RAW TRANSCRIPT -- NOT YET REVIEWED FOR CORRECTIONS BY CASSINI PERSONNEL

**FTS-NASA-VOICE**

**Moderator: Trina Ray**

**January 29, 2008**

**1:00 pm CT**

Coordinator: Today's conference is now being recorded. If you have any
objections you may disconnect at this time. Ms. Hendrix, you may begin.

Amanda Hendrix: Thanks. Hi everybody and welcome to the first CHARM
Telecon of 2008. I'm pleased to be sitting here with Don Mitchell from
Johns Hopkins Applied Physics Lab outside of Baltimore Maryland.

And he's going to be talking with us today about energetic particles and
Saturn's rotating magnetosphere. Don is a member of the Cassini MIMI
team which he will be talking about today and he has a lot of experience
and interest in working with energetic neutral atom imaging in the solar
system, both in Saturn system and I think in the Jupiter system and
elsewhere.

So I guess without further ado we'll go ahead and have Don get started.

Don Mitchell: Okay, hello everyone. If you've all got the first page of
this thing up, it shows a series of images of Saturn's magnetosphere
which I'll actually return to later, so I'm not going to explain it now.

But this is one rotation in the life of Saturn's magnetosphere as imaged
in energetic neutral atoms.

So I'll go on then to the following slide which shows the spacecraft,
and various instruments on it. And if you have the Power Point version
you will have seen a couple of pop ups where the INCA and the CHEMS and
LEMMS sensors are circled.

The INCA is a stand alone bracket on the side of the spacecraft and this
is the Ion And Neutral Camera. It's in one of the three sensors that
make up the MIMI instrument, and that's the one that does the energetic
neutral atom imaging.

And then the CHEMS and LEMMS sensors which are both (ends that you)
charged particle instruments. They measure the charged energetic ions
and electrons in the vicinity of the spacecraft.

So now we'll move on. Next slide just is reminder of the system we're
looking at and not the part that I'm going to talk about so much, but
the moons above with prominent, most prominently Titan and then the
large icy moons and then some rocks.

And below you see the layout in the inner magnetosphere - what's the
inner magnetosphere for me is the ring system for Saturn and the ring
(unintelligible) and where some of the icy moons lie.

And in particular the relationship of the E ring to the moon Enceladus
which I'm sure you've heard quite a lot about in the past. And it's an
important aspect of the magnetospheric environment for us.

Moving to the next slide, this is just a slide which describes very
broad brush what the magnetosphere is composed of, its large magnetic
bubble surrounding the planet Saturn, each magnetized planet has one.

It's the region dominated by the planet's magnetic field and around
which the solar wind flows. There's a - for Saturn there's a very
complex interaction between the icy satellites and rings of Saturn and
the plasma that's trapped in the magnetic field.

Also the neutral gases in that region and of course the solar wind which
is - which shapes the outer flow reaches of the magnetosphere.

In particular, Enceladus with its strong emission of water into the
magnetosphere at about four (unintelligible) radii dominates the neutral
and ionized gas populating the magnetosphere inside the 10 RS and
actually a large part of the plasma in the outer magnetosphere as well,
although they are also atmospheric and ionospheric contributions in the
outer region. A little bit of solar wind contribution as well.

Typical constituents are water product constituents with some - a little
methane, and of course the solar wind, hydrogen also from the solar wind
and helium double plus although not much of that. Unlikely the case that
works for example where the solar wind is probably half - makes up half
the content of the magnetosphere.

Moving on, this is Enceladus again. As I said, very important to us and
particularly because of the jets that are emanating from the south polar
region. They put out both the micron sized ice particles that make up
the E ring, but also more importantly to the magnetosphere large
quantities of gas, water products that provide both the neutral gas
which is very important to the evolution and condition of the
magnetosphere.

And also very - the prominent - the most prominent source of ionized
particles in the magnetosphere as we go to plasma. So the next slide
shows a schematic that was put together by the magnetometer team,
Michele Dougherty, (unintelligible) science back in 2006 and it shows
the gas neutral cloud coming out of the south end of Enceladus.

And before me, because it's ionizing in place it deforms the magnetic
field in the vicinity of the moon, and that was really how the
magnetometer team was able to compound the fact that this cloud existed
and then other teams jumped in and found more evidence for it.

Again, the next slide shows the same figure, and it also shows some
measurements from the CAPS plasma team that indicates that there's, the
plasma inside of about 10 Saturn radii \-- and in this diagram the
radius is shown in decreasing - from large radii to small radii as time
goes from left to right.

You see that inside of about 9 or 10 Saturn radii, there's a very strong
increase in the density of ions, water product ions relative to
hydrogen, and in fact that continues much further out in the
magnetosphere. The cutoff in this figure was really more of a latitude
cutoff, it turns out, than a radial distance cutoff.

And so if you stay in the equatorial plane, that dominance of water
product is maintained, instead it went further out in the magnetosphere.

So again without Enceladus, Saturn's magnetosphere would be a very good
replace, probably hydrogen dominated and not water product dominated,
and the mass of the ions makes a lot of difference as to how the
magnetosphere behaves.

Amanda Hendrix: So that's really interesting to me though, and maybe
you're going to talk about this in a little while (unintelligible) but
that Titan is more important, relative - I mean Enceladus is really
turning out to be the important....

Don Mitchell: Yeah, that's a very good point. Titan was expected to be a
much more important source of materials for the magnetosphere, certainly
much further out in the magnetosphere and a very dense atmosphere, but
it turns out not to be.

There was expectations that there would be a considerable amount of
nitrogen in the magnetosphere and there's very little nitrogen. There's
a lot of oxygen which again is coming from Enceladus. I don't think
anybody anticipated coming into this how strong a source Enceladus was
and how weak a source that Titan would turn out to be.

So the next slide is rather complicated and I'm not going to go through
it in gory detail, but it shows a schematic of what our kind of
understanding of Saturn's magnetosphere was coming into the Cassini
mission.

And it's substantially held up there, there's some tweaks that need to
be done to this figure, but it's not too far off. So it shows the inner
part of the system dominated by this dust and gas from Enceladus and
then the Titan influence on the magnetosphere is probably overstated in
this figure.

It shows that Titan neutral (porous) and it's kind of a violet color,
and as far as we can tell that neutral (porous) really isn't there, or
if it's there it's very weak. And the hot plasma which is shown in red
is shown to be - it's trapped on the magnetic field and it extends out
into a large sheet, and is shaped in the outer part of the magnetosphere
by the magneto pause which is the current sheet that separates the solar
wind from the internal magnetic field dominated magnetosphere.

There are a lot more elements here which I just don't want to clutter
your minds too much with, but the one thing that's indicated that I will
point out is the ring current, which is the current that flows in the
equatorial plane around Saturn in the direction indicated by the arrow
in the sort of middle lower part of the diagram.

And that current is buried by the plasma in the system, it deforms the
magnetic field and it's a very important part of the whole
magnetospheric system, the way it's unique in Saturn.

We'll be getting into that a lot more.

Amanda Hendrix: And it looks from this slide like that current extends
from within the main rings of some, all the way out almost to Titan?

Don Mitchell: It does look like that here but it doesn't actually do
that.

Amanda Hendrix: Oh, okay.

Don Mitchell: And that's part of the inaccuracy of this diagram. At the
time of the Voyager fly bys it was thought that the energetic particle
population basically went all the way up to the ring edge, and some
energy ranges that's still true.

The very high energies, there's a draft particle population that goes
all the way in the ring edges and the ring - of course inside the rings
it's empty because the rings absorb all the particles.

But in fact in the ring current energy range which \-- actually there
are two energies and I'll into that a little more later \-- but in the
energetic particle range which I'm talking about which tends to be from
10 kilovolts hydrogen up to several hundred kilovolts there, the region
inside of about 6 RS is almost empty of those particles.

And that's because the gas density from Enceladus is so high that charge
exchange losses, the ions pick up electrons from that neutral gas and
once they pick up electron become a neutral, they are no longer
magnetically trapped and just leave the system.

And that loss process is so much stronger than the source processes of
those particles that they just can't survive in that region. So it's
really quite empty of those particles.

So the ring current sort of in the 7 RS so technically, so the next
slide again just emphasizes one more time how important Enceladus is,
this shows a view of Enceladus which is exposed such that you can see
the E ring quite well and you can see the almost the active contribution
of Enceladus to the E ring. It certainly peaks at the orbit of Enceladus
and then it just diffuses away from that by random motion of the
particles.

And the gas does pretty much the same thing. It's the distribution of
gas, looks very much like the distribution of the E ring except it's a
little bit more extended in the direction perpendicular to the
equatorial plane so that it goes above and below the equatorial plane a
little bit more than the E ring particles do.

So the next slide shows the gas jets from Enceladus, the - just a
reminder the water product plasma, and then the gas cloud is imaged by
the UVIS measurement instruments on Cassini.

This is UV emission from the oxygen in the gas cloud and so you can see
its relationship to Saturn and the rings and so on, and so it dominates
the inner part of the magnetosphere.

The next slide is a different view of the magnetic field, much more
sketchy, but it shows where the trapped low energy plasma in the field
lies, reaching from the low energy plasma does go well in all the way to
the ring edge because it doesn't get lost by charge exchange.

It does charge exchange with a cold neutral gas but it comes from the
cold neutral gas, so as charge exchange occurs, and an ion is lost, it
leaves behind a cold ion, that ion gets swept up in the rotating
magnetic field and brought up to the same energy as the rest of the ions
in the inner magnetosphere.

And so it's basically exchanging particles, and there's no net loss or
gain in that process. So the plasma at the very low energies can exist
quite comfortably in that region and just goes through this constant
turnover in the identity of actual particles that are there at any given
time.

There's this sort of disc-like, if you look at this as an object of
revolution, the magnetic field sort of forms a disc, the sheet of
current which is the ring current and evolved into the tail current on
the night side.

And in the other part of this diagram, which I want you to pay attention
to is the arrow that shows rotation, because unlike in the case of
(Eros), rotation is a very important part of the dynamics of this
system, and show more evidence for that in a bit.

So going on, this is the data we got during the Saturn orbit injection,
which shows just a spectrogram of energetic particles. Energy is in the
vertical dimension, time going horizontally and I've shown a few
fiducials for radial distance on the bottom.

So you can go across in and out of 6 RS distance, and of course between
those two arrows we're at lower altitudes. Inside of that, those
energetic particles are simply gone, and that's where the water products
twist out if you (unintelligible), if you click it again you see the
diagram that I showed you earlier of the UV emissions of the water
product cloud and that's basically where it fits into this diagram.

And it's this cloud that is eating away those energetic particles, not
allowing them to exist there. So then if we move on again...

Amanda Hendrix: I'm sorry, can you go back to slide 12, so inside 6 RS
you've got a lot of neutrals, and not energetic particles because
they're getting eaten up by the neutrals, but what's happening at the
low end?

Don Mitchell: Okay, good question, it looks like another gap in the
energetic particle population that's just around, about three-quarters
of the way across the diagram to the right.

And labeled below, this is where the spacecraft was climbing in latitude
and went out of that disc-like structure that I talked before into the
magnetic field that's above that which contains very few particles that
that region is essentially not, doesn't close through the equatorial
plane, these magnetic field lines connect out into the solar wind.

And any particles that get on to them can simply stream along the
magnetic field and escape the system. So the (lobes) are typically empty
of energetic particles in any magnetosphere and the equatorial region
tends to be full of energetic particles.

So that's not an unusual characteristic for any magnetosphere to have.

Hendrix: If you'd stayed at the same latitude all the way out then
wouldn't have seen a drop out in the energetic particles.

Don Mitchell: That's correct. Now, the - however it brightens up again
in this region labeled sub-storm, and there are dynamics going on, it's
not a static kind of a place, but in addition to the rotation there it's
very sporadic dynamics going on.

And as in Earth where we have sub-storms in the magnetotail, we labeled
this as sub-storm whether it's really exactly the same phenomenon or not
we're not sure at this point.

But certainly there is reconfiguration of the magnetic field and an
energization of particles and this happens in the analog iterate is
called a sub-storm. It's where this current sheet, there's a break down
in the instability that goes off in the current in the current sheet.

A phenomenon called reconnection goes on, this releases energy which is,
has been stored in the current and in the stretch descended magnetic
fields. The fields relax and become more like dipolar fields.

So you effectively move even though the spacecraft is still climbing in
latitude, the but the current sheet all the sudden expands over it, so
you see because it's relaxed, you see that the content of the current
sheet show up at higher latitudes when this phenomenon goes up.

Amanda Hendrix: So does that - I mean that's at 20 RS which is also
where Titan is, but it doesn't have anything to do with Titan in this
case?

Don Mitchell: Probably not, yeah, Titan - one researcher, Chris Russell
has postulated that Titan does affect the frequency of sub-storms and
that you get more sub-storms when Titan is in the tail region.

He may be right, I think that's something that we'll be looking at
(unintelligible). So moving on from this, this is just a diagram
attributed to Stan Cowley which looks at a cut through the equatorial
plane of the magnetosphere and looks at the flows of the plasma.

And it shows that both the Saturn there, the flow of the plasma is
rotational, it's rotating about the planet and this is because the
magnetic field has its flip points in the ionosphere of Saturn.

That ionosphere is spinning with the atmosphere and there's an electric
field that's created in that motion which is translated along the
magnetic fields up to the plasma in the equatorial plane and cranks it
up to the same rotational velocity as the flip points of the magnetic
fields.

So the whole magnetosphere rotates essentially connected coupled to the
ionosphere of Saturn, and this happens at Jupiter even more strongly, it
happens at Earth, but Earth it's a slower, you know 24 hour rotation
where this is just under 11 hours rotation period and a much bigger
system.

So the velocities are much larger here and it's a much bigger factor in
the flow patterns of the particles at Saturn than it is at Earth. But I
was going to say Jupiter is probably even more dominant than it is at
Saturn.

Now if you go far enough out in the system and on the day side you run
into the magneto pause separation current that separates the solar wind
from the internal magnetosphere and the core rotation stops there
because the ionosphere has no influence on the solar wind out there,
it's not magnetically connected any more.

And on the night side there's also a breakdown it's basically you're
getting into such a long lever in the connection from the ionosphere to
the equatorial plane, that the torques exerted by other forces \-- again
mostly dominated by the solar wind flow \-- end up being stronger than
the torques that the ionosphere can exert.

And so the flow patterns break down and change direction. But there's
this very large region going all the way out to Titan's orbit which is
typically rotation dominated and even much further than that as we'll
see.

So just wanted to impress upon you how much of this system is rotating
and connected to the ionosphere.

The next is a diagram from a science paper that's just showing the
atmosphere and you have to imagine the ionosphere above this. It's
there, it's just not imageable.

But there's some slippage between the atmos - sort of friction between
the atmosphere and the ionosphere that causes the ionosphere to rotate
and the ionosphere then causes the magnetosphere to rotate. Each of
those couplings may have a little bit of slippage in them, it's like a
slippy clutch in a car or something, and so you don't necessarily expect
the magnetosphere to rotate exactly at the same velocity as the
atmosphere rotates.

So if we go on to the next slide, you'll see a profile of the
atmospheric rotation curve as a function of latitude. This is mostly in
the southern hemisphere where we've been getting most of the data.

And you see the pattern of different flow speeds, either jets of
different, the atmosphere, of different latitudes. We don't see this
pattern imposed on the magnetosphere and part of the question we have is
\-- or at least we haven't resolved it if it is \-- and so this slippy
clutch is sort of integrating out the bind structure in the atmospheric
jet.

The next slide shows some relevant rotation velocities plotted against
the atmospheric rotation curve, so that again the jets are represented
by the data symbols as well as the lines on this plot.

And then in broadly, the colored vertical band shows the range within
which the rotation of the magnetosphere has been pinned down. The
Voyager period is on the right edge of that band, and that - based on
SKR \-- that's Saturn kilometric radiation and I'll talk more about that
in just a moment \-- but you see that the left hand side of that band is
also determined by driving kilometric radiation period, but from
Cassini, and Ulysses got intermediate values.

So part of the puzzle has been the variation that we see in this, and
then more recently (Anderson and Goldreich) have - based upon radio
science data have shown that what drives that rotation period for the
interior Saturn which is quite different from that of the magnetospheric
period. So this shows that the interior is only loosely coupled to the
magnetosphere.

Going on, this shows the - how the SKR data has been used to derive the
rotation periods for Saturn, the Voyager on the - is the shorter period
and then during Cassini era we've gotten longer periods for the rotation
of Saturn.

The SKR data in the upper left of the black box shows that it is just a
series of repeating bursts, and they're irregular, but if you look at
them over a very long period of time there's a very well defined
frequency that falls out.

And how - and that's the frequency that was for a long time used as a
proxy for the internal rotation rate of Saturn and it's been within the
Cassini era and a little bit before that with Ulysses where it's been
indicated that this is not really a constant, it's a varying frequency
and so it can't possibly be what the internal rotation rate of Saturn
is.

And angular momentum conservation would require that to be much more
steady than what we've seen from this measure of rotation.

The next diagram is, you know I'm not going to go through in any
details, just a summary of rotation period measurements that have come
out since the 1980's to the present.

And you - the period whether you look at it now as mean seconds or in
fractional hours, is right around 10.8, but it's actually changed
anywhere from 10.3 or so up to 10.9 or so.

So there's quite a bit of variability, part of that is measurement error
and part of that is true variability. The next slide shows the evolution
of that period over the course of the Cassini mission.

And so even over the course of the Cassini mission there's been enough
variation in the rotation rate so that the SKR data couldn't be fit with
one particular frequency.

So using that information, the team lead by (Don Grenetta) and (Bill
Fereset), I always use the SKR data to derive a polynomial fit to that
curve and to derive a coordinate system that represented what the
instantaneous day to day rotation rate of Saturn is over the course of
the mission so far.

And you can use this then as a variable coordinate system to organize
all the rest of the observations that we make in the magnetosphere. Some
of them are well organized by it and some aren't, but a lot of the
energetic particles tend to be well organized by it, and that's what I
want to continue with.

That was I guess, what, slide 19. We'll go on - the next slide, this is
some magnetic field data from Voyager where it was first realized that
this rotation modulation of magnetospheric quantities was not restricted
to just the SKR radiation but was also in the magnetic fields in the
middle of the magnetosphere.

The next slide is a diagram and if you're on Power Point you can click
it twice. A diagram of this, what's called the camshaft model that was
derived from that magnetic field data which postulates that there's - in
the inner magnetosphere there's kind of a stiff part of the plasma that
goes around like a camshaft and is tied tightly to Saturn's ionosphere
and rotates therefore with Saturn and disturbs the rest of the
magnetosphere outside of that camshaft region as it rotates around.

Part of what this diagram indicates is that the rest of the
magnetosphere is labeled as sub-co-rotating and what that means is that
if you measure the velocity of the plasma in the magnetosphere, it's
rotating around Saturn always, but it's rotating at a speed that is
slower than this period that's indicated by the SKR radiation.

So if you were hooked to that plasma, you get a longer rotation period
by quite a bit, it would be oh, 11 1/2 or 12 hours rotation period
rather than 10.8 hours rotation period.

So this cam shaft going around at 10.8 hours, the rest of the
magnetosphere plasma is going around slower than that, but this thing's
causing a ripple in the rest of the plasma which has, and that ripple
actually is pretty substantial and ends up affecting the energy of the
plasma that's going slower than it is and producing currents in the
magnetosphere that rotate around with this cam. Right now we don't have
a more sophisticated model than this, the model itself is kind of a
cartoon model.

Amanda Hendrix: Where is that camshaft, what RS?

Don Mitchell: Well good question, we, and actually let me go onto the
next slide to try to answer that.

Amanda Hendrix: Okay.

Don Mitchell: There's - this shows at the top view, this is now in
Saturn longitude system, so it goes around 360 degrees around the
planet.

You can see the organization of the SKR intensity and you see this kind
of sine wave in the top panel, the sine wave of the Saturn kilometric
radiation and that's kind of what defines this system in the first
place, it's not too surprising that that fits it well.

But then the next one down shows the magnitude of one of the components
of the magnetic field vector, and you see that it's also well organized.

Now you've got a radial \-- this is data that Don Burnett from Iowa put
together \-- it's got a radial distance range on the magnetic field
perturbation, and that runs from 3 Saturn radii which is about as far in
as we can measure it because we don't go inside that with the spacecraft
very often.

After 12 Saturn radii, well organized by this system of itself and
radii. Beyond 12, it does something very different and that's a puzzle
that I'm going to talk about a little bit but not a lot because we have
to spend probably two more CHARM telecons talking about that one.

And then the third panel shows the electron density and you can see the
electron density is also well organized by this. Now this is probably
getting to the guts of where the cam is coming from. This is not just
electron density but it's also ion density, it's the net neutral plasma
field so electron density is proxy for the ion density.

But this is organized well between the distances of 3 and 5 RS. So this
is much more restrictive, if you go beyond 5 RS, the electron density of
the plasma density is not well organized by this system, yet the
magnetic field is much further out.

So this is likely much closer to the core of the cam that we're talking
about. This is a density enhancement that's essentially traveling around
at this rotation speed. Now at these low latitudes, low latitudes means
both equatorial crossing distances in a dipole field, this plasma is
connected very tightly with the ionosphere, the coupling tends to be
strong at low latitudes and weaker at higher latitudes.

So the atmosphere and ionosphere can exert a much more rigid coupling to
the plasma close to the planet, which this is, and keep it locked in the
rotation period of the ionosphere.

Further out as I mentioned the plasma's going slower than that, but this
part's not. This part's rotating fast right, with the same period as the
SKR. And so this is like a pressure bulge on one side of the planet
that's moving around with the planet, and maybe the source of this
ripple that goes off through the rest of the magnetosphere.

Amanda Hendrix: So on this lower panel, you're measuring electrons in
the 3 to 5 RS region. Isn't that where we didn't have a lot of
electrons, or these have a different energy range?

Don Mitchell: This is, yes, this is where we don't have the high energy
stuff.

Amanda Hendrix: Oh okay.

Don Mitchell: But this is the low energy plasma that as I mentioned,
this is the stuff that first is going to be happy to be there because
it's actually being created from the gas cloud that Enceladus is putting
out.

Enceladus puts out this gas cloud, sunlight ionizes the gas and so it's
created in place and it's cold, it's not hot plasma.

Amanda Hendrix: Okay. Got you, okay.

Don Mitchell: Okay, it's confusing, it is, but there's a very, very
large range of energy we're talking about here and the behavior at the
low energy is really very different from the behavior at high energy.

So moving on, this is the a cartoon of the model that Don Burnett put
forward based on the data that was shown in the previous slide, and it
shows again this disc rotating Saturn of course inside the - it's got
kind of a hole in the middle, and that hole in the middle is where rings
are and there's no plasma at all in there.

But outside the rings, there's the disc rotating around, and he's
showing outflow at one particular longitude, and if you look at the next
slide, that diagram is repeated and then there's a sort of an equatorial
cut through that diagram as he envisions it with outflow on one side and
presumably inflow on the other side and then the circulation cells which
are like eddy vortices based upon that flow of the plasma out on one
side.

And there's a complicated process by which that flow is maintained, but
basically as plasma runs around those eddies on the inside where they're
running toward the outflow region, it's the part of the eddy that is
closest to Saturn, there's more addition of material in that region
because the cold gas density is higher in that region and so sunlight
ionization will produce more plasma in that region.

So on the Saturn side of those eddies, there's mass being added to those
eddies, and that tends to make it then fling and by centrifugal
acceleration it gets flung out and outflow through the outflow channel.

And this whole pattern then rotates with Saturn and this outflow channel
presumably is the cam flow. There's a lot of dots to connect before we
can pin that down.

Again, we're at the cartoon level of physics of this stuff right now.
And the next diagram's just a pretty diagram. This is one that (Marty
Kibbleson) from UCLA put together in a science article.

And in this article she doesn't go into her model too much, but one of
the points that she has talked about in presentations about this is that
she's - what she's demonstrating in this diagram is simply that
Enceladus is so important to the magnetosphere, a point I've tried to
make as well.

But that she - her model also says that the - if a special place in the
ionosphere, a special longitude in the ionosphere that has a different
conductivity, a higher conductivity than the rest of the ionosphere
which would account for this preference for a particular longitude in
this rotating system.

Of course right now we don't have any idea how such a conductivity
anomaly could be attained.

Amanda Hendrix: Could that maybe a - some sort of atmospheric storm or
something?

Don Mitchell: It could be, and that's something that - yes. It's
definitely a possibility, nobody's made such a correlation, I think
it's - there's a lot of cross instrument analysis that's going to have
to be done to try to establish the correlations that you're talking
about and we simply haven't reached that point.

We're still trying to figure out what's going on. But anyway, moving on,
just one last model I wanted to mention, it's actually not in press yet,
but it's been submitted and it's been presented in meetings by
(unintelligible) also at UCLA which tries to take the model by the
various models that I've shown you previous to this and integrate them
into a model that explains some of the energetic particle behavior.

And in this diagram, what it shows is this asymmetric population of
energetic particles which are rotating with the planet, and involvement
from the magnetotail where the instabilities in the magnetic field tend
to be more ripe for particle acceleration and so the - trying to
establish a link between acceleration mechanisms in the magnetotail,
very common to all magnetized planets and this rotational modulation.
It's a long convoluted connection, but there's some evidence that it
exists.

So again, one of the mysteries is how a cam rotating down at 3 to 5 RS
can cause things out at - as you will see, out as far as 40 and 60 RS to
be modulated at the rotation, the same rotation period. That's just very
hard to do, coupling through the whole magnetosphere, but it's a fact,
so we have to try to connect all the regions.

So in the next section what I wanted to talk about, you can skip the
stuff at the top of this slide and just go to the bottom. I'm going to
be talking about the typical behavior of energetic particles in the tail
current sheet, so that was the tail region on the previous slide. The
high latitude extension of those populations because there's some
interesting - again these things have to be connected to the ionosphere
through their high latitude extensions of the field.

And so not everything is happening in the current sheet and we can get
some important clues at the higher latitude extensions of these magnetic
fields. And then the appearance of - oh, yeah, low altitude, accelerated
ion and electron beams in the tail.

These are associated with the aurora which is also going to be tied up
in this whole story. And reconnection accelerated electrons, particle
jetting and acceleration, that's a long way of saying basically the
sub-storm type phenomena that I was talking about before, where you get
reconfiguration of a magnetic field, relaxation of stresses in the field
which are translated into heating of the particles.

The next figure which is slide 28 shows three panels, the top one is
energetic electrons, the second one is sort of medium energy protons and
the third one is the magnetic field strength.

And you can see little wiggles in the magnetic field strength, and now
the scale here is, there's - there are vertical dotted lines which show
distance from the planet in Saturn radii. So you see this data is going
much further out up to 40 Saturn radii and beyond.

And the only thing I want you to take away from this is that there are
wiggles there and those wiggles, as you might expect, are at this
rotation period of the planet.

And they are ubiquitous so you just see them every time we go out in the
tail, that same pattern is there. So this - moving to the next slide,
this summarizes an analysis that was done of those wiggles for two
years' worth of data.

And you can see that there are some of the periods that are at longer
periods than the rotation of Saturn as determined by these Saturn
kilometric radiation which is the solid horizontal line, but that most
of the data is actually lying just about on that line.

And it's actually a function of the signals and noise that we see in
that wiggle. When the wiggle is well established, and we get a good
signal to noise on the analysis of the frequencies associated with the
wiggles and the particle data, they line up right with the SKR period.

And this is again, in this period in this distance from 20 Saturn radii
out to 60 Saturn radii, much further from the planet and yet that same
period is going on.

And if we in the next slide \-- slide 30 \-- (slide) that data down into
the equatorial plane in a coordinate system that's determined again by
the Saturn kilometric radiation, we've done this for various epochs
during 2006, 50 day epoch.

And you see that the data lies roughly on the same spiral pattern in the
equatorial plane. So if you were to look at this, if you were to sit at
any particular radius, which is a circle in each of these diagrams,
you'd see that same period walking along, you'd see it coming around
once per rotation. You'd get a data point.

But if you look at them as a function of radius, they're not lying
radial, they're lying on a spiral. So there's like a lag in the
appearance of the peak of the particle intensities the further and
further you get out in the system.

And that data was all organized by the same spiral up until day 200 of
2006, and then after day 200, the spiral jumped to a different location.
This is, this sort of puzzled us, but I think this can be explained by
differences between the two hemispheres, the southern hemisphere and the
northern hemisphere.

Because all the data up until day 200 and including day 200 was taken
when the spacecraft was pretty close to the equatorial plane, and
because of the solar wind pressure on the magnetosphere the equator of
the magnetic field to the position of the current sheet in the tail, is
actually displaced away from the geographic equator by the solar wind
pressure.

Whereas southern summer, that's on Saturn, the solar wind is coming
basically from the same direction as the sun, it's blowing from the
south on the magnetosphere and forcing the current sheet a little bit
toward the north.

And so if you're in the equatorial plane you're going to be, any time
you encounter the current sheet it's going to be because the current
sheet bounced down from further north of you to over you and then
bounced back up if you want to look at it as flapping current sheets.

So after day 200 we started moving into the - out of the Saturn's
equatorial plane and in fact in I think, well let me go down a couple
slides before I talk about that. The next slide just again shows that
the - how well established that period is for electrons, for hydrogen.

And especially for oxygen, one of the other peculiarities of what we see
is that even though this plasma is all coming from Enceladus and it's
water products, so the water products will give you oxygen and hydrogen
primarily, maybe some OH.

The hydrogen and oxygen don't behave the same. The hydrogen - they both
show that period of 10.8 hours, but it's a much better established
period for the oxygen than it is for the hydrogen, another puzzle we
don't really know the answer to, but I'm sure it screams out in the
data.

The next slide shows you a series of these plasma sheet encounters in a
magnetic field data and in energetic electron data, and if you click
that slide one more time? That's funny, never mind, it worked on mine,
anyways it's not important.

But this shows what I was talking about. It shows the Saturn's equator
and the location of the current sheet is in green, the equator, the
equatorial plane is indicated in red down at the bottom.

And then you see a series of these encounters with the plasma sheet both
in the magnetic field data and in the energetic electron data. And the -
to get those encounters when you're sitting in the equatorial plane, it
must mean that that green current sheet is flapping up and down,
flapping down to the equator and then back up periodically.

So going on to the next slide, this was a special orbit for Cassini
because this was the transition from the equatorial orbits to higher
inclination orbits, and during this particular orbit there was a Titan
encounter where you see the kink on the inbound pass which changed the
inclination of the orbit at that point.

The inbound orbit was in the equatorial plane, below the nominal
position of the current sheet. The outbound orbit was at higher
inclination about the same distance above the nominal position of the
current sheet.

So this was a good test to see if this was a current sheet that was
flapping up and down, which is exactly what Jupiter's current sheet
does. Current - Jupiter has an offset dipole and so the - like Earth,
and so the magnetic equator wobbles around as the planet rotates and
flaps up and down.

And so you naturally get a period of - at the period of Saturn, of
Jupiter's rotation due to this flapping. But if you encounter the
current sheet from below or from above, you'll see 180 degree phase
shift between the flaps.

In other words, looking at this diagram we're looking at, if it were -
if the current sheet were flapping or were an offset disc around Saturn
and rotating around, you'd see at one phase of the rotation that the
disc would be down near the inbound track, and then 180 degrees later it
would be up near the outbound track.

And so you should expect to see the encounters in the inbound track be
180 degrees out of phase with those in the outbound track.

Now if we move on, you'll see a series of encounters on the inbound
track from \-- this is in the magnetic field now and the plasma data,
everything lines up nicely.

And the next one \-- that's just a repeat of an earlier slide, we can
skip that \-- oh that's the one we had there, doesn't matter. And this
again shows the inbound and outbound tracks, sorry for the repetition
here, this is sloppy editing.

But if we move on one more slide, you'll see what the energetic electron
data looks like in SKR coordinates for this inbound track and for the
outbound track. And you'll see that the peak in the energetic electron
data is not separated by 180 degrees in longitude, it's separated by
more like, I don't know, 30 degrees in longitude.

And going on one, another slide I just basically did a superimposed
epoch of all that data and averaged it up so you could see it more
clearly, so you can see it's a very well established peak on the inbound
track, and then I drew a line where the peak should be on the outbound
track if it was flapping with the current sheet up and down.

This is slide 38, going to slide 39, you can see that peak shifted only
a little ways. It's nowhere near where it ought to be if it was a
flapping current sheet. So it's not a flapping current sheet, and a lot
of people have tried to explain it based on the flapping of the current
sheet, but it's not going to cut it.

So this cam that's going around in the inner magnetosphere is causing a
particular longitude to light up way out in the magnetosphere,
presumably due to this ripple it's sending out through the whole
magnetosphere.

But it's actually got a stronger pattern out here than you do in the
intermediate distance. So...

Amanda Hendrix: And just to interrupt real quick, when you say flapping
current sheet which isn't (unintelligible), that's due to the
(unintelligible) magnetic field?

Don Mitchell: Yeah, which Saturn doesn't have. So you don't expect to
see...

Amanda Hendrix: Okay, oh yeah, yeah, yeah.

Don Mitchell: Forgot to mention this, but you know I think you all know
probably.

Amanda Hendrix: You see that in Jupiter.

Don Mitchell: Yeah, Jupiter's got a token magnetic field, it does flap.
Saturn has an aligned dipole, it shouldn't flap, but it does show this
periodicity way the heck out there. So people have been trying to come
up with reasons why Saturn really has a tilted dipole that's somehow
shielded in the inner magnetosphere and appears to be aligned, but is
expressed far out in the magnetosphere as a tilted dipole.

And what this data says, no, that's not what's going on, it's not a
tilted dipole, either inside or outside but it's a very asymmetric field
nevertheless.

So moving on. Now I mentioned that the high latitudes have a different,
or a complementary tale to tell. We've talked about things happening in
the equatorial plane or very near the equatorial plane. This illustrates
a series of days of data during which the latitude of the orbit was up
at about 30 degrees.

So we were well off the equatorial plane now and well away from even the
magnetic equatorial plane. And so up here we didn't know what to expect,
but if you go to the next slide you see a very periodic structure in the
energetic electron data.

And this is actually one of the cleanest events that we have over the
periodicity. It's even better established here than it was in the
equatorial plane. So it shows that this communication from the
ionosphere out to deep in the magnetosphere because these field lines
will map out to crossing equatorial crossing points of 30 or 40 Saturn
radii.

There, this period is very well established here.

Amanda Hendrix: So real quick, this is on 41, the vertical dash lines,
that's the RS?

Don Mitchell: Yes, I'm sorry.

Amanda Hendrix: Okay, so they're all up at the (unintelligible).

Don Mitchell: Yes, those are the spacecraft distances that the
equatorial crossing distances of the field line is even further out than
these. Because we're off the equator and then the dipole - stretched
dipole but it's going to be crossing well beyond this.

And at this latitude and for this orbit, well really what we're doing is
we're almost following one field line out. We're pretty much on the same
(flux tube) going out over many days at a time.

So we're able - it's rotating, it's not the same in the sense that
it's - we're seeing two (flux tubes) in a rotational sense, but they are
so similar in radial, if that makes sense.

Amanda Hendrix: So, but the periodicity really seems to pick up outside
of like 12 RS.

Don Mitchell: Yes, inside a 12 RS, things are weird. And that's why
we've done our analysis mostly on data from - this analysis on data from
20 to 60 RS, because the phenomenology is really repeatable in that
region.

There's the region from 3 to 5 is special because of the cam or whatever
it is, the density anomaly. From 3 to 12 in the magnetic field data it's
special because it has particular twist in the field that's repeated
through that region, and it's well organized by this coordinate system.

So between 12 and 20 somewhere in that region, there's a sort of a real
change in the behavior, in the shape of the magnetic field, in the phase
of the components. They're still modulated at that frequency, but it's a
new regime, it's just a different place there.

So the connection there is going to be interesting to understand,
whether that connection is translated through the equatorial region as
this cam up here is to be, or whether it's translated through the
ionosphere, high latitudes through coupling between different latitudes
on Saturn. And, but in any case there's a real sea change between 12 RS
and 18 or so.

So moving to the next slide, this just shows if you, the data from the
previous slide and dimmed it in this SKR longitude systems. Again, it
all peaks at not surprisingly at around 200 degrees longitude which is
where the electron peaks in those low latitude passes that I showed you
in the tail.

Things like these electrons are connected to the tail so they shouldn't
be in the same longitude region. And if you, next slide, line them up on
that spiral, it shows the shift in spiral, not the original spiral, and
this is the later epoch which has the shifted spiral.

So again it's in the northern hemisphere. The other - the data before
the shift was in the southern hemisphere. This thing must be some kind
of a twist between the, in other words the enhancement shows up at a
different longitude in the southern hemisphere than it does in the
northern hemisphere, about 30 degrees different in longitude.

It's got to be a twisted structure that's rotating around at this period
with one hemisphere leading the other hemisphere.

Okay, and if I look at one of these enhancements and blow up some data
from - this is from the INCA sensor actually, the INCA's ion neutral
camera, it uses - most of the data you'll see here is from the neutral
part of it, but once we turn off the collimator plate which sweeps ions
out and don't let them in, then we let the ion in and we measure ions.

It's very sensitive ion measurement and much more sensitive than
anything else we have on the spacecraft for measuring energetic ions and
so we can see things at that intensity level that we can't with the
other instruments.

And you see that there's a well correlated enhancement in the ions
associated with that energetic electron, even at these high latitudes.
The little sine waves that are going on underneath this diagram at the
bottom panel, that's spacecraft attitude.

And we're in one of the downlink rolls here so this spacecraft is
rolling and you can see a modulation of the ion data at that roll period
which says that the ion data is not the same in all directions, it's got
both direction preferences.

And if we move to the next slide, you can see that expanded. Now it's a
time intensity plot with a whole bunch of different energy channels
shown. So we've got the lowest energies at the bottom of this bottom
panel, and the highest energies at the top of this bottom panel.

And you can see on the left hand side of this panel there's a repeating
intensity variation which is - this is caused by a flow of the plasma.
As the plasma flows past the spacecraft, if you're looking into the flow
you get a higher intensity than if you're looking away from the flow.

So as the spacecraft rolls, you see first looking into the flow and then
looking away from the flow, looking into the flow, looking away from the
flow. So this is a measure of the flow of the plasma and it's going
around in that same co-rotation direction, but not at the full
co-rotation speed, going slower just like all the rest of the plasma
does. Then there's this enhancement which it's much harder to see that
flow than I thought it would be, and it's weaker, the flow ends up to be
in the enhancement of the ion.

And then the flow analysis has to be picked up when it gets down to the
weaker intensities after you're out of that part of the ions that
correlated well with the energetic electrons.

There's another feature in here which are these little tiny little dots
that appear also at the spacecraft roll period, and with a very
different spectral signature than the rest of the data.

Those are beams that are coming up along the magnetic field, they're
exactly field aligned, and they are traveling away from Saturn. These
are energetic protons that are being accelerated in the auroral zone.

And we're seeing these things spit out along the fields out and into the
tail. So we don't know how important the source of these are for the
general ion population in the tail, but they are energetic and they are
intense.

They are sporadic, we see them come and go, but they are a fascinating
feature of this whole system and another clue to the coupling between
the ionosphere and the magnetosphere.

It's that coupling and the slipping in the coupling, the slipping clutch
mechanism that generates electric fields and waves that can accelerate
these particles at low altitudes and then you see them flowing out along
the magnetic field.

So that's all I was going to talk about with the energetic particles,
and I have a series of conclusions about this which I can go through or
not, or I can just go on. Should I read these?

I've got the energetic - I've got the neutral imaging stuff still to
come.

Amanda Hendrix: Why don't we go through this quickly and then go...

Don Mitchell: Okay, so this sort of summarizes what I talked about with
the charged particles. All charged particles regardless of species \--
that is oxygen and hydrogen and electrons the major species \-- exhibit
this strong period say of about 10.8 hours which is the same SKR period.

And the period of these are evident over time periods of more than a
year. That is at least we've seen them ubiquitously which, all the time,
we've seen. The indices are evident at both low and high latitudes, the
equatorial plane or at these higher latitudes.

A regular spiral pattern appears if you map them into these longitude
systems that is established by the SKR data. The most recent one is
called the SLS system which is Saturn Longitude System Three.

They keep updating the polynomial (sys) to the SKR data as we gain more
time in the mission, so that they can extend the validity of it because
it is variable system and you can't extrapolate it very far without
getting into trouble. So there will be a Four and there will be a Five
as more data gets added to the mix.

(Unintelligible) time flows in the magnetotail are predominately cross
tail, in the same direction as co-rotation but they're well below the
rigid, what would be rigid co-rotation velocity.

Reconnection flows are much stronger \-- and I didn't show this stuff so
I'm not going to - I shouldn't talk about it here. It's more flows that
go on and I didn't want to get really confused.

Field aligned electrons or ions and electrons probably accelerated in
the auroral and sub-auroral (unintelligible) here independent of the 10
hour and 45 minute periodicity. I didn't point that out on that slide,
but you could see those beams whether you were in the ion enhancement or
in the weaker period.

We're sort of asynchronous with that, so these - the generation of these
field aligned electrons and ions, little intense field aligned beams
that are coming out, is not so well organized by this SKR period,
they're sort of asynchronous with that.

And then the last one ignore because we don't show you anything on that.
Now the next slide, 47, I'm going to change gears and talk about
energetic neutral atoms, and we showed that in the opening slide, some
images from that.

Energetic neutral atoms are created when ions that are trapped on the
magnetic field and gyrating about the magnetic field encounter a cold
gas atom, pick up an electron from it and become energetic neutral.

There's very little energy exchange in that process. It just - about 30
EV is the typical energy loss of the ion in that process, and this is an
ion that's got 10 kilovolts to hundreds of kilovolts.

So it's inconsequential. But what is consequential is that the ion no
longer is charged, so it's no longer trapped on the magnetic field, so
now it just dips off into space, literally in whatever direction it
happened to be going when that charge exchanged.

So it's kind of a random process, and you, any place where you have a
mix of energetic ions and cold gas density of certain, you need some
threshold gas density that really see emission to get enough intensity.

But anyway, you mix cold gas and energetic ions and you will get
emission of energetic neutral atoms. So the Ion And Neutral Camera on
Cassini was built to produce images in that emission.

So if you just pretend these things are photons instead of neutral
atoms, you can think of this mix emitting photons and you can take a
picture of it. Well they're emitting neutral atoms and we take a picture
of it, just using those as the emissions.

So moving on to the next slide, this shows what that mix is at Earth,
just to illustrate because this is where we first starting using the
technique. At Earth you've got the hydrogen geocorona which is the
extended exosphere.

Hydrogen goes out to many Earth radii, and falls off with distance from
the Earth and you've got the ring current. Earth's got its ring current,
Saturn's got its ring current, Jupiter's got a ring current.

When you mix those two, you should get energetic neutral atom emission
and the lower left panel shows what we our simulated image of that would
look like.

If you go to the next slide, this is actually a movie for those of you
who have a movie link. And you can see an image of the ring current. The
instrument that we took it with, this is not INCA but it was a clone of
INCA, which was flown on an Earth based spacecraft, it's on the left.

And on the lower right you see an image of the Earth's ring current.
We've gone in the limb of the Earth and magnetic field lines and just
represented dipole field lines at different local times.

That's 4 Earth radii and 8 Earth radii just for scale and orientation.
And you can see kind of why they call it a ring current, it's a ring,
and this is the energetic neutral atom emission, so I'm not going to,
for those of you who had the movie you could see that it's dynamic, it's
changing a little over time.

A lot of the change that you saw in that series of images was just
because of the orbital motion of the spacecraft. But it was also
evolving, the intensity was decaying as we went through a magnetic storm
and into the recovery phase of the storm.

So using the same technique as Saturn, we actually built and flew the
Saturn instrument, the INCA sensors before we did this one instrument.
But this one got data before we got to Saturn, so we could have
developed our display techniques and our understanding at Earth.

So let's go to the next slide, this is a little bit of a non-sequitur
but this is basically a artist's conception of what the relationship of
the ring current to the magnetotail (valve) shaft and the plasma sheet,
all things I've mentioned, and the solar wind of course, just so you can
get another view of where these things lie in relationship to each
other.

The ring current is drawn in orange and sort of extends out into yellow
and really goes out to the orbit of Titan, so you see Titan indicated
there again for scale, not a particular player in the ring current, but
it's something actually that I use a lot for a fiducial just so people
can see scale in some of the images that we've produced.

So the next slide reminds you that you have these trapped particles,
with magnetic field lines of Saturn and connecting into the ionosphere,
and the next slide is shows you another aspect, remember for energetic
neutral atoms, I said you have to have energetic particles, that was in
the artist's concept I showed you the plasma sheet and the ring current.

This shows you measurements again from the view of this instrument, UVIS
instrument...

Amanda Hendrix: Ultraviolet imaging\...

Don Mitchell: Yeah, of the emission from in this case hydrogen atoms in
the Saturn system, shows you the distribution of neutral, of cold
neutral gas. So earlier I showed you the oxygen gas which is
concentrated much closer to Saturn, the hydrogen gas is distributed much
more broadly in the system.

And this gives us our kind of medium to charge exchange of gas with the
energetic particles and produce the images that we produce. So we get
them both from the oxygen cloud and from this hydrogen cloud.

And the next slide shows you a typical image of the Saturn magnetosphere
when viewed from the side in the distance, and off to the right and a
little bit below the plane, it's again southern summer.

And this is a image taken with the ion neutral camera Saturn of this
mix, the mix of the energetic ions and the magnetosphere charge
exchanging with that cold gas cloud.

You see that it's much brighter on the night side, on the left side than
it is on the day side. That's partly because it is brighter on the night
side than it is on the day side and that's something we found by
analyzing a lot of these images.

So there's also - tends to be a left-right asymmetry in any of these
images, and that's because of the rotation. As - again as - just as when
we were looking at the plasma inside the tail and the spacecraft was
rolling, when we looked into the flow, we saw a brighter signature than
when we looked away from the flow.

Well the same thing's happening here, the left hand side of this image
is coming towards you, that's part of the rotation that's rotating
toward the viewer, and the right hand side is rotating away from the
viewer.

That tends to brighten the left hand side and dim the right hand side of
these images. So you always see this bias for if north is up, the left
hand side is going to be brighter than the right side typically, unless
dynamics dominate and you get something that's just so much brighter on
the right and so much dimmer on the left intrinsically, in other words
it's not symmetric, intrinsically, but it's very asymmetric
intrinsically. This effect can be overcome, but in these images it's
just, you can think of this as a symmetric ring current and it looks
brighter on one side because it's rotating towards you.

Amanda Hendrix: It doesn't have to do with the way the sun is, very
much.

Don Mitchell: No, it doesn't. It doesn't. There is, probably that effect
is in here too but the dominate effect in this image is probably just
the rotation. We've seen the same thing looking from, this is looking
from the dawn side, we've looked at the magnetosphere from the dusk side
as well.

And you see the brightest part is on the summer side there. So it's
because that's rotating toward you when you're sitting on the dusk side.

So the next one is actually a movie, for those that have the movie, for
those of you that have the PDF there should be some still frames from
the movie, which are similar to the ones that were at the beginning of
the talk.

And this movie shows a bright intensification moving around at a
rotation period that's keeping pretty much pace with the pink or purple
or whatever violet arrows, the coordinate system which is rotating with
Saturn.

And that is the coordinate system established by the SKR radiation. So
you see that over several Saturn rotations, there is intensification in
the ring current, and this is the ring current that's going around.

Now the orbit of Titan is the outermost dashed circle there for scale,
and I've also shown the orbits of Rhea and Dione there for scale. Rhea
is about 8 1/2 Saturn radii and Dione is about 6.

So this shows you several features, it shows you that yeah, you get
rotation with the planet and it's very obvious. It also shows you that
inside the orbit of Dione you get very little emission, it's pretty well
empty.

And there is some smear in this and that's just intrinsic to the
measurement, it's - you know, this is a weak emission and there is
scattering in the instrument and we only have so much angular resolution
and so much counting for (unintelligible) these images.

So they are limited by that, but this is all consistent with basically
no emissions inside the orbit of Dione which is if you can think back
far, that's the empty place where the gas cloud chased away all the
ions, and there are no ions. There's plenty of cold gas, but no ions so
you don't get any neutral atoms.

You'll only get them where you have both. And so this shows, this is a
nice - this is the only way we have of getting a truly global picture of
the ring current that's instantaneous and you can put together in these
kinds of movies.

We fly through the ring current all the time, we measure it in situ, but
it's very difficult to get this kind of information, what's going on in
other parts of the system simultaneously without this kind of imaging
overview.

It's very complimentary to the situ fly by data.

Amanda Hendrix: So how did you get this, this is from way out of (Apple
F)?

Don Mitchell: This was way out at (Apple F), this is from a high
latitude orbit, so where the (Apple F) is, I think it should show on
here, the lower left it shows the coordinates of the spacecraft, so
we're at 30 RS and we're at 52 degrees latitude local time near 1800 so
we're very close to dusk by the end of the movie anyway.

And it gives an L value which is meaningless at this distance and this
latitude but it's a very high L.

So this is a very good viewing geometry for us, you get to look down,
not directly from the north pole, which would be very nice to do but
hard to maintain, but any time we're above about 30 degrees latitude we
get pretty good viewing.

Saturn's very nice for this because the cold neutral gas is density
confined near the equatorial plane, not strictly confined there but it's
certainly got (unintelligible) fall off below the equatorial plane, and
the same is more or less true for the energetic particles.

You can kind of think of this emission as confined to a disc. And that
makes the projection, you know your ability to interpret what you're
seeing much easier than it is at Earth where you don't have either the
energetic particles or the, especially the cold gas confined to a plane.

And Earth's cold gas is spherically symmetric. So that makes it much
more complicated because you, the line of sight measurements in a
(unintelligible) along the line of sight, and it's like looking at an
x-ray, if you don't know what you're looking at and it's a soft tissue
x-ray, you have no idea where anything is in relationship to anything
else. If you do a CAT scan, you're good, but we can't do that.

The next slide is actually just that same series of stills that I
started out with, series of stills from the movie that shows different
phases actually not from that movie, because it doesn't have the same
coordinates on it, but you can see that the time starts out at day 55,
11 hours and ends up at day 55, 21 hours, 41 minutes, which is one
Saturn rotation later.

And you've seen this, the intense blob of emission has rotated back to
the same position it started out in. Now again, the background plasma's
not rotating at that speed. The background plasma is going at a slower
speed.

It's going, it would take it 11 1/2 to 12 hours or even longer, 14 hours
sometimes to get around Saturn. So what you're seeing is a moving and
acceleration mechanism, not the plasma going, you didn't just heat the
plasma in one place and then let it rotate.

You're - it's the heating mechanism that's moving around. So that's one
of the interesting things we've learned in this process.

The next is another movie, and this is actually during one of the HST,
Hubble Space Telescope observing periods, you'll see a brightening on
the night side which is the lower right, and then a big intensification
in the lower right, which is on the night side, which then takes off and
rotates around.

So these things are not always just sitting there rotating around
forever, the - and it goes black for a little while, while the
instruments turns down its voltages and then turns it back up.

But this is a very complicated structure. If you have followed it at all
or were paying attention to what people were doing the SKR data from
other presenters, not me, you would have noticed that the SKR data is
very messy.

But they were able to get this very well designed coordinate system out
of it, but the emission is not always from the same longitude, it
bounces back and forth by almost a hemisphere in longitude, it's all
over the place.

It just averages out to one of these locations. And so if you take a
long term average you get a well defined frequency. I think the same
thing is true with this data, and we haven't fully established that yet,
but I think that there's a hemisphere that likes to light up.

And this again it may even be more than a hemisphere, it's more like
there's a quadrant that doesn't like to light up. But the location of
the brightness can show up anywhere in that region.

And it probably just depends upon the particular vagaries of the
instabilities in the current sheet that are going on for any particular
rotation where it starts up. But it's got a tendency to show up in one
small, you know relatively small longitude range.

Now once it's established on any given sequence of rotations, it tends
to stay in the same place. So move forward again, now what we did with
this is, and I'm not going to go through all the text here so you can
ignore that unless you want to read it later.

But we did a series of movies in which we, instead of using the actual
intensities that were measured, we normalized the data to the peak
intensity in each image so that we were able to follow the peak in the
emission as a function of time regardless of how intense it actually
was.

And then track that data and find out how these blobs go around and
where their peak rotations are, how they rotate. And the next slide
shows another example where we got a little symbol to show where we
located the peak for this particular normalized frame of a movie.

And then we tracked that peak over time and use it as a - able to
develop statistics on the motion of these things. And the next slide
shows what the angular speed of these blobs are.

So I'm showing you two examples. I've shown you one example, the movie,
where the blob is rotating around, and it's pretty much kept pace with
the SKR coordinate system. And then I showed you another one where a
blob was created on the tail side and it took off and started rotating.

If you follow them in general, some of them go at rigid rotation period,
which is where the vertical line is in this diagram. But most of them go
slower than that, so they don't actually keep up with the SKR coordinate
system, because a lot of them really are created, you accelerate a bunch
of particles and then they're locked into the background plasma and do
rotate at that lower rate.

And this diagram shows that an awful lot of them are rotating at
anything from co-rotation period to something well below co-rotation.
What has tend to happen though is after a rotation, a new blob shows up
ahead of the old one.

And so although their velocities around Saturn are too slow to catch, to
stay at this SKR period, you get a new region forming which
reestablishes itself where the old one started in the SKR frame, and
then it may keep up or it may drop behind.

But then you get another new one that forms eventually, and so you, if
you follow that over time, they tend to form in the same location in SKR
longitude, but they always, they often fall back.

Some don't, some stay up and those are the ones which are probably
keeping a real active acceleration mechanism going the entire time. The
ones that are falling behind is where that acceleration mechanism sort
of peters out, you're left with the particles that are still glowing and
still going around but dying because nothing's sustaining them.

And those will rotate at a lower speed, and then you knock off a whole
bunch of new ones that haven't been (phased) of the ones that had
dropped behind.

And if we go, the next slide just shows a bunch of derived co-rotation
velocities from both the CAPS instruments and our MIMI instruments,
we're showing that in general that the dashed line is where rigid
co-rotation at the SKR rate would be. And everything tends to go slower
than that.

So on the next one, just there for, to show you the process we went
through to try and analyze this statistically. We put - we took, when we
were looking at the system more or less from the side, we took repeated
frames, I'm having a hard time explaining it.

We normalized them spatially - so each rotation spatially - so each
vertical stripe in one of these images is a particular image compressed
horizontally and stretched to the same distance vertically so that
regardless of how far we were from Saturn we stretched it to the same
size.

And then - so you can look at this as just a bunch of stacked images
turned on edge over time over many days. This was our best example, it
was - we had about seven days of continuous viewing and rotation which
is very unusual, because with before the probe drop off and they weren't
allowing the spacecraft to articulate.

So it was - happened to be when we had good viewing so we got a nice
steady view. I'd love to go back to another period like that, but that
won't happen. Anyway, if you stack all these images up over time, and
then you draw a line down the middle of them, and down the middle should
be where Saturn is, the center of them.

The reason we did that is because of this day-night asymmetry that's
imposed by the rotation. We didn't want that influencing the analysis so
we took a cut right at Saturn where the velocity of rotation is neither
towards you nor away from you.

And so you should be able to see more or less undisturbed intensity
variation. And if you follow that line across here you'll see that
you're going to get a periodic result out of this.

And then if you analyze the period of that on the next slide, you see
long periodograms based on that data. And what you see is something I
mentioned earlier that the hydrogen is very poorly organized, but the
oxygen is extremely well organized.

I haven't shown you any movies of oxygen because they're really not very
good. The oxygen gets scattered very badly in the instruments and it -
for reasons I can't get into now.

But anyway, we can't get good images in oxygen. However, we can follow
the intensity variations in the oxygen quite well, and what this shows
is that the oxygen is just very well organized and it's right at that
10.8 hour period in the energetic neutral atoms.

Now it's not too surprising, we showed that we got that in the energetic
particles too, but recall the energetic particle data that we analyzed
was between 20 RS and 60 RS. This submission is happening between 8 RS
and roughly 20 RS, so this is the middle magnetosphere, this is that
region that includes this transition between 12 and 20 which is all
weird and screwed up.

That weird and screwed up stuff may be what's accelerating the
particles, you know, it's going to be a lot of twisting of the fields,
it's going to be a transition from a dipolar to a stretched disc like
field.

It's a very dynamic region, and we get a lot of particle acceleration
there. What this seems to indicate is that the oxygen responds much more
effectively to this periodic acceleration mechanism than the hydrogen.

They both do respond to it, you can see it clearly in some of the
movies, but if you average over time, the hydrogen's really not - that
doesn't fall out very well whereas the oxygen falls out very well.

Okay, and then the next slide just shows if you follow the periods that
the peak period in these long periodograms over a couple year period,
you'll see that the oxygen sits right on that SKR rotation period and
the hydrogen is all over the place.

It sort of averages out to the same, but it's messy. And then so this
really generates more questions than it answers, but some of those are,
why do the energetic neutral atom signatures as well as many others,
SKR, energetic particles in the 20 to 60 RS region, magnetic field 3 to
12, plasma density between 3 and 5 show up persistent periodicity at a
well defined slowly varying frequency.

This is a very broad range in radius, it's a very broad range in
latitude that it connects to the ionosphere, the coupling is fairly
strong through the ionosphere at low latitudes, it's fairly weak at high
latitudes.

Why do we get exactly that same frequency? I mean if you look at the
co-rotating plasma, it gets more and more sub-co-rotating the further
you go out. It shows the couplings weaker and weaker, and yet this
frequency is rock solid throughout the magnetosphere.

It's still a puzzle. The driving mechanism must be only loosely coupled
to the interior, that is we're not looking at the interior rotation
period of Saturn which you know the SKR from Voyager used to be used as
the interior rotation period of Saturn.

I think we're pretty convinced that's not the case. The driving
mechanism is not likely bound to the magnetosphere, plasma velocity
doesn't keep pace with the disturbance, and so the ionosphere's got to
be where it's all connected.

But you've got this coupling between different latitudes is still a real
open question how that occurs. And then the next slide, is there a
preferred latitude in the ionosphere that couples closely to a
magnetosphere feedback loop that then reinforces the disturbance at that
latitude?

So if you have an ionospheric conductivity anomaly, how do you maintain
it? Well one way of doing that is, if you have a conductivity anomaly,
it will result in a longitudinal variation and field aligned current
that run under the magnetosphere.

Those field aligned currents then communicate from the ionosphere to the
magnetosphere, trigger an instability in the magnetosphere that can
accelerate particles, and the pressure variation created by the
accelerated particles can reinforce that same current system feeding
back to the ionosphere and further ionizing the ionosphere in that
location.

Then you've got a feedback loop that may be established and may connect
the low altitudes to high altitudes and also may spread it to different
latitudes. But that's still hypothesis.

(Southwood) and (Cry) have both come up with other mechanisms that
bounce on into here. We still don't have an answer, but energetic
particles, especially oxygen tend to be energized on every rotation,
preferentially at a particular SKR longitude and probably preferentially
local time as well. That's less well established.

This is likely followed by a current structure that we were just
describing, set up in the magnetosphere by your choice, ionosphere,
unbalanced load which is what I call Don Burnett's mechanism for the
heavy side ion, you know his people put the question to him why that
longitude, and he said well some longitude's got to do it.

It's like an unbalanced load in a washing machine, you spin it, how does
it decide where to put the out of balance thing, it's just a random
perturbation that can start it up, but once you start it up it can
amplify.

Let's see, probably not a magnetic anomaly. For the magnetic anomaly it
would have to be tied to the interior and therefore couldn't vary in
frequency the way this does.

This triggers and instability in the middle magnetosphere and energizes
particles, so I think I've got some more here, but I think I've probably
pretty much worn it out.

The next is another nice movie which if you want to view you can, it's
more of an edge on this movie, it goes on for a number of rotations, it
was really the first time we had firmly established this data.

Let's see if there's anything else here that's good. No, I'm going to
stop here, I could go on for quite a while.

Amanda Hendrix: Okay.

Don Mitchell: I think I've...

Amanda Hendrix: Well this is great though, thank you very much. It's
quite a - it looks like quite a big topic and quite a lot still to
unravel. Just a lot of different aspects.

Don Mitchell: Still a bit, yeah. Well I could have just talked about the
stuff that we really know but I think that's less exciting than the
stuff that we're still trying to figure out.

Amanda Hendrix: Yeah, I know. Do people on the line have questions? I
think I'm going to ask - I have one, I have a couple questions but I
think I'll just ask one. And that is, since it does seem like such a big
problem, you know like such a big system and so many aspects to it, what
do you think would be the best way to - you know if you had your dream
spacecraft and your dream instrument, what would you do to try to get at
this whole big problem?

Don Mitchell: Well, we'd be - well my dream spacecraft would be at least
two, maybe three.

Amanda Hendrix: Okay.

Don Mitchell: Well first of all, energetic neutral ions technique
benefits greatly from having an in stereo viewing because you're, you've
got line of site integration that your intensities, that you're looking
at and the, sorry, so with a stereo view you can strain the problem much
more.

You can tell where along the line of sight a particular intensification
is going on. I would certainly want a next generation of instrumentation
that would have higher sensitivity and higher angular resolution so that
we could resolve some of this stuff better.

Right now we're seeing these big blobs and we can't resolve any kind of
active regions within them that may be, you know that you might be
making the information.

But the other thing we'd like to have would to be to have in situ data
complementary to the energetic neutral ions. So it would be nice to have
a view point from high latitude, kind of a....

Amanda Hendrix: Looking down.

Don Mitchell: Yeah, looking down the system, at the same time that
you're in orbit and finding out what's going on locally so you can start
to get - one of the things that we haven't done, made much progress on
so far is connecting these very dramatic intensifications that we see in
the ring current that are rotating around with what we see when we fly
through the system.

We see lots of stuff, we see lots of injections, but we don't usually
see things that we can establish around a scale, what we see in the
energetic neutral ion images.

We see a lot of smaller scale stuff, but the very large scale structures
are not things that we have been able to identify in situ data well.

Man: I have a question.

Amanda Hendrix: Yes.

Man: I'm watching the rotation around Saturn and if I have it figured
right it looks like it was coming towards us on the left and going away
on the right, but the, it was red on the left and blue on the right
which seems - is the Doppler effect not a consideration here, or how do
I have that figured wrong?

Don Mitchell: Let's see, I'm not sure I follow your question entirely,
but I think what you were saying was that you were seeing one of these
things rotating around that was brighter on the left because it was
coming towards you and then dimmer or more toward the blue on the right
as it was going away from you, is that correct?

Man: Yeah, that's correct, and that's what I was wondering, it seemed
unless I had something figured wrong, it seemed like coming towards us,
so the red shift it should be just the opposite there, but...

Don Mitchell: Ah, okay. Yeah, sorry, it's a bad choice of color bar I
think. Red is just more intense, and blue is less intense. It's not a
measure of the energy of the particle that we're looking at.

Man: I see.

Don Mitchell: So what we're really looking at is a cut in energy, we're
looking at a specific energy band, so on the left and right we're seeing
the same energies because that's what the instrument's choosing to look
at.

Man: I see.

Don Mitchell: And then as they're coming towards you - what's really
effectively happening as it's coming toward you, you're moving lower
energy particles into the pass band of the instrument, because you're
adding the velocity of the flow to those particles.

And as you're looking on the right where the particles are going away
from you, you're moving higher energy particles into the pass band that
you're looking at, that energy band.

And so the variation in intensity is because there are fewer higher
energy particles and more low energy particles.

Man: I see, thank you.

Don Mitchell: Uh huh.

Amanda Hendrix: Any other questions out there? Okay, well Don thank you
very much for a really interesting talk, and everybody on line please
tune into the next CHARM telecon on February 26th where we will hear
about the latest results. It'll be sort of the Huygens third anniversary
CHARM telecon, we'll hear about the latest results from the descent
imaging instruments team.

So thanks everybody for calling in, and we'll talk to you next time.

Don Mitchell: Thank you.

Amanda Hendrix: Bye.

END