**FTS-NASA-VOICE**

**September 27, 2005**

**1:00 p.m. CDT**

Coordinator Welcome and thank you for standing by. I would like to
remind all participants this conference is being recorded. If you have
any objections, you may disconnect at this time. You may begin.

Trina Thank you, everyone. I'd like to welcome you to the CHARM telecon
for September. Today, we have one of the science planners and operations
engineers from the Magnetometer team. Nick is going to be joining us. I
won't even attempt to pronounce his last name, because I know I'll
butcher it. He's got some other topics sprinkled throughout his
PowerPoint, but he definitely has a whole bunch about what the
magnetometer has been seeing in the last year.

With that, I'll go ahead and turn it over to you, Nick.

Nick Thank you. Thanks very much. Hello, everyone. It's a pleasure to be
invited along to give this presentation. I'm going to be talking to you
today about mainly magnetic observations made of the planet Saturn, by
the magnetometer on board the Cassini spacecraft in the first year of
its mission.

My name is Nick Achilleos. I'm a Science Planner and Operations Engineer
based at Imperial College, which is, in a sense, mission control for the
magnetometer instrument. We can monitor the health and safety of the
instrument from Imperial, and we can also command it remotely through
secure links that we have with JPL. So it's a very nice way of
participating in the mission for us.

I've got the names of the people at the Imperial College team on the
cover page here. I guess most importantly, our principal investigator is
Professor Michele Dougherty, and our operations team leader is Steve
Kellock. Peter and Tim are our operating and data processing guys. Cesar
and Chris are involved in the science, and we have many more
co-investigators at various places around the globe that you can see
there.

Today, I'll start off by talking in very general terms about the Cassini
mission and the journey to Saturn. I'll also show some images from the
approach to Saturn, because people always want to see the pretty
pictures of Saturn, and so they should, but I also want to -- I'd like
you to take away with you, if you could from today's talk, the fact that
not only do we have the visible planet of Saturn, but Saturn is
surrounded by an invisible environment that's measured by the field and
particle instruments on Cassini, and that environment is just as
important in terms of the science that we can learn.

I'll describe magnetic field measurements from the first orbit of
Cassini and also some of the early orbits. Then I'll look at images very
briefly from the Huygens' probe mission to Titan. And although I haven't
listed this aspect of the talk here, I'll also talk a little bit about
the observations made by the magnetometer at the icy satellite known as
Enceladus.

So if we proceed to the next slide, this is just to give us a bit of
context. The top two panels here sort of show us the extremes, if you
like, in our knowledge of Saturn. On the left here, we have one of
Galileo's original sketches of the planet, when he was looking at it
through his telescope, and his interpretation; that was almost that
Saturn has these lobes or ears adjacent to the body of the planet. We
now know that of course this is a low-resolution image of the fantastic
ring system that the planet has.

On the right image there, you can clearly see that ring system. This is
an image from the imaging science subsystem on Cassini during the first
approach to Saturn. Of course there are many spectacular images like
this easily available to the public.

On the bottom here, we have portraits of the two guys after who the
mission is named. We have Giovanni Cassini on the left here. He was born
in Genoa, in Italy, but he spent his final years at the observatory in
Paris. He studied, among many other things, the moons of Saturn and also
the famous gap in the rings. The Cassini Division is named after him.

On the right, Christiaan Huygens was born in the Netherlands. Again, he
was a true Renaissance man and studied many different areas, but his
contribution to planetary science included studies of Saturn's rings,
and he's credited with having discovered Saturn's moon Titan.

The next slide shows us a schematic view of the trajectory of the
spacecraft from its launch at the Earth in late 1997, up to the arrival
at Saturn in July last year. You can see that on the way the spacecraft
is planned to fly past a few of the planets of the inner solar system,
Venus firstly and then the Earth. Then by the time these flybys have
been done, the spacecraft picks up a lot of momentum and energy of
motion from the gravity assist of these planetary flybys. There was a
Jupiter flyby in late 2000/early 2001. By that stage, the spacecraft
would then be well on its way to Saturn, and it arrived there the 1^st^
of July 2004.

The next slide shows us a view of the spacecraft. To give you an idea of
scale here, the high-gain antenna that you see on the top is about four
meters in diameter, and this is the antenna through which the spacecraft
does most of its communications with the Earth.

You can see the rim of the Huygens probe, which is tucked behind the
main body of the spacecraft in this picture, the sort of middle-right
there. Our instrument, the magnetometer, is situated on this long
11-meter boom that you can see extending out from the left-hand side of
the spacecraft in this picture. Of course the reason we have the
magnetic sensors on this boom is so we can hold them as far as possible
from the main body of the spacecraft, because, of course, the main body
of the spacecraft, there's lots of electric currents flying around,
which generate magnetic fields, and so we want to try and avoid the
contamination by those fields as much as possible.

The next slide again gives you an idea of scale. This is the spacecraft
being assembled, surrounded by human-sized technicians. I guess it's the
size of something like a small school bus.

The next slide, page six, shows you probably the most critical maneuver
in the whole mission, which was the arrival at Saturn and the insertion
into the first Saturn orbit of the spacecraft. This is known SOI or
Saturn orbit insertion.

What we're looking at here is a view down onto the northern hemisphere
of Saturn. Of course you can see the plane of the rings, the rings in
the system, which orbits Saturn itself. The sun is way off vertically
upwards in this picture. You can see Saturn's sunlit face, and you can
also see the shadow of the planet on the rings.

The parabolic curve that intersects through the middle of the picture is
the spacecraft trajectory. This is Cassini's arrival at Saturn. At the
top, at 00:47 Universal Time on July the 1^st^, the spacecraft ascended
up through the gap in the rings, between the so-called F and G rings.

A short while later, which is marked by the red segment of the
trajectory, the spacecraft fired its main engine, more or less against
its direction of motion. The reason for this was to slow down the
spacecraft enough so that it could be captured by Saturn's gravity into
orbit around the planet. That engine burn lasted for something like one
and a half hours, and at the end of it the spacecraft was in orbit
around Saturn, and there were a lot of happy and relieved scientists and
engineers.

If you look toward the bottom of the picture, you can then see the
descending ring plane crossing, which occurred about four hours or so
after the initial ascent. So Cassini now plunges back down through the
ring plane, and by this time it's in orbit around the planet and has
commenced its four-year tour of Saturn.

If we go on to slide number seven, this is an image taken by the ISS,
Imagining Science Subsystem, during the first approach to Saturn. You
can see the banded structure of the clouds on the main body of the
planet and of course the ring system, with the Cassini Division here,
very obvious.

Moving on to slide number eight, this is just a bit of detail, fine
detail of the rings, and images like this I think are going to keep ring
dynamicists very busy for many years to come. But the main point I want
to make here is that you can see these gradations in color in the rings.
There are white and also sort of brown or tan colored segments of the
rings. This is thought to be because principally the rings are made of
water ice, just like you put it in your drink here on Earth, and the
sort of organic compounds known as fall-ins. So you have icy segments of
the rings and also more dirty sections.

The Cassini Division itself is due to a so-called orbital resonance with
the satellite Mimas. I'm sure someone else has probably spoken to you
about that in more detail, but essentially the idea here is that it's
very difficult for material in the rings to stay there, because the
orbital period of material in the rings is an integral multiple of the
period of the satellite Mimas. So Mimas pulls on this material at the
same stage in its orbit, and tends to clear that particular position of
ring material.

If we go on to slide number nine, this is detail of the F ring. It's a
very interesting ring. It has a very braided or ropelike appearance. If
you look very carefully on the inner side of the ring, you can see these
dark diagonal slashes. This is, I believe or I am told, because the
satellite Prometheus orbits very close to the F ring, and at times can
draw, literally pull ring material out of the F ring, and then the
differential motion of the particles in the ring. Remember, the
particles which orbit Saturn at larger distances will generally have
larger orbital periods. So the differential motion of particles in the
ring then draws out those radial features, due to Prometheus, into these
diagonal features.

If we now go on to slide number ten, we can start talking a little bit
about the magnetic aspect of Saturn. We'll start off in very general
terms by looking at planetary magnetic fields in general. The first
picture here is something that probably all of us are familiar with.
It's just a simple bar magnet. It's also drawn in around the bar magnet,
which has a north and a south magnetic pole, the so-called magnetic
lines of force.

This is a very powerful representation in the way we represent these
field patterns, because it shows us not only the direction of the field
but also the strength. Now the direction of the field is given by the
tangent to the lines of force that you see drawn there. So if you place
a compass at any point along one of those magnetic field lines, its
needle will be tangent, locally tangent to that curve.

You can see that the field lines are crowded close together, very close
to the poles, which indicate field strength is relatively high. Then as
you go to large distances from the bar magnet, the field lines are far
apart, less dense, if you like, and that means that the field strength
is smaller, weaker, with the larger distance from the source, as one
would expect. The good news is that the magnetic field to the planet, to
a good first approximation, looks as though there's a giant, strong bar
magnet somewhere near the center of the planet.

The bottom panel here shows us the case for the Earth. The blue disk is
the body of the Earth, and you can see the magnetic field lines, which
have been drawn in the diagram here, and they this dipole-like pattern
corresponding to the bar magnet.

Also shown here is the magnetic axis, which is the axis of symmetry of
the field itself, in other words, the line joining the north and south
magnetic poles is a bar magnet. Also shown is the rotational axis of the
Earth, the axis around which the Earth rotates, and you can see that
there's an angle between the two. In general, we expect this to be the
case, and the angle between these two axes is known as the dipole tilt.

If we go on to slide 11, there's a table here, which makes some
comparisons between the properties and magnetism and rotation for three
of the magnetized planets in the solar system, mainly the Earth, Jupiter
and Saturn. If we start off by looking at how heavy these planets are,
if we set the mass of the Earth equal to one unit, Jupiter is more than
three hundred times as heavy as the Earth, and Saturn is about 95 times
as massive. If the Earth's radius is, again, set to one unit, Jupiter's
radius is more than ten times as large and Saturn's is just under that.

When we look at the periods of rotation, something very interesting is
seen, because the Earth, as we know, rotates once every day, every
24-hour period. However, Jupiter and Saturn are much faster rotators.
They have periods less than half a day.

That's important because for their size these planets are very rapid
rotators. If you're sitting at the equator on the Earth, the Earth's
rotational motion carries you around at roughly one kilometer a second,
and for these giant planets that figure increases to something like ten
kilometers a second. So the rotational motion of these giant planets is
very rapid.

If we look at the intrinsic field strength of the planets, this can be
measured by a very simple parameter known as the dipole moment. It's
just equal to the field strength at the equator, on the surface of the
planet, multiplied by the cube of the planet's radius. If we set this
equal to one for the Earth, Jupiter's intrinsic field is 18,000 times as
strong as the Earth's. So Jupiter has a very strong field indeed.

But Saturn is not exactly weak either. It's 550 times as strong as the
Earth's field. So the giant planets are very strongly magnetized in
comparison to the Earth.

Now if we look at the dipole tilt angle, remember, this is the angel
between the rotational and the magnetic axes. Both the Earth and Jupiter
have tilt angels of about ten degrees, and Saturn mysteriously has a
very small tilt angle. Current measurements constrain this to be less
than one degree in the case of Saturn.

In other words, Saturn's magnetic and rotational axes are almost
parallel. That's a bit of a mystery, because the theories of how these
fields are generated say that a planetary field, which has that degree
of symmetry, will not be able to survive on time scales as long as the
age of the solar system. So this is sort of a mystery of Saturn's field
that we hope to try and unravel, as we get more and more accurate
measurements of the field's strength surrounding the planet.

If we go on to slide number 12, we sort of explore in a bit more detail
the fact that these giant planets have very strong magnetic fields. One
of the ingredients for generating these strong fields is the rapid
rotation that we've already talked about.

Another ingredient is the nature of the interior of the giant planets.
Here we have a schematic view of Saturn, which has been cut away to
reveal the structure of its interior. The planet is thought to have a
rocky core, which is surrounded by a shell of hydrogen in a very exotic
state. Essentially, it's thought that there is a layer of hydrogen
surrounding the core of Saturn, which is in what's called a liquid metal
form.

The pressures at the interior of Saturn are enormous. They are more than
40 million times the atmospheric pressure at the surface of the Earth.
Under these very extreme conditions, the bond between the atoms in
hydrogen molecules and even the electrons and the nuclei of hydrogen
atoms weaken significantly, and you get effectively hydrogen in a
conducting state and you also get hydrogen in a liquid state.

Now you're never going to find hydrogen in this state in laboratories on
the Earth, because we just can't reproduce the extreme physical
conditions required to make it. But in theory, this form of hydrogen is
thought to exist in the interior of Saturn, and it's thought to play a
very important role in the generation of that planet's very strong
magnetic field.

If we move on to slide 13, this is just an introduction before we look
at data of the magnetic environment of Saturn. Now Saturn, in this
picture, is that tiny disk that you can see near the center, surrounded
by the rings. So you can see that in terms of the effect of Saturn's
magnetic field on its environment, the region of influence is very large
compared to the planet itself.

The reason that some of these structures that we're going to talk about
in this picture here comes about is through the interaction between the
planet's magnetic field and the solar wind, which you can see streaming
in from the left in this picture here. Now this solar wind is like a hot
soup composed of charged particles, mainly negative electrons and
positive protons. The solar wind is continually streaming out from the
sun all the time, and it moves radially outwards through the solar
system and flows past the planet.

Now because the solar wind consists of charged particles, these guys can
be deflected by strong magnetic fields, and of course Saturn has one.
The Earth has one as well, as does Jupiter. So essentially what happens
is the field of Saturn deflects these frozen particles and they are
forced to flow around the planet.

But the story gets a bit more interesting, because of the fact that the
solar wind itself is moving so rapidly, the particles in the solar wind
at Saturn's orbit moving with velocities of typically 400 or 500
kilometers per second. In fact, that's much faster than the speed at
which pressure waves can propagate through the medium.

What that means is that essentially the solar wind is a fluid that is
moving supersonically past Saturn. So when you have a fluid moving in
that fashion past an obstacle, the only way the obstacle can slow it
down is through the formation of a shock wave. It's analogous to a
supersonic aircraft moving through the Earth's atmosphere and the
formation of a sonic boom, of a bow shock in front of it that leads to a
sonic boom as it sweeps over observers on the ground.

The same sort of thing happens here. You can see the bow shock boundary
on the left that's formed by this interaction, as solar wind material,
which is also known as plasma or ionized gas, moves through the bow
shock boundary it becomes strongly heated and compressed and greatly
slowed down. Just on the other side of the bow shock is the region known
as the magnetosheath. As you'll see, the magnetic field in the
magnetosheath is very turbulent and very disordered compared to the
other agents that we see here.

As we move closer to the planet, we encounter the second main boundary
in this interaction, which is known as the magnetopause. Now the
magnetopause you can think of as a thin skin, which, in general,
separates the magnetic field of the planet from the magnetic field and
the plasma in the solar wind. So, in general, solar wind material is
flowing around the magnetopause boundary as it sweeps past the planet.

If we look inside the magnetopause, this region is known as the
magnetosphere. By definition, it's the region where the dynamics of the
fields and the particles are dominated by Saturn's internal field, as
well as the field due to some of these external forces that we see here.

If we look at the representation of the field lines, we see that on the
side of the planets facing the sun, the dayside, we have these
dipole-like shapes. On the night side, the dipole-like shapes persist as
you look at regions very close to the planet, but as you get further
from the planet you see that the field lines are drawn out into this
long tail-like configuration, which, appropriately enough, is known as
the magnetotail.

Before we move on, probably the most important thing to point out about
these boundaries is that they are not static boundaries. They don't stay
still. The density and the velocity of the solar wind are always
changing, and as they change so too do the locations of the bow shock
and magnetopause boundaries. In general, a solar wind, which is denser
and flows more rapidly is better, is better at compressing Saturn's
field, and so a faster, denser solar wind means a smaller magnetosphere
and more compressed bow shock and magnetopause boundary.

Let's move on to slide 14 and start talking a little bit about
magnetometer science. On this slide here you can see photographs of the
two magnetic sensors that together comprise the Cassini magnetometer
instrument. On the left we have the Fluxgate magnetometer or FGM. This
is a magnetometer, which measures magnetic fields using a system of
coils. It's situated about halfway along that 11-meter boom that we
talked about coming out from the side of the spacecraft.

On the right is the helium magnetometer, and that little ruler there in
that picture is about ten centimeters long, to give you an idea of how
compact these sensors are. The helium magnetometer measures magnetic
field through the effective magnetic field on the absorption properties
of helium gas. Helium gas in the sense there is illuminated bipolarized
infrared light from a lamp, and by measuring the absorption line seen
from that gas the direction strength of the magnetic field can be
inferred.

What's important about the helium magnetometer is it can be operated in
what's known as scalar mode. In this mode -- was that a question?

Trina It sounds like there's a question, or does somebody have an open
line? Nick, I'll go off to the operator and see if we can isolate that
line. Why don't you go ahead and go on.

Nick As I was saying, the helium magnetometer can be operated in a
special mode known as scalar mode. In scalar mode we can no longer
measure the direction of the magnetic field, but we can measure the
field's strength very accurately, up to about one part in a billion. So
what we do is when we get very close to Saturn, we switch the helium
magnetometer into scalar mode to get very accurate measurements of the
field strength due to Saturn's internal source, and by doing so we hope
to be able to map the very complex components of the planet's internal
field, in other words, components more complex than a simple dipole
pattern.

If we go on to slide 15, this is actual magnetometer data from Cassini
taken during the interval between June 27^th^ and the 29^th^ last year,
as the spacecraft was making its first approach towards Saturn. What
we've shown here in the top three panels as a function of time are the
three components of the magnetic field, and this is just to emphasize
that the magnetic field is three-dimensional quantity. It has strength
and it has direction, but we'll concentrate, for the purposes of this
talk, on the bottom panel, which is the total field strength in
nanotesla units as a function of time, as well as time on the horizontal
axis.

We've also shown the distance or the range of the spacecraft from Saturn
in units of Saturn radii. One Saturn radius is 60,000 kilometers, and
you can see that during this interval is three days. The spacecraft went
from a distance of 50 Saturn radii from the planet to a distance of
something 20 Saturn radii. So it covered a lot of distance and got
closer to the planet during that time.

So what's happening in terms of the magnetic field? You can see that if
you look at the first half of this time it's all that we're studying,
that the magnetic field exists in one of two very distinct states. First
of all, you have a relatively low quiet magnetic field of about half a
nanotesla. Then this is interrupted at regular intervals by a slightly
stronger and quite noisy magnetic field trace. This is because the noisy
field corresponds to the magnetosheath region, and the quiet field
corresponds to the solar wind.

During the first half of this interval, Cassini is making brief
excursions into the magnetosheath as it approaches the planet. Of course
the reason it does this is because Saturn's bow shock boundary is always
moving back and forth. So as the spacecraft approaches the planet,
sometimes it finds itself in the solar wind; sometimes it finds itself
in the magnetosheath. So essentially, those jumps in field, which are
marked here by the S labels, S1 through S7, are where the spacecraft
crosses Saturn's bow shock.

As time progresses and the spacecraft gets closer to the planet, you can
see another transition in the properties of the field. The field starts
to show these very large spikes. So we start off in the field of the
magnetosheath after the S7 crossing, and then we start to see some very
brief jumps in the field to higher field strength. Then during the time
marked by the MP labels, we see a final transition into the very high
relatively smooth field, and then by that stage of course we're inside
Saturn's magnetosphere itself, where we find the strongest magnetic
field measurements.

In essence, what we're seeing here is firstly a transition from the
solar wind to the magnetosheath, and then from the magnetosheath to the
magnetosphere. We've had more than ten orbits of Saturn since this data
was taken, and we see these kinds of crossings in all of the data, but
of course in detail their properties are different.

Now if we look at slide number 16, this is that same data that we saw,
but visualized in a different way, for people who are more, I guess,
geometrically minded. What we are looking at here you can think of as a
view onto Saturn's orbital plane, the plane it which it orbits around
the sun. Saturn is this red dot that you can see at the origin.

The distance along the horizontal axis is the distance from the planet
towards the sun. For the x-axis here, it's positive towards the sun. The
vertical axis is essentially the distance perpendicular to the x
direction. So essentially we've collapsed the y and zed coordinates into
a single distance here.

The parabolic curve that you can see going through the picture here is
the spacecraft trajectory. The spacecraft moved from the right-hand side
up near Saturn and then back down through the left-hand side of that
trajectory. You can see blue dots on the trajectory, which our time
techs spaced at intervals of one day. Of course the blue dots are
crowded closer together as we get further from the planet; because the
spacecraft moves more slowly the further it gets from the planet.

You can also see that hanging off the spacecraft trajectory we have
these blue lines, and these are representations of the field measured by
the magnetometer. In particular it would be the VHM, the vector helium
magnetometer. The length of the lines indicates the strength of the
field, and the direction of the lines, as you might have guessed,
indicate the field direction.

You can also see two dashed curves that have been drawn here. The red
dashed curve is a model of the average position of the bow shock, and
the purple curve is a model of the average position of the magnetopause.
The red crosses indicate where the spacecraft saw the bow shock, and the
purple triangles indicate where it saw the magnetopause. So you can see
that compared to the average positions that one would expect at Saturn
for these boundaries, the Cassini data saw indications of a relatively
expanded magnetosphere during the inbound pass of this orbit. It other
words it encountered the bow shock and the magnetopause earlier than one
would have expected.

On the way out, the story is rather different. The spacecraft, as you
can see by the triangle and the cross symbols, encountered the
magnetopause and the bow shock earlier than expected on the way out,
which indicates that the magnetosphere was relatively compressed on the
outbound pass.

However, you can see that, at the very end of the orbit shown here, we
again see red crosses, and so the bow shock was once again seen where
one would have expected to see it on average. So that indicates that
late on the outbound path of this first orbit, the magnetosphere
expanded once again to something like its average position. So this
indicates the magnetosphere of Saturn, as with the other planets, is a
very dynamic place as is continually in motion.

Before we leave slide 16, I'll also point out that the field
measurements, which are these blue lines that you can see attached to
the trajectory, seem to run more or less parallel to the magnetopause
boundary. If you look at the purple dash curve and compare the field
measurements that you see there, you can see that they are almost
parallel to the magnetopause boundary. The next slide, 17, hopefully
explains to us why that should be the case.

In this slide here, we have Saturn as that blue dot. We have a cartoon
representing Saturn's magnetopause surface surrounding the planet. On
the right here, we have the solar wind flowing in towards the
magnetopause from the right.

What we're going to do here is we're going to follow the motion of two
parcels or blobs of plasma in the solar wind. The first parcel is
towards the top of the picture here, and this guy has a very simple kind
of trajectory. Essentially, he sails right past the magnetopause and
never hits it. So he has a nice linear trajectory, and you can think of
these as snapshots of the same blob, spaced by regular intervals in
time. So it's just a nice linear trajectory, and he sails right past
Saturn and its magnetosphere.

His friend, towards the bottom of the picture, however, gets into
Saturn's magnetopause surface much closer to the sub-solar point. So of
course this parcel of plasma has to flow around the magnetopause
surface, because it can't penetrate it. So it has a very different kind
of trajectory. It gets closer to the magnetopause surface. It then
decelerates and starts to leak around the magnetopause surface. So
subsequent snapshots of this guy might look like something as shown in
the picture here.

What's interesting about plasma, such as solar wind, is that they are
almost perfect electric conductors. What that means is that if you have
two blobs, which are connected by the same magnetic field line, as these
two guys are, there's a physical law that says blobs of plasma, which
start out moving on the same magnetic field line must remain connected
by that field line. So you can see the only way that these two guys can
stay magnetically connected, as they approach Saturn's magnetopause, is
if the field line itself distorts and wraps itself around the
magnetopause surface.

You can see that what this means is that essentially the field pattern
drapes itself around the obstacle of Saturn's magnetosphere, and
appropriately this is known as magnetic draping. It's a consequence of
this property of fields and plasmas moving together, which is known as
frozen-in slow, and we'll see some more examples of that in the later
part of the talk.

If we move on to slide 18, this is a very busy plot, so I won't talk
that much about it, but I just wanted to point it out because it's a
very nice example of how we can compare datasets from different
instruments onboard Cassini and do science in that fashion, doing
correlated studies. This was work that was done by Emma Buntz, one of
our co-investigators at the University of Glasgow.

The time interval that you see here represents the outbound path of the
first orbit of Cassini. The panel in the middle here, which is marked B
nanotesla, is the total field strength of the function of time.
Essentially, what I want to point out is that in the time interval that
Emma has marked on the top as a hot plasma injection, you essentially
see very strong disturbances, not only in the magnetic field data, but
in data from other instruments onboard Cassini.

If you look at the second panel, this is data from the radio and plasma
wave subsystem, which measures electric fields due to waves in the
plasma environment of Saturn, and you can see that at the same time as
the magnetic field disturbance there is this very strong burst of radio
waves, across a very wide range of frequencies as compared with other
times, when it's a lot quieter.

The bottom two panels are data from the MIMI and CAPS instruments
onboard Cassini, which measure the energy of particles in Saturn's
plasma environment. You can see that in both cases the increased counts
in one case, and then the increased color scale in the bottom panel. The
population of energetic particles dramatically increased again during
this hot plasma injection event.

This injection itself comes about because material in the solar wind
sometimes leaks in to Saturn's magnetosphere through so-called
reconnection events, and hot plasma moves in closer to Saturn and, at
the time shown, it happened to engulf the Cassini spacecraft. This is
why you see such disturbed measurements, not only in the magnetometer,
but in all the other instruments as well.

If we move on to slide 19, this is magnetic data taken from this first
flyby of Titan by the spacecraft. Titan orbits Saturn at a distance of
20 Saturn radii. Remember, one Saturn radius is about 60,000 kilometers.
What's interesting about that is that Titan is very close to the
boundary of Saturn's magnetosphere. It's generally quite close to the
magnetopause.

What's also interesting is that Titan takes about 16 days to do one
orbit around Saturn. However, the material in which it is immersed, the
magnetospheric material in which Titan is immersed at that distance,
rotates around Saturn much more quickly than Titan. Essentially, what
this means is that magnetospheric material or plasma is continually
sweeping past Titan at a velocity of something like 100 kilometers per
second. So you have this very rapid flow of magnetized plasma past an
obstacle.

As for the case of the solar wind and Saturn's magnetosphere, you expect
to see some sort of effect on the magnetic field itself. In fact, this
is what you see here, the purple curve passing near Titan is the
spacecraft trajectory. In this case, the spacecraft is moving from left
to right, closer towards Saturn. Saturn is off to the right in this
picture.

Material in Saturn's magnetosphere situated at Titan's orbit is sweeping
past Titan, and its direction of motion here is up out of the page
towards us, the observer. You can see, at large distances from Titan,
the field measurements, which are indicated once more by these lines
hanging off the spacecraft trajectory, generally point in the same
direction. They are slightly inclined towards Saturn itself.

You can see that close to Titan itself the field becomes very strongly
distorted and, in fact, it looks as though the field is like fronds in a
fringed curtain, and that this curtain is draping itself around the
Titan obstacle. In fact, that's pretty much what's happening, because in
order to flow around Titan the material has to leak around the sides and
over the top of the moon, and because the field and the plasma moves
together the magnetic field also drapes itself around the Titan
obstacle.

Slide 20 shows you a bit more detail of that interaction. Mainly the
important thing to notice here is that you have the obstacle of Titan.
Titan has this extended rather thick atmosphere, which represents an
additional obstacle to the flow of Saturn's magnetospheric material. You
can see that there are these field lines, which move with the material,
and which drape themselves around the Titan obstacle as material flows
past.

So material far from Titan, at the top and the bottom of the picture,
moves relatively quickly and drags field lines with it. You can think of
these field lines as rubber bands. Material which impacts on Titan
closer to its center moves more slowly, decelerates and leaks around the
side of Titan, and so the field line there are bent further back towards
the direction of the magnetospheric flow.

In this picture here you can also see an extreme form of this draping,
which is in the region on the right here behind Titan, known as the wake
region of this flow. You can see that above Titan's orbital plane the
field points towards Titan, and below the orbital plane it points away
from Titan.

If we look at the next slide, number 21, if we just concentrate on the
right panels here, this is, again, the magnetic field measured by the
magnetometer, the first Titan flyby. The red traces here are the
measurements by the Cassini magnetometer, and the blue curve here is a
model of this interaction between Saturn's magnetosphere and Titan,
which was put together by Harko Backus, a student at the University of
Cologne. You can see that for all the components of the field, Harko's
model is doing a very good job of producing the large-scale features of
how the field changes with time.

Now the close approach to Titan occurred when this top component of the
field, the so-called BX component, dramatically changed sign. You can
see that just past the time of about 15 hours 30 minutes there's this
dramatic sign change in the BX component. This occurred because the
spacecraft crossed this wake region of Titan and, as we saw, the field
changes direction as one moves through that wake, and that's exactly
what the spacecraft is seeing here.

If we move on to slide 22, this is a just a few more images of Titan
that I'd like to show. Again, this is from the Imaging Science System on
Cassini. You can see the very famous clouds of Titan here, in this image
here. The radius of Titan is about 2,500 kilometers, and that makes it
the second-largest moon in the solar system, second only to Jupiter's
satellite Ganymede. Titan's atmosphere, as you'll see, is nitrogen-rich,
but also it contains methane.

If we go to the next slide, 23, this is data taken by the INMS
instrument onboard Cassini during that first Titan flyby. We can see
here that we have density of different molecules in Titan's atmosphere,
as measured by INMS, the Ion and Neutral Mass Spectrometer, as a
function of altitude above Titan's surface. So here we're looking at
about 1,200 to 1,600 kilometers above Titan's surface.

What we have is three profiles of three different molecules in the
atmosphere. You can see that nitrogen is the most abundant molecule, N2.
The next most abundant then is CH4 or methane, and then this followed by
H2, which is molecular hydrogen.

The red symbols are data taken during the inbound path, and the blue is
taken during the outbound path of that flyby. You can see that in
general there seem to be these wavelike disturbances that, in general,
the profiles indicate a fairly steady composition for Titan's
atmosphere.

On the bottom of the graph, I've shown data from the Voyager UVS
instrument. Remember, Voyager flew by Saturn more than 25 years ago. So
the fact that these data are more or less continuous with the INMS data
indicates that Titan's atmosphere is fairly stable in terms of
composition over long time scales.

Moving on to slide number 24, these are the famous pictures that
appeared in the media that were taken by the Huygens probe as it
descended towards Titan January 14^th^ of this year. You can see
different landscapes on Titan here. You can see these ridges or
channels, which are thought to have been once fed by flows of liquid
methane or methane rain on Titan. You can see towards the bottom of
these pictures a much smoother landscape, near which the Huygens probe
landed and was thought to have the texture of something like wet sand.
So it's almost like a huge beach, where the liquid is dominantly liquid
methane.

The next slide shows an image of Titan's surface, this time taken by an
instrument onboard the Cassini orbiter. Cassini has a radar instrument,
and the radar instrument operates a wavelength, which effectively allows
it to peer through the thick haze surrounding Titan and onto its
surface. You can see on the right here the rim of a rather large crater
that was discovered on the surface of Titan. That's more than 400
kilometers wide. It's fairly rare that these sorts of features are seen,
and so it's thought that there is a dynamic continual reshaping of the
surface of Titan, due to liquid flows.

Slide number 26 finally shows us some magnetometer data that was taken
during a flyby of the satellite Enceladus by Cassini earlier this year.
Enceladus orbits Saturn at a distance of about four Saturn radii. So
it's much closer in than Titan.

The panel on the left here shows you the magnetic field vectors, which
were measured by the magnetometer during the first flyby in February
this year. You can see that at closest approach to Enceladus there was
this strong distortion in the field. Again, you see a pattern that looks
as though the filed is draping itself around Enceladus.

What's interesting about this is that the field draping indicates
there's an obstacle, which is much larger than Enceladus itself. On the
basis of this measurement we did, the project allowed a much closer
flyby of Enceladus the 14^th^ of July this year. So that very close
flyby, which was about 750 kilometers above the surface of Enceladus,
all the instruments on Cassini saw some kind of interaction due to the
presence of Enceladus in Saturn's magnetosphere.

On the right, there is a schematic diagram of what's happening with the
field. As hot plasma in Saturn's magnetosphere flows past Enceladus,
because of this frozen-in condition that we've been talking about, the
field distorts itself and drapes itself around the Enceladus obstacle.
The obstacle shown in this picture here is this haze of gas, which is
essentially steam, water surrounding Enceladus, but observations taken
since then indicate that the source of the out-gassing on Enceladus is
asymmetric. There seems to be a lot more steam coming from the southern
hemisphere of Enceladus. So we're refining this picture all the time.

The idea here is that Enceladus, the strong out-gassing from the moon,
from its icy surface being broken up, Enceladus is continually adding
mass to the plasma in Saturn's magnetosphere. As water molecules leave
the surface of the satellite they become ionized. Once they become
ionized or charged, they can be affected by the magnetic field and they
get swept up with the other flow of magnetospheric material.

Investigation of Enceladus is very interesting and ongoing, and if we
move on to the final slide here, one could also say that about the
exploration of Saturn in general. Thank you for listening.

Trina Are there any questions? I guess I had a question, Nick. Of the
two instruments that you described, which one is more sensitive?

Nick The helium magnetometer is more sensitive, when it's operated in
scalar mode, because then it can detect field strength better than one
part in a million. If you operate either instrument in vector mode, they
can measure field strength of something like one part in 10,000. So the
helium magnetometer in scalar mode I think is the most sensitive in
those sorts of terms.

Trina For example, when you were flying by Titan, the TA data that you
showed on page 21, is that coming more from the vector magnetometer or
more from the helium part way at the end, the vector way at the end?

Nick The data on page 21 comes from the helium magnetometer in vector
mode, the helium magnetometer essentially from up to a field strength of
256 nanotesla. It can measure field in vector mode, and as soon as you
get to a field stronger than that, by which time you're something like
four Saturn radii or closer to the planet, you need to switch over into
scalar mode, because the vector mode saturates and you can't get any
further vector measurements.

However, the Fluxgate magnetometer takes vector measurements over a very
wide range of fields. So we always have something to compare the helium
magnetometer measurements with.

Thanks Thanks.

Matthew Matthew Ota of the Saturn Observation Campaign. What effect does
the magnetosphere have on the spokes that are seen in the rings?

Nick My understanding, which is sketchy admittedly, is that the spokes
in the rings are not always seen. In fact, I think that the ISS team
reported seeing spokes in the rings for the first time quite recently
for Cassini. So then the question remains, what causes the appearance of
these dark spokes in the rings?

One explanation I've heard people talk about in a very abstract kind of
way is that it might be due to the presence of charged particles of dust
being trapped in the magnetic field, and essentially levitating
somewhere above the surface of the ring plane. That's about the extent
of my knowledge on that.

Are there any more questions?

M Thanks, first, for the hour. It's been very interesting. I have a
question though about an event that occurred when the Cassini probe was
passing Jupiter. As I recall it, there was a coronal mass ejection
roughly timed so that there was \... action \... the sun, and what the
magnetometer was able to detect on the Cassini probe relative to the
magnetometer on the Galileo probe. Could you describe those events?

Nick Yes. The flyby of Jupiter occurred in late 2000/early 2001.
Essentially, what happened during the event that you're describing is
that Galileo and Cassini were able to determine simultaneous
measurements of the position of Jupiter's magnetopause boundary.

That was very interesting, because it indicated that the passage of the
coronal mass ejection past Jupiter was compressing the sub-solar portion
of the magnetopause at the time the observations were taken. In other
words, we got a very rare view by two spacecraft of a planetary
magnetopause, and not only that, but we caught the magnetopause in the
act of being compressed by a transient solar wind event, such that the
sub-solar portion of the magnetopause measured by Galileo was a lot more
compressed compared to what Cassini was seeing.

M Thanks very much.

M I have a question. If you could refresh my memory, you mentioned that
you were seeing some of the same data with Cassini that you saw with
Voyager 20-something years ago. What was that data again?

Nick Essentially, Cassini saw the same set in general, the same kinds of
things. As the spacecraft makes the transition of Saturn's
magnetosphere, we see an encounter with the planet's bow shock and with
the magnetopause. However, the Cassini observations during the first
orbit indicate that Saturn's magnetosphere was relatively expanded
compared to what Voyager saw during the inbound path of the orbit, and
relatively compressed to what Voyager saw during the outbound path.

M Thank you.

M I have a question on the question of why it's unexpected to have the
rotational tilt correspond with the magnetic pull.

Nick Again, my knowledge here is quite limited, but my understanding is,
from talking to the field modeling guys, that if you have a field
configuration where the magnetic and rotational axes are parallel, that
field is not sustainable by dynamo action. It should just be decay away
on a time scale, which I believe is somewhat shorter than the age of the
solar system. So you wouldn't expect that field to survive on very long
time scales. So it's quite a mystery why the field of Saturn is so
axis-symmetric.

It indicates either that Saturn's field is being generated in a way that
we don't understand, or that there is some kind of process happening in
its interior that causes our external observations of the field to
appear more axis-symmetric than what they are in reality.

M I'm afraid I still don't understand why something spinning like a
dynamo, what would trigger that to dissipate, if the planet is rotating
symmetrically?

Nick I think you need the combination of -- I believe in the dynamo you
have a combination of the rotation of the planet and the convective
motions in the interior. So you're always sort of breaking and forming
the field, in a sense, to generate it. My understanding is that a
consequence of that is that you expect there to be a measurable angle
between the magnetic and rotational axes. I think that's the extent of
my knowledge. You'd have to ask one of the field modeling guys to learn
more about that.

Rachael This is Rachael from Virginia. I have a question I hope you can
answer. In doing outreach with a lot of elementary schools, I seem to be
getting the same question a lot and I can't answer it. The question is
if you count Saturn's rings or if you include Saturn's rings, would
Saturn have more mass than Jupiter? It's great that the kids are
beginning to think about this.

Nick My feeling would be no. I've never done the calculation, but my
feeling would be probably not, because Saturn itself is something like
100 times the mass of the Earth. Jupiter is 300 times the mass of the
Earth. So, essentially, what that means is that you would have to have
twice the mass of Saturn contained in its rings, if the whole system was
to exceed Jupiter's mass, and I don't think that's very likely.

Rachael The mass of Saturn's rings are roughly the mass of one of the
small moons, maybe bigger than Hyperion, but not much. So the answer is
no. You had it right on, Nick. It's great that they are asking a
question like that.

Nick Yes, it's very good.

Rachael I mean Saturn's ring look like they contain just a whole bunch
of mass, but it's just broken up into very small particles and spread
out over a very large area.

Nick Right. In fact, I think the \... of Saturn is supposed to have a
density less than water. So if you had an ocean large enough, Saturn
would float in it, right?

Rachael Yes.

Trina Do we have any more questions?

M I have a question about Titan. I'd like to know the influence of the
magnetic field on the chemistry and the atmosphere.

Nick Titan, the observations that we've taken indicate that Saturn's
field penetrates into Titan's atmosphere somewhat. It doesn't penetrate
completely, because currents flow in Titan's ionosphere and they sort of
shield Titan against Saturn's field partially but not completely. So you
do have penetration of the field into Titan's atmosphere to a certain
extent.

What that means is that energetic particles in Saturn's atmosphere can
flow down along field lines and actually intercept Titan's atmospheric
material at certain patches on the surface of Titan. In essence, what
that means is that you have an extra source of ionization, in theory,
for Titan's atmospheric material. So you have the solar ionization
source as well as ionization from this sort of magnetospheric rain of
energetic particles that follow the fields to which Titan is connected.

M But it's negligible, I guess, compared to the moon itself then.

Nick That's not very well known. People need to do more detailed
calculations of the interaction between the magnetosphere and Titan
itself, to get a better idea of that.

Trina Are there any other questions? I'd like to thank Nick very much
for joining us. We know it's quite a bit later for you there in England
than it is for us here in the States, so we appreciate you staying up
late and telling us about the magnetometer results.

Next month we'll be joined by Dr. Bill Kurth. He'll be talking about the
radio and plasma wave experiment and the results from that experiment
over the course of the last year or so.

Take a look for the e-mail I just sent out regarding the 508 compliance,
and the passwords, and linking to the password-protected site. If you
have any questions, go ahead and contact
<Charm_leads@cdsa.jpl.nasa.gov>. With that, we'll say --

Jane Trina, this is Jane. I will send the announcement out to the Saturn
Observation Campaign members. So those of you that are on the phone fear
not; I'll be sending you that information shortly.

M Will the SSA people be getting that information, too?

Jane They should. Kay Ferrari is on my distribution, and she typically
forwards it back out to her Solar System Ambassadors, but I'll pick up
the phone and give her a ring to make sure.

M Thank you.

M You didn't send it to us directly.

Jane Right. I send it to what I call a CHARM distribution, and then we
have the Solar System Ambassadors and the Saturn Observation Campaign,
and it sort of explodes from there, and museums also. Anita will send
out to the museum people.

M I got in here just as you were talking about that, and I didn't quite
understand what was going on. It sounded important.

Jane It all will be revealed in the e-mail. I went ahead and put the
password and the user name in the e-mail for folks, so that they have a
record of it.

Trina Thanks, everybody, and have a great month.