**FTS -- NASA - VOICE**

**October 25, 2005**

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

Coordinator I just wanted to remind all parties that this conference is
being recorded. If you have any objections, you may disconnect.

J. Jones I guess that's my cue. Thanks, everybody. I'd like to welcome
you to today's Charm telecom. My name is Jane Houston-Jones from Cassini
Outreach. Your usual coordinator, Trina Ray, is on vacation this week,
so I'm filling in for her. Can everybody hear me? I hope so.

M Yes.

J. Jones Great. The only housekeeping reminder I have is to mute your
telephone by pressing your mute button or star six if you feel that
there's going to be any background noise that the rest of us can hear.
We'll all appreciate that. I asked our speaker how he would like to
receive questions and he said if you have a question, it's best to just
go ahead and say it right then or at an appropriate pause in the action
so that he can answer the question at that time.

These telecoms are provided to all of our outreach groups, such as the
Cassini Saturn Observation Campaign and the Museum Alliance and the
Solar System Ambassadors each month by the Cassini Mission. So we'd
really love to know when you use this material for programming of your
own. You can either let us know or let your outreach coordinator,
whether it's Anita, Kay or me for the Cassini Saturn Observation
Campaign. Let us know so we can let our science folks and our mission
people know how much these talks help you out.

You should have all received the URL's where the talks are stored. If
you have a problem you can probably just send me an e-mail while we're
in the talk and I can probably help you out. That's J.
Jones.h.jones@jpl.nasa.gov.

Now, onto the talk, this month, CHARM Telecon is entitled, "Highlights
of Radio and Plasma Wave Science Results from Early in Cassini's Orbital
Tour," a nice, short title. Joining us today to talk about some of the
results is Dr. William Kurth. He is the Deputy Principal Investigator of
the RPWS, Radio and Plasma Wave Science Instrument on the Cassini
spacecraft. He's also the RPWS operations team lead and a research
scientist in the Department of Physics and Astronomy at the University
of Iowa. The University of Iowa in Iowa City is where much of the flight
hardware and software was developed.

> With that, I'll give it to you, Bill.

Bill Kurth Thank you very much for that nice introduction. I'm going to
say at the outset that this is the first CHARM telecom that I've done,
so I suspect many of the listeners have much more experience at this
than I do. If it's okay with you, I'd like to be as informal as
possible. There are a number of issues that may come up that you have
questions about. If you want to ask those questions before we go too far
downstream, that would probably be good. If you have questions, there
are probably others and we might as well get them cleared up.

I'm going to go right on to the second slide, which lists topics that I
plan to talk about today. One of the main points I'd like you to take
away from this discussion today is the breadth of science that we can do
with this thing we call the RPWS. We're going to talk about Saturn
kilometric radiation, which is a primary radio emission generated in
Saturn's magnetosphere. We're going to talk about Saturn electrostatic
discharges, which is a radio emission that's generated by lightening in
Saturn's atmosphere. We're also going to talk about dust impacts. You
may be curious about how we actually detect dust with a radio
instrument, but hopefully I'll be able to explain that to you.

Before we actually start, if we can go to the next slide, I have a
picture of a model of the Cassini spacecraft with a number of booms and
appendages highlighted that talk about the various sensors that this
instrument uses. It was once remarked to us that our interface control
document for our instrument is the largest of any instrument on the
payload and it's because we have all these things sticking out of the
spacecraft. There are three electric antennas. These are made of
beryllium copper. They're very long tubes, an inch and one-eighth in
diameter and ten meters long. Virtually everything I talk about today
was detected using this particular set of antennas. They measure the
electric component of waves and plasmas and radio waves.

There is a set of magnetic antennas that you can't see much detail of,
but they're tucked up under the high gain antennae. There are three of
them. They're orthogonally oriented. They're designed to detect the
magnetic component of waves and in this application, basically plasma
waves. I'll say a few words about the different between radio and plasma
waves.

Finally, there's a langmuir probe. This is just a five centimeter sphere
that has a bias voltage applied to it. We measure the current going into
the probe and use that current to tell us about the density and
temperature of electrons in the plasma where Cassini is.

The prime objective for this experiment is to study naturally occurring
waves in space plasmas. We talk about radio waves, we talk about plasma
waves and it's a very important distinction to us. It may not be to you,
but to give you some ideas of the distinction, a radio wave is a wave
which propagates from a source and interacts very little with a medium
between the source and the receiver. When you listen to an AM or FM
radio station on your car, you're basically detecting radio waves
because the emissions are generated at the transmitter. They propagate
we call freely to your antennae on your car and detect the signal and
listen to the program.

A plasma wave is more of an in-situ type of thing. It's the result of
the mixing and stirring of the plasmas, which make up the space around
Saturn, between the planets and pretty much most of the natural
universe. This plasma is composed of charged particles, electrons that
are negatively charged and ions that are usually positively charged. In
many respects these waves, these plasma waves, you can draw analogies
between them and waves in, say, the atmosphere. We hear sound waves
because those are local compressions and refractions of the density of
the air that allows us to listen to sounds.

In a plasma, instead of listening to pressure variations we're basically
detecting oscillating electromagnetic waves that are caused by the
motion of these charged particles with respect to each other. If you're
immersed in a plasma and there's a disturbance in that plasma, there
will be waves of a large variety of what we call modes; that is waves
that the waves propagate.

For modes that actually look a lot like sound waves, they're
compressional to other waves that actually look a lot like radio waves,
except that they're restricted by being strongly influenced by the
medium in which they're propagating in. If you have questions about
that, now would be a good time to ask. In principal, I don't think
you'll need to know that for the rest of the talk, though.

Let me go to the next slide. This is an artist conception of Saturn's
environment. We call it a magnetosphere. A lot of people complain about
the use of the word sphere in this long word because it's not really
spherical in shape, but it's meant to define a region which is dominated
by the magnetic field of a planet, in this case Saturn. This picture, by
the way, I believe was created by the MIMI team. You'll find a lot of
features in here that are studied by the MIMI instrument, but I stole it
because it was a nice picture.

Saturn's magnetosphere is a region in space that's dominated by the
magnetic field of Saturn and charged particles, plasmas and energetic
particles that are basically attached to those magnetic fields. When we
enter the region around Saturn, and this region extends approximately 20
or 30 planetary radii, Saturn radii upstream of Saturn, then we're
moving into a regime that is very exciting to a number of us that study
fields and particles because a lot of what's going on in the Saturnian
system is driven by interactions between the pieces that you're more
familiar with; the icy moons, Titan, the rings and the planet itself.
These interactions are largely through the magnetic field of Saturn and
through these charged particles that populate this region.

One of the features in particular that I will be talking about is
located very close to the North Pole of Saturn here. It's called Aurora
and SKR. You see a wiggly line coming out of the northern part of the
globe of Saturn there. That's going to be, actually, my first topic of
discussion. I think that's probably all we need to talk about for that
slide, so let's go on to slide five.

Now we're getting into amateur graphics. These are my graphics. This is
meant to depict a single magnetic field line among many that threads
through the planet as its source in the core of the planet, but in
particular threads the Auroral region; in this case in the southern
hemisphere of the planet. The little blue arc that you see there is
meant to represent the southern lights on Saturn. In fact, there are
also northern lights that probably look very similar to the ones that we
see in the south. Most of the images of Aurora that you'll see in this
talk are of the southern hemisphere, though, because of the viewing
geometry, primarily from the Hubble Space Telescope because it's
southern summer on Saturn at this point in time in Cassini's orbit.

I've drawn a little star somewhat above the surface of the planet.
That's meant to represent a source region for these radio waves. We
believe, because we've studied radio waves, planetary radio emissions
not only from Saturn, but Jupiter, earth and even Uranus and Neptune,
that there's a very common planetary radio emission phenomenon at each
of the magnetized planets that happens to be the brightest radio source
associated with any of these planets. In fact, Jupiter's radio emission
of this nature rivals the sun in intensity.

It's a common feature of planetary magnetospheres. The only planet at
which we really had a chance to fly through the source region is at
Earth, and we've been doing that for decades now and there are still
aspects of this radio emission that we don't fully understand. The fact
that we can go to Saturn and study these emissions in detail and, in
fact, later in the mission when the inclination of Cassini's orbit
increases, we hope we'll actually fly through Saturn's radio source
region.

It's important to understand that the source region is on a magnetic
field line that threads through the Auroral region because we believe,
based on what we understand from the Earth, Jupiter and now Saturn, that
the same processes that generate the Northern Lights, that is energetic
charged particles crashing into the atmosphere and causing the
atmospheric ions to be excited and then de-excite, releasing photons -
that that process is also wrapped up in the generation of these radio
waves. In fact, you can think of these radio waves that we'll be
discussing as simply another wavelength at which we can observe the
Northern and Southern Lights on Saturn.

Now if we go to the next slide, if you're looking at a PowerPoint
version, I believe these four panels will come up consecutively on four
consecutive clicks. If you're looking at the Adobe version of this, it
should all show up at once. Basically, these four panels labeled A, B, C
and D are progressively greater and greater resolution observations of
the same phenomenon.

> The format of this presentation is what we call a frequency-timed
> spectrogram or dynamic spectrogram. The vertical axis in all cases
> here is frequency. We're looking in the frequency range of several
> tens of kilohertz, generally above the audio frequency range, up to
> about 100 kilohertz. The first panel goes up, actually, to several
> hundred kilohertz. The horizontal axis is time, and that varies from
> one of these displays to the next. The first one labeled A covers
> two-and-a-half hours or so of time. The colors represent intensity of
> waves. Things that are red are very intense and the darker blues
> represent basically the receiver background or lack of emissions.
>
> So if we start in panel A you'll see a white square there or
> rectangle. As you move to panel B it's basically a blowup, if you
> will, of the region that's highlighted in panel A, except when you go
> from panel A, which uses what we call low rate measurements on the
> Cassini RPWS, panel B comes from a special receiver called a wideband
> waveform receiver where we actually record wave forms on the antennas
> and bring those wave forms down in a digital fashion to the earth.
> Having the actual waveforms as opposed to just spectral information
> allows us to tell a lot more about these radio emissions.
>
> You'll see a white rectangle in panel B, which covers the whole
> frequency range, but a short period in time. Panel C shows the
> expansion of that white rectangle into high resolution. Continuing on
> to panel D, the rectangle in panel C is expanded to show pretty close
> to the ultimate resolution in both frequency and time of the RPWS in
> this particular mode.
>
> The whole point of this expansion from low resolution to high
> resolution is to demonstrate that these radio emissions that are
> generated as part of the Aurora process on Saturn are composed of a
> bewildering array of narrow-band features that drift downwards in
> frequency and upwards in frequency and more diffuse emissions that
> don't appear to have a whole lot of structure. There are some features
> that have very sharp upper cutoffs and others that have very sharp
> lower cutoffs.
>
> One of the things that we don't understand very well, even at the
> Earth, is the nature of all these narrow-band emissions. One aspect of
> these radio emissions that is well understood is that they're
> generated at a frequency that's related to the magnitude, the strength
> of the magnetic field at the source. In a planetary magnetic field,
> the closer you go to the surface of the planet, the stronger the
> magnetic field. Radio emissions that are generated at higher
> frequencies come from positions that are closer to the planet than
> lower frequency emissions that are generated further away from the
> planet.
>
> When we see these narrow-band emissions that drift upwards or
> downwards in frequency, it tells us that something is moving along the
> magnetic field in the source region. If it's a down going drift, it
> means that something is moving up the field line away from the planet.
> If it's drifting upwards in frequency, it means that something is
> drifting down towards the planet.
>
> What these somethings are is the real question. One way to think about
> it is a tiny radiating center like a flashlight, perhaps, that moves
> up and down the field line and changes its frequency as it does that.
> The whole spectrum that you see in panel A is composed of literally
> thousands of these miniature radiating sources all doing their own
> thing at the same time and combining to give you that overall
> spectrum.
>
> You go down to the higher resolutions you can actually see the
> emissions from individual radiators as they move up and down in this
> source region. The real question is, what are those radiators and why
> do they work the way they do? That's a question that we've been asking
> ourselves concerning the earth's emissions for a long time. We really
> don't have a very good answer to that just yet.
>
> If we go to the next slide it's another example of some of the fine
> structure we see in this Saturn Kilometric Radiation. This is slide
> seven. I don't know how this is going to work, but we actually have an
> audio track that's created from these data. Now, obviously you can't
> hear frequencies in the range of 50 or 60 kilohertz, so we've taken
> some liberties with the science here and shifted the frequencies down
> into the audio range so we don't have to sit here for the 30 or 40
> minutes that it takes to cover this entire set of data in this
> particular slide. We've also compressed time.
>
> I'm going to try to play the audio on my computer and we'll try this
> once and see if you can actually hear it on the telecom. If you can't,
> there are WAV files that are associated with this. They don't have
> very good names, I see, but the list of WAV that's on the Web site,
> there are five WAV files listed there. The bottom one has a size of
> 717 kilobytes, that's the one that we're going to listen to now. It
> probably would not work well for everybody to listen at the same time
> to their own recording. Let's first see if I can play this on my
> computer and if you can't hear it, then we'll dispense with that as an
> experiment. Here we go.
>
> Can any of you hear this?

J. Jones Yes.

M Just in time for Halloween.

J. Jones That was great. That came out just fine.

Bill Kurth Good. I was glad to hear the Halloween comment because you're
certainly welcome to download this and set it out in your yard and play
it on Halloween night. It struck us when we first heard this that that
would be a perfect application. As you can see, we have a lot of fun
with these. It takes a little bit of work to generate these, but often
times they're quite entertaining to listen to.

I should say, the last slide in the presentation has a couple of links
and they take you to a page on our Web site here at the University of
Iowa. We have what I like to call a sing-a-long with Mitch version of
this where we actually have a cursor moving across the spectrogram so
you can kind of keep track of where things are as you listen to it. It's
a Java program, so you have to follow the instructions on the Web site
to make sure you have the right version of Java on your computer and
make sure it's enabled and so on. After I'm done you might want to try
to go look at a couple of those and see if they work for you. If they
don't, you can send me a message and I can see if I can get some help
for you, but it's somewhat useful to see the cursor move across as you
listen so you can kind of pick out the features that you're listening
to.

If we go to the next slide, and this one is another, much smaller case,
but at higher resolution, you'll notice on the frequency access that now
we're talking about frequencies up around 325 kilohertz. This is
actually near the center of the Saturn kilometric radiation spectrum
most of the time. What's remarkable about this particular spectrum is
that you see this narrow-band tone that kind of wanders across the lower
third of the spectrogram. Evidently, shooting out of that at least three
times, and it looks like there's a fourth one starting towards the end
of the data set, are these very rapidly drifting features that move to
higher frequencies with time.

Amazingly enough, we have a very similar observation that comes from the
Earth's magmutunetosphere, this very same type of thing happening. In
fact, it's quite possible that these three emissions that go shooting
off to higher frequencies are triggered from the lower frequency one.
This triggering process we don't understand very well, but it's a
well-known phenomenon that goes on in the earth's magmutusphere at much
lower frequencies, not for radio waves but for some of these plasma
waves that I talked about at the beginning, where the propagation of the
waves is strongly influenced by the plasma and the magnetic field. So we
don't understand how these are created, but it tells us that the process
is a very complex one and one that we simply don't understand.

Now, this particular one we also have sound for and it's a much shorter
cut. It's the third one on the list of WAV files and it's also the
shortest. It has a size of 126. I'll see if I can play that again. Could
you hear that one okay?

J. Jones Yes.

Bill Kurth Good. So, again, what we think we're listening to here,
especially with these three rapidly rising tones, are very small
radiation sources that are moving downwards towards the planet at
actually a fairly high rate of speed. These would be traveling at, say,
the sound velocity in the plasma. I don't think there is anything else I
was going to say about this particular one.

Shifting gears a little bit, I want to go back to this idea that these
radio emissions are tied to Saturn's Aurora. The next slide shows three
Hubble Space Telescope images of Saturn, of the planet itself.
Superimposed on these three images are ultraviolet images of the Aurora
on Saturn also taken by HST, but a different camera and at a different
time, so this is meant to show you where on the planet the Aurora are
found and the different types of forms that these lights take as they
evolve.

Since the blue Aurora images here are taken in ultraviolet light, this
is not what you would see through a telescope or through HST looking in
the visual range. I guess you can kind of understand this. Even though
the Aurora on the day side of the Earth, if you look at normal visual
wavelengths you don't see those because they are drowned out by
sunlight. There are UV emissions that are there nevertheless.

This set of three images of the Aurora, starting from the bottom and
working upwards, are taken about every other day starting with the
bottom one. The bottom one shows a very thin, not very bright Aurora. In
fact, this is kind of the typical way the Aurora appear in the HST
images most of the time.

The central image shows you basically an explosion of Aurora. The Aurora
is no longer a very thin, narrow line. It's expanded pollward from what
turns out to be the dawn side of the planet. It's quite a bit brighter.
It covers a lot more area on the planet. The third image also shows a
somewhat more intense Aurora than we see in the first image, but it's
started to quiet down somewhat.

These three images were part of a set that were taken in January of 2004
in a campaign that we ran using both Cassini and the HST, except for one
stretch of images that were taken in the space of part of a day. We
basically had an image of the Aurora from HST about once every other
day. While Cassini was in this solar wind, a plasma that blows off the
sun past all the planets at a speed of roughly one million miles an hour
basically measuring this solar wind before it runs into Saturn's
magnetosphere.

> Then we observed with the RPWS the intensity of radio emissions. HST
> measured the Aurora and some of the other instruments on Cassini, the
> magnetometer and the plasma instrument and the energetic particle
> instrument all measured aspects of the solar wind that would then
> interact with Saturn's magnetosphere. The idea is to try to understand
> what controls the variations in the Aurora's you see in this picture.
>
> If you go to the next slide we show a sequence of images across the
> top of the Aurora. These are taken basically every 96 minutes or so.
> If you look at them closely you can see bright regions that look like
> they're rotating with time. In fact, this was a discovery of this
> campaign, that using the Hubble images we can actually see that the
> Auroral bright spots tend to rotate in the same direction of the
> planet, but at much slower rates. Instead of rotating at about
> ten-and-three-quarter hours per rotation they rotate at, say, 50% to
> 75% of that rate for reasons that we don't understand for sure.
>
> In the bottom panel we see another one of the spectrograms that shows
> the evolution of the Saturn kilometric radiation as a function of
> time. The correlation is somewhat difficult to explain. There is an
> Aurora in the first image, but we don't see much in the way of radio
> emissions for that first image. We do see Aurora that are rotating
> kind of away from the dawn region and it turns out that Cassini is off
> on the dawn side of the planet, so these Aurora are moving away to the
> other side of the planet from where Cassini is. Shortly after the last
> image is taken we no longer see the radio emissions anymore. We think
> that has to do with the fact that the bright Aurora, which probably
> are on field lines that are generating the brightest radio emissions,
> kind of moved out of our field of view.
>
> The reason why we don't see radio emissions associated with that first
> image is a little bit fuzzier, but it has to do with the fact that
> these radio emissions are beamed and possibly very narrow beams
> whereas the light from the Aurora you can see from basically any
> direction as long as you can see the polar regions of the planet. The
> radio emissions are not beamed in all directions with equal intensity.
> They're thought to be beamed in very sharp beams. So if Cassini
> doesn't happen to be in the right direction, even though there are
> very strong radio emissions, it won't see them.
>
> The next slide basically summarizes the entire campaign, where we have
> images every other day roughly of the Aurora and a long-time history
> of the Saturn kilometric radiations in the bottom panel. Now, the time
> scales change quite a bit here, so when you look at the bottom panel
> it's the brief bursting emissions at the higher frequencies that is
> higher than the panel that are the Saturn kilometric radiations.
>
> You'll notice that there's a brightening of the Aurora on, say,
> January 18^th^ that comes about the same time we see a brightening in
> the Saturn kilometric radiation below. Then January 26^th^ and 28^th^
> are when we see these very bright Aurora I showed you in the image
> earlier. They occurred at a time when the radio emissions are actually
> at their brightest for this entire time interval. So we are clearly
> seeing a correlation between the intensity of the Aurora and the
> intensity of the radio waves that are generated by a process that's
> related to the generation of the Aurora.
>
> The next slide simply shows these two images from January 26^th^ and
> 28^th^ and how they line up with the radio emissions that we see
> during those two times.
>
> If you like to see charts and plots, the next slide actually shows
> some quantitative comparisons. The top panel shows, with the thin
> lines, the intensity of these radio emissions as a function of time.
> The black dots show kind of an integrated power that must've been
> input into the atmosphere to account for the Auroral light that was
> measured by HST. You can kind of see when the radio emissions are
> intense those dots are higher and when the radio emissions are less
> intense those dots aren't quite as high.
>
> The bottom panel actually shows the variation between the Auroral
> input power based on the HST images and the radio power that's emitted
> in the form of Saturn kilometric radiation. There appears to be a
> correlation there. The middle panel is actually a measure from the
> CAPS instrument of the pressure in the solar wind. That's basically
> how hard is the wind blowing. The interesting thing here is that when
> the wind blows harder, the Aurora get brighter. So that's a
> fundamental discovery of this campaign.
>
> Are there any questions? Either everybody is understanding what I'm
> saying or they're all asleep. They must be asleep.
>
> The next topic I'd like to address is the rotation period of Saturn.
> If you've ever thought about how astronomers come up with the rotation
> period of a gas giant like Jupiter or Saturn that has no landmarks
> that can be seen, just clouds, you might scratch your head and say,
> "How do they do that?" Well, it turns out these radio emissions that
> you've been listening to are at least one way. In fact, the official
> rotation period for the four outer planets all come from measurements
> of periodicities in the radio emissions that we've been talking about.
>
> This next slide shows a spectrogram that has a time period of about
> five days and a set of arrows that are basically spaced at regular
> intervals. You can just tell by the brightness of the radio emissions
> that they appear to brighten and go away and brighten and go away in a
> periodic fashion. This is another one of those places where the
> animations on the University of Iowa page would be useful. There are
> all sorts of things that go on for this particular animation.
>
> In particular, what we believe happens is that these radio emissions
> brighten up during part of the phase of the rotation of the planet and
> get dim for another part of the phase. Again, we have a sound byte
> that we can listen to. I'll see if I can play it for you. It's not
> pleasing to listen to, but let me play it.
>
> So it's not perfect, but it's very clear when you listen to that that
> you're hearing something that's periodic. I'll play that again since
> it's short.
>
> Now, this type of periodicity was actually observed by the Voyager
> spacecraft when they flew by Saturn in 1980 and 1981. The radio
> astronomers associated with the Voyager project analyzed the
> periodicity and said, "This must be the rotation period of the deep
> interior of the planet." Now, why did they say that?
>
> I've tried to point out that the radio emissions are associated with
> the magnetic field. In particular, the magnetic field that thread
> through the Aurora. The magnetic field of a planet is generated by
> convection of conducting materials deep in its core.
>
> The idea is that since the radio emissions are tied to the magnetic
> field, the magnetic field is generated deep in the interior of the
> planet, that by measuring the periodicity and the radio emissions
> you're measuring the rotation period in the interior of the planet.
> This is how the period of Jupiter, Saturn, Uranus and Neptune have all
> been determined. In fact, it's also the technique by which pulsar's
> rotation periods are measured. The frequency of a pulsar is related to
> the spin rate of the neutron star underlying the pulsar.
>
> All this is fine and dandy. If we go to the next slide you get kind of
> an idea of how this works, particularly at Jupiter. At Jupiter the
> magnetic moment of the dipole is offset from the rotation axis by
> about ten degrees. The same is true for Earth. For Uranus and Neptune,
> the angle between the magnetic dipole axis and the rotation axis of
> the planet is actually larger than that.
>
> If you have radio emissions that are kind of beamed off in particular
> directions by the magnetic field you can imagine that as that planet
> rotates, a particular observer would be in the beam for part of the
> rotation but not in the other. In fact, that's exactly what happens at
> Jupiter.
>
> The problem at Saturn is that that magnetic dipole access is as close
> as can be determined based on the Voyager, the Pioneer 11 observations
> and even early Cassini observations - it's almost exactly aligned with
> the rotation axis of the planet. This raises the first question, "Why
> on earth is there a spin modulation of these radio emissions when the
> magnetic field appears to be symmetric with respect to the rotation
> axis?" We don't understand the answer to that question to start with.
>
> The real issue comes up, if you go to the next slide, this shows three
> different determinations of the radio modulation period. The first
> curve in green shows the peak, primary peak in the modulation period
> as measured by Voyager I and II back in the early 1980's at ten hours,
> thirty-nine minutes, twenty-four seconds plus or minus seven seconds.
>
> The yellow curve is based on the periodicity measured by the Cassini
> RPWS as it approached Saturn basically the year prior to arrival in
> orbit. The surprising thing is that this has a period that's like six
> minutes longer than the one measured on Voyager I and II. Ever since
> we've been in orbit we've been monitoring this. Currently, the most
> recent period that we've measured is yet another minute longer than
> measured on approach.
>
> Now, for any of you who understand the principal of conservation of
> angular momentum, you'll say, "This sounds like a problem. How on
> earth could Saturn's deep interior, where all the mass is, be slowing
> down this rapidly?" It turns out that this is not a unique observation
> to Cassini. There's a spacecraft that's been in solar orbit since I
> believe the mid-1980's. It's called Ulysses. It has a radio receiver
> that, in many respects, is similar to the one on Cassini. It can
> detect the most intense of the Saturn kilometric radiation over a
> large part of its orbit around the sun.
>
> There are a couple of people who noticed that the period that they
> measured with the Ulysses spacecraft did not match the period that
> Voyager I and II measured back in 1980 and 1981. In fact, those two
> guys are members of the Cassini team. In fact, there are variations
> that go on time scales of years that amount to something on the order
> of one percent of the rotation period is measured by Voyager I and II.
> This is a real problem.
>
> You might say, "Well, why is it a real problem?" First of all, knowing
> the rotation period of a planet seems to be one of the first types of
> things that you'd like to know about a planet. Second of all, if
> you're an atmospheric scientist and you want to study the winds on
> Saturn and you don't have a crater on the ground that you can kind of
> look at and understand how fast the clouds are moving past that crater
> to measure their speed, you'd like to have the rotation period of the
> interior of the planet to know what the wind speeds are relative to
> that and take that as a frame of rest.

Andy Ingersoll, who's one of the atmospheric scientists on Cassini, says
that it's hard to understand how fast the winds are blowing based on the
Voyager I and II period of ten hours, thirty-nine minutes, but if you
slow down the planet by another percent or so, then it gets even more
difficult to explain how fast the winds must be blowing relative to an
interior that's moving slower.

It's really important to understand what the rotation period of Saturn
is. I think the best way to characterize what these radio emissions are
telling us is that we don't really know what the rotation period of the
planet is. We suspect that the modulation of the radio emission is
somehow associated with the rotation of the planet, but we really don't
believe that the rotation period of the planet is varying by one percent
on timescales of decades or even less.

There must be some reason why the radio period varies without requiring
the planetary interiors rotation period to vary, but we don't understand
that. One of the ideas has to do with there may be correlations in the
period relative to the strength of the solar wind. We've already seen
that the radio emissions and the Aurora are strongly influenced by the
pressure of the solar wind. There's a mechanism by which it's been
suggested that maybe the variation in the wind speed or the pressure of
the wind might have something to do with changing this period, but
that's not very well understood.

Another idea has to do with the fact that Saturn's magnetic field is so
perfectly aligned with the rotation period or with the rotational axis
that perhaps this doesn't allow the radio emission to be as precise an
indicator of the rotation period as we might want.

I think that one of the greatest puzzles that we have today, that we're
hoping that Cassini data taken throughout the rest of the mission will
help us answer is one, why does this radio modulation period vary the
way it does and two, if we understand that can we then determine what
the true rotation period of the deep interior of Saturn is? I think
that's the big question.

The next slide is kind of a summary of what I've talked about relative
to the Saturn kilometric radiation. It's a radio emission that's
produced as part of the process which produces Aurora. It's intensity
correlates with the intensity of the Aurora and both are stronger when
the solar wind pressure is larger. The fine spectral features in the
spectrum are due to tiny radiation sources moving up and down the
magnetic field lines, but we don't really understand how they're
generated. They are very, very similar to spectral structures we see at
Earth and in fact, Jupiter. The period of the SKR is thought to be
related to the rotation of the deep interior of the planet, but it
varies and we simply don't understand that variation.

Any questions?

Next slide is meant to introduce the subject of lightening at Saturn and
how we detect that with the RPWS instrument. I think the best way to
think about this is imagine yourself driving in a thunderstorm, and I
understand even those of you out in Pasadena had this opportunity not
too long ago, with an AM radio on in the car. You very quickly decide to
switch to FM because the static on the radio drives you crazy. Those
cracks and pops on the AM radio station that you hear during a
thunderstorm aren't really interference; they're actually radio
emissions from the lightening strokes.

The energy that's released in a lightening bolt is not restricted to the
visible wavelengths that you see with your eyes, but there's a full
spectrum of radio emissions that are generated and propagate in all
directions from the lightening stroke, some of which go up. If they're
high enough in frequency they can propagate through the ionosphere of
the planet and the radio detector, like we have on Cassini, can actually
detect them.

In fact, as we flew by Earth during our long interplanetary cruise to
get to Saturn we actually detected lightening from thunderstorms at
Earth by the radio emissions that they generated. Obviously I wouldn't
be talking about this if we weren't also detecting radio emissions from
lightening at Saturn.

Bill Kurth The next slide shows one of these frequency time
spectrograms, like I've been showing you but it's got a very different
character than the ones I've shown you so far. We're in a frequency
range here that's well above the Saturn kilometric radiation. In fact
the red at the very bottom of this spectrogram are radio emissions from
Saturn, the Saturn kilometric radiation. There are a few solar radio
bursts here called Type 3 radio bursts for reasons that I don't really
understand but they're well known. We know that they come from the sun
and we even understand quite a bit about how they're produced.

What's left on this spectrogram are these short vertical stripes that
show up, more or less sporadically, throughout this spectrogram. They
were originally called Saturn electrostatic discharges when they were
observed by a similar instrument on Voyager and we're quite convinced
now that they're associated with lightning in the atmosphere.

> So the first thing I need to explain is why do they appear this way in
> this type of a display. And in order to do that, I have to explain to
> you that the way the radio on Cassini works is that it doesn't listen
> to all of these at the same time because if it did and you had a
> lightning stroke, you would actually see radio emission at all
> frequencies, at least above some lower frequency limit at the same
> time. So you'd see long vertical stripes.
>
> But the way this instrument works is it actually sweeps in frequency
> from low frequencies to high frequencies every few seconds. So at any
> given instant in time, it's tuned to a particular frequency. If you
> happen to be tuned to say, six megahertz, when there's a lightning
> flash, you'll see a signature in the six-megahertz channel because
> that's where you happen to be listening and lightning is a very
> broadband radio emission. So that's where you would detect it. The
> next time a lightning stroke comes along and you happen to be looking
> at ten megahertz, you detect it at ten megahertz.
>
> In fact, the fact that you see vertical streaks here is related to the
> fact that we're often in the process of moving from one channel to the
> next higher channel during the lightning stroke. So we actually see a
> bit of the stroke in the first channel and a little bit of the stroke
> in the next channel and sometimes in rare cases, we see it in a third
> channel. So in fact, the vertical length of these vertical stripes
> that you see help us identify the duration of the lightning stroke and
> it's typically on the order of about 50 milliseconds or 1/20 of a
> second.

Ken R. Excuse me. This is Ken Renshaw,, solar System Ambassador from
Arkansas. Are all the splotches on that window there, are those all
lightning is it something else mixed in with that?

Bill Kurth The ones that are red, I'm pretty much convinced are
lightning. Now it is possible that some of the point-like blue and green
dots are simply noise in the background of the receiver. Especially down
at the bottom where you kind of see the rastorization of that noise, I
think that's receiver background noise.

Ken R. So probably any of the vertical things that are red, particularly
the elongated ones, would be lightning then?

Bill Kurth Yes.

Ken Right. Thank you.

Bill Kurth As I mentioned, this phenomena was discovered by the
Planetary Radio Instrument on Voyager 1 and there's quite a bit of
debate early on about what these could be. But to make a fairly long
story short, the next picture shows an image of Saturn taken by Voyager
and a kind of blue-highlighted oval, which is kind of an artist's
conception of a storm system, which might explain the radio emissions
that were seen by Voyager.

Basically, the observations that Voyager had was that on a very regular
basis, every time the planet rotated, they would see these bursts and
they would build up to a peak and then they would fade out and then you
wouldn't see them for a while. Then you would see them again and then
they would go away again. By looking at the time period over which the
bursts were visible, the investigators determined that there had to be
an extended storm that was not a point source but extended by about 60
degrees in longitude around the planet to explain the signature that
they were seeing. And the localization to low latitudes had to do with
the fact that the repetition rate between these clusters of lightning
strokes was not at the radio rotation period of ten hours, 36 minutes,
24 seconds that Voyager measured for the radio emissions but actually
was ten hours, ten minutes. Because there's a very fast jet stream near
the equator of Saturn moving in a prograde sense, it means that cloud
systems near the equator actually only take about ten hours, ten minutes
to rotate around the planet whereas if you go to different latitudes, a
cloud system would take a longer period of time.

> So the localization of the lightening source, both in longitude and at
> least in the extent of longitude and the fact that it was located near
> the equator was based on the timing of when these bursts were seen. So
> that was the Voyager experience.
>
> The next slide kind of shows the current status of detection of
> lightning using the same technique, by the way, from Cassini. And the
> first thing that we can say is that lightning is not nearly as regular
> or as repeatable as it was during the Voyager era. What this slide
> shows is the number of events or lightning strokes per Saturn rotation
> for what we would call storms that have been observed since we've been
> in orbit. They're labeled A, B, C and D. They're fairly arbitrarily
> labeled but they're identified by the fact that we see a cluster of
> these FEDs that kind of come and go every rotation of the planet for a
> while and then we don't see any for a while. Then we see another set
> of emissions for a period of several days or more. Then we don't see
> any more. Then we see them again.
>
> And then there is this long period from late in 2004 till spring of
> 2005 where we didn't see any identifiable lightning strokes to where
> we got to this storm labeled D fairly recently. So that's kind of a
> surprise to us. We expected that once we got into orbit that every
> day, that is every Saturn day, we would see lightning and we would see
> a pattern that was similar to what Voyager saw. It turns out we don't
> see that.
>
> You'll notice on here that each of these storms has a somewhat
> different repetition period. We don't see the ten-hour, ten-minute
> repetition that Voyager observed. We see somewhat longer ones. And in
> fact, this tells us that the latitude of the underlying storm system
> must be different than what was seen by Voyager because of the
> variation in wind speeds with latitude.
>
> C is the one that seems to be studied the most of this set, probably
> because it had perhaps the longest duration and the largest number of
> bursts. In fact, some work that's been done between members of the
> RPWS team and the imaging team found that there is a correlation
> between when we could see the SED, the electrostatic discharges, the
> radio emissions and this storm, which has been called the Dragon Storm
> by some members of the imaging team. Basically, the repetition period
> for the storm C radio bursts is the same as the rotation period for
> this feature in the atmosphere. So the atmospheric scientists said
> this might be what you would expect to see for a convective storm,
> like a thunderhead, simply speaking, in Saturn's atmosphere. We're
> just seeing kind of the top of the cloud, so to speak, and it's
> basically an indication that there's a large convective storm deep in
> the atmosphere below this cloud.
>
> Now some of the problems with the observation is that we actually
> started seeing the radio emissions, we could see the radio emissions
> starting before this cloud came up over the horizon, which seems to be
> a problem. Also the radio emissions tended to go away when the storm
> was kind of right below Cassini, as viewed from Cassini. Turns out
> Cassini was often the dawn direction, so that corresponded to a time
> when this storm would go into the sunlit hemisphere, from night into
> day. Some fairly recent work has suggested that maybe the way to
> explain these phase issues, the fact that we can see the radio
> emissions before we see the cloud and the fact that the radio
> emissions stop when the cloud goes into the day side, can probably be
> explained by propagation arguments. That is, even though the source of
> these storms is not in view from an optical point of view, when we
> start seeing the radio emissions, the radio emissions probably
> propagate around the planet because they're focused somewhat by the
> planet's ionosphere before they eventually leak out and escape so that
> Cassini can detect them.
>
> And the ionospheric density is greater in the day side so when the
> storm moves into the sunlight hemisphere the ionospheric density is
> larger and it's harder for these radio waves to propagate to Cassini
> then. So I don't think we know that that's the correct explanation but
> it's at least a plausible description of why we see the timing between
> the radio emissions and this cloud the way we see them.
>
> The next slide is a series of images taken by HST. Basically shows the
> changing of the seasons on Saturn. Some of our team members have been
> wondering why it is the Saturn electrostatic discharges are so
> different in their occurrence now than they were during the Voyager
> era. It turns out that the season at Saturn during the Voyager flyby
> was closer to what it appears in this lower image, that is, the rings
> were nearly, not quite, but nearly edge-on to the sun when Voyager
> flew by. And when Cassini arrived at Saturn, the rings were fairly
> wide open with the sun illuminating the southern side of the rings and
> the southern pole of the planet. If you think about the fact that the
> rings cast shadow on the atmosphere and the ionosphere, it suggests
> that perhaps it has something to do with the shadow of the rings on
> the planet that controls some aspect of this lightning story.
>
> One idea that we had was that the stark contrast in temperature
> between a parcel of atmosphere that is in the ring shadow, in the
> bottom picture it's a very deep shadow, and as it rotates around to
> the dark side it's also being dark so maybe there's temperature
> contrast between that shadow and surrounding atmosphere that drives
> convection. Whereas if you look at the pattern of what ring-shadowing
> would be in the upper image, it's a more distributed shadow over,
> basically, the majority of the northern hemisphere of the planet. So
> the shadowing on both the atmosphere and the ionosphere is quite a bit
> different between these two seasons and maybe that has something to do
> with it.
>
> Fortunately, as we continue on in the Cassini mission the season is
> going to slowly approach that as it was during the Voyager era so
> we'll be moving back to a season where the rings are more or less
> edge-on to the sun. If this idea that seasons have something to do
> with the occurrence of lightning or at least our ability to detect it,
> we would expect to see more and more Voyager-like patterns of
> occurrence of the Saturn electrostatic discharges.
>
> So to summarize, the lightning observations, Saturn electrostatic
> discharges, are radio emissions that are produced by lightning in
> Saturn's atmosphere. There's been a correspondence observed between at
> least one group of these radio emissions and a convective cloud
> feature. The occurrence of these radio emissions is much more
> irregular than was the case during the Voyager epoch. We think maybe
> the seasons on Saturn have something to do with these differences.
>
> Any questions? Okay.
>
> The last topic I'd like to cover is dust and this is probably one of
> the more intriguing aspects of the RPWS investigation. It's actually
> something that we planned for but only because we were surprised when
> Voyager II through the ring plane at Saturn back in 1981. We
> discovered that there was a very large effect when we did that and we
> had to rule out all possible plasma and radio wave phenomena that we
> just couldn't explain. Basically what happened when Cassini went into
> orbit at Saturn is we flew through the ring plane twice, basically
> between the very narrow F ring and the G ring, which is typically not
> visible in most of the images that you see. So we went through what an
> astronomer would say was a clear zone in the rings between the F and
> the G rings. There's an artist's conception of Cassini doing its main
> engine burn. It's actually over the ring system during this time
> period.
>
> But to get to that position, inbound it had to fly through the ring
> plane and outbound it flew back through the ring plane. In the next
> image, which is an actual Cassini image of the ring system, you'll see
> a little X out there past the extension of the ring ansae. That's
> where, spatially, the ring plane crossing occurred. The next slide
> with the blue background shows kind of a sketch of the trajectory and
> the location of the two ring-plane crossings. Maybe the best thing to
> do is play the recordings that we made during each of these ring-plane
> crossings. So let's see, the first ring-plane crossing is actually the
> first .wav file on that list on the Web page and I'll try to play that
> now.

(13-second recording)

> Now that sounded like static or probably more precisely, if you've
> ever driven your car through a hail storm, it sounds more like hail
> hitting your car. And in fact, there's another recording which is the
> second .wav file. It sounds somewhat similar.

(16-second recording)

> So we maintain that those signals are the result of very tiny dust
> particles, micron-sized dust particles hitting the spacecraft as we
> fly through the ring plane. And in fact, if you go to the next slide
> you can actually see a wave form. This is the voltage on the antennas
> as a function of time and this is only a hundred-millisecond time
> period. Each one of these spikes that you see in the wave form we
> think is an individual dust particle hitting the spacecraft. So you
> can see that, in this particular slide, depending on how you count
> them, there's 20 or 30 impacts and this is a fraction of a second.
>
> We actually measured on the order of a thousand impacts on the
> spacecraft, per se, during these ring-plane crossings. Is there a
> question?

Ken K. Yes. Can I ask a couple questions here?

Bill Kurth Sure.

Ken K. My name's Ken Kremer. I'm an Ambassador here. The size of the
dust particles -- roughly what is it?

Bill Kurth We think around the order of a few microns and that's similar
to the size of the particles in cigarette smoke.

Ken K. What number, like less than ten microns?

Bill Kurth Less than ten microns, probably, yes.

Ken K. More than one.

Bill Kurth More than one. In fact for the data that you just listened, I
think the RMS, the root mean squared, average particle size is on the
order of three or four microns.

Ken K. And you determined the size of this particle from this data?

Bill Kurth Well, that's an interesting question. In principal, yes. The
voltage or the magnitude of the voltage spike that you see is, at least
in part, a function of the mass of the particle. Unfortunately, it's
also a function of the target material because, well, let me go to the
next slide. It kind of gives you an idea of what's going on when these
occurs. This is a schematic of the spacecraft. This shows impact point.

Ken K. Number 29, you're talking about?

J. Jones Yes. Slide 29.

Bill Kurth Yes, 29.

Ken K. I don't see the spacecraft here. ... the antenna I see, okay.

Bill Kurth Yes, there's an antenna and then there's kind of a curved
surface that's meant to represent the spacecraft body. The impacts that
we're talking about have relative speeds on the order of ten kilometers
per second. In fact during the SOI ring-plane crossings, it was 16
kilometers per second. There's so much kinetic energy in these impacts
that not only is the impacting particle itself vaporized but also a part
of the spacecraft. When it's vaporized it forms a gas that has the
temperature on the order of 100,000 degrees, which means that part of
the gas is ionized. And once you've ionized the gas then the electrons,
being more mobile, will kind of leave the impact area more rapidly than
the ions do. So you set up an electric field between the spacecraft and
this expanding cloud.

There's actually a couple of different models for this. Another model
that may be appropriate to think about is that if these impacts are
asymmetric with respect to the pair of antennas we use as a dipole, then
one antenna will collect more electrons than the other. The net result
is that we see this voltage pulse on the antennas. The exact model that
explains that is, I think, somewhat uncertain at this point in time. The
model that says that the voltage impulse has to do with an asymmetric
collection of electrons, in principle can tell you something about the
mass of the impacting particle. But there's lots of uncertainties. We
did this analysis on Voyager and we came up with a few microns as the
most likely size.

> It turns out the other way that you can analyze this is something that
> was derived by a group in France who realized that the spectrum, kind
> of the ensemble average spectrum of these dust particles hitting the
> spacecraft, ought to produce a spectrum that goes as frequency to the
> minus 4 power. There's a break point in the spectrum and the frequency
> of that break point is associated with mass of the particle. And I
> don't think I can explain that without using some equations. But there
> is an independent method that basically uses a spectrum of these
> impacting particles and that is generally recognized in the people who
> know about these things as giving a more reliable value, partly
> because you measure that spectrum with a monopole so you're basically
> measuring this electric field that's depicted in the slide 29 as
> opposed worrying about what fraction of the electrons you actually
> detect.
>
> But it turns out that the particle sizes I've given you are using this
> technique that we think is somewhat more reliable, that is, using the
> spectrum of the impacting particles, using the monopole antenna.

Ken K. I've got a couple more questions. Okay. .... I'm going to guess
that the amount of dust varies. Did you see more dust in the ring plane
and less at the highest point and intermediate amounts as you were going
above and below?

Bill Kurth That's right. In fact one thing that I hope you could hear
when you heard these recordings was a variation from fairly low-impact
rates to a peak and then going back down to a lower rate. That, in fact,
is meant to convey and these are actual data, we've time-compressed and
did some things like that but the time history is accurate. The vertical
distance over which the bulk of those impacts occurred is something on
the order of 2000 kilometers and the half-width at full power is on the
order of a couple hundred kilometers, to give you some idea of what the
thickness of this region is.

And the density, if you know what the cross-section is and the velocity,
you can calculate what the density is. To put it in terms people can
kind of think about, in the very peak of this region we're probably
seeing something on the order of one dust particle per room the size of
an office, maybe a little bit bigger. Some of the others
\--.\[interrupted by question\]

Ken K. Where? At the ring-plane crossing itself?

Bill Kurth Yes. We've also made measurements of these things out in the
E ring, where the densities are somewhat lower but there it's kind of
like finding one dust particle in the size of an auditorium. I don't
know if you've been in Von Carmen Auditorium \--.

Ken K. Yes.

Bill Kurth Basically an auditorium's that got 30, 40 feet by 30, 40 feet
by 30, 40 feet. That kind of gives you some impression of how dense
these dust particles are.

Ken K. How ... is it at the highest point?

J. Jones Can I interrupt for a second? The fellow who's asking the
questions, could I have your name so that I can make sure I get it
correct on the transcript?

Ken K. Sure. Ken Kremer. K-R-E-M-E-R.

J. Jones Great. Thanks a lot, Ken. Okay, back to Bill Kurth.

Bill Kurth Okay. You were asking?

Ken K. Oh, yes. At the highest point, above the ring, what was the
density at that point compared to the \--?

Bill Kurth Oh, gee. I don't have that number off the top of my head but
\--.

Ken K. It's less, I would guess, though.

Bill Kurth Much less.

Ken K. A lot less.

Bill Kurth The impact rate in the ring-plane crossing, as I said, was of
the order of a thousand per second whereas at the highest point above
the rings, and I can't remember exactly how far above the rings we were
at the top, although that one slide gives you kind of a feeling for
that. I would say we were seeing dust impacts maybe once per second or a
few seconds.

Ken K. So 1/1000 or less.

Bill Kurth Yes.

Ken What is the time compression of that audio?

J. Jones Could the speaker please give their name, please?

Ken Yes. Kenneth Renshaw

Bill Kurth Again, I don't think I can give you that exactly although
there might be a comment on the University of Iowa Web site where the
animations are. But I believe the total duration for each one of those
recordings I played was about five minutes, I believe.

Ken Okay. Thank you.

Bill Kurth Possibly ten. Five or ten minutes. Any other questions?

Ken K. Yes. Can we go back to slide 25? What I would like to know is
where did it pass over? Do you have like an overhead view where it would
have crossed over in the ring plane?

Bill Kurth Well, if you go to slide 26. Oh, you mean at the highest
point.

Ken K. Along the whole point, right where ... one side. That's never
been quite clear exactly where that happened.

Bill Kurth I don't have a slide that actually shows that here.

J. Jones There will be one on the Cassini Web site and if the person
asking the question will tell me their name, I'll send it to you.

Ken K. This is Ken.

J. Jones Oh, okay. Ken Renshaw. No, which Ken? Ken Renshaw.

Ken K. Kremer.

J. Jones Okay, sorry. Ken, I'll send you the Cassini press release that
had that all that. It had some animation and some of those distances for
you.

Ken K. ... is what I wanted to know, too.

J. Jones Okay. Okay and how about if we, soon, let Bill finish his talk
because we've been going an hour and a half already. But ask your
question, though.

Ken K. one last question. The spokes, is there any relationship to the
dust ... spokes?

Bill Kurth Well, that's a very interesting question. For these dust
particles, I think no. There is a theory by Gertz and Morefield that
suggest that one mechanism for generating the spokes is meteorite
impacts on the rings. These would have to be much larger particles,
maybe marble-sized or larger. The idea is that you release enough energy
in some event that you can elevate some of the particles above the ring
plane and they would by nature be fine particles, particles that could
have a charge that's fairly large compared to their mass so they could
be affected by electrostatic forces. So one of the ideas was that,
indeed, there might be some event like impactors on the ring itself that
would result in elevating fine dust particles above the ring plane and
that those have something to do with spokes.

Ken K. Thank you.

Bill Kurth Okay. I only have one more slide, well, no. Slide 30 is kind
of a summary of the dust measurements, the fact that we're an effective
dust detector. We use essentially the entire spacecraft as a target. Now
we're not a dedicated dust instrument. The cosmic dust analyzer in
Cassini actually measures the mass and the velocity and even the
elemental composition of dust. So in fact, they are the dust
investigation on Cassini. But the fact that we're not too particular
about the attitude of the spacecraft and we have a large collecting area
means that there are time periods when CDA can't look in the right
direction to see the prevailing flux of dust coming in. So we think that
the RPWS could be complementary to the measurements that are made by the
CDA.

Finally, on the last slide are these links that I mentioned that you can
go to at your leisure. I think I'm going to end there. If you have
questions, I'll certainly try to answer them.

J. Jones This is J. Jones again. I did want to mention to everybody if
they go to the main Cassini Web site, which is saturn.jpl.nasa.gov, on
our front page for the last few days we've had a "Eerie, bizarre sounds
of the Saturnian System" feature. What we've done is we collected some
of these sounds from Enceladus, Titan, Saturn, including some of these
that you just heard in this CHARM talk for your Halloween enjoyment. So
I encourage everybody to go and play all those fun sounds in your office
and annoy all your cubicle mates because they're really fun. So that's
my final announcement.

Ken K. Yes, I see it's the second one, second headline here.

J. Jones Yes. It's not the first because there's something new at
Cassini every day so it's two down from the picture of the day. But it's
still on the front page. And it's another place to go and get those
sounds. Plus, on Bill Kurth's link to the space audio page there's just
tons of really fun sound files for all of you to learn more about.

Does anybody have any other questions?

Pete G. I have a question. This is Pete Goldie.

J. Jones Hello, Pete.

Pete Hello. It has to do with something I hope I didn't zone out and
miss, but what are you picking up when you go by Titan as far as SEDs?
Are you picking up lightning there?

Bill Kurth Actually, we are looking for those. I'm sure that you would
have heard it by now if we had anything that we could report. But it
turns out that Voyager set some upper limits, set an upper limit in
space having to do with the amplitude of the emissions versus their
occurrence rate. It could be you could fairly intense emissions on Titan
but Voyager just wasn't close enough when that flash when off if the
occurrence rate is low. Or if the occurrence rate is high then amplitude
of the emissions had to be fairly low otherwise Voyager would have
detected them. So there's already a fairly severe limit on the basis of
the single Voyager I flyby of Titan that suggested that lightning is not
found in Titan's atmosphere.

> But obviously with the 44 close Titan flybys and I'd have to say that
> this instrument is better equipped to do this measurement than Voyager
> was, it's certainly one of our objectives to see if we can find some
> evidence of lightning in Titan's atmosphere. A couple of our
> investigators are actively doing that. But you haven't heard any
> announcements yet and there's probably a reason for that.

Pete G. Is there a reason why Titan's atmosphere would not be generating
lightning?

Bill Kurth Let's see. I'm not an expert on lightning but one of the
things that people seem to think are important for the type of lightning
that we're familiar with at Earth is a polar molecule like water. And I
don't think that occurs in Titan's atmosphere. You also need some way of
separating charges, convective cloud systems, and there have been some
clouds seen on Titan, actually before Cassini arrived. But since we've
arrived, there have been a set of clouds near the southern pole that
have kind of been seen to move around and dissipate and regroup other
places.

Pete G. So carbon dioxide is not sufficiently polar to serve as the
polar molecule in the atmosphere?

Bill Kurth I don't know. I'd hesitate to express an opinion on that.

Pete Thank you very much.

Bill Kurth That's one of the reasons I think it's interesting to go to
these other locations to study lightning because we have some
preconceived notions about how lightning works at Earth and we can go to
places that don't have an abundance of water like we have in our
atmosphere and if we happen to find lightning there then we have to
explain what it is that makes lightning work there if we don't have
water.

Ken Yes. This is Kenneth Renshaw again. I was wondering, you were
mentioning the differences between the electromagnetic detection radio
waves and the plasma. Is it detected in the same way or is there some
difference in there?

Bill Kurth From an instrumental point of view, we use the same antennas
and the same type of receivers. Generally speaking, there is a dividing
line in frequency between plasma and radio waves.

Ken Oh, I see, the frequency.

Bill Kurth But that's not a fixed line in the sand. It depends on the
local conditions. There are certain resonant frequencies in the plasma.
Probably the most important one for this discussion is something called
the plasma frequency. That's the frequency, if you took an electron and
displaced it from its, let's say it has an equilibrium position with
respect to the ions in the plasma, and you just let it go, it'll execute
simple harmonic motion about its equilibrium position. It's fairly
simple to work out the frequency of that. It's just like a pendulum and
the natural frequency is basically a function of the square root of the
electron density.

Ken I see.

Bill Kurth So for higher-density plasmas, the dividing between the
plasma wave regime and the radio wave regime gets higher and higher.

Ken Okay, appreciate it. Enjoyed the discussion.

Bill Kurth Thank you.

Dale This is Dale Manquen. I was wondering about your comments about the
bending of the waves, rotation rate, seeing over the horizon and things
like that. How much similarity or is there any similarity between our
ionosphere -- I'm a ham radio operator and I think in terms of
ionosphere and bending.

Bill Kurth Well, I think that's kind of the whole idea of how we see
these radio emissions from the lightning before the storms actually come
over the horizon. There is an ionosphere at Saturn. The details of how
it compares to the Earth's, I'd hesitate to try to comment on. But the
whole idea of trying to explain how we can see the radio emissions from
lightning before the storm has come up over the horizon is basically
ionospheric skip.

Dale Thank you.

J. Jones Thank you. Anybody else have any questions? Hello?

M We're here. Some of us are here.

J. Jones Okay. Well then, if there's no more questions, I think we'll
thank Dr. Kurth for a great talk. I'm not sure who Trina has lined up
for next month but it'll be continuing excitement from the Cassini
mission. So with that I think I'll say goodbye to everybody unless,
Bill, you want to wrap up or anything.

Bill Kurth I'd just like to thank the listeners for their attention.
Feel free to send me an e-mail message if you have other questions.

Pete Actually that's a question. What is your e-mail, please?

Bill Kurth william-kurth@uiowa.edu.

Pete And just the last part again, please?

Bill Kurth uiowa.edu.

Pete Very good. Thank you.

J. Jones Thanks everybody. You can learn more about the RPWS instrument
if you'd like to from the Cassini Web site, just from the main
saturn.jpl.nasa.gov under the Spacecraft section, which is one of the
main navigation bars on the Web site. You can go to the Cassini Orbiter
Instruments; go to RPWS and there's a spacecraft page and then there's
actually the science team's Web site that has all these links that are
on the last slide.

And with that, I think I'll let everybody go back to what they were
doing before they were doing this.

Bill Kurth Goodbye.

J. Jones Okay. Goodbye everybody.

M Thanks, J. Jones.

J. Jones Sure. Goodbye.