B ring microstructure
UVIS team meeting,
Berlin June 8, 2009
Miodrag Sremčević
LASP, CU Boulder
Small scale structure
In B ring
●
●
Small scale structure is very effectively
observed in UVIS HSP data with either
spectral methods (FFT and wavelets)
or using autocorrelations.
Examples so far:
- self-gravity wakes in A ring (~10-50m)
- overstable waves in A and B ring (~150m)
- double stars show as positive correlations
over structured ring (observed 10m - ~km)
Example: selfgravity wakes in A ring
FFT
Autocorrelation
Reminder on UVIS ring geometry
(B,φ) determine line of sight
(and were used to explain optical depth
variation of A ring self-gravity wakes)
 V = angle in the corrotating ring
plane → occultation track motion
orbital
motion
occultation
track
towards
star
φ
V
radius
Important
information:
for
SG
wakes
the
autocorrelations vary with the angle in the co-rotating
ring frame (i.e. SG wakes are pitched ~110deg)
How about B ring?
●
●
●
low optical depth (tau=~1) shows selfgravity wakes signature remniscent of A
ring
Mid-range optical depth (tau=~2) shows
overstable waves
Higher optical depth (tau>2) shows
interesting structure. But only fraction of
stars are penetrating thick B ring.
B ring autocorrelations
Correlations do not show significant change
with the angle in the corrotating ring plane!
Spica (Alp Vir) shows the same trend: although
occultations have poorer radial resolution and smaller
B angle: no dependence of correlations with angle in
the ring plane!
Problem with these observations: they are double stars
which always casts doubts about the underlying
structure. However, taken with other observations they
Imply no change in autocorrelations shape with angle
in the ring plane!
Self-gravity wakes?
Compared to A ring SG wakes these
are 5-10 times larger (no indication of
that in N-body simulations)
● Most crucially: no dependence on
the ring plane angle → no pitch!
● Absence of negative coorelations
● It is still possible that SG wakes are
on top of the new structure (5-10m
correlations do show some small
dependence): this is the case for
overstable waves
●
Difference in transparency
A ring SG wakes are half transparent, half
opaque, but new structure (shown with 6m
resolution) is nowhere transparent!
Isotropic correlations
Single ring particles would produce
needed isotropic “triangular”
correlations
● However, the implied size would be
enormous: ~100m! This would give
incredibly large ring mass and is
unlikely!
● Overstable waves have isotropic
correlations, but have wave (sine
shape) form → clearly not met here.
●
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)
● New study by Salo & Schmidt (2009)
shows that viscous instability is
valid mechanism, but still requires
somewhat more elastic particles:
- Bridges et al (but small particles)
- or Hatzes et al collision model
●
Conclusions
Isotropic correlations indicate radially
aligned structure of ~100m in size
● Absence of pitch angle rules out
Self-gravity wakes
● So far only candidate: viscous
instability induced irregular structure
● We need more occultations
covering specific geometry
ring plane angle ~0 degrees →
|v_phi-v_kepler|<|v_radial|
●
Future ring occultations
Wealth of new possible specific
observations (and all require very
specific and unique geometry):
● Super-occultations (tracking ring
particles)
● B ring microstructure
● Propeller occultations
● F ring turnarounds to investigate
small scale structure
● C ring microstructure