The Plasma Cam - the Institute of Geophysics and Planetary Physics

1
Periodicity in Saturn’s Magnetosphere—The Plasma Cam
2
J. L. Burch, A. D. DeJong, J. Goldstein, and D. T. Young
3
Southwest Research Institute, San Antonio, TX, USA
4
Received XX XXX, 2009, accepted XX XXX, 2009, published XX XXX, 2009.
5
Plasma ion data from the Cassini Plasma Spectrometer (CAPS) are examined for all
6
orbits from July 14, 2004 through Dec. 31, 2006. In order to eliminate effects of
7
incomplete angular coverage, data are only used from the CAPS anode that is closest to
8
viewing into the corotational flow and within 20° of that flow. The data are plotted in the
9
SKR-based SLS3 longitude system. The result is a cam-shaped distribution in radial
10
distance and SLS3 that has an outer lobe extending to ~25 Rs at SLS3 longitudes in the
11
range 0° - 45°. This outer lobe maps closely to the inner extent of a previously observed
12
spiral pattern of periodic ion enhancements, which had the magnetic signature of
13
plasmoids at distances >35 Rs. The plasma cam and the plasmoid spiral emanating from
14
it are responsible for plasma periodicities observed at radial distances beyond ~15 Rs in
15
Saturn’s magnetosphere. Citation: Burch, J. L., J. Goldstein, A. D. DeJong, and D. T.
16
Young (2009), Periodicity in Saturn’s Magnetosphere—The Plasma Cam, Geophys. Res.
17
Lett., 35, LXXXXX, doi:10.1029/2009GLXXXXXX.
18
18
19
20
Introduction In
order
to
investigate
Saturn’s
plasma
population
and
possible
causes
of
21
periodic
phenomena
within
it,
ion
data
from
CAPS
(the
Cassini
Plasma
22
Spectrometer)
[Young et al.,
2004]
were
analyzed
on
a
statistical
basis
for
the
period
23
July
14,
2004
through
Dec.
31,
2006,
which
is
the
total
time
period
of
the
initial
24
near‐equatorial
orbit
phase
of
Cassini.
25
An
important
constraint
on
the
study
is
the
incomplete
angular
coverage
of
the
26
measurements.
The
field‐of‐view
of
the
CAPS
Ion
Mass
Spectrometer
(IMS)
covers
a
27
fan
of
8°
x
160°,
and
this
fan
is
moved
both
by
the
CAPS
actuator
and
by
frequent
28
spacecraft
maneuvers
(the
latter
mostly
in
connection
with
imaging
operations).
29
These
motions
certainly
complicate,
and
could
even
invalidate
the
results
of
a
long‐
30
term
statistical
study.
However,
the
fact
that
to
a
close
approximation
plasma
31
corotates
with
the
planet
(with
some
lag
that
increases
with
distance)
allowed
us
to
32
evaluate
statistically
the
ion
fluxes
as
functions
of
radial
distance
(or
dipole
L
shell)
33
and
SLS3
longitude
[Kurth et al.,
2008]
by
selecting
only
those
data
acquired
with
34
particular
sensors
that
were
viewing
closely
into
the
corotational
flow
direction,
35
thereby
effectively
removing
effects
of
spacecraft
and
actuator
motions.
36
An
important
result
of
the
study
is
a
cam‐shaped
locus
of
plasma
that
fills
a
37
circular
region
inside
L~15
Rs
for
most
longitudes
but
displays
an
outer
lobe
that
38
extends
to
nearly
25
Rs
in
the
SLS3
longitude
range
between
0°
and
45°.
This
39
particular
description
as
a
cam
is
chosen
because
the
observed
distribution
has
the
40
appearance
of
the
mechanical
device
of
the
same
name,
which
converts
rotational
41
motion
into
displacement
once
per
revolution,
for
example
in
automotive
engines.
42
The
outer
lobe
of
the
cam
is
observed
to
overlap
the
region
identified
by
Burch et 43
al.
[2008]
as
the
inner
locus
of
periodic
ion
fluxes
observed
by
CAPS
over
the
time
44
period
Dec.
29,
2005
and
Sept.
7,
2006.
These
periodic
fluxes
were
observed
by
45
Burch et al.
[2008]
to
extend
along
a
spiral
path
in
R
and
SLS3
longitude
that
46
reached
nearly
50
Rs
and
along
which
magnetic
signatures
consistent
with
47
plasmoids
were
observed
within
the
periodic
ion
fluxes
at
all
distances
beyond
48
about
35
Rs.
Other
similar
observations
of
plasmoids
in
this
region
have
been
49
reported
by
Jackman et al.
[2007]
and
Hill et al.
[2008].
Together
the
plasma
cam
50
and
the
spiral
path
of
ion
events
observed
beyond
the
cam
can
explain
all
of
the
51
periodic
plasma
phenomena
observed
at
radial
distances
greater
than
~15
Rs.
52
Periodicities
in
Saturn
Kilometric
Radiation
(SKR)
have
been
known
since
the
53
time
of
the
Voyager
flybys
[Desch and Kaiser,
1981],
while
similar
periodicities
in
54
the
Voyager
magnetic‐field
data
were
analyzed
by
Espinosa and Dougherty
[2000]
55
and
Espinosa et al.
[2003a,b].
Espinosa et al.
[2003b]
first
used
a
“camshaft”
analogy
56
to
explain
periodic
magnetic
perturbations
observed
by
Voyager
with
the
cam
57
action
being
supposed
to
result
from
a
magnetic
anomaly
fixed
in
the
planet’s
frame
58
of
reference.
However,
the
plasma
cam
reported
herein
is
the
first
observed
59
phenomenon
that
actually
has
the
appearance
of
a
cam.
60
The
spatial
overlap
of
the
plasma
cam
with
the
spiral
path
of
ion
events
and
(at
61
greater
distances)
plasmoids
reported
by
Burch et al.
[2008]
supports
their
62
suggestion
that
magnetic
flux
tubes
heavily
laden
with
plasma
stretch
into
the
night
63
side,
ultimately
leading
to
magnetic
reconnection
on
closed
field
lines
in
the
tail
64
region
of
Saturn’s
magnetosphere
as
suggested
for
Jupiter’s
magnetosphere
by
65
Vasyliunas
[1983]
and
Kivelson and Southwood
[2005].
66
67
68
Observations The
data
were
selected
and
averaged
in
the
following
manner.
First,
only
Saturn
69
geographic
latitudes
between
‐10°
and
+10°
were
considered.
Next,
for
each
four‐
70
second
energy
sweep
of
the
IMS
the
energy
closest
to
the
water‐group
(mass/charge
71
=
18)
corotation
energy
for
that
radial
distance
was
chosen.
All
eight
IMS
anodes
72
(which
together
sample
the
8°
x
160°
fan)
were
evaluated
as
to
their
look
directions
73
with
respect
to
corotation.
Only
those
anodes
viewing
within
20°
of
the
corotational
74
flow
were
used.
Of
these,
the
anode
viewing
closest
to
the
corotational
flow
75
direction
was
selected.
These
selected
anodes
provided
the
basic
data
set
used
in
76
the
analysis.
77
For
the
plot
shown
in
Figure 1,
average
count
rates
(counts/62.5
ms)
were
78
computed
in
bins
1°
wide
in
SLS3
longitude
and
1
Rs
wide
in
dipole
L
value.
Each
79
average
typically
contains
counts
from
several
different
anodes
from
different
80
locations
within
the
spatial
bins
depending
on
the
individual
look
directions
of
the
81
anodes.
82
The
results
in
L
vs.
SLS3
longitude
plotted
in
Figure 1
show
a
cam‐shaped
region
83
with
a
lobe
extending
beyond
25
Rs
at
SLS3
between
0°
and
45°.
It
is
possible
that
84
this
cam
lobe
is
related
in
some
way
to
the
electron
density
peaks
observed
by
85
Gurnett et al.
[2007]
near
SLS3=330°
at
3‐5
Rs.
If
so,
then
there
was
a
lag
of
~52°
86
(the
difference
in
SLS3
locations
of
the
Gurnett
et
al.
electron
density
peak
and
the
87
midpoint
of
the
outer
cam
lobe)
over
~21
RS
(the
distance
between
the
3
–
5
RS
88
electron
density
peak
and
the
outer
extent
of
the
plasma
cam
lobe)
or
~2.5°
per
RS.
89
A
quantitatively
similar
lag
has
been
observed
at
larger
distances
in
the
Cassini
90
energetic
particle
data
by
Carbary et al.
[2007]
who
found
that
28‐48
keV
electron
91
fluxes
peaked
along
a
spiral
pattern
in
the
SLS2
longitude
system
with
a
lag
of
~3.4°
92
per
RS
of
radial
distance
between
10
and
60
Rs.
93
As
shown
in
Figure 2 (c and d),
the
outer
cam
lobe
intersects
the
magnetopause
94
in
the
afternoon
hours
when
SLS3=100°
is
at
noon.
As
noted
by
Kurth et al.
[2008]
95
in
their
derivation
of
the
SLS3
longitude
system,
SKR
intensities
tend
to
peak
in
the
96
prenoon
hours
when
SLS3=100°
is
at
noon.
Thus,
any
relationship
between
the
97
interaction
with
the
cam
and
the
magnetopause
and
the
generation
of
SKR,
if
one
98
exists,
is
not
simple
and/or
involves
a
delay
of
a
few
hours.
99
Also
shown
in
Figure 2
are
green
pixels
that
locate
the
midpoints
of
the
periodic
100
plasma
events
shown
in
Figure
1
of
Burch et al.
[2008].
Also
shown
in
Figure 2
is
101
the
spiral
path
in
R
vs.
SLS3
coordinates
derived
by
Burch
et
al.
as
a
fit
to
a
larger
102
data
set
(as
shown
in
their
Figure
2),
with
those
events
lying
beyond
35
RS
103
exhibiting
the
magnetic
signatures
of
plasmoids.
The
spiral
path
of
the
periodic
104
plasma
events
are
seen
in
Figure 2
to
intersect
the
outer
lobe
of
the
plasma
cam,
105
and
the
four
panels
of
Figure 2
show
how
the
plasma
cam
and
spiral
rotate
through
106
Saturn's
magnetosphere
and
intersect
the
magnetopause
as
the
planet
rotates.
107
107
108
109
Discussion and Conclusions A
statistical
study
of
plasma
ion
fluxes
in
Saturn's
magnetosphere
covering
the
110
entire
~2.5‐year
period
of
the
Cassini
near‐equatorial
orbit
has
revealed
a
cam‐
111
shaped
region
of
plasma
when
plotted
in
the
L
vs.
SLS3
coordinate
system
(Figure 112
1).
For
most
SLS3
longitudes
the
plasma
fills
a
circular
region
within
~15
RS
while
113
in
the
SLS3
west‐longitude
range
between
0°
and
45°
the
plasma
extends
farther
114
outward
to
near
25
RS.
As
the
planet
rotates,
the
cam
intersects
the
magnetopause
115
once
per
Saturn
day
(Figure 2).
It
is
notable
that
when
the
SLS3=100°
meridian
is
at
116
noon,
which
coincides
with
the
average
peak
in
SKR
intensity
[Kurth et al.,
2008],
117
the
plasma
cam
intersects
the
magnetopause
in
the
afternoon
hours.
Since
the
SKR
118
generation
is
known
to
be
at
low
altitudes
along
magnetic
field
lines
that
intersect
119
the
magnetopause
in
the
prenoon
hours
[Gurnett et al.,
2007],
it
is
unclear
what,
if
120
any
connection
there
is
between
the
interaction
of
the
plasma
cam
with
the
121
magnetopause
and
the
generation
of
SKR.
However,
it
can
be
stated
that
the
plasma
122
cam
will
produce
periodic
plasma
events
at
all
L
shells
between
~15
RS
and
~25
RS
123
and
that
the
ion
events
lying
along
the
spiral
path
identified
by
Burch et al.
[2008]
124
and
plotted
in
Figure 2
will
extend
the
periodic
events
outward
to
~50
RS
on
the
125
night
side.
126
The
plasma
cam
was
observed
for
the
first
time
in
SLS3
coordinates,
which
are
127
derived
from
SKR
measurements.
Therefore,
there
must
be
some
connection
128
between
the
two
phenomena.
However,
encounters
of
the
plasma
cam
with
the
129
magnetopause
are
not
consistent
in
any
straightforward
way
with
the
generation
of
130
SKR
in
the
pre‐noon
hours
when
SLS3
=100°
is
at
noon
[Gurnett et al.,
2007]
131
because
at
this
time
the
outer
cam
lobe
intersects
the
magnetopause
in
the
132
afternoon
region.
133
Another
outstanding
question
is
what
produces
the
observed
plasma
cam.
An
134
interchange‐driven
two‐cell
convection
pattern
locked
in
SLS3
as
suggested
by
135
Gurnett et al. [2007]
is
one
possibility.
In
the
Gurnett
et
al.
model,
outflow
from
such
136
a
convection
pattern
occurs
in
the
SLS3
range
of
330°
as
derived
from
wave
137
measurements
of
total
electron
densities
at
R
=
3‐5
RS,
which
peak
near
this
138
longitude.
If
this
outflow
is
in
some
way
responsible
for
the
outer
cam
lobe,
a
139
corotation
lag
of
about
2.5°
per
RS
would
be
estimated
to
occur
between
4
and
25
RS.
140
Burch et al.
[2008]
suggested
that
the
ion
events
and
plasmoids
observed
along
141
the
spiral
path
plotted
in
Figure 2
were
produced
by
reconnection
on
closed
field
142
lines
that
are
stretched
outward
by
plasma
loading
in
the
longitude
range
lying
at
143
the
base
of
the
spiral.
The
overlap
of
the
outer
lobe
of
the
plasma
cam
with
the
base
144
of
the
spiral
shown
in
Figure 2
provides
further
evidence
for
this
suggestion
with
145
the
plasma
cam
being
responsible
for
the
plasma
loading.
A
possible
conceptual
146
model
would
then
involve
the
following
steps:
147
148
149
150
1. Plasma
loading
of
magnetic
flux
tubes
occurs
in
a
restricted
range
of
SLS3
longitudes
(source
not
yet
known);
2. These
flux
tubes
rotate
with
the
planet
but
with
a
lag
of
~15%
at
distances
near
25
RS;
151
3. When
the
plasma‐laden
flux
tubes
rotate
into
the
tail
region
their
further
152
stretching
ulitmately
leads
to
magnetic
reconnection
of
the
type
proposed
153
by
Vasyliunas
[1983]
for
Jupiter,
and
this
reconnection
forms
the
154
plasmoids
that
were
observed
beyond
~35
RS
by
Burch et al.
[2008].
155
4. Corotation
lag
causes
the
ion
events
and
associated
plasmoids
to
lie
along
156
a
spiral
path
in
R
vs.
SLS3
coordinates,
and
this
spiral
path
rotates
with
157
the
planet;
158
5. As
the
plasma
cam
rotates
across
the
night
side,
the
spiral‐path
ion
events
159
and
plasmoids
first
intersect
the
magnetopause
in
the
post‐midnight
to
160
dawn
quadrant
with
results
that
have
not
yet
been
observed;
161
6. Later
the
outer
lobe
of
the
plasma
cam
intersects
the
magnetopause
on
162
the
day
side
with
consequences
that
may
include
SKR
generation;
163
however,
the
cam's
intersection
with
the
pre‐noon
magnetopause
occurs
164
when
the
SLS3
longitude
is
near
0400
LT
(Figure 2b)
rather
than
at
noon
165
as
might
be
expected
from
the
SKR
observations
[Gurnett
et
al.,
2007];
166
7. Rotation
of
the
plasma
cam
and
the
spiral
path
of
ion
events
and
167
plasmoids
are
responsible
for
all
periodic
plasma
events
(and
perhaps
168
magnetic‐field
events)
that
are
observed
beyond
~15
RS
in
Saturn's
169
magnetosphere.
170
170
Acknowledgements.
This
research
was
supported
by
JPL
Contract
No.
959930
171
with
Southwest
Research
Institute.
Helpful
discussions
with
Dr.
Michelle
Thomsen
172
are
gratefully
acknowledged.
173
174
175
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[email protected];[email protected]; [email protected]).
210
211
Figure 1. Average
ion
counts
per
4
seconds
from
anodes
closest
to
the
corotational
212
flow
but
within
20°,
at
energies
equal
to
the
corotation
energy
at
each
radius,
and
at
213
geographic
latitudes
from
‐10°
to
+10°.
214
Figure 2. Rainbow
pixels
show
the
plasma
cam
as
in
Fig.
1.
Green
pixels
show
total
215
counts
per
4
seconds
for
all
8
anodes
at
midpoints
of
periodic
plasma
events
from
216
Figure
1
of
Burch et al.
[2008].
Also
plotted
is
the
spiral‐path
fit
to
the
total
data
set
217
in
Burch
et
al.
The
SLS3
longitude
is
noted
in
each
panel.
In
comparison,
as
noted
by
218
Kurth et al.
[2008],
SKR
is
observed
to
peak
when
the
SLS3=100°
longitude
is
at
219
noon,
and,
as
noted
by
Gurnett et al.
[2007],
SKR
is
generated
at
relatively
low
220
altitudes
along
magnetic
field
lines
that
pass
near
the
magnetopause
in
late
morning
221
local
times.
222
FIGURES
FIGURE
1
FIGURE
2