**NASA**

**November 30, 2004**

**1:00 p.m. CST**

Moderator Good afternoon. I would like to inform all participants that
today's conference is being recorded. If you have any objections, you
may disconnect at this time. I would now like to turn the meeting over
to Dr. Crary. Sir, you may begin.

Frank Hello. My name is Frank Crary and I work with one of the
instruments on the Cassini spacecraft. I was asked to give a talk about
some of our initial results. I wasn't entirely clear who the audience
would be, so please if you have any questions just jump in and I'll
answer things as we go along.

Trina Hello, Frank. Sorry I'm late.

Frank Hello, Trina. I just got started without you.

Trina As you can hear, I'm totally out of breath. I've been running from
meeting to meeting.

Frank I know the feeling.

Trina Do you know if they have started the recording yet?

Frank Yes, they did.

Trina Excellent. You've clearly introduced yourself and you can just go
ahead since you got started without me. Excellent.

Frank The title of today's talk is Saturn's Magnetic Bubble. If
everybody is looking at the presentation, on the second slide you'll see
that I've immediately dropped into jargon, which is to say I'm going to
be talking about Saturn's magnetosphere. I want to start off by talking
a little bit about what a magnetosphere is mostly focusing on the Earth.
That is because I use it as the standard against which we're comparing
what we've seen in Saturn. Then I want to talk a little bit about the
instrument I work with so you'll have some idea of what we are measuring
and what I'm talking about and then go into four of our first results.
One was the solar wind and how it affects Saturn's aurora, and this we
actually did in January while we were still approaching Saturn; then
describe our first pass close to Saturn and through Saturn's
magnetosphere; then say something about the ionospheres we saw over
Saturn's rings and during our first Titan encounter.

On the third slide, I just have sort of a summary of what a
magnetosphere is, what parts there are to it, and what interesting
things there are about it, and maybe introduce a little bit of the
jargon I'm probably going to be using later on. The idea is that the
solar wind, a flow of ions and electrons from the sun that is something
around 500 kilometers per second, is encountering the planet's magnetic
field. Since the solar wind flow is supersonic that forms a shock wave
upstream of the planet and behind that the flow is deflected around the
planet by the planet's magnetic fields. In the process, the planet's
spheric magnetic fields get compressed around noon on the sunward side
and stretched out into what is called a magnetotail on the nightside.

Because I couldn't find a good figure for it, I have some hand-drawn
sketches on the fourth slide where on the top left I'm showing what the
Earth's magnetic field would look like if there were no solar wind at
all; just the sort of dipolar field you see from a bar magnet. On the
top right and the bottom, I have two different ways that the solar wind
can push on and affect the magnetosphere. The top right is if the
magnetic field embedded in the solar wind points in the same direction
as the Earth's magnetic field in that case they just bump into each
other. The solar wind flows around and as I said, the Earth's magnetic
fields get compressed on the dayside and stretched out on the nightside.

On the bottom, I've sketched a more interesting case, which is where the
solar wind magnetic field and the Earth's magnetic field point in
opposite directions. In addition to being compressed on one side and
stretched on the other, you get a spot at noon and around midnight where
the two magnetic fields cancel each other out and that opens up the
space inside the planet and results in all sorts of things. First of
all, what we call the magnetosphere is that region where the planet's
magnetic fields are sort of shielding out the solar wind, deflecting it
around and creating this bubble in the solar wind.

Inside you have things like trapped radiation belts; something called a
plasmasphere, which is a cold and dense plasma near the equator which
comes from the Earth's ionosphere. I should say cold and dense are I
guess relative terms.

M Excuse me. I have a question.

Frank Sure thing.

M This case example where the magnetic field of the solar wind is going
one direction and then in the lower part where it's going the opposite
direction, does that occur against the Earth's magnetic field on a
routine occasion, or is this just hypothetical?

Frank It's a little bit hypothetical because I've drawn them with the
solar wind magnetic field pointing almost directly north or directly
south and that actually almost never happens, but you get something very
similar when it's tilted northward and when it's maybe 45 degrees off
northward or 45 degrees off southward and that happens quite frequently.

M So you've observed this phenomena or at least somewhat?

Frank At Earth this is something that people have been studying and
measuring since the early '60s. It's very well documented. I'm hopping
back and forth between slides three and four. Some of the affects in the
solar wind are produces aurora; it can enhance the radiation belts; it
can cause what are called magnetic storms where there is a strong
perbation to the Earth's magnetic field. That was actually first
observed in the 1800's when somebody noticed that the change in the
magnetic field was inducing voltage spikes in telegraph lines and
noticed that it correlated very well with sunspots.

M I think you answered my question because what I was really wondering
is if this was a hypothetical or if both cases had been observed with
the Earth.

Frank Both cases are quite real. If I were to draw it entirely
realistically where there was a tilted field that wasn't quite purely
north or purely south then it becomes a very complex three-dimensional
shape. I just didn't want to try to draw that even.

M I appreciate that. Thank you.

Frank Deep inside in the protected region of the magnetosphere you have
the radiation belts; you have this plasma from the Earth's ionosphere,
which is oxygen, ions, and electrons. I called them cold and I called
them dense. For the ions and electrons that you find in the space around
the planet or in the solar wind, they are. A thousand ions per cubic
centimeter is, relatively speaking, a whole lot even though the math is,
I think I worked that out to be 2x10^-26^ grams per cubic centimeter, so
that's really quite tiny compared to deep in the atmosphere. Similarly,
the temperature, which is about one electron volt, the energy an
electron gets when it drops through one volt electric potential, is
actually about 11,000 Kelvin, so that is only relatively cold.

Those things can create a fairly substantial affect on the Earth.
Generally, those X points, those points in my figure on slide four where
the solar wind and the Earth's magnetic field cancel out, those are
associated with electrical currents that are typically hundreds of
millions of amps, although they're very, very diffused. A very strong
current might be one amp per square kilometer, so it's very diffused.
It's spread over such a large area that you get very large electrical
currents and very large voltages involved, which is the thing that
accelerates electrons into the atmosphere and produces the aurora.

The two other things I wanted to say here were that close into the
planet the ions and electrons are very tightly coupled to the Earth's
magnetic field and the Earth's magnetic field is itself tightly coupled
to the Earth's ionosphere and the ions and electrons there. This results
in a circulation of the plasma from west to east at the same rate as the
Earth rotates. I'm going to say a little bit about what that means. We
see the same thing at Saturn and I'll be talking about that more later,
so I just wanted to introduce the term.

The other thing about these two open and closed configurations is it's
usually this open solar wind and Earth magnetic field opposed
configuration where you get all of the activity. When this happens,
variations in the solar wind pressure in the flow speed or a shock in
the solar wind can have very dramatic affects on the Earth and that is
typically when you see bright aurora, when the aurora moves southward,
or when there are very sudden enhancements in the radiation belts.

The fifth slide is a figure of the Earth's magnetosphere looking on it
sideways with the sun off to the left and that was true in my other
diagrams as well though I forgot to say it. This one was actually drawn
by an actual ....

M Excuse me. I wanted to point one thing out is that chart four that was
hand-drawn; I don't think that was on the PDF figures. It's only on the
PowerPoint.

Frank I'm sorry about that if people were missing out or confused.
Anyway, slide five is the same sort of thing but a little more
realistically and a little more complicated, but it shows I think a lot
better the inner-region that is the dark shaded colors, which is the
plasmasphere; the stretched out magnetic field lines downstream in the
tail; the bow shock and so on.

Moving on to the next slide, I have a comparison between Earth, Jupiter
and Saturn, some of the various things that affect their magnetosphere.
The first thing that is important is that Jupiter and Saturn both rotate
much faster. We know actually from the Voyager as well as the Galileo
spacecraft at Jupiter that one result of this, as the plasma is
circulating east - west and rotating with the planet it feels a fairly
strong centrifugal force that can have a lot of affect on the dynamics.
Because the Earth doesn't turn as quickly that is much less of a factor.

The other thing is that Jupiter's magnetosphere that, as I've said,
we've now had several spacecraft study, including one orbiting it for
several years, we're at the point where we think we have an idea of what
is going on there. The Earth has been very well studied. What we see
with Saturn is the rotation rate of Jupiter and Saturn are similar, but
the magnetic field strength of Saturn and the Earth are more similar and
Jupiter's is tremendous. One of the things we're trying to study with
Cassini is how these different parameters affect what happens around
Saturn; how Earth-like it would be or how Jupiter-like. Is the rotation
rate the big deal or is the magnetic field strength the big deal? The
theories that have been used at Earth to explain all of the measurements
that have been made say they're both important, but it's very hard to
study them independently.

> I'm not going to talk about it much, but one peculiarity of Saturn is
> that its magnetic pole is exactly the same, as far as we've been able
> to tell so far, as its rotational pole, its geographic pole. That
> isn't true of Jupiter or the Earth. For the theories of how planets
> have magnetic fields, how dynamos work, this is a little bit of a
> quandary because the theories that as far as everyone can tell are
> very, very solid says that a dynamo has to have a little bit of
> asymmetry, a little bit of a tilt, and the previous spacecraft haven't
> seen it at Saturn. This is something that Cassini is going to be
> studying in quite a bit of detail, but I'm actually not going to be
> talking about it. I just wanted to mention it at least.
>
> Another important difference is the distance from the Sun. The farther
> out you get the lower the density of the solar wind gets, so it isn't
> pushing on the planet's magnetic field as strongly. The speeds
> actually remain about the same, so the pressure exerted by the solar
> wind really depends on the density, which is down by about a factor of
> a hundred from the Earth. How much and to what extent that compensates
> for the magnetic field being weaker is something we're going to be
> trying to study. We've started studying by mapping out where those
> boundaries I talked about earlier are. Where is the bow shock in the
> solar wind? Where is the magnetopause where you start excluding solar
> wind and only having stuff that is going on internal to Saturn's
> magnetosphere? On every orbit, we hope to cross those boundaries a
> couple of times and start learning statistically where they are, what
> are the shape of the boundaries, and how that depends on solar wind
> conditions.
>
> Finally, one big difference between Jupiter and the Earth is that
> inside of the Earth's magnetosphere the only source of plasma is hot
> ions leaving the Earth's upper ionosphere. At Jupiter, there is a
> volcanically active satellite, Io, that is a source of an enormous
> amount of neutral gas in the space around the planet and that gas gets
> ionized. It produces a tremendous source of sulphur and oxygen ions.
> At Saturn there are embedded internal sources. This is gas that
> evaporates off or is sputtered off by energetic particles off the
> surface of the rings and the icy satellites or which escapes from
> Titan's atmosphere.
>
> The estimates of how much of a source that is put it as a fairly low
> number, though there are recent measurements originally by the Hubble
> Space Telescope and more recently by Cassini's ultraviolet
> spectrometer that suggest the neutral gas in the space around Saturn
> is a lot denser and a lot more abundant than people thought. That
> could profoundly change what we think about these internal sources.
> That goes back to whether Saturn would more closely resemble the Earth
> or Jupiter.
>
> With all that said, the next slide is just an overview of all of the
> orbits that Cassini is going to make around Saturn. I'm sorry to say
> in this figure the sun is to the right. In the ones I showed earlier,
> it's to the left, so I apologize for that. What I want to show is
> simply that the orbit of Cassini around Saturn is going to take it all
> over the map; sunward of Saturn where the compressed nose of the
> magnetosphere is, and far away from Saturn; on the nightside into the
> magnetotail.

Mathew This is Mathew ... at the Saturn Observation Campaign. I'm
looking at the slide and I see the tremendous variety not only of the
ellipticity of the orbit but also the planes. Judging by this drawing,
it looks to me like you've got a really good opportunity here to get a
good map of the magnetosphere of Saturn. Is that correct?

Frank That is exactly what I was saying. One of the things that's really
special about this is those highly inclined orbits we're going to be up
over the poles and we're going to be flying through the magnetic field
lines that connect the magnetosphere to Saturn's aurora. At the Earth
what happens is those electrical currents that I was mentioning, they
tend to close through the Earth's atmosphere and the ionosphere, but
when they become too intense they become unstable and generate beams of
high-energy electrons accelerated through about 10,000 volts. Those
electrons hitting the atmosphere are what produces the aurora. This has
been studied quite a bit at the Earth exactly how that instability
happens, how things break down and how the electrons get accelerated,
but it's only been studied at the Earth.

Galileo at Jupiter and all of the other spacecraft that flew past
Jupiter or Saturn or Uranus and Neptune, for that matter, though that
was only Voyager, were all at relatively low latitudes. They never got
up close and at high latitude where the physics that really produces the
aurora is going on. Cassini is going to be our first chance to really
learn if the way things work at Earth is also the way they work at
another planet as far as aurora go. I think that's everything I wanted
to say about the orbital tour.

I wanted to do just a brief two-slide description of the instrument I
work on, Cassini Plasma Spectrometer, or CAPS, and how it works, what we
measure, because I'll be showing you a lot of data from it. All of our
studies are very much a collaboration with other instruments. To
understand this sort of thing you need measurements of the plasma, which
is what CAPS measures, but also of the magnetic fields and the radiation
belt particles and also electric magnetic waves that are generated in
the magnetosphere and interact with the plasma.

> We work with all four of those instrument teams, occasionally with
> some of the others as well. For example, the ultraviolet spectrometer
> makes very good measurements of Saturn's aurora that we're in the
> process of trying to connect to some of our measurements of what the
> solar wind was doing. That's actually, in a sense, the first topic I
> was going to talk about after I describe the instrument.
>
> Anyway, what we do is we measure the ions and electrons that are in
> the immediate environment of the spacecraft and just sampling them all
> through all of those many orbits that were in the previous figure. We
> measure the number of electrons flying into the instrument as a
> function of both their energy, their direction, and for the ions, what
> they're made of, if they're oxygen or protons or nitrogen or something
> else. We cover a range from one electron volt to 50,000 electron
> volts, which is the range that most of the ions out there are in. Some
> of the electrons may be at lower energies. A few of the ions are at
> high energies, but the bulk of the plasma is what we're measuring.
>
> We look into almost a full hemisphere. That gives us very good angular
> coverage, but it does occasionally mean we have problems making
> measurements because we do it as the same time as the cameras are
> taking pictures and sometimes they need to point in a direction that
> isn't as good for us. Anyway, the whole point of that is to take our
> measurements and determine things like the density of the ions; their
> flow speeds; temperatures; if there are accelerated beams of
> electrons, we see that; and we do that using three sensors, one for
> the electrons and two for the ions. The ion sensors are split between
> a high-resolution one and one that simultaneously makes a measurement
> of composition. I don't want to say too much more about it other than
> the fact that the composition and the angular information are really
> the big advantage we have over Voyager, plus the whole four years
> we're going to be orbiting compared to the two quick fly-throughs.
>
> Slide nine is just a picture of the instrument and I'll tell you
> briefly how it works. We use a set of curved metal plates. For
> example, in the top right with the red line going into it is a diagram
> of our electron sensor. It has basically two hemispheric metal plates.
> We apply a voltage to them and what happens is an electron will fly
> in. If it's coming from the right direction and it's got the right
> energy, the electric field between the plates will bend its trajectory
> to match the bend in the plates and it will get through. Anything with
> a different energy or coming from a different direction gets filtered
> out and then we vary the voltage to scan through different energies.
> We also have a motor that rotates the instrument back and forth to
> give us some of our angular coverage.

M I tried to do a little homework before this teleconference, but I
wanted to clarify the directions of motions of this instrument. You just
mentioned that it can rotate, but it's limited because it's tied with a
cable to the spacecraft. Is this a single-axis rotation, or is there
also two-axis rotation, or do we depend on the spacecraft itself to get
into certain orientations for this instrument?

Frank It's a single-axis rotation and you've caught me because I was
trying to avoid working in three dimensions again. One of our angles, if
you can imagine instead of that red line I've drawn going into the
electron sensor something flying straight into the page along your line
of sight from your eye, it would go into the instrument and bend on the
far side of it because it really is a hemisphere. This is just a cut. It
would land on our detector in a different place. The angle in and out of
the page of my slide is measured by where the electrons hit the
detector. If you could tilt the page left and right, that is the plane
we rotate in and that is what we scan out the second dimension with.

M That means that that center line that is going down from the top to
the bottom that is the axis of rotation.

Frank There is something that is a circle with the bottom right and the
top left parts blackened out. That is the axis rotation, not going
straight into the page.

M Thank you.

Trina Frank, let me make sure I've got it right. If you had this page in
your hand and you rotated it flat clockwise and counter-clockwise, the
red line of the electrons would sweep back and forth and that's the
rotation, or do I have it wrong?

Frank That's the rotation and its particles going in and out of the page
or at 45 degrees going right and into the page or something that we
measure with the location of the electrons hitting the detector.

Trina Okay, Thanks.

M I have a question. On the previous page, it mentioned comparing it to
Voyager this equipment has 200 seconds and the Voyager has 96 seconds.
What is that about?

Frank The thing is if we weren't turning the actuator at all we'd get a
measurement angle in one direction and energy once every two or four
seconds. Two of the sensors do that measurement once every two seconds.
One of them is a little bit slower. It does it every four seconds. We
have to turn the instrument back and forth; typically we operate at 180
degrees. To do that we only scan at one degree per second because it
isn't a very big motor that is driving us and that takes us, counting
the slowing down and stopping at each end, about 200 seconds.

When we're reasonably confident we know what direction to look in, for
example, during Titan encounters, we are fairly sure that all of the
ions are going to be coming from the direction the spacecraft is flying.
We're flying into them and seeing it all in the face. We limit our
actuator range and get time resolutions of about 55 seconds, so we can
vary that.

> Actually, it was quite interesting to look at some of our initial data
> because we were seeing things in the two to four second resolution,
> just quick energy spectrum after spectrum, that didn't really show up
> well in the Voyager data. We're now kind of scratching our heads and
> trying to figure out if things in Saturn's magnetosphere were
> different when we had our first pass or if it's simply that having the
> two-second resolution makes a lot of difference.

The one other thing I wanted to say about the instrument, the piece of
it that is in the center of the diagram on slide nine is our composition
instrument. There the ions go in just the same sort of trajectory as the
red line shows for the electron sensor above it. You can see the gray
fan, which is the range of angles that can get through. The difference
here is after they've gone through those curved plates, and you can
follow the dot dash line which is the trajectory of them, after they've
gone through that an been filtered so that they have known energy, we
then put them through what is called a time of flight measurement where
we measure the time it takes them to cross a known distance. That is
actually a pretty short time, about 90 nanoseconds for a typical proton.

> Anyway, because we know what energy they are having filtered to tell
> what energy they are knowing their speed because we know how long it
> takes them to cross known distance, the energy is just one half mass
> times velocity squared. Since we know the velocity, we can work out
> what their mass is and that lets us tell the difference between a
> proton, an oxygen ion, a nitrogen ion, things like that. That is a big
> advantage trying to understand, for example, if we're seeing plasma
> that has escaped from the atmosphere of Titan, which would primarily
> be nitrogen or carbon compounds, or if it's water that evaporated off
> the surface of the icy satellites and was subsequently ionized. That
> is actually one of the surprising things that we discovered. I'll talk
> a little bit about composition in a moment.
>
> With enough said about the instrument, the first of the four
> discoveries, though I don't know if we understand it well enough to
> call it a discovery, I want to talk about is a study we did of the
> solar wind and Saturn's aurora. What we really wanted to do here is
> see what it was like. The Earth's aurora is, as I described earlier,
> very well known to be driven by the solar wind. Changes in the solar
> wind are really what set off the dynamics in the Earth's
> magnetosphere.
>
> Jupiter, on the other hand, has this big source of plasma inside it.
> It has very strong magnetic fields and it rotates very rapidly. All of
> those combine to produce aurora around Jupiter that are really
> determined by what is going on inside the magnetosphere. There is a
> little bit of evidence that Jupiter's aurora is affected by the solar
> wind, but it tends to be a weak affect and what is really going on is
> more of an internal process.
>
> We wanted to see which is it for Saturn? Is it solar wind driven? Is
> the aurora and presumably the associated dynamics produced internally
> the way it is at Jupiter? To do that, we used the CAPS and the
> magnetometer and the other particle and field instruments on Cassini
> to measure the solar wind and see what it was doing while at the same
> time the Hubble Space Telescope was taking UV images of Saturn. We got
> enough time to take an image of Saturn every other day roughly and on
> the next slide give a little bit of the details.
>
> We were about 550 Saturn radii upstream, which was just close enough
> that we could really do the comparison. We were seeing the same solar
> wind that was reaching Saturn. We timed it to spend about a month
> doing it, so we covered a full solar rotation and we're confident
> going in that we'd see at least a couple of shocks on solar wind, a
> couple of interesting events, that ought to drive aurora if Saturn is
> Earth-like.
>
> The Hubble Space Telescope (HST) measurements were taken by an
> instrument called STIS (Space Telescope and Imaging Spectrograph),
> which is really a fantastic imaging spectrograph that unfortunately
> failed earlier this year, which is really a sad loss for those of us
> who have worked with it. Anyway, it took eight orbits worth of data to
> cover a full rotation of Saturn on January 8^th^ then went on to take
> an image about every two days for the rest of the month.
>
> In the next slide, slide 12, I've got on the right-hand side, and I
> hope it's showing up a little better on your monitors than on mine,
> one of the STIS images where you can see in the ultraviolet the disk
> of Saturn and the rings primarily reflecting sunlight and you can see
> in the south this bright oval which is the main the aurora oval of
> Saturn. On the left, although some of the letters and numbers are too
> small to read, I have a plot for the whole month the solar wind
> density and speed measured by the CAPS instrument and the magnetic
> field in the bottom plot. Total strength is in black and the various
> components, what direction the solar wind was pointed are shown in the
> colors.
>
> As we expected, we did catch a couple of interesting events, one on
> the 16^th^. You can see about a third of the way to the right in the
> middle panel a big jump in the solar wind speed and then later toward
> the end of the month on the 25^th^ another even bigger jump in solar
> wind speed, another shock wave passing us, and that the magnetic field
> strength jumps enormously well and becomes very disorganized, the
> direction changing quite substantially.
>
> On the next slide, slide 13, I've got a close up of one of those
> events. This is the one that was really captured extremely well. The
> timing of the HST observations was as good as you can get with one
> image every other day. What I'm showing is four close-up images of the
> aurora over the color scale changed a little to make it clearer along
> with the speed and the magnetic field. This is for the shock that hit
> the spacecraft on the 25^th^. Based on the speed the estimated arrival
> time all of the bottom two plots are shifted to give the arrival time
> at Saturn. The shockwave is there a little bit into the 26^th^.
>
> The first image from Hubble Space Telescope is on the left. It shows a
> not terribly bright oval that is kind of typical of the data we got
> during undisturbed periods; that was before the shock reached Saturn
> indicated by the first blue line. The second image and the second blue
> line showing when it was taken was about 10 or 12 hours after the
> shockwave reached Saturn and you can see the aurora lit up. The oval
> on the dawn side of the planet became much brighter.
>
> About a day later, we had another image from Hubble. The oval is
> organizing itself and fading, but it's still quite bright. In fact,
> the brightest points are the brightest that were seen that whole
> month. Then in the final image, late in the 30^th^ the aurora has gone
> back more or less to normal.
>
> Slide 14 is a quick summary of what we think we learned. We saw very
> convincingly that Saturn's aurora is strongly by the solar wind. Even
> away from the shocks and the big events, we had a very good
> correlation between the brightness of the aurora and solar wind
> pressure. Even when it wasn't a big shock that was really triggering
> things, if the solar wind is pushing on Saturn's magnetic field a
> little harder the aurora would be a little brighter.
>
> The interesting thing, and I really didn't emphasize it, is we didn't
> see a big affect from the direction of the solar wind. Compared to
> Earth where you have those geometries where the solar wind magnetic
> field really opens up the magnetosphere; that is very odd. The one
> thing we're thinking about is in all of our data it looks like the
> north-south component of the solar wind's magnetic field was always
> quite weak. It was never pointed due north or due south. It was almost
> never pointed within 70 degrees of due north or due south, so it could
> be that the solar wind is simply different, 10 AU from the Sun
> compared to at 1 AU, and therefore, it drives Saturn's magnetosphere
> in a different way.
>
> We saw a very definite response to a shock hitting Saturn. The aurora
> brightening and shifting in latitude is similar to what is seen at the
> Earth. The only problem we have is at the Earth a big brightening of
> the aurora generally means filling on the nightside and a shift to
> lower latitudes. At Saturn, for the case we have the good data on, it
> actually filled on the dawnside and shifted to a higher latitude, so
> there is definitely something different going on. As soon as they saw
> this, a lot of theorists who try to explain this sort of thing at
> Earth were all over it and trying to come up with answers. I think
> they're still at it, but it's definitely something to test the
> theories they have about the Earth if they can explain both this and
> what happens at the Earth with the same theories then we're definitely
> onto something.
>
> Going to slide 15, I want to talk about our first look at Saturn's
> magnetosphere.

M Excuse me. What is the definition of a shock? That's an increase to
supersonic or decrease to subsonic?

Frank Actually, it's supersonic on both sides of the shock, but it's
generally characterized by an increase in the velocity. The whole flow
is supersonic mostly because the Sun's corona is absurdly hot. That gets
it going and speeds it up to supersonic speeds, but every now and then
you have an event on the sun called coronal mass ejection. That is
something that releases a lot of mass, a lot of energy. The energy
release is associated with solar flares, but the extra mass and the
extra energy also cause a very fast stream in the solar wind.

> As it catches up to the material that has already gone on ahead of it,
> it steepens into a shock. That is typically what we were seeing. You
> also get reverse shocks, but we didn't see many of those. They don't
> happen as much because if you have a fast stream catching up with a
> slow stream it will steepen. If you have a slow stream behind a fast
> one, they just spread out and don't really interact very much. Those
> are the gradual decreases you can see in the velocity I have plotted
> on slide 12.

Before I go on from the solar wind aurora stuff, are there any more
questions about that?

For Saturn's magnetosphere, we first started seeing it about 50 Saturn
radii away when we crossed the planet's bow shock on June 27^th^, and we
actually saw several crossings because the solar wind was variable and
depending on how strong it was the bow shock either moves in or out. It
moved in and out across the spacecraft a couple of times. The same was
true of the magnetopause, which we crossed for the first time a day
later at 33 Saturn radii. Both of these were a little bit further out
than would have been predicted by Voyager, which honestly didn't
surprise me too much. With something as variable as this the fact that
you can't pin down its exact location from a couple of passes is really
no surprise. That just means we have to collect a lot of these and get
good statistics on both the shape and the location of the magnetopause
to understand how it's responding to the solar wind.

Before I go through what we saw inside Saturn's magnetosphere, I want to
go to slide 16, which has two very strange looking color plots that I'll
try to explain because I'm going to show a whole lot of them as we move
forward. I might as well spend some time on the first one so you know
what you're looking at. In these plots, the colors code for different
fluxes in particles. Blue and black mean hardly any particles being
measured at all. The lighter blues to green to yellow and finally to red
indicate more and more intense fluxes of particles.

Vertically, each row of pixels represents a different energy and each
column a different time, so you're seeing time advancing from left to
right. At the very beginning of the plot, in the top panel we're seeing
intermediate flux of electrons at a low energy of one to ten EV and in a
somewhat lower population of electrons maybe 200 EV. Going right to left
at the line marked "A", there is a sudden intensification of both.

Trina Somebody has an open mike, if they could take a look.

Frank Does that clearly enough explain what I'm showing in these plots?
Is there anybody who is totally baffled by what I'm trying to display
here or how it's being displayed?

Ray Yes, this is Ray Warren in Colorado. You're looking at time across
the horizontal access, but what is it we're looking at, Saturn itself?

Frank No. The directional information is hidden in here because we
couldn't do four dimensions in a plot, but the one on top is pretty much
looking almost directly away from the spacecraft. That is good because
there is some contamination in the data from the spacecraft's surface.
Where solar ultraviolet photons hit the spacecraft's surface they kick
off low energy electrons, and our cleanest electron data comes when
we're looking straight away from the spacecraft and we don't have to
worry about that very much. Generally, the electrons have enough energy
that they're just coming from all directions. They're moving much faster
than the spacecraft, so we don't see a preference for them to come from
the direction the spacecraft is moving.

Typically, each individual electron has a speed much greater than the
average flow speed, so they are just zinging around in all directions.
There is a preference for whether they're moving along or perpendicular
to the magnetic field, but generally, wherever you look you see
electrons and there isn't a whole lot of difference which way you look,
at least not for the regions we were in.

Ray One more question please. There is that black section right in the
middle and then you've got these it looks like wings coming from each
side and dipping down in that black section.

Frank You're seeing a number of things. I'd rather go right to left
because it's just easier for me to think about. As we're getting closer
to the planet, first of all, at the boundary mark "A" when we were about
14 Saturn radii out all of a sudden the intensity of those hundreds of
EV moderate energy electrons went through the roof. Then starting around
the boundary mark "B" we could see that there were really two distinct
populations; one, you've got these energetic electrons and now with
fluxes they were quite substantial; also, electrons with a lower energy
starting off at 20 or 30 EV but then gradually becoming cooler as we
approached the planet. That is the red wing you see going down the
closer in we get to Saturn.

The other thing to note is that those energetic particles up toward the
top of the panel simply disappear inside of a certain distance. Finally,
we get to that black region. That is when we turned the instrument off.
We were going into the main engine burn to put the spacecraft into orbit
around Saturn and we had a lot of concerns. We use a lot of high
voltages inside the instrument, as much as 15,000 volts, and we simply
didn't feel safe leaving them on during a big engineering activity where
all of this gas and combustion products were being vented out of the
rocket. So, we shut down. That is where you see the black in the data
and then we started back up half way over the main rings. We got on
about halfway over the B-ring. We have a little snippet of data there
that I'll show in more detail in a moment. Then, again, going out and
away from Saturn, we generally saw the reverse of what we saw going in.

In the bottom panel, you're seeing the ion data. Here the ions are, I
suppose you'd say, looking westward. We expected all of the flow to be
rotating with the planet the same way it does deep inside Earth's
magnetosphere, so we were expecting the flow to be I suppose you would
say west to east. If you were sitting on the spacecraft with your head
pointed north and looking toward Saturn, we were expecting the flow to
come in from off to your left. That is what it was doing and the
spectrum that I'm showing is the spectrum that was looking straight into
that flow.

Again, left to right, we were seeing virtually no ions at all until we
crossed that boundary marked "A". Then we started seeing a few of them
with the flux gradually picking up indicating that the plasma is getting
denser as we go in. You'll notice fairly clearly that there are two red
regions in around the center of the plot; two different energies where
we were seeing a high flux of ions going into the instrument. What
you're seeing is in fact that all of the ions are moving more or less
with their average flow speed. Their energy, one half mass times
velocity squared, is therefore going to depend on the mass of the ion.

That bottom red streak we identified in our composition data as protons.
The upper red streak with about 16 to 18 times the energy we identified
as either oxygen or molecular ions associated with water. It's simply
the different masses of ions that are all moving with the same speed
that give you those two discrete peaks. Again, outbound, leaving Saturn,
we saw more or less the same thing. In this case, the fluxes were lower.
You're only seeing greens and yellow, which are about a tenth the number
of particles coming into the instrument.

> That was simply because the spacecraft had turned and we were no
> longer quite as favorably oriented to look into the flow and see the
> ions coming straight into the instrument.

I think I described all of the regions. There is a summary of the five
regions on slide 15, one slide before those two spectrograms, but I
think I just spoke through everything I have on that slide except for
the fact that the region between roughly the lines "A" and "C" was
highly variable when you look at it up close. This plot that you're
seeing covers 24 hours going in from about 20 Saturn radii away from the
planet and out to about 15. When you look up close, as I'll show in a
minute, it is constantly changing. There is a whole lot of variability
in transients that are quite interesting for reasons I'll get into in a
moment.

Slide 17, right after the color figure, shows for the same period the
electron densities and temperatures that we calculated from the flux.
You can see what I already told you reading off the spectrograms that
boundary "A" corresponds to a quite substantial and quite sharp jump in
density. The line "B" denotes the region where we start seeing both a
dense, cold collection of ions and a hot, tenuous one coexisting. Then
the hot component goes away at that boundary "C", in addition to all of
the transient activity going away.

On slide 18, I describe what we think is going on with that first
boundary, the one marked "A", where the densities went up very, very
quickly. We think that resembles what is called the Earth's plasmapause,
the outer boundary of the Earth's plasmasphere. That is really a
boundary in how the ions and the electrons circulate or flow around
Saturn.

In the next slide, slide 19, I'm showing a theoretical model of what the
flow might be like inside Saturn. The detail is in the tail toward the
top of the figure and here the sun is down. I hate to keep showing
figures with the sun in different directions, but I took the
illustrations from a whole bunch of different people and no one seems to
do it consistently. Anyway, the lines show stream lines where the
authors who put together this model believed the plasma would flow. They
predict like at Earth that near the planet the ions and electrons simply
circulate around east to west, but as they go further out and they start
either running up against the compressed magnetopause on the dayside or
flowing outward and down the magnetotail on the nightside you get to a
region marked in red where you transition from circulating around the
planet to eventually simply flowing downstream.

> The idea here is that any ions that are produced close to the planet,
> any new ions from neutral gas associated with the rings or outflow
> from the planet's ionosphere, any source has a chance to build up
> because it's basically just flowing around and around in circles and
> the source can just build up over time. Whereas, outside of that red
> line, any source only has enough time to build up for about one swing
> around the planet, something like ten hours. Therefore, you expect to
> see low densities because of the short lifetime outside of that red
> line and high densities inside of it.
>
> That has been observed at the Earth. In fact, recent spacecraft have
> actually done images of it where they can actually image the dense
> plasmasphere and see those boundaries to low density regions and at
> least at the Earth, the sharpness of the boundary is a good indicator
> of how recently there was a disturbance in the magnetosphere because
> you can't support very sharp boundaries against diffusion. Plasma
> would normally tend to diffuse across the boundary and you'd have a
> more gradual build up, but if you suddenly compressed the
> magnetosphere and compressed this region of circulation in the
> plasmasphere, you just strip away everything outside and for a brief
> time you have a very, very sharp boundary. At least that is what has
> been observed at the Earth. That could indicate that there was some
> activity in the solar wind just before we took these measurements. We
> wouldn't be able to tell because once we crossed Saturn's magnetopause
> we weren't in the solar wind anymore and we can't really say anything
> about what was going on there.
>
> The other interesting thing to note is this boundary should have been
> much further out if you use simple scaling laws from the Earth, but
> again, there are reasons to think that those simple scaling laws are
> tuned to the Earth's magnetosphere and typically solar wind conditions
> at Earth, which may not apply to Saturn at all. So, moving on, the
> slide 20 ...

M Question on this slide before we move on. Which is the direction of
revolution of Saturn itself in relation to this diagram? Is it to the
left or to the right?

Frank I'm sorry. You're looking down on the axis of rotation.

M That is why we have this asymmetrical look to the diagram. In other
words, if Saturn is going around the Sun as we're looking at this
diagram, it is heading toward the left, so that's why you have that
offset flow.

Frank It isn't really Saturn's motion that is doing that. What is
happening is, although they aren't shown, the solar wind flow outside of
the magnetopause boundary is pretty much from the bottom of the page to
the top. If you can imagine the shear across the boundary, on the
right-hand side at dusk the normal east-west circulation
counter-clockwise here that matches Saturn's rotation is causing the
plasma to flow in the same direction the solar wind is going, so there
is no real shear on that side, but on the left side their flowing in
opposite directions. You've got the co-rotating circulation inside the
magnetosphere heading toward the Sun and the solar wind flow on the
outside heading away from the Sun. It's really that shear in the way it
interacts with Saturn and Saturn's magnet fields that give you that
teardrop asymmetry.

M Those arrows show the direction of rotation of the magnetic field
components?

Frank It's the direction of the plasma flow. If you were to imagine
Saturn's magnetic field lines as a hoop of wire attached to the planet,
as the planet rotates they also sweep around in the direction indicated
by those arrows.

M Thank you.

Frank I don't know if that was as clear as I'd like to be, but I hope it
was okay.

M Question. I'm confused. Let me try it a different way. On slide 19,
we're looking down at the top of Saturn?

Frank Yes.

M The planet is rotating counter-clockwise?

Frank Right.

M That clears it up. Thanks.

Frank That was what we think that outer boundary and the dense region we
entered into were all about. The other thing I want to talk about is the
region inside of about ten Saturn radii and what we think is going on
there. I can see I put some of these figures in the wrong order because
it would have been easier if I would have had slide 21 first. There you
are seeing the same sort of color spectrograms that I showed before, but
now it's only covering a one-hour period. In the ions on top, you can
very clearly see the two populations of oxygen on the top at the higher
energy and protons at the bottom at the lower energy, but you can also
see intervals, a couple early on, one at about 35 minutes after the
hour, and one at about 15 minutes after the hour where the ion densities
dropped significantly. Those correspond, looking on the bottom, to
periods where these electrons between about one and ten EV disappear.

> The event at 48 to 50 minutes after the hour is a nice big dramatic
> one. They disappear and there is a huge intensification of the 100 to
> 1,000 electron volt particles, and less clear in the figure at the
> highest energies, you can kind of see streaks I guess you would say,
> enhancements in energetic particle flux going from left at the lower
> energies to right at the higher energies. There is one of those from
> about ten minutes after the hour to maybe 18 minutes at the very top
> that I'm showing in green. If you look carefully, you can pick some
> more of those out.

In many cases, you can see something similar in the ions, except it's
sloping the other way. This is something that has been seen both at the
Earth and at Jupiter. It is a result of an additional drift motion by
ions and electrons that depends on their energy. It's really driven by
the fact that they are attached to curved magnetic field lines, and as
they move up and down the magnetic field, they see an additional
centrifugal force, but the real important thing is that it's a known
characteristic way this seeing energetic ions first and low energy ions
later, a known characteristic way that a sudden injection of energetic
particles will spread out and evolve with time. We're interpreting these
as the same thing at Saturn that we've seen at Jupiter and the Earth;
evidence that there are a lot of short duration localized injections or
sources of acceleration and at least at Jupiter, that is understood by
having a source of plasma within the magnetosphere.

> Here for Saturn the reasonable source, considering the fact that we're
> seeing hydrogen and oxygen, is the E-rings or the icy satellites
> because they are in this region and because the neutral gas that's
> evaporated and sputtered off by energetic particles has been observed
> in the ultraviolet on numerous occasions. The basic theory to explain
> what we're seeing is that as that plasma builds up it's also rotating
> with the planet, circulating around, and because the planet rotates
> rapidly, there is a strong outward centrifugal force. Because of that,
> you get an instability that is in affect putting a dense fluid on top
> of a less dense one. You'll get drips and blobs of dense plasma from
> the inner plasmasphere accelerated outward and less dense blobs going
> inward and a whole lot of turbulence and radio transport of the
> plasma, removing the material from the source region.
>
> In contrast, if you're inside of the source region you don't see that.
> Our current guess is that is why we see all of these energetic
> electrons and all of these transient events going away when we cross
> about four Saturn radii from the planet. We think that is crossing the
> source of all of this neutral gas and plasma. In fact, four Saturn
> radii is a very natural place for that because it's where the E-ring's
> density peaks; it's where the orbit of Enceladus is; it's in the
> middle of the icy satellite region. I want to go on from talking about
> ...

M Question.

Frank Okay.

M Slide 21, I thought earlier you said something about if you look at
the 48-minute point on the bottom part of the slide for electrons
between one and ten electron volts where it disappears. Am I
interpreting this thing right where the red marks are indications are
the density of those electrons and when there is a gap there that means
they dropped off at that time?

Frank Yes, that is exactly it.

M Then I thought you said that if you look above where the ions are at
that same time in the 100 to 1,000 that there was a burst. On my
drawing, it doesn't show it that way, so maybe I am confused.

Frank Actually, it looks like the ions are dropping out.

M That's what I see too, so maybe my screen is out of adjustment, but
what was it you said about ...?

Frank No. It's up above 1,000.

M I don't see it either there in the blue area. It still looks kind of
much the same on either side of that band.

Frank I personally have learned a whole lot about more human perception
than plasma physics in working on some of these figures. What I've
learned is if I make a figure where you can clearly see the main proton
and oxygen peaks the faint stuff gets washed out. If I try to make a
figure where you can see the faint stuff, there would be nothing but
solid red between ten and 2,000 EV. This isn't really the best figure to
show what I'm trying to show, and I apologize for that.

M On this example also, my other question was; at what radii out from
the planet is this representative of?

Frank This was 1,800 hours inbound so that is about 8 Saturn radii.
We're just getting in to the icy satellite region. I think we would have
passed Rhea but not Tethys, I think.

M On the ion chart, you were differentiating ion species by energy?

Frank Yes.

M I missed that. What was that?

Frank The ions actually have relatively low temperatures, so primarily
what we see is the energy of the particles is simply the ram energy that
they have because the average flowing plasma is moving faster than the
spacecraft. That means the main peak we see is at the kinetic energy of
something with the flow speed so one half mass times flow speed squared.
Since that depends on mass, you see peaks at different energies for
different mass ions with the oxygen peak being about 16 times higher
than the proton peak, as it ought to be.

M Thanks.

Frank The next thing I want to go on to is composition. In slide 22, I'm
showing you a mass spectra that we took. This is actually integrated
over a relatively long time because we don't have quite as much
sensitivity as we might want. Anyway, where it says TOF Channel that
means time of flight channel. It's simply a measure of how quickly the
particles moved across the system.

> You see one peak marked "N^+^", which you get for the time it takes
> nitrogen at a given energy to cross our mass spectrometer. You see a
> different peak at a longer energy for oxygen because at the same
> energy the oxygen with a higher mass has a lower speed, so it takes
> longer to fly through. This is just showing the raw data to prove that
> we really did measure nitrogen and oxygen.

On the next slide, I say a little bit about what that means. We, in
addition, with a little more work, although it's not well resolved in
that spectrum I showed, we can differentiate between a mix of not just
oxygen but OH^+^, H~2~O^+^, H~3~O^+^, molecular ions associated with
water. These are the sort of things that have been observed around
comets by several spacecraft at Haley and by the Deep Space One
spacecraft, Borrelly. With all that water around and being in the middle
of the E-ring and where the icy satellites orbit, there is a natural
source of water, protons as well, so this isn't really a surprise.

> What we're going to be working on is the relative abundances because
> these ions run into each other and chemically react with each other
> and how the abundances vary with radius tells us quite a bit about how
> quickly plasma is being transported out of the source region and also
> about perhaps the gas that it's coming from because both oxygen and OH
> neutrals have been seen in the UV, as well as protons. There are
> probably other molecules there. There is probably water vapor there,
> but the distribution of the neutral clouds is another thing we can get
> out of the relative variations in these densities.
>
> We see a little bit of H~2~^+^ molecular hydrogen ionized, which may
> come from Saturn's ionosphere, but we haven't really pinned that down
> or investigated it too much. We're swamped with so much data and so
> many things in it.
>
> The nitrogen, however, is kind of interesting. First of all, we only
> see it at a relatively small abundance, about 5% level, compared to
> the oxygen and the protons, but we can also identify that it is not
> nitrogen ions coming directly from Titan. That is because ions gain
> energy when they diffuse into a stronger magnetic field. They're
> adiabatically compressed and we can say that nitrogen ionized near
> Titan and then diffusing inward ought to pick up an energy of at least
> 1,000 EV. All of the nitrogen we're seeing isn't even a tenth of that.
> That is consistent with the nitrogen being ionized locally right near
> where the spacecraft is.
>
> The two ways we can explain that, there are some theories going around
> about how neutral nitrogen from Titan could move inward and implant
> itself on the surfaces of icy satellites and be later evaporated off,
> sputtered off and ionized. Or, another possibility is ammonia because
> looking at the Voyager images of Enceladus, a lot of planetary
> geologists believe that pure water ice would be a little too stiff at
> those temperatures. It wouldn't deform and move like a glacier does
> well enough to explain some of the things they were seeing in the
> images. They promptly came out with a theory that there might be a few
> percent ammonia mixed in because that would act as just enough
> antifreeze to let them explain the geology of Europa. If they were
> right then that is a potential source for the nitrogen we're seeing.
>
> The final thing about composition is what we saw over the main rings
> and this is really the next thing I want to talk about, getting away
> from the magnetosphere and start talking about ionospheres. Before I
> do, slide 24 is simply a little bit of a teaser. It's the same two-day
> plot spectrogram of electrons and ions that I showed earlier. This is
> for the second pass, of what the project calls the "Rev A Periapse",
> through the system.
>
> Accounting for the different distances, it looks very similar. Here we
> only got into about six Saturn radii compared to 1.3 on the first
> orbit, so distance from planet as a function of time is totally
> different. Region-by- region, we see a lot of the same things again,
> so I just wanted to flash that up without really talking about it and
> move on to the rings.
>
> First, I'll say a little bit about where we were. We were about 17,000
> kilometers above the B-ring when we started taking data, so this is
> over the main ring system. Closest approach was 1.3 Saturn radii, but
> we moved outward a bit from there. As I mentioned, right around that
> time, inbound closes approach we had a burn by the spacecraft's main
> engines to put us into orbit and there were science observations over
> the rings outbound. This produced a gap in our data and that was the
> black streak somebody commented on in an earlier viewgraph that was
> when we turned down our high voltages.
>
> Another side effect of that is that our mass spectra was a little bit
> off calibration because we can't take our really high voltages up like
> turning on a light switch. It's worth noting that the spacecraft was,
> when we were taking data, both in eclipse behind Saturn and over the
> dark side of the rings. The sun was on the south side of the rings and
> we were on the north side.
>
> Very quickly, slide 26 is a picture of the trajectory going in looking
> down on Saturn's north pole with the red line being the orbital
> insertion burn and if you can make out the blue and the yellow marks
> below Saturn, that is where we were taking our data. That was the
> eclipse period.
>
> Going on to slide 27 is again some more of these time energy
> spectrograms of the ions and the electrons we saw while we were over
> the rings. Talking about the electrons first, at the very beginning of
> that plot that covers 30 minutes, the low energy red stuff, just
> ignore that. That is actually photoelectrons produced by the
> ultraviolet hitting the spacecraft, but at about three minutes into
> this period we went into eclipse and they went away. There was no more
> UV light hitting the spacecraft, and we saw pretty much nothing until
> about 16 minutes in when we started seeing a fair density of
> electrons. This period until about 22 minutes was while we were over
> the Cassini Division. As soon as we crossed into the A...

Trina That is so cool, Frank! I'm sorry; I just have to say that is so
cool.

Frank I like it too.

Trina So, the B-ring is on the left and the A-ring is on the right and
the green is the Cassini Division?

Frank Bingo. The funny thing about that is essentially we were seeing
the electrons when we were over the thinnest rings. The lower the
optical depth of the ring the more electrons we saw. That might seem a
little bit backwards, but since we were over the nightside of the rings,
any heating of electrons, any ionization that is associated with
ultraviolet light if we are over the B-ring, it isn't getting through to
the side of the rings we're on.

> All of those processes would be shut down. As soon as we go over the
> Cassini Division, there is still material in there. There is still
> enough to give you evaporated water and now there is ultraviolet light
> getting through to start really ionizing it. Similarly, when we go
> over the A-ring, it isn't nearly as optically thick as the B-ring nor
> as thin as the A-ring, so you see something intermediate in there.

Now, on top with the ions, first of all, the lack of data before five
minutes is simply our instrument turning on. The ion side takes longer
than the electron side to get up. The lack of counts, lack of particle
flux after about 26 minutes is actually a spacecraft turn to unfavorable
orientation, so the whole time we were over the rings, and we were on
and in a position to see something, we were seeing ions.

Now, I mentioned that our mass spectrometer was not quite up and running
in a calibrated state, so we have to do something a little bit indirect
to figure out what these ions are. There are a couple of white lines
that you can kind of make out, actually three, going across the plot,
two in the middle and one toward the bottom. What we can say is from
everything we know about the Earth's magnetosphere and from what all the
theories predict for Saturn this close to the planet, the plasma ought
to be very, very tightly tied to the rotation rate of Saturn. It ought
to be rotating absolutely rigidly at the same rate as the planet. If so,
we know what energy various masses would have just from their ram energy
relative to the spacecraft just from one half mass times flow speed
squared.

The energies where we see counts are a perfect match, and even when you
look in detail at the shape of these peaks, it's a perfect to a thermal
Maxwellian population of ions at that co-rotating flow speed composed of
stuff with a mass of 16 and a mass of 32. We only get the mass, but the
best thing we can think of is atomic and molecular oxygen ions.

> Those same masses were measured by another instrument, the Ion and
> Neutral Mass Spectrometer; though, because of the spacecraft turns and
> pointing requirements, there good data appeared after ours ended. We
> aren't the only ones seeing it. They're making that detection in a
> completely different way, so we're very confident we were seeing both.

In slide 28, I say a little bit about what it means, seeing O^+^ over
the main rings. Again, atomic oxygen ions are something you get from
water. You just have to photoionize it and you can get it fairly easily.
Because we expect all of these ions to have a short lifetime, they're
bouncing up and down the magnetic field lines with about a 12-hour
period, that means a couple of times a day they are going to cross the
ring plane. Those ions were over the B-ring. It's very difficult to
imagine how any of these ions could be formed and stay around for more
than 12 hours or so.

That means they don't have a chance to run into each other and
chemically react. There is no way for that sort of chemistry to produce
O~2~^+^. You can't get O~2~^+^ by ionizing water, so we believe the most
likely explanation is that there is a neutral oxygen atmosphere
associated with the rings. There are ways to product that. Ultraviolet
light or energetic particles hitting solid ice can knock off molecular
oxygen. This has been, either observed or inferred, both from telescopic
observations of moons of Jupiter and Saturn, and also measured in the
laboratory. This is a reasonable thing to happen.

Our current quandary is that those data and those lab experiments also
say you ought to be kicking off molecular hydrogen. We just don't see
that. We don't see any trace of hydrogen ions. We're trying to
investigate whether there is some way to preferentially remove hydrogen
from the system and support a more oxidized state. I should stop a
minute and ask if there are any questions.

Then I'll move on very quickly and just say a little bit about Titan
since we've now had our first close encounter with Titan and we've
gotten to see a little bit of what it looks like. Figure 29 shows a
theorists drawing of what all of the plasma in Saturn's magnetosphere
would look like when it ran into Titan. In blue, magnetic field lines
that are getting hung up in the ionosphere because it's electrically
conductive and draped around Saturn. In red, stream lines of the flowing
ions being deflected around it. The black curly Q things are the
expected specially ionized particles.

Now we're trying to see if what we measured actually matches any of
that. In fact, some of it does pretty well. I don't have any word slides
about this. Oh, I just found a word slide. I forgot I put that in. Slide
30 is an overview and I'm afraid it didn't turn out too well. The
notable things are we still see those two peaks, the protons and a
heavier ion. The energy is consistent with a nice, reasonable flow
speed. You see 120 minutes or so of data going in towards Titan and then
the energies of the ions gradually go down. There is some stuff in
Titan's ionosphere at closest approach that I'll talk about later. The
important thing for this slide is right after we get out of Titan's
ionosphere, boom, velocities go up to actually almost twice what they
were before the encounter and then gradually decline. What we saw going
in and what we saw going out were rather dramatically different.

In the electrons, similar things, but the main thing there is there is a
large population of ten to maybe a couple of hundred EV electrons there
that would be quite efficient in ionizing gas in Titan's atmosphere
sustaining an ionosphere even on the dark side of Titan, on the
nightside, and also exciting some UV emission that should be observable
by the Cassini instrument. These disappear right around closes approach
when we're actually in Titan's atmosphere and that's probably a matter
of Titan's atmosphere, even though we were pretty high up and it's
pretty tenuous, it could very well be enough to have absorbed all of
these guys.

Going to the next slide, I want to say a little bit about those flows I
mentioned, the way that on one side, the side that was actually away
from Saturn, the flow slowed down very gradually. This is the sort of
thing we'd expect for a very diffuse cloud of neutral gas producing
ionization over a wide range of distances from the planet. Adding fresh
ions to a plasma tends to slow it down and this sort of gradual slowing
is quite consistent with that.

What we saw on the other side, a rapid increase to almost twice the
pre-encounter and presumably, therefore, the upstream flow speed is
actually what you would expect for ordinary flow past an obstacle. When
it flows around and over the flanks it sped up. We believe that really
matches some of the theoretical predications that said that because the
flow of plasma and magnetic fields across Titan induces an electric
field and it induces one that points away from Saturn, that fact has
been used in a number of theories to say why for a number of different
reasons you would expect all of the source of fresh plasma, all of the
ions escaping from Titan's atmosphere, to do that on the side facing
away from Saturn. This seems to be holding together and we think we
understand what is going on with that.

On slide 32, you can see that a bit better; the gradual flow slowing and
the sudden increase in a close up of the one hour centered on closest
approach. The reason that you see the flux of ions pulsing in and out is
that we were using that actuator to rotate the instrument back and
forth, so every time we crossed the direction the flow was coming from
we saw a flux. When we looked away, we didn't, back and forth, on and
off, on and off. That's unfortunate because we'd love to be taking data
all the time, but it's actually quite necessary for us to get the
direction of the flows as well as the speed.

I just want to move on pretty quickly to slide 33 which is a real close
up, plus/minus ten minutes, and this is when we were flying through the
ionosphere in Titan's atmosphere, actually going down to what is
probably the densest part of the atmosphere. Starting with the ions,
again, you're seeing the pulsing because of our actuator motion, so we
went to a limited range and a faster sweep time so you see more looks in
the right direction.

> Again, we have this same thing in our energy spectrum where we see one
> peak for each different ion species, so we're seeing the particles
> with the ram energy relative to the spacecraft and since they all have
> the same flow velocity, they all come in at an energy that depends on
> their mass. You can see a whole chain of different energies, a whole
> string of different peaks.

If you look at my last figure, slide 34, where I've just shown a line
plot of a single time, you can see many, many discreet peaks. We're
getting good fits to these spectra assuming seven different ion species,
all behaving pretty reasonably at a temperature of 150 to 200 Kelvins,
which is what it ought to be according to the theoretical models of the
structure of Titan's ionosphere. You can even see there is probably more
going on at higher energies and higher masses, but there you are talking
about species with a mass above 100, probably fairly complex, moderately
complex hydrocarbon and nitrile molecular ions. There are a whole bunch
of species it could be, so they all merge together.

We're really happy to see this and we think we're going to get some very
good constraints on the ionosphere composition just from these energy
spectra without even looking at our mass spectra. Combining the two, we
should be in great shape.

The other thing to note in that previous slide, in slide 33, I already
mentioned the higher energy electrons now in the bottom panel
disappearing. It's also worth noting that the ionosphere minus 15
minutes to closest approach looks very different from the same period
after closes approach. This may be because closes approach was over dusk
local time, so we're moving from the dayside to the nightside of Titan,
but at the moment we really don't understand that asymmetry.

The other thing that at the moment we're still debating amongst
ourselves, but if it's true it would be especially nice, there are four
vertical streaks where you can make out multiple dots right around
closest approach, maybe minus two minutes to plus two minutes. In fact,
they look very similar to the ion data. What we're thinking is there
isn't really anything that limits our electron sensor to electrons. Any
particle with a negative charge would go through and would be seen.

> It's quite possible both for theoretical and experiment grounds to
> form negative ions under the right circumstances. It's quite possible
> that what we're seeing there is in fact negative ions in Titan's
> ionosphere, which is something that people hadn't thought about
> before. If we can really pin that down that's going to count as a
> discovery that is really going to have some of the atmospheric
> chemists scratching their heads, but I'm looking forward to doing that
> and confusing them.

I believe that is all of my slides and everything I had planned to say.
Why don't I stop there and answer whatever questions anybody has?

W I'm going to have to leave, so I just want to say thank you very much.
I've enjoyed it immensely.

M Can you just accept congratulations for giving a wonderful talk?

Frank Thank you very much.

M It will take some digesting, but you've really given us a full meal.
Thank you.

Frank Alright.

Trina If there are no questions, Frank, I would like to echo what
everyone else said. Thanks very much. We really appreciate it. It was
our first magnetospheric ... so you had to do the whole what is a
magnetosphere, etc.

> I wanted to give an advertisement for our next CHARM teleconference.
> Typically, we do these things on the last Tuesday of the month. Of
> course, the last Tuesday in December is just after the holidays,
> December 28^th^, but we've decided to go ahead and have the
> teleconference because, of course, we will have just released the
> Huygens probe a few days before and we'll be gearing up for the
> Huygens mission. Even though it is the holidays, we thought that it
> would be worthwhile to go ahead and do the teleconference at our
> normal time. It will be available via an audio download if you aren't
> able to make it in on that day. It seems since there were so many
> interesting things going on in the mission in late December that it
> seemed a shame not to have our teleconference, especially with such an
> exciting topic to be at hand. That's our advertisement for next month.
>
> Again, Frank, thank you very much for today.

Frank I think I did hear one question.

Trina Oh, question? By all means ...

M Could you just in a few words summarize any surprises that you've
gotten out of your work?

Frank Nitrogen, I was expecting a whole lot more nitrogen. It's the
dominant constituent in Titan's atmosphere and I think we had been
expecting a good strong density of nitrogen ions in and around Titan's
orbit and also diffusing in closer to Titan. We're seeing nitrogen, but
we've only been able to identify it at maybe the 5% level.

M Any thoughts as to why?

Frank Possibly. When I was talking about the sharp boundary we saw at
about 15 Saturn radii where the density suddenly increased, we've been
attributing that to outside being open stream lines where the plasma
just doesn't stick around very long and even if you have a good source,
doesn't have a chance to build up. If that is true then Titan is
probably spending most of its time in that kind of region and no matter
how big of a source it is it may not be enough to make up for the short
lifetime. If so, that is going to have a number of people slapping
themselves on the forehead and saying, "Oh, I should have thought of
that."

Also, there are theoretical models, simulations of the magnetosphere
that should have predicted that. That would mean that the theoretical
models, ones that are also used to study the Earth, aren't getting the
dynamics of the tail and how quickly plasma is lost down tail. They
aren't getting that right because they weren't predicting low nitrogen
densities either. That is going to be very interesting to figure out
exactly what is going on there. I think that was my biggest surprise.

M Thank you very much.

Frank No problem. I was glad to do it.

Trina If there are no more questions, everybody have a terrific holiday
since we won't see you until December 28^th^. Frank, again, thank you
very much. We really appreciate it.

Frank No problem. Talk to you later. Good-bye.