Nanograin densities outside Saturn's A-ring
Robert E Johnson, Wei-Lin Tseng, Meredith K Elrod, Ann M Persoon
NNanograin Density Outside Saturn’s A ring
Robert E. Johnson , Wei-Lin Tseng , M.K. Elrod , A.M. Persoon Engineering Physics, University of Virginia, Charlottesville, VA 22902; 434-422-2424; [email protected]; http://orcid.org/0000-‐0001-‐7798-‐5918 Physics, NYU, NY, NY 10003 National Taiwan Normal University; No. 88, Sec. 4, Tingzhou Rd., Wenshan District, Taipei 11677, Taiwan (R.O.C.): 886-2-77346402; [email protected] NASA Goddard Space Flight Center, Greenbelt, MD, 20771. 757-725-3014 [email protected] CRESST, University of Maryland, College Park, MD 20742 Dept. of Physics and Astronomy, University of Iowa, Iowa City, IA, 52242 [email protected]
Abstract
The observed disparity between the radial dependence of the ion and electron densities measured by the Cassini plasma (CAPS) and radio (RPWS) science instruments are used to show that the region between the outer edge of Saturn’s main rings and its tenuous G-‐ring is permeated with small charged grains (nanograins). These grains emanate from the edge of the A ring and from the tenuous F and G rings. This is a region of Saturn’s magnetosphere that is relatively unexplored, but will be a focus of Cassini’s F ring orbits prior to the end of mission in September 2017. Confirmation of the grain densities predicted here will enhance our ability to describe the formation and destruction of material in this important region of Saturn’s magnetosphere.
Introduction
As the enormously successful Cassini mission winds down, the focus will be on two regions in the Saturnian system that have mostly been avoided due to possible hazards. From November 2016 through April 2017 Cassini will carry out ~20 orbits outside the edge of Saturn’s main rings (~2.1R S ; 1R S = Saturn’s radius) and close to the tenuous F ring (~2.23 R S ) focusing on the environment between the main rings and the tenuous rings. Subsequent to these orbits, referred to as the F ring orbits, Cassini will then change to orbits that are inside the D ring (< 1.1R S ) in order to study Saturn’s extended atmosphere and its interaction with the rings, the so-‐called proximal orbits. Here we show that, in addition to the presence of grains observed in the tenuous F and G rings, plasma measurements indicate that there is a significant density of small charged grains that permeate the region outside the visible edge of Saturn’s A ring. On Saturn Orbit Insertion (SOI) in 2004, Cassini’s plasma instrument discovered a surprisingly robust density of ionized oxygen molecules over the rings (Young et al; 2005; Tokar et al. 2005) produced by photoionization of an oxygen ring atmosphere. Because of the low radiation flux over the rings (e.g., Cooper et al. 2016), O is formed primarily by solar UV-‐induced decomposition of the icy ring articles with a concomitant, but much more extended, H atmosphere (Johnson et al. 2006). These molecules are scattered from over the rings into Saturn’s magnetosphere and into its atmosphere by ion-‐molecule interactions (Johnson et al. 2006; Luhmann et al. 2006; Bouhram et al. 2006; Martens et al. 2008). Cassini subsequently crossed the ring plane outside of the A ring where it observed a much higher density thermal plasma. In spite of the high background radiation (Tokar et al. 2005), plasma ion densities and composition were extracted (Elrod et al. 2010; 2014) indicating that the newly discovered ring atmosphere extended well beyond the outer edge of the A ring. Although Cassini never came closer than ~2.4R S , the analysis by Elrod et al. for orbits with a periapsis inside of that of Mimas (~3R S ) confirmed that the ring atmosphere was primarily produced by the solar UV decomposition of ice (Johnson et al. 2006). That is, because the tilt of the ring plane to the solar flux varies during Saturn’s orbit, an atmosphere formed by solar UV should exhibit a seasonal dependence (Tseng et al. 2010). The variation in the CAPS thermal plasma O data from near southern summer solstice to equinox clearly exhibited such a dependence (Elrod et al. 2012; 2014). A seasonal dependence was also seen in the energetic particle data (Christon et al 2013; 2014) and the plasma electron data (Persoon et al. 2015). Although there is now strong evidence for a seasonal varying ring atmosphere, the region between the outer edge of the A ring and inside the F ring, through which Cassini will pass during its F ring orbits, is not well described (e.g., Cooper et al. 2016). We show here that, in addition to the ionization of molecules scattered from over the rings and the ionization of neutrals diffusing inward from Enceladus, this region is permeated by charged nanograins likely coming from the edge of main rings and the tenuous rings. Cassini Plasma Data
The near equatorial CAPS ion densities (Elrod et al. 2012; 2014) and the corresponding electron densities extracted from the RPWS data (Persoon et al. 2015) for the Cassini SOI orbit are displayed in Fig. 1a as a function of radial distance, R , from Saturn. These both exhibit striking spatial morphologies, but the densities disagree by up to an order of magnitude unlike at larger radial distances where the RPWS and CAPS charge densities are reasonably consistent (e.g., Persoon et al. 2015; Elrod et al. 2012; Sittler et al. 2006). In Fig. 1b we also show results from a later orbit during which Cassini penetrated into this region. The large drop in the ion density from SOI in 2004 to that in 2007 was attributed the change in the orientation of the ring plane to the solar flux as Saturn approaches equinox, the seasonal variation discussed above. More important to the work here, an unpredicted, steep decrease in the ion density with decreasing distance from the edge of the rings is seen in both Fig.1a and 1b. It is also seen in Fig. 1b, that at the smaller R the free electron density dropped below the limit needed to stimulate the RPWS upper hybrid resonance. These are points we will return to below. The physical processes occurring in this region were examined in Tseng et al. (2013). Here we revisit that work focusing first on the use of the near equatorial plasma data to predict the presence of a significant density of as yet unobserved small charged grains in the region between the outer edge of the A ring and the G ring and then on the role of these grains in quenching the ions. Although the effect of rains on plasma transport in this region has been discussed (e.g., Sakai et al. 2013; Roussos et al. 2016), the data in Fig. 1 was not considered. (a) (b) Fig. 1 Ion and electron densities within ~ 0.5R S of the equator (a) SOI orbit; (b) a 2007 orbit. Lines: electron density extracted from the RPWS upper hybrid resonance (Persoon et al. 2015); blue diamonds: total ions from CAPS: O + W + dominated by O at SOI and W + at 2007 (Elrod et al. 2014); red square: total H + W + from CAPS dominated by W + (Sittler et al. 2006) Nanograin Grain Density
The
region
between
the
G
ring
and
the
outer
edge
of
the
A
ring
is
primarily
populated
by
two
neutral
gas
sources,
the
gas
torus
produced
by
the
Enceladus
plumes
and
the
extended
ring
atmosphere,
with
contributions
from
the
ice
particles
and
small
moons
in
the
F
and
G
rings.
However,
combining
models
of
these
two
principal
sources
with
the
ionization
rates
and
models
for
diffusion
cannot
explain
the
plasma
observations
in
Fig.
1.
Although
for
most
regions
of
Saturn’s
magnetosphere
the
ion
and
electron
densities
extracted
show
reasonable
agreement,
this
is
not
the
case,
for
instance,
within
the
Enceladus
plumes
where
the
disparity
was
attributed
to
the
presence
of
small
charged
ice
grains
(Morooka
et
al
2011;
Hill
et
al.
2012).
Here
we
propose
that
the
disagreement
in
Fig.
1a
also
indicates
the
presence
of
a
significant
density
of
charged
grains
with
sizes
well
below
the
limit
detected
by
the
dust
instrument
on
Cassini.
In
this
region
the
electron
temperature
is
low,
so
that
grains
are
predominantly
negatively
charged
(Jurac
et
al.
1995;
Kempf
et
al.
2006).
The
RPWS
upper
hybrid
resonance
is
sensitive
only
to
the
free
electrons.
Unfortunately,
the
electron
instrument
on
the
CAPS
suite,
which
detected
negatively
charged
clusters
at
Titan
(Coates
et
al.
2010)
and
small
charged
grains
near
Enceladus
(Hill
et
al.
2012)
did
not
obtain
electron
densities
due
to
spacecraft
charging
and
the
significant
background
radiation
(e.g.,
Schippers
et
al.
2009).
Because
of
this
background,
only
the
heavy
ion
densities
were
extracted
from
the
CAPS
measurements,
the
O and
the
water
group
ions,
W + (H O + ,
H O + ,
OH + ,
O + ).
Rough
upper
limits
were
placed
on
the
light
ions
(H ,
H + )
indicating
they
were
a
small
fraction
of
the
near
equatorial
positive
charge
(Elrod
et
al.
2014)
consistent
with
models
(Tseng
et
al.
2011).
he
ion
density
in
Fig.
1a
is
dominated
by
O with
temperatures
close
to
the
pick-‐up
energy,
indicating
the
ions
are
short-‐lived
and
freshly
formed,
primarily
by
ionization
of
O in
the
extended
ring
atmosphere.
The
ion
densities
in
the
2007
data
in
Fig.
1b
are
dominated
by
W + with
a
temperature
that
is
a
fraction
of
the
pick-‐up
energy.
This
plasma
is
formed
primarily
by
ionization
of
water
molecules
in
the
Enceladus
gas
torus
with
a
significant
but
smaller
contribution
from
the
O ring
atmosphere.
It
is
also
seen
in
Fig.
1
that
there
is
a
sharp
drop
in
the
heavy
ion
charge
density
with
decreasing
distance
from
the
edge
of
the
A-‐ring.
This
radial
dependence
is very different
from
that
predicted
by
models
of
the
ion
source
strength
and
radial
diffusion,
indicative
of
a
strong
quenching
process
(Tseng
et
al.
2013).
That
is,
accounting
for
both
the
ring
and
Enceladus
sources
the
ion
production
rate
varies
slowly
with R across
this
region,
so
that
the
loss
processes
and
not
the
source
processes
determine
the
observed
radial
dependence.
Since
the
difference
between
the
ion
and
free
electron
densities,
as
well
as
the
rapid
quenching,
can
be
due
to
the
presence
of
negatively
charged
grains,
we
estimate
their
density
and
size
distribution
in
anticipation
of
the
F
ring
orbits.
The
ion
density
goes
through
a
maximum
as
suggested
by
the
peak
in
Fig.
1a
and
by
the
much
lower
density
at
~3.8Rs
where
the
CAPS
measurements
are
roughly
comparable
to
the
RPWS
electron
densities
(e.g.,
Sittler
et
al.
2006).
Such
a
maximum
is
consistent
with
the
ionization
of
a
robust
O atmosphere
emanating
from
the
edge
of
the
A-‐ring,
but
rapidly
quenched
close
to
the
ring
source.
This
is
similar
to
the
observation
that
the
plasma
density
produced
by
the
Enceladus
gas
torus
peaks
outside
the
neutral
source
region
(Sittler
et
al.
2006;
Tokar
et
al.
2008).
Below,
we
first
estimate
the
grain
density
consistent
with
the
observation
in
Fig.
1a
and
then
show
they
act
to
quench
the
ions.
Assuming
the
difference
between
the
ion
and
electron
densities, Δ n q ,
is
due
to
distribution
of
small
negatively
charged
grains,
then Δ n q depends
on
the
density
of
grains, n g ,
and
the
charge
per
grain.
The
average
charge
per
grain
in
turn
depends
on
the
grain
radius, r g ,
and
the
plasma
environment.
Based
on
the
plasma
properties,
models
suggest
the
grain
potential
is
~
-‐1
to
-‐2eV
in
this
region
(Jurac
et
al.
1995;
Kempf
et
al.
2006).
As
these
very
small
grains
are
likely
singly
charged
(Hill
et
al.
2012),
setting n g ~ Δ n q gives
a
rough
upper
bound
to
the
small
negatively
charged
grain
density.
At
SOI, Δ n q peaks
at
~
1000/cm just
inside
the
G
ring
orbit.
Grain
size
distributions
are
predicted
to
be
steeply
varying,
so
that
the
number
density
of
grains
with
radius
greater
than a is
estimated
as n g ~ n g0 ( r g / a ) -‐
α with
α
~
4-‐
5
and n g0 a
normalization
density
(Kempf
et
al.,
2008).
The
peak
in Δ n q occurs
near
3R S where
the
Cassini
Dust
Analyzer
(CDA)
gives
a
density
for r g >
~
1 µ m grains
of n g ~3x10 -‐9 /cm .
Using
this
to
normalize n g ,
we
find
that
a
density, Δ n q ,
of
singly
charged
grains
with
a
minimum
grain
radius, r gmin ~
1 µ m [ n g0 / Δ n q ] can
account
for
the
difference
in
the
observed
charge
densities.
For Δ n q ~
1000cm -‐3 at
2.7R S , r gmin ~
1.3 nm for
α
=
4
and
~5 nm for
α
=
5.
Since
the
distribution
is
strongly
peaked,
the
dominant
grain
size
is
within
a
factor
of
a
few
times
these
estimates
for r gmin .
It
also
means
that
these
estimates
do
not
change
enormously
if
the
larger
grains
are
more
highly
charged,
as
one
would
expect
to
be
the
case.
Therefore,
the
difference
near
the
peak
of
the
CAPS
data
for
SOI
can
be
accounted
for
by
singly
harged
nanograins
with
a
size
distribution
like
that
discussed
above.
Nearer
to
the
F
ring, Δ n q ~
100cm -‐3 .
In
the
absence
of
CDA
measurements
in
this
region,
we
use
the
same
normalization
for
the
grain
size
distribution,
in
which
case r gmin again
does
not
change
significantly
due
to
the
steep
size
distribution.
These
densities
are
much
larger
than
the
E
ring
grain
densities
in
this
region
and
much
larger
than
the
grain
densities
extrapolated
by
Sakai
et
al
(2013)
to
the
edge
of
the
A
ring
(~0.01-‐0.03
cm -‐3 )
in
order
to
explain
the
drag
on
the
measured
ion
flow
speeds.
The
above
estimates
indicate
that
the
ion
and
free
electron
differences
in
Fig.
1a
could
be
accounted
for
by
a
density
of
very
small,
as
yet,
unobserved
nanograins.
However,
this
conclusion
must
be
consistent
with
the
observation
of
the
steep
radial
gradient
seen
in
the
ion
data.
Using
an
electron
temperature
of
~
-‐2eV,
Tseng
et
al.
(2013)
estimated
that
loss
due
to
electron-‐ion
recombination
gave
ion
lifetimes
>
~10 s
assuming
a
free
electron
density
equal
to
the
ion
density.
Using
the
lower
free
electron
densities
in
Fig
1a,
lengthens
this
time.
However,
ions
can
also
collide
with
and
neutralize
on
grains.
Because
the
grain
potential
is
much
smaller
that
the
ion
energies,
and
ignoring
the
attractive
force
between
the
positive
ion
and
negatively
charged
grains,
the
ion-‐grain
collision
cross
section
can
be
estimated
from
the
grain
radius:
σ col ~ π r g2 .
As
the
measured
ion
flow
speeds
at
SOI
are
of
the
order
of
the
co-‐rotation
speed
(Elrod
et
al.
2012)
we
use
an
average,
That there are micron size or larger grains in this region is, of course, well-‐known from the CDA measurements and by light scattering in the tenuous rings. But the difference between the electron and ion densities, and the steep drop in the ion density with decreasing R , is very different from the radial dependence of the ion source term. Here we show that these plasma measurements can be explained by the presence of negatively charged nanograins that reduce the free electron density and act to collisionally quench the ions. Both the SOI and 2007 data indicate this region is permeated with such grains and their density increases with decreasing R . These grains are likely sourced from the edge of the A ring as well as from the F and G rings. This is consistent with studies indicating that collisions of small icy bodies inside 3.0Rs produce debris (Tiscareno et al., 2013; Attree et al., 2014) and it is even the case that small objects can form from such debris, as seen at the edge of the A ring (Murray et al., 2014). Orbiting debris is known to deplete the energetic ion and electron fluxes (e.g., Cuzzi and Burns, 1988; Paranicas et al., 2008; Cooper et al. 2016). More recently Roussos et al., (2016) suggested that energetic particle signatures inside the orbit of Mimas appear to be shifted by plasma transport affected by the presence of charged grains. Here we have used the Cassini CAPS and RPWS data to add to that picture by estimating the charged nanograin density in this region. Since the F ring orbits will occur as Saturn approaches the northern hemisphere summer solstice, we expect plasma densities of, roughly, the same order of magnitude as those estimated from the SOI data. Unfortunately, the CAPS instrument will be off. Therefore, ion densities must be derived from the Ion Neutral Mass Spectrometer (INMS), but, since the electron densities are expected to be much larger than those in the 2007 data, the upper hybrid resonance should be observable. Grain densities might be detectable by the INMS, by energetic particle absorptions or, possibly, by plasma waves generated by impacts on the spacecraft (e.g., Schippers et al. 2014). The results presented here give, for the first time, an estimate of the nanograin density in the vicinity of the F ring, which can be used, along with the expected Cassini measurements, to constrain modeling of the formation and destruction of material in this important region of Saturn’s magnetosphere. Acknowledgements
REJ acknowledges support from NASA via the Cassini mission through SwRI and JPL. The Cassini RPWS data are in the Planetary Data System. W.-‐L.T. acknowledges support from MOST 104-‐2112-‐M-‐003 -‐006 -‐MY2 in Taiwan (R.O.C.). AMP acknowledges support by NASA through contract 1415150 with the Jet Propulsion Laboratory
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