PARTICLE SIZES AND
DISTRIBUTIONS MEASURED BY
DIFFRACTION IN
OCCULTATION DATA
Tracy Becker
UVIS Team Meeting
Jan 8 - 10, 2014
Pasadena, CA
Thursday, January 16, 2014
Particle Sizes from Diffraction
✤
Sharp ring edges in the A ring
✤
Rev 9 misaligned solar occultation by the F ring
Thursday, January 16, 2014
Sharp Ring Edges
GSC5249-01240 Occultation
HST Faint Object Spectrograph
✤
Diffraction “spikes” are seen
at the sharp edges in the rings
in stellar occultations
✤
The magnitude and radial
extent of the signal are
indicative of the particle sizes
and their fractional optical
depth
Bosh et al., 2002!
Thursday, January 16, 2014
Sharp Ring Edges: Detections
✤
Thursday, January 16, 2014
We have detected strong diffraction
spikes in ~50% of the stellar
occultations at the Encke Gap, Keeler
Gap, and outer A Ring edge
Sharp Ring Edges
HSP Ramp Up Effect
✤
✤
Why don’t we see them in
every occultation?
✤
Strength of Stellar Signal
(S/N)
✤
HSP Instrumental Effect
✤
Elevation and Azimuthal
Angles (Jerousek et al.
2011)
Elevation Angle
Detection
Dependence
Detections not correlated with
azimuthal or temporal
variations in the rings
0o-30o
Thursday, January 16, 2014
30o-60o
Elevation Angle
60o-90o
Sharp Ring Edges: Model
✤
Ring Edge Model:
✤
✤
✤
BRIEF ARTICLE
Utilizes geometry of the occulation
THE AUTHOR
BRIEF ARTICLE
BRIEF ARTICLE
(1)
(2)
calculates the I/F using a power-law size distribution
THE AUTHOR
�
�2
� amax
THE
Maximum contributing
particle
5 sin
cmθ)
I
e
2J1 (ka
2 size:
AUTHOR−τ
µ0
F
−τ
µ0
I
e
=
F
4πµ0
�
=
4πµ0
πa
amin
sin θ
−τ �
�
�2
amax
�
�
µ0
2πθ)
I
e 2J (ka sin θ) 22 2J1 (ka sin
amax
1
k=
=2
πa n(a)da
n(a)da
πa
F
4πµ0 sin
sin θλ
amin
θ
amin
ad
Thursday, January 16, 2014
2π
k=
λ
n(a) = Ca−q
2π
k n(a)da
=
= Ca−q da
λ
n(a) = Ca−q
(3)
n(a)da asdsad
bb = whaa
a = particle size
τ = optical depth
µ0 = sin B
λ= wavelength (150 nm)
θ = scattering angle
I
=
F
Sharp Ring Edges: Model
Model
dependence on
minimum particle
size
Thursday, January 16, 2014
Sharp Ring Edges: Model
Model dependence
on slope of power
law size distribution
‘q’
Thursday, January 16, 2014
Sharp Ring Edges: Model
Encke Gap
Keeler Gap Both Edges
A Ring Outer Edge
Thursday, January 16, 2014
Sharp Ring Edges: Results
Region
Encke Gap Inner Edge
Encke Gap Outer Edge
Keeler Gap Inner Edge
Keeler Gap Outer Edge
A Ring Outer Edge
q (amin - 5cm)
2.7
2.7
2.7
2.7
2.9
amin
3 mm
3 mm
1 mm
1 mm
<1 mm
✤
General Trend: Minimum Particle Size Decreases and Size Distribution
Slope Steepens from Encke Gap to Outer Edge of the A Ring
✤
Consistent with results from Colwell et. al self-gravity wake study
Thursday, January 16, 2014
Sharp Ring Edges: VIMS
Outer Edge of A Ring GamCru89
Keeler Gap and A Ring Outer Edge AlpOri026
VIMS data:
Radial extent of diffraction signal bigger (theta ~ lambda/a)
Thursday, January 16, 2014
Sharp Ring Edges: Beyond A ring
Thursday, January 16, 2014
Sharp Ring Edges
✤
Current Work:
✤
Working with Rebecca Harbison (VIMS) to compare models and
results
✤
Analyzing all stellar occultations to determine particle size and
distribution at each edge in A ring
✤
Determine if there is any change in particle population due to
recent encounter with satellite
✤
Begin process on sharp edges elsewhere in the ring system
(Huygen’s ringlet, Maxwell ringlet, etc.)
Thursday, January 16, 2014
F Ring Solar Occultation
✤
Thursday, January 16, 2014
Misalignment of Rev 9 solar
occultation decreased solar
signal --> diffraction detected at
F ring
F Ring Solar Occultation: Model
Thursday, January 16, 2014
F Ring Solar Occultation: Results
✤
New Results!
✤
particle size range from 5 50 microns, with power law
size distribution slope of 3.5
✤
Range of sizes and
distributions will likely fit
the data - looking to
characterize the parameter
space
Thursday, January 16, 2014
F Ring Solar Occultation:
Solar Port FOV
✤
Solar scan planned for
March 2014 characterize the
sensitivity of the solar
port FOV along the slit
Thursday, January 16, 2014
F Ring Solar Occultation:
Solar Port FOV
✤
Verify pointing in my model
to know the exact decrease
in signal at Sun’s location in
the FOV
Rev 9 Solar Occultation
!"#$%&"
*1%"
'()*"!$+,-"./"($+0"
Thursday, January 16, 2014
Angular Separation of Sun in model:
(x,y) = (.004, .0110)
F Ring Solar Occultation:
Other Solar Occultations
✤
Rev 43 Solar
Occultation by
F ring
Thursday, January 16, 2014
F Ring Solar Occultation:
Other Solar Occultations
✤
Rev 55 Solar
Occultation by F
ring
Thursday, January 16, 2014
F Ring Solar Occultation
✤
Current Work
✤
Include results from upcoming solar scan in model
✤
Look at ISS images to constrain location and width of rings at time of solar
occultation
✤
Finish model tests
✤
explore parameter space: find range of particle sizes and distributions that
can fit the data
✤
apply model to other solar occultations
✤
write paper
Thursday, January 16, 2014
Summary
✤
✤
✤
Stellar Occultations Ring Edge Analysis
✤
Particle sizes decreases and size distribution steepens from Encke Gap to
outer edge of the A ring
✤
sub-mm particles in the outer edge of the A ring
F Ring Solar Occultation
✤
steep size distribution with particles a few microns to ~ 10s or 100s of
microns
✤
contribution from strands significant - average particle size 50 mic
Plan: Complete analysis on both projects
Thursday, January 16, 2014
Thursday, January 16, 2014