Ring Science Update
Josh Colwell, Richard Jerousek, James Cooney,
Richard Jerousek, Tracy Becker, Kevin Baillié
•
•
•
•
•
Occultation Statistics
Self-Gravity Wakes
Faint and Narrow Ringlets
"Ghosts" in the Rings
Rev 9 Solar Occ Analysis
UVIS Team Meeting June 18-20, 2012, Braunschweig
1
Extracting Information from
Statistical Moments in UVIS
Stellar Occultation Data
Data Analysis Updates
• We have scrubbed a number of minor
issues in calculation of the area of the
region in the ring plane from which
light is collected: included stellar
diameters; used spectral type to
adjust effective wavelength for each
star; put semi-circular end-caps on
each track.
• We have scrubbed a handful of occs
with problematic calibration.
UVIS Team Meeting June 18-20, 2012, Braunschweig
3
UVIS Team Meeting June 18-20, 2012, Braunschweig
4
BetCen77_1
BetCen077_1
C ring
A ring
B ring
Cassini Division ramp
looks like inner A ring and
not like the rest of the
Cassini Division
So we see different "populations" in
different regions of the rings reflected by
their different autocorrelation lengths.
But what do these lengths signify?
• In the cartoon picture, autocorrelation length
is determined by a mean particle size.
• Organization of particles (for example in selfgravity wakes) introduces additional length
scales that may be geometry-dependent.
• Any structure can introduce excess variance
which is interpreted as a "larger"
autocorrelation length scale.
UVIS Team Meeting June 18-20, 2012, Braunschweig
7
Variations in Autocorrelation
Length
• Autocorrelation length is calculated by
comparing the excess variance to the size
of the area from which light is collected
each integration period.
• Pick featureless regions of the rings that
have low optical depth for analysis: C ring
83,000 km and C ring plateau P5.
• Not all occs give the same autocorrelation
length for these regions.
UVIS Team Meeting June 18-20, 2012, Braunschweig
8
C Ring autocorrelation lengths at
different values of elevation angle.
C Ring autocorrelation lengths at
different values of elevation angle.
AC Length correlated
with optical depth in
main C ring, but
drops in plateaus
(which have higher
optical depth).
Small-scale structure
is tracking with
optical depth, or
segregation of
particles by size is
occuring in main C
ring.
Cassini Division Ramp
Model curves (solid)
and data show at
least three distinct
particle "populations"
in the C ring and
Cassini Division.
C Ring Ramp
C Ring Plateaus
UVIS Team Meeting June 18-20, 2012, Braunschweig
12
UVIS Team Meeting June 18-20, 2012, Braunschweig
13
UVIS Team Meeting June 18-20, 2012, Braunschweig
14
UVIS Team Meeting June 18-20, 2012, Braunschweig
15
UVIS Team Meeting June 18-20, 2012, Braunschweig
16
UVIS Team Meeting June 18-20, 2012, Braunschweig
17
UVIS Team Meeting June 18-20, 2012, Braunschweig
18
UVIS Team Meeting June 18-20, 2012, Braunschweig
19
UVIS Team Meeting June 18-20, 2012, Braunschweig
20
UVIS Team Meeting June 18-20, 2012, Braunschweig
21
UVIS Team Meeting June 18-20, 2012, Braunschweig
22
A Ring autocorrelation lengths at
different values of elevation angle.
Occ Stats Summary
• Persistent correlation of autocorrelation
length scale with line-of-sight distance.
• At larger LOS:
–see larger piece of rings in each measurement;
–projected star disk in ring plane is bigger;
–increased probability that the star is faint.
UVIS Team Meeting June 18-20, 2012, Braunschweig
24
VIMS/UVIS Self Gravity Wake Results
Josh Colwell and Richard Jerousek
Self-Gravity Wakes
crκ
Q≈
1
Gσ
λcrit = 4π Gσ / κ ≈ 1− 100 m
UVIS Team Meeting June 18-20, 2012, Braunschweig
2
2
26
Azimuthal View Angle
Dependence on Opacity
UVIS Team Meeting June 18-20, 2012, Braunschweig
27
Self-Gravity Wake Properties
UVIS Team Meeting June 18-20, 2012, Braunschweig
28
Granola Bar Model (Colwell et al.)
Pasta Model
(Hedman et al.)
UVIS Team Meeting June 18-20, 2012, Braunschweig
29
Pasta Model Parameters: OccultaCons with Any B Angle.
Pasta Model Parameters: OccultaCons with Any B Angle.
Pasta Model Parameters: OccultaCons with Any B Angle.
Comparison Between α Scorpii Occultatons
Comparison Between α Scorpii Occultatons
In order to find the magnitude of the error in our determinaCon of T, differences in I between either side of the trans-­‐Encke region, ΔI, were found and compared to the change in T between occultaCons at various viewing geometries at a radial locaCon of r=134600km, in the trans-­‐Encke region.
Error bars were calculated by:
ΔT=ΔI/I0xT
The magnitude of the systemaCc error compared to the scale on increasing T with increasing B angle seems to indicate that instrumental ramp up does not play a significant role in the determinaCon of self gravity wake parameters.
The distribuCon of ring opening angles, φ, for each set of occultaCons is ploZed below for r=95,000km. Differing distribuCons in viewing geometry may sCll play a role in the discrepancy between best-­‐fit pasta model parameters between each set.
The distribuCon of ring plane incidence angles, B, for each set of occultaCons.
Pasta Model Parameters: OccultaCons with 15<B<50 Angle.
Pasta Model Parameters: OccultaCons with 15<B<50 Angle.
Pasta Model Parameters: OccultaCons with 15<B<50 Angle.
AnalyCc Granola Bar Model Parameters: All OccultaCons
S-­‐G Wake Summary
• Something strange is going on.
• Differences between VIMS and UVIS do not seem to be (enCrely) due to:
–UVIS ramp-­‐up behavior
–observaConal geometry differences between VIMS and UVIS
• We cannot explain remaining opCcal depth differences between UVIS and VIMS observaCons of Alpha Scorpii (Antares). Upcoming joint occ of Sirius will give us another check.
42
Faint and Narrow Rings
• Systematic search for ringlets in gaps
throughout the ring system.
• Several ringlets detected, but
generally only in a handful of
occultations.
• Working on assembling everything
into a coherent presentation.
UVIS Team Meeting June 18-20, 2012, Braunschweig
43
Ghosts Update
• Paper submitted to Nature June 4.
Returned unreviewed yesterday.
• Reformatting for either GRL or Icarus.
UVIS Team Meeting June 18-20, 2012, Braunschweig
44
Goal: Determine the Particle sizes responsible for forwardscattered light from F Ring during mis-pointed solarocc Rev 9
Core
Strands
Lines represent the time at which the solar port boresight
would cross the strands and F Ring core if strands are
located 150 km to either side of the core
Detection is likely from the particles in the Strands
Model the Occultation
Determine when FOV is over rings (lights up in plot)
Optical depth is a continuous function of distance from
core
Sensitivity of the FOV is a continuous function of distance
from boresight
UVIS Solar Port FOV Sensitivity
Across the Slit
Along the Slit
Fit the sensitivity with linear and polynomial fits, depending
on ‘x’ & ‘y’ distance from the Solar Port Boresight
Data from Solar Scans in 2003, 2006, & 2007
2003 T Scan of Sun
2006 Solar Scan Along Slit
Applying Model to the Data
Ignore - C Kernel issue
- closed “shutter”?