that allow observation in ram direction using the ACT. Control of s/c pointing is desired when possible to optimize phase space coverage during approach periods and flyby.

  20. Change definition to a cylinder extending > -15 Rs with a radius of 10 Rs about the saturnian -X axis. Same remarks apply as for survey (4).

  22. Use MAPS Tour Analysis Tool to define roughly expected location of flank regions. Need s/c orientation so that CAPS FOV includes plasma flow direction.

S. Maurice, "CAPS Orbit Analysis Tool (T18-5)", LANL, Nov. 1997.
S. Maurice, "MAPS Tour Analysis Tool (T18-5)", LANL, Nov. 1997.