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.
SOI - 2 year
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.
CAPS will be on and operating in a ride-along mode during Saturn insertion.
Plan to obtain pitch angle distributions using ACT.
SOLAR WIND, MAGNETOPAUSE AND BOWSHOCK
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:
12±2 hr. in LT; > 40 Rs, ±30 deg lat.
s/c: z-axis perp. sun-Sat. line in any direction.
12±1 hr. in LT; 25-40 Rs, ± 15deg latitude.
s/c orientation: z-axis perp. sun-Saturn line.
12±1 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
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 ~ ½ of the phase space survey can be achieved as
ride-along during DFPW or ORS modes.
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.
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
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.
Current definition OK (> 60 deg lat.,< 8 Rs, 08-14 LT)
Spacecraft orientation same as for auroral zone.
Suggest using < 8 Rs and ±1.5 Rs above/below equatorial plane (rather than
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.
RING CONNECTION REGION
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.
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.
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).
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.