Transcription
I'm going to talk about my neatest fears in general, and the earth being a specific example that we've studied in most detail. But actually, the earth is a more complicated partly because we know more about it, so therefore it seems more complicated, but also because it is actually a little bit more complicated in terms of dynamics and structure than some of the other ones. But before we do that, um, what I would like to know is what your area is, and this is not, you know, I read your CV and I, we know as the people running this, but you don't know each other, maybe what the breadth of fields of research that you will do. So those of you who do solar stellar physics and other stars, wave your hands. And yet, okay, okay, good. Look around and see who your fellows are. Okay. Heliosphere, Astra spheres, solar wind. Okay, good. Reasonable number, that's good. Now how about Earth ionosphere magnetosphere? Okay, right. Exoplanet or planetary magnetospheres? He's put his hand up for every single one. You can have multiple interests; that's okay. They speak this often. Okay. Space physics, plasma physics, anywhere basic theoretical underlying physical theory is that right? Okay, good. Okay. Others have I missed anything? What have I missed? What, who else had some other quest, other others? Wow. Okay, okay, good. Yeah, yes. Climate. Oh, good point, very good point. Yeah, atmospheric and indeed the last lecture of the school by David Brain will be about planetary climate, yeah, including you. Any other said I missed? Okay, great. So the point I'm really trying to make here is there's a very broad group of people here with very broad interests. So although you may be thinking, "Oh my golly, this is so basic for other people," it's not so basic. Okay, okay. So let's think about magnetospheres. We, this diagram, you ought to look, the way you're supposed to read this is this whole magnesia of Mercury fits within the earth. This whole Magnus forever fits within Jupiter, and then this whole mighty sphere of Jupiter is small on the scale of the heliosphere. And if you think about how the planet, the magnetosphere of Mercury, it scales with the heliosphere, it's many orders of magnitude. In the middle of the night, I was trying to work out the number of orders of magnitude; I think it's about seven orders of magnitude. And yet, Moggy Kittleson has written a paper arguing that the structure of the interaction of the interstellar wind with the heliosphere is actually not a lot different than the interaction of the magnetosphere with with Ganymede. And so she'll be talking about the interactions with moons on Monday, but the basic physics of the interaction of a plasma with a body, whether it has a magnetic field or doesn't, is really, once we're dealing with the general processes, can be scaled from object to object.
Okay, so we have some discussion from Sabina about planetary magnet magnetic fields, and we know that Earth and Sasson are and and Jupiter pretty dipolar fields. And in the case of the Earth and Jupiter, the tilt is about 10 degrees from the spin axis, and that with Sampson we have a tilt that is basically zero, very closely aligned as Sabina described, associated with the Scimitar ization due to flows inside. But also notice that I put in here obliquity, that is tilt of the spin axis with respect to the oil plane, and the seasonal in effects on the earth and sunset are going to be substantial, with very limited seasonal effect at Jupiter because on the spin axis of Jupiter is pretty much in the orbital plane, the ecliptic plane, indeed of the planets. Now when we look at Uranus submission, of course, the offset tilted dipole approximation is very poor; it's a highly non-dipolar complicated magnetic field, and of course Uranus is tipped on its side, so huge seasonal effects. But these magnetospheres are much more complicated, and I hope to just briefly measure them at the end. Sadly, we know very little about them. So let's start off with a rate. Yes, if all of these objects, their angular momentum is huge, and so they remain pointed in some direction. So for the earth, it's Polaris and oh, sorry. Okay, so how does the spin axis of Uranus vary in its orbit? It remains pointed at some star, Earth, it's Polaris, Uranus, I don't know, it's some blah blah blah blah, stop. Okay, good question. So yes, the seasonal effects are huge, and I hope to come to that at the end. Okay, so let's go to the radiation belts, and here we are. We have 1958 Van Alen here in the middle with Pickering, head of JPL, and von Braun, the rocket man, with Explorer one. And I'm showing you this because we didn't talk about the radiation belts and how cool it must have been to have been on that very first rocket experiment that the US did. And so you need to go here and see this because it's, this is me down here next to the rocket. Go check out these rocket parts; they have this Explorer one. This is where they commanded them. Look, it's so cool. Go, that go to Kaitlin out Carol and check it out and have your picture taken. Me making a silly, just yet. Okay, you know what I meant. Okay, let's talk about the radiation belts a little bit, and um, Thomas Gombo she went through the adiabatic invariants that describe particle motions around the field, bouncing up and down and then drifting around. And these timescales, you can see this is for the earth, our gyro frequencies of a very short bounce periods, a little longer, and then the drift periods are as much as minutes. And the important point here is that if you have fields that vary slowly or relatively constant compared to these djaro and bounce and drift, then you will, the corresponding invariant will be conserved; that's why it's called an invariant. But of course, if you have variations in the fields on these timescales, you will violate those invariants. Okay, and this will become important as we will discuss a bit further. And of course, for a dipole, remind you that we define an L shell as the the description of this magnetic flux tube, the furthest crossing point is our furthest point from the central planet. For a diaper field is aisle shop.
So let's think about the radiation belts, and this was introduced a little bit here is a plot showing as a function of L shell and energy. So this is energy across here for one to 14 MeV electrons, for example, and this is elf L here. We have this distribution of flux here, and you can see we have protons and electrons, a little different, and you can see this is described usually in this canonical picture of different rating in a radiation belt and out of radiation belt in these two regions. So when you look at a variation as a function of time, so this is L shell again, but limiting the energy range in this case greater than 2 MeV electrons, you see this outer belt and the inner belt, but they vary with time, coming and going over the days here and there's a whole variety of time scales on which things vary basically associated with perturbations in the in the magnetosphere. So I have a question for you. We're sitting on a satellite; we have a detector that, or we're virtually sitting on a satellite; we have a detector that measures particles in a range, say 4 to 6 MeV electrons. What are the physical processes that might cause the measured flux as a function of time to change? Ideas, precipitation. What causes the particles in that energy range to precipitate? So you might have change in their pitch angle. What could be causing change in their pitch angle? That's the angle. All right, wave particle interaction. So you can have wave particle interactions that takes those particles that you're measuring and scatters them into a loss cone, and they disappear into the planet's atmosphere. What else could be happening? You're measuring as a particular energy range. All right. So when you're going like this, you mean what I think what you're trying to say is transport in and out of your physical location, right? So you're sitting in a particular location, and these, the energetic particles could be transported out of your physical location, right? So this transport in in or out or past you or your spacecraft could be moving through a boundary, right? Okay. So what else could be changed the number of flux in your your window? Right, very good indeed. So either you're passing through the boundary of boundaries passing over you, and of course with a single spacecraft, it's very hard to separate those two; that's why we have multiple spacecraft going out into space like Cluster and Thermos and MMS and so on. Okay. Any other ideas of how you're looking at a particular energy range in a particular location that flux? We've had loss of particles due to precipitation; we've changes in the energy distribution, right? So we're looking as a particular energy band; you could lose energy, maybe it's radiated in some way or giving energy to waves, in which case it would get out of your past bound, or there could be some way in which particles are accelerated and heated and put into your energy range. Okay. So you need to, when you see a variation as a function of time, you have to think about the variety of processes you've got: source processes, lost processes, energization processes, and then boundaries, physical transport processes.
So here we have a canonical picture, worthy to inner and outer radiation belts. The outer belt, you have particles transporting from the outside of the magnetosphere, conserve the first adiabatic invariant, and you move into more energetic, sorry, higher magnetic field, right? Higher magnetic fields, you'll end up moving into faster djaro motion, your, and then then there's also wave heating that will heat them up. So as the particles come in from the outer mat used where they heat up, and we end up with these energetic main radiation belt on the outside. The inner radiation belt, the protons are produced by cosmic rays interacting with the atmosphere, and then the point though is that as these particles come in, energized from outside, at some point waves scatter them into the lost code, and they get lost; they precipitate down onto the atmosphere. Okay. So um, there's a whole load of ways in which particles are accelerated in the out of magnesia; I'm not going to go into this for the earth, but as they get, they are brought in and populate this radiation belt. Now I just want to show you the RBSP, the most recent Van Allen probes spacecraft data, the the RBSP instrument that was built in those labs that you visited at last; they were built and put on these two spacecraft, we have, and here you see their orbits put into this coordinate system. So this is distance from the planet and vertical distance; you can see that there are two radiation belts here, the bright red here and the lighter red in here, and then these are different epochs as a function of time. This is time for different and energy ranges, and you can see there was a lot of excitement that back last September, up to September's ago, there were three radiation belts, one, two, and three, and you can then think about what combinations of energization, transport, loss could have led to this configuration, particles coming in from the outside being lost due to wave particle interactions leading to precipitation. Now I want to move on to other planets, and of course I want to talk about Jupiter because 1959, around the time of Explorer 1, we were starting to get observations of radio emission. This is 10.4 centimeter radio mission from Jupiter, and what we're seeing here is synchrotron emission of tens of MeV electrons. So these electrons are gyrating around the magnetic field, and they send out, these are relativistic electrons, and they're sending out synchrotron emission, it beams perpendicular to the magnetic field. So over here, you're going to see stuff emitted here, and then at high latitudes, you'll see some for those particles that bounce up to high latitudes, synchrotron emission. How it actually works, you know, I don't, but it's these are relativistic, the actual physical process of how you convert kinetic energy of the electron into radiation, I don't know how it works, right? So this is radio emission that is emitted, and now I don't know exactly how it works. No, no, this is not collisional; this is just relativistic electrons gyrating around the magnetic field and emitting radio emission, photons. Or I'm hearing three people talking at once. Finish what you're saying, sorry. Right, I agree. Okay. The question though was what is the physical mechanism of of general of emission of a radio mission? I don't know how that works; it's being accelerated and easy, yeah, right. Okay. I think what we're finding our collective knowledge here, wisdom is telling us, gyrating electron that is is relativistic being accelerated because it's going around results in radio emissions. Okay, and the details are in a textbook somewhat. Okay, so good enough. But the value is indeed in that this tells you it has a magnetic field. We knew back in 1959 that Jupiter had a magnetic field; we knew how strong it was; we knew that it was tilted by 10 degrees because the radio mission wobbled with the 10-hour period. Okay. So we are also able to look at radio emission from a variety of different ways. This is the VLA radio telescope south of here; this is the Cassini spacecraft on its way to Saturn making images of this radio mission, rather do different wavelengths. But what's interesting is that you can see there in the picture, angle distribution of electrons in this region is not just a simple distribution; we have two populations, one which are highly column confined to the equator with a very pancake distribution, but there are also some that are tend to be populated at high latitudes where in fact you're removing the ones at near 90 degrees because there's a moon there that absorbs them near the equator. Okay. But there's another factor about radiation belts, which is they're nasty and dangerous, and indeed these relativistic electrons are very damaging to sensitive electronics. And so when the Juno spacecraft gets there in two years' time, it'll go into orbit; it'll go over the radiation belts, back under, over here, and then come around avoiding the radiation belts but getting very close to the planet. The problem is though, because of the flatness, that rotational oblate nurse at the planet, the orbit will naturally process from being equatorial to higher angles, and at some point, somewhere around we think about orbit 30 or so, it will begin to go through these radiation belts and die. We anticipate that it won't survive going through the radiation periods. Okay. But this is going to be an important mission; I'll tell you a little bit more about it later in terms of the magnetosphere, trying to understand what's going on.
Finally, let me just mention for these radiation belts at Jupiter, unlike the earth where you saw that very dynamic changes that are BSP probes measured due to all sorts of timescales due to the changes in the magnetosphere, there's basically just a factor of two at most variation over decades that we've been observing these emissions, and the biggest effect was back here in 1994 when there was a dramatic change due to the Shoemaker-Levy 9 comet going through the radiation belts and all the debris interacting with these radiation belts. Ok, ok. So let's talk about magnetospheric size. What determines the size of a magnetosphere? Well, we know that we have supersonic flow coming in from the Sun, we out from the Sun coming into the planet; we have an obstacle which is the magnetic field, and we want to think about what determines this subsolar magnetopause standoff distance here, distance from the center of the planet to the magnetopause at the subsolar point, which we will call m or MP. And there's an upstream bow shock because this is supersonic; it has to go through a transition here. The bow shock tends to be about 1.3, in the case of the Earth, times the magnetopause distance. And this bow shock is where you take kinetic energy of this supersonic flow; we talked a lot about the solar wind being supersonic and having a lot of kinetic energy, but you're converting that into thermal energy as it and then deflecting on the plasma around. So upstream here, in fact, the pressure here is in fact about a little bit less than the upstream conditions, but it's really much the same as the upstream conditions. So let's think about what we're doing; we have a roughly dipolar magnetic field. I'll point out that this is this whole derivation of magnesia size and dynamics as is discussed in full at some of the earlier lectures from previous years, but I'm gonna summarize this and go through it fairly quickly. If you put this dipole and you put it into an external field in the opposite direct, in the same direction, you will end up with currents around it that would end up with a field around here that's about three times the dipole. If you put in an oppositely directed field, the image dipole, whoops, I haven't wait a minute, I got this wrong, this is, but yeah, this is a southward external southward field; you end up with twice the dipolar field on the boundary. So usually what we do when we derive the size of the magnetosphere, we take this magnetosphere, this is from Cravens' texts, but they're all the textbooks have the same sort of derivation. You have kinetic energy of the solar wind; you have the magnetic energy of the internal dipole; you have the ram pressure balancing the internal magnetic field pressure. And so you write this equation for this balance of the kinetic energy and the magnetic field energy, and notice that I've got the surface magnetic field B0, which is the equatorial value of the field, and then because it's a dipole, drops over as 1 over R cubed, and so if we square it, it'll be 1 over R to the sixth, and I've normalized it to the planetary radius RP. But what we need to do is we need to double the strength of the field here to account for these currents on the magnetopause. This is the sort of fairly standard mirror dipole approximation, and you end up with a formulation of this balance between Rho V squared and the internal field pressure. You solve for the magnetopause distance, and this is what you get, a familiar perhaps expression describing how what the standoff distance depends on, which is the internal field strength and the upstream magnetic field. But of course, notice this 1/6 power, which will dampen this and make it a less sensitive to variations that things inside. Yes. Okay, indeed, and we will be looking at this. So I've been a little sloppy here; there are better derivations of this in their previous discussions; you we explained with this in the in the labs, but I'm keen to do comparisons when you're looking at very different situations, both there magnetospheres we know and potential magnetosphere, say, of exoplanets. So we take this situation; we have a a at this description of basically the ram pressure balancing the internal field, and so you end up with this expression here, which is often called the Chapman-Ferraro distance, going back to the 1931 paper that they wrote back then. Okay. So question, what happens when these things vary? So let's start off with, how does the solar wind vary with distance from the Sun? Ten seconds, talk with your mates. Okay. What is the answer? Whatever else, quit. Great. Okay. How does the solar wind vary with distance from the Sun? Approximately with distance, not too much time, but distance pretty much constant once you get beyond the Alfvén point. Okay. Okay. So now how does this quantity, one over the sixth root of density times B squared, which is this quantity in Sonya, how does that vary with distance? One foot. Right. So what we're finding is that if for a given magnetic field we took a dipole or a planet and we move it around in the solar system, then you can see it's actually not a very strong dependence on distance, but if you go all the way up to Uranus and Neptune, then this quantity gets to be quite substantially different from, say, 1 AU of the earth. And so what does that mean for the size of the magnetosphere? If we were to take Earth out to Uranus and aperture, it'd be big; it would be a bigger bigger size. Keep that in mind when we get to the outer magnetosphere. Okay. So we think about Jupiter; we have a very strong magnetic field, huge magnetosphere; we actually also will talk about the dumping of material into it, but you take this whole Manny's fervor of Earth, it would fit within the planet more or less. Okay. So let's think about what happens; we do our scaling relation; we go look at our planets; we look at the strength of the field up there, equator, not all that different, bigger at Jupiter, much less at Mercury, but but otherwise pretty similar to the earth. You calculate the standoff distance using our Chapman-Ferraro relation up here, and indeed this is what you observe. So you will see that for four of the planets, it pretty much matches up, and for Jupiter and Saturn, however, we have a substantial difference, almost a factor of two difference between the theory and the practice for Jupiter and a number that is not clear how different.
It is, and this is something that I think Nathan is struggling with. He's working on it. Yeah, well, yeah, yeah. Okay. Um, so this is using average conditions, and I believe I was there. I think I can remember the conditions were a little strange. We had a very high mark; it's the highest marking of a shop that I published in a paper, observed in this whole system. So, mark 1700 Chua. We had a very high ram pressure, and so that makes sense that it's a little smaller on that particular one occasion where we flew through. Excellent question. Any other questions?
Okay. So the question obviously is why, and the answer is another way of putting this is to draw these together with where the the size that you would get for a—this is the dipole standoff distance—and the answer is you've got hot plasmas that inflate the magnetosphere of Saturn and particularly Jupiter inside. So when you take for the dipole and you have a ram pressure, the size of the magnetopause varying is the sixth root of Rho V squared, and where Jupiter it's more like a third or maybe a fourth; some debate about the exact number here. If you then increase the solar wind by a factor of ten, you get a fairly small change. The Earth's magnetosphere is quite rigid in its response to the solar wind, but for Jupiter it's much more compressible, and you've got a factor of two variation in the size of the magnetism.
So let's think about the plasma inside. You remember our base expression; it's actually on the board right there. And KT over B squared to u naught. And so really this is telling us the amount of thermal energy in the plasma compared with the magnetic field, and in the plasma sheet around Jupiter we have numbers that are substantial tens, and so very much dominated by a hot plasma in here, whereas it high latitudes in fact it's a very low beta plasma with a fairly empty—so when we think about the compressibility, we have to include not just the Rho V squared and balancing the magnetic field, but inside we have this substantial pressure that is of the plasma inside. Yeah. Okay.
So the question is this: because Jupiter is more compressible, does it respond in an interesting different way than the Earth? And the answer to that is yes, very much so. It is very compressible, and indeed you see big changes. Now what it turns out is that's probably by the time the solar wind gets out, Jupiter tends to be kind of bimodal, and you get compression regions and then a relaxation, and the magnetosphere responds accordingly. Yeah, it's an interesting response. Yeah, yeah. Okay. The radio emission. Okay, look at the scale; this is a hundred RJ, radius of Jupiter. The scales of the magnetopause, the radio emission is at three feet deep, deep in—you can't even really draw it on the scale; it's about the size of these dots, this dot in on either side. Okay, it doesn't see the solar wind really to speak on. Ah, what if you had a hot Jupiter exoplanet and moves in close to the star, then you might see strong response. Okay. Now there's—as we will talk about—there's many different kinds of radio emission from Jupiter. We just talked about Synchrotron emission that's in close, which is due to those energetic electrons, but there are other kinds which may be better for diagnostics of exoplanets. Good question. Why is it important for Jupiter, not for us? Hang in there, dude; we're just getting to this.
Okay, so um, if we look at all the planets, you'll see about these giant planets, they have moons out there that are embedded in the magnetosphere, and this has important consequences. Quick question though: what's wrong with this diagram? Kind of cool pictures. No, it's nice. Not to scale. True. Good, good. Yep. Not just go—what is wrong? What is off? The colors are off. Yeah, good, good. You're warming up. Think about how the solar wind moves out from the Sun; these lines—what are these lines? Magnetic field, right? Sector boundary. How does the wind go? Radial. So it's not too bad for the Earth, but ooh, the graphic artist kind of got things wrong for Jupiter, right? I just toss that out; there's a—the solar wind pulls the tail behind it; it should be over. Yeah. Okay, we'll come back to these things.
Okay, so let's talk about plasma sources for the Earth. Things are actually fairly complicated for the Earth. We have an ionospheric source; we have a bit of a solar wind source, and there's a problem of course that if you have hydrogen, protons are protons, and protons, helium is a little easier because usually the solar wind, two alpha particles, doubly ionized. Internal sources are singly on Earth, and then oxygen, singly ionized oxygen comes from the atmosphere, and I think that maybe Rod will talk a little bit more about sources from the planet coming out, but they've come out and they get mixed in with the magnetosphere; they get heated, accelerated, and ultimately passed back and through the magnetosphere, but it's a small amount, five kilograms a second in terms of total plasma. So the Earth is very much dominated by the magnetic field rather than the plasma source being an important factor. We go to Jupiter; that's very much not the case. We have these moons, Io, Europa, Ganymede, and Callisto, and particularly pesky Io, that volcanic moon that is spewing out sulfur dioxide and volcanic gases, is a big producer of plasma. Indeed, we have about a ton a second, so that is a big truckload every second being dumped into the magnetosphere, and it's caught up in the magnet, becomes ionized, caught up in the magnetic field. You have a rotating donut or torus of plasma, sulfur and oxygen ions that are emitting in the ultraviolet. So this is a real image taken by the Cassini spacecraft as it flew past Jupiter on its way to Saturn, and you can see the UV glow of sulfur and oxygen ions glowing due to actually electron impact excitation. Yes, every 10 hours the magnetic field rotates, and so this is a 10 R for a 1 wobble, total mass of about two megatons. Okay. Oh, an important point here is actually the time frame for removal of this plasma; it's quite long, 10 to 20, 10—sorry, 20 to 50 days. So quite long; you put it in here and hangs around for a long time. Think of that compared with the cycle, cycling time at the Earth; the cycling time of stuff through the Earth is more counted in in minutes to hours rather than in many days. Okay, very different system.
So what is the energy come from to make stuff radiate? Well, the energy comes from ion pickup. Marquis Kilson will talk a little bit more about this. We have electrons bombarding and neutral atoms. So in this case we do have collisions; electrons bombarding the neutral gases that come from here, ionizes them, the inner magnetic field that is into the board here. You have an equatorial plane; you have a—on that jar, it's like this with a large ion gyro radius; little electron going like this; you get effectively a little bit of a charge separation that can lead to a pickup current, but more importantly you have to think about the magnetic field couples the the plasma to the spinning planet; you're extracting energy from the planet, and the ion gains gyro motion as it as its picked up. Another process that happens is charge exchange. So there are some collisions; it's not entirely collisionless, but when you've got a time frame of weeks, you can have fairly low level of collisions and still have an important effect on the physical chemistry of the system, and so what you do is you have a neutral atom, so you have an ion, say a plus, comes in, hits a neutral atom of b, you ionize the b, starts gyrating, and the a becomes neutral, no longer confined by the magnetic field and comes whizzing off as an energetic neutral atom. An energetic neutral atom imaging of a system is now being used to try and understand plasmas, particularly at the outer planets, and to see what's going on. It's one way you can see stuff by looking at the energetic neutral atoms that come away. Again, you're extracting momentum from the plasma that is coupled to the spinning planet, so it's the way in which energy is added into the system.
So let's look at—this is a messy diagram; it's got a lot of stuff on it, but let me talk you through it. We look at Jupiter; we have Io at about six RJ. At Saturn we have Enceladus at about four Saturn radii. They both spew out stuff; in the case of Io it's sulfur dioxide, sulfur—the gift—it's sulfur monoxide; they get broken up to produced sulfur and oxygen neutrals. This is a big cloud that gets spread around; neutral atoms and molecules away from Io around Io's orbit, and I've got Europa's orbit out here. So you can think of Europa actually being orbiting in the effluence that comes from Io. At Saturn we have plumes from Enceladus spewing out water products and and ice particles that spread out into the system, and the neutral clouds from Enceladus get puffed out. There are neutral, neutral collisions; actual collisional processes; there's a dense enough amount of this to spread things out, and there's charge exchange processes that lead to puffing out the cloud away from Enceladus. So rather than being confined at four, it actually spreads all the way out throughout the magnetosphere, and in fact it can be observed either in the UV or from Herschel. So we know that there's a standing cloud around Saturn. Now if you look at the total amounts of neutrals, it's relatively small at Jupiter, but the total amount of ions is substantial. Oops, sorry, you go back. Um, but what's important is to notice this; look at the bottom line here. In the case of Jupiter you have a lot of ionization, so ions dominate over neutrals by a lot, fifty to one; many more ions than neutrals. The ions quickly ionized, electron impact ionization, and remove the the neutrals; they become ionized and trapped in the magnetic field. In the case of Saturn the neutrals dominate over the ions by a hundred to one, so you can barely call this a plasma; it is an ionized plasma, and the lines do follow plasma physics, but you have to realize that there are a lot of interactions between the ions and the electrons and the neutrals that basically it's less classical physics, more atomic and molecular physics. So in some ways you might think of this being a bit more behaving like an Astrophysical cloud, gas cloud than a magnetosphere.
So when we look at the torus, the ionized part, and here is those UV emissions again; UV emissions observed by Cassini. We tip the torus on the side; you see it wobbling about. These are different emission lines; this is sulfur emission lines, oxygen emission lines, a bunch of more ones over here. The aurora, by the way, of the planet are in the middle here, but what you see is this dense region where there are up to a couple thousand particles per CC close to Io, sprinting out, whereas at Saturn you have much less; you have maximum densities on the order of 100 per CC, and the total product of plasma here is tons, whereas over here it's more like tens of kilograms. A lot of UV emission; the energies are substantial in here. Over at Saturn the energies are much less, and there's no UV emission to speak of. If we look at the composition at higher energies, we can observe—it's easier to actually get the composition at higher energies—and what we see at Earth is stuff that comes from the solar wind and then products that come from the Earth's atmosphere. Jupiter we have stuff from the solar wind again, a lot of these products of sulfur and oxygen from Io, and then out at Saturn we have the water products, that substantial amount of water products that add to material that comes inside, penetrates in this hole away.
So let's think about the flow, the mass flux. We have a source at—you know—we have a neutral source; we have ionization, electron impact ionization, and then we lose some particles due to fast neutrals that that charge exchange process, and then some of the plasma is transported out, and indeed what we get is about 50/50, about half of the material is lost as fast neutrals, half the material transports and moves out. In the case of Saturn again we have a strong neutral source, but most of those particles, 95% of them are lost as fast neutrals, so it's a very neutral dominated system, and only about 5% of those are transported out into the magnetosphere. If we look at energy, this is a little bit more complicated, but I'll talk you through it. You, we have this pickup process that happens of the ions, puts energy into the ions; they pick up energy; they then couple to the electrons. What do we call that process of transferring energy from ions to electrons? I've got coupling here; ion-electron coupling. There are processes between charged particles, but they don't actually hit each other, but because they're charged they can move fairly large distances but still transfer momentum. Coulomb collisions, right? If you heard those, Coulomb collisions that act at larger distances than those billiard balls that bump into each other. So Coulomb collisions transfer the energy from the ions to the electrons, and I like to think I learnt plasma physics from this guy Bruno Coppi at MIT, and they said—Italian—and he would talk about—it's the trucks and the bicycles, right? Senator bicycles—I'm not a very good accent—you get the idea. And you have, in fact, the mass ratio is about right; the ions are the trucks, the electrons are the bicycles, and if you're a cyclist you have a visceral sense of the relationship of masses between these two, and you don't want to get hit by a truck, right? So the ions that can conveying their momentum to the electrons. There's also some electrons, hot electrons; throughout the universe there are these hot electrons, hole electrons just seemed to be all over the place; that is the distribution function is usually not a Maxwellian; usually it has a tail at higher energies, and all sorts of ways in which those high-energy tails can be produced. I usually just say it's waves; you know, plasma physicists will come up with a hundred and one fun ways of making hot electrons, right? And they go off and they do all their equations, and basically they just make hot electrons. Okay, so you can in—you could delve into the details of how it happens, but there are always ubiquitous hot electrons in are it in plasmas, and this is a way, you know, energy is added to the electrons. And so in the case of Jupiter the energy comes from ionization through the ions and then is radiated out as UV emissions, and I'll bet you there are many Astrophysical plasmas where this is very similar sort of processes occur. At Saturn, of course, most of the energy is carried away by these fast neutrals, spread out into the system. So in fact this whole process of—and Io spewing out neutral material, these plumes that send out water—are in fact extracting momentum from the spin of Saturn and sending it out as fast neutrals into this big extended neutral cloud, and the magnetic field and the plasmas are just an intermediary in this process. So finally, to just look at the plasma sources summed over all the planets, we have these very strong sources at Jupiter and Saturn, and the rest are pretty minor by comparison.