Small-scale Ring Structure
Observed In Cassini UVIS
Occultations
Miodrag Sremčević
LASP, CU Boulder
UVIS team meeting,
Boulder January 5, 2010
Cassini UVIS stellar occultations
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High Speed Photometer (HSP) 110 – 190nm
>100 stellar occultations of the rings
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1 - 8ms sampling rate (mostly 1ms used)
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Rings radial resolution from few to ~50m
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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)
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.
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
β Persei Rev116 occultation
At occultation turnaround (R=131,436km):
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Radial resolution (in 1ms) dR < 1cm
Ring orbital motion
= 17.0 km/s
Occultation azimuthal speed = 16.2 km/s
==> Azimuthal resolution dL =~ 80cm
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Projected star diameter =~ 20cm
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Fresnel zone =~ 300cm
β Persei Rev116 light curve
Stellar level
~transparent
gap
~opaque
SG wake
Optical depth histogram
Size of the SG wakes and gaps
Correlations: looking for particle signature
Is it possible to observe
individual particles?
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Limiting factor is Fresnel zone (2 – 3 m)
VIMS γ Crucis occultations (R104 & R106)
have Fresnel zone ~120m, and projected star
~200m, yet the overstable waves λ=~150m
are observed
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!
Extra Slides
(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
What is no structure signature?
[p r r −⟨p⟩][pr −⟨p⟩]
∑
C r =
2
∑ [p r −⟨p⟩]
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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
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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
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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