Small-scale Ring Structure
Observed In Cassini UVIS
Occultations
M. Sremčević
L.W. Esposito
LASP, CU Boulder
J.E. Colwell
University of Central Florida
2009 AGU Fall Meeting
San Francisco, December 18, 2009
Miodrag.Sremcevic@lasp.colorado.edu
Cassini UVIS stellar occultations
●
●
High Speed Photometer (HSP) 110 – 190nm
>100 stellar occultations of the rings
●
1 - 8ms sampling rate (mostly 1ms used)
●
Rings radial resolution from few to ~50m
●
Signature of structure down to meter scale
(1) A, B rings: self-gravity wakes (<50m)
(2) A, B rings: overstable waves (100-200m)
(3) B ring: irregular axisymmetric (~100m)
(4) C ring: weak structure (<few 10m)
2/12
Example occultation (β Cru Rev98)
NOISE
LEVEL
F RiNG
B RiNG
C RiNG
A RiNG
Throughout main rings (C, B, Cassini, A rings) the noise by
far exceeds expected (Poisson) values.
3/12
β Persei
occ. track
“turnaround point”
=tangent to rings
= ∞ radial resolution
Orbital
motion
~20km/s
For highest resolution:
Match occultation track azimuthal
motion to the orbital motion of the rings.
Occultation moves as ring particles.
Background: synthetic image from a UVIS occultation
A ri
ng
Special occultation geometry
4/12
β Persei Rev116 occultation
At occultation turnaround (R=131,436km):
●
●
Radial resolution (in 1ms) dR < 1cm
Ring orbital motion
= 17.0 km/s
Occultation azimuthal speed = 16.2 km/s
==> Azimuthal resolution dL =~ 80cm
●
Projected star diameter =~ 20cm
●
Fresnel zone =~ 300cm
5/12
β Persei Rev116 light curve
Stellar level
~transparent
gap
~opaque
SG wake
6/12
Optical depth histogram
7/12
Size of the SG wakes and gaps
8/12
(2) Overstable waves in inner A ring
Wavelet transform
Coadded wavelet
Optical depth
9/12
Overstable waves time series
~150m
10/12
Overstable waves autocorrelation
UVIS
Local
N-body
simulation
(H. Salo)
11/12
Summary
First time directly resolved A ring SG wakes
● Unique occultation with 80cm resolution
(only diffraction limited)
● ~30% of the ring is transparent (τ<0.05)
~30% of the ring is opaque
(τ>1.5)
~40% of the ring in intermittent state
● Opaque wakes are seen as large as 200m
Purely transparent regions are bit shorter
● Only handful more opportunities until 2017:
highest observation priority!
12/12
Extra Slides
What is no structure signature?
[p r r −⟨p⟩][pr −⟨p⟩]
∑
C r =
2
∑ [p r −⟨p⟩]
●
●
No structure == no correlations
(stellar counts are independent and only the
medium occulting the star can induce correlations)
Anything different from 0 in the correlation plot
indicates ring structure
4/14
Autocorrelation C(Δr)
(1) Selfgravity wakes in A ring
ΦV=32°
ΦV=60°
ΦV=73°
ΦV=83°
ΦV=99°
ΦV=106°
ΦV=112°
ΦV=131°
ΦV=148°
Correlation lag Δr [km]
ΦV=occultation track angle in the corrotating plane.
Strong dependence on occultation geometry.
5/14
2D autocorrelation
UVIS
data
N-body
simulation
(H. Salo)
6/14
Autocorrelation C(Δr)
(3) Irregular B ring structure
ΦV=41°
ΦV=61°
ΦV=74°
ΦV=78°
ΦV=100°
ΦV=102°
ΦV=105°
ΦV=111°
ΦV=116°
Correlation lag Δr [km]
Very weak dependence on occultation geometry!
10/14
Irregular B ring structure
Structure has ~100m radial scale
● Axisymmetric (no angle dependence in occultations)
● Irregular in appearance (these are not waves)
● Very distinct from self-gravity wakes
● Viscous instability?
- In 80-ies proposed as mechanism to produce
irregular B ring structure
- Fell out of favor since the needed theoretical
criteria were not met (ring particles turned to
be too inelastic)
- Salo & Schmidt (2009) show that viscous
instability is a valid mechanism,
but still requires more elastic particles
●
11/14
Porco(2005)
Double star occultations
γ Lupi
secondary
No signature
of the γ Lupi
secondary
Narrow ringlets
(~few 100m) in
inner A ring make
ring self-similar
==> double star
signature
No structure here
and thus no
secondary star
signature
(SG wakes are
much smaller
than double star
separations)
12/14
(4) Mysterious C ring structure
γ Lupi
secondary
●
●
Secondary star signatures in many occultations
indicate presence of structure in C ring.
Possibly ~axisymmetric narrow ringlets or
waves on tens of meters radial scale
13/14
Extra: UVIS occultation geometry
(B,φ) determine line of sight
(and were used to explain optical depth
variation of A ring self-gravity wakes)
ΦV= angle in the corrotating plane
(describes occultation track motion)
orbital
motion
towards
star
φ
occultation
track
integration
area
ΦV
radius
Extra: Selfgravity wakes in A ring
Strong correlations
up to 100m
both positive and
negative
FFT
Autocorrelation
Extra: SG wakes - UVIS vs local N-body
Alp Vir Rev8 ingress
UVIS HSP detector view
(N-body simulations by H. Salo)
simulation
Sig Sgr Rev11
simulation
Extra: Overstable waves tilt angle ~0
Extra slide: Pan wakes tilt angle (~1')
Extra: overstable waves
local N-body simulations (H. Salo)
Extra slide: OS power spectrum
UVIS
Local Nbody
simulation