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This is about science produced by the California Institute of Technology and originally broadcast by station KPCC in Pasadena California. The programs are made available to the station by the national educational radio. This program is about solar wins with host Dr. Albert Hibbs and his guest Dr. Marcia Neugebauer of Caltex Jet Propulsion Laboratory. Here now is Dr. hips. Although the volume of space in between the planets in our solar system is an extremely empty vacuum compared to the atmosphere we're familiar with here on the surface of the earth. Nevertheless it's not completely empty constantly streaming through it are the particles that boil off from the surface of the sun. This is done if you use cloud of particles has been given the name of the solar wind. And here to tell us about it is Mrs. Marsh and I about of Caltex Jet Propulsion Laboratory. Marcia Perhaps you want to start out by defining a little better than I have. What is the solar wind. Well the solar wind is a name given to the atmosphere of the sun
which is dreaming outwards through when to put a tarry space continuously. It's not really boiling off. It leaves the sun even faster than it. A better analogy might be with a rocket engine jet propulsion. Where's the jet force does this light question of us such as on comet tails or is it something even stronger than that. Now that the sun's atmosphere the sun's corona is so hot that the gas is just like a rocket engine the gas expands outward supersonic Clee the analogy to our rockets not as well as about 5 solar radiation from the center of the sun whereas the earth is it two hundred fifteen salary. So I most of in a planetary space is going supersonic clay. I see so that the sun itself is like a train with a rocket engine and then the outer corona has an office on then beyond that there's a supersonic stream out. Yes but the sun doesn't get propelled by and I mean first of all uniform
in all directions for a small but it's uniform in all directions and also. The mass flow isn't that great. The sun doesn't get rapidly a lot lighter because of this gas it's constantly shooting out. It's probably lost only about one percent of its total mass during its whole lifetime due to its dollar will buy the solar wind how fast when you say supersonic where how fast is it that it's moving when it gets out near where we are or as I assume it can just keep going out into space or it doesn't stop and I just continue Well it may stop me. OK well at least it was moment by the time he gets out of the earth it's still moving out at fairly high clip. Yes it's going about a few hundred kilometers a second few hundred kilometers means that it takes two to four days to get from the sun to the orbit of the earth. When it gets people are still arguing about how far it goes before it starts going slower it
slows down again and it some estimates say this happened at 5 astronomical units and some people are saying at a thousand. As you know the unit nobody's ever observed this so you know I thought. Ali that's an arm of the unit would take it far beyond the solar system Wright was still going along that fast what. What forces would slow it down other than just gravity which is a rather see one last year lighted. Mechanism is that the ions in the solar wind collide collide with the interstellar gas and exchange their charge with ice in the interstellar. And this was act actually slow down the particles just to get things in charge you would get a fast neutral particle a slow ion on it so it would bounce something else away. Yes the momentum would continue as they were a galactic magnetic field will get back pressure on the solar wind too. So it's like blasting a rocket engine into something where there's already a little bit there but very
little and that's why it's going so so fast. There's a little back pressure what is the density of the particles in the solar wind how well on an average day it's something like 5 ion pairs per cubic centimeter I say I am perished I did say earlier that the solar wind is a completely ionized guess it's mainly hydrogen but it is so hot that. Proton and electron have become separated completely ionized in a dense state about five proton electron pairs per cubic centimeter. That's how does that compare with density of air here the surface of the earth. It's about 1 10 billion billionth of a factor of 10 to the 19 left. So it's a very hard vacuum. It's better than any vacuum we know how to make in the surface of the earth by many writers as a magnitude thing they're out. How do we know what kind of observations have ever led us to discover that there was such a
thing since it's so such a rarefied wind. Well first it was inferred that something had to occasionally travel from the sun to the earth because it was noticed early in the century. That following a solar flare on the Sun two days later there would be all sorts of noise in the Earth's magnetic field compass needles would bounce up and down the be what's called a do you magnetic storm. It could not have been light from the sun which caused this to happen because it would get here in a few minutes and had to be particles which were traveling at about a thousand kilometers a second. Then back out. In the 1950s Professor Bearman the Max Planck an institute in Germany. Decided that this flow of particles from the sun was probably continuous instead
of just a now and again thing falling flares he base this conclusion by observing that a certain kind of comet tail always pointed almost directly away from the sun. I see instead of being a curved tail that we're used to in drawings it was another kind of yes. Some comets actually have two tails. The one kind is a dust tail which the solar wind does not interact with much of the others. The tail of ionized gas which is solar wind pushes around to this point almost right away from the sun it can push it around because the solar winds ionized too is probably the direct interaction mechanism what links the two people that this theory isn't completely worked out yet but it almost certainly has something to do with the magnetic field carried by the solar wind. This is when the solar wind is a very highly ionized gas with a very high and I trickle connectivity when it leaves the sun. Part of the sun's magnetic field is trapped in this gas and can't get out of it. And part of
the sun's magnetic field is carried out into space by the solar wind and this. So when this I win that hits a comet. Both the particles themselves and this tracked magnetic field act on a comet tail. So with this observation he concluded that there was a solar wind around not just in fires where the results were observed on Earth but all the time until years later he noticed when he did find the tail of a comet of the state it was a steady phenomenon as of now as the sun didn't come and go. That's correct. And since then we have also observed the solar wind from space vehicles just within the last. Few years. What kind what will a artificial satellite do or do you have to something you never get further from the earth and that the solar wind cannot penetrate close to the earth because of the Earth's magnetic field this is a whole field in itself. This interaction You mean like how to kill your
atmosphere when our solar wind the Earth's magnetic field is you left the earth with just gradually get weaker and weaker and weaker but you could always if you had an very very good instrument you could theoretically always measure it but turns out that the. So when pushes this in on the sunward side of the earth pushes the years back towards the earth and confined it to a cavity which has been named the magneto sphere the geomagnetic cavity. See so there's a definite boundary between the earth's magnetic field which stops someplace out in space in the magnetic field that's coming along with the solar wind and runs into is that that's right in this cavity that the magneto sphere you might first hand think it was a ball but turns out that it it's teardrop shaped. People don't really know how long the tail was called the do you magnetic tailor there and then the tear in it. It may go on for many
hundreds of thousands of miles we just have never made measurements yet that far from the earth that were conclusive. I see but at least on the side of the earth toward the sun it's pushed in toward how far out is it from the earth. This boundary of the Earth's field on an average day about 10 are its radio which would be about sixty three thousand kilometers 40000 miles out and the does it stay there pretty definitely Or does a wobble back and forth. When it is Rosie it stays pretty much put. But as a satellite some salads with very high Apogees ones that go very far from the earth in a planetary probes as they leave the earth. They will go through this surface and often they go through it several times and one trip away from the earth they go through it. Then the surface will catch up with them again Thumbelina find themselves inside the Earth's field for a little while and they'll be back and
interplanetary space. So it has some sort of motion which isn't very well understood yet. What are the causes of these motions are what the size of them are. But you have to get outside of this wobbly an invisible wall. The earth's magnetic field move you to measure the sun's actually have to get it even farther than that because there's another complication outside of what's called the magneto pause this end of the Earth's magnetic field. There is what's called the earth's bow shock which is a shock wave which is set up in this stream just like in a supersonic aircraft. There's a shock front set up ahead of its wings that steers the blowing air around it. The Earth's geomagnetic feel the dimming that field has a shock front ahead of it and this is typically. Three your fourth radio in front of the mag need a pause again so it's
maybe at 14 Earth radio away. You have to get even beyond that before you can see the undisturbed interplanetary medium. Well then I suppose that out a probe like the one that went to Venus certainly well outside of this fair and then toward the sun it was possible to make find way some nice clean measurements on the solar wind. Did you get any measurements from it. Yes in fact this was one of the first experiments that made fairly detailed measurements that were clearly far enough away from the earth not to be affected by the year. This is America to have. How close did it get to the sun during the time you were making measurements and how long did it. Well it's still flying but we can't use Well how long. It worked for about four months from the end of August 962 until the end of December that year. We got useful data during that whole period of almost the whole period of four months. It got
it operated at about two weeks after it passed Venus and they got in to about point seven seven tenths of an astronomical unit so that we could see how the solar wind varied over that distance. And that's about where Venus is about seven you think got only slightly inside Venus seven times as far as we are. What sort of instruments can you use on a spacecraft to measure this extremely rarefied stream of particles. Well there are several kinds of have been used they all have several properties in common the particles in the solar wind have so little energy that you can't use ordinary detectors like Geiger counters or things like that because the particles don't have enough energy to get through the window. The geiger counter. So you have to do something that just completely open to space. And all detectors so far have you some form of electrostatic analysis has pushed the particles around with an electric field in a certain way and then. I
certainly want with a certain range of energies will get through your detector an electric field will push all others particles out of the way. Then you measure how many particles each energy theoretically could also do a magnetic analysis by pushing the particles around with magnetic fields. But then but then you have to measure the particles themselves or some extremely sensitive detector on him. How do you actually count of particles of human measure an electric current of yes we actually have a collector to measure the total number of particles which enter this collector per unit of time one of the big challenges in making the current detector. Yes your transmitter has to measure. Micro micro amperes are hundreds of a micro micro ampere So they're very small and paired what you get in a light bulb. So the instrument then takes this these charged particles and distorts them or changes
their paths with an electric field and the ones that manage to negotiate the change paths get collected and I measured it I started to say that we could also do this with a magnetic field. But people haven't done this yet for positive ions because it would take a very large magnetic field. That because they're so heavy. Yes they have high momentum for this velocity and you want to also measure what is the magnetic field in this plasma that what is a magnetic field has been carried out from the sun by the solar wind. You don't want to send a sensitive bang a timer out there to measure magnetic field of your plasma detector. So you would set up such a big magnetic field you'd smart the measurements of the other. That's right so so far. People have used magnetic analysis for electrons but it's been like just static so far for positing the heavier positive ions. What about the numbers you got from these measurements. Are the numbers you've given us before about speeds and densities.
The ones you measured on merit too and those you pair with theory and other measurements and sort it in other words. Did you get a consistent experimental picture from basically from the solar wind measurements on to. Well explain as far as comparing one experiment against the other. All the differences you can say well this if you observe any differences you can say something has changed back on the sun you can always blame anything on the sun rather than blame it on your instrument. And things haven't changed that much during 962 we saw velocities which ranged from. Close to 300 kilometers a second to maybe up to almost a hundred kilometers a second while on the EMP satellite which was a year and a half later when the sun was close to cellar minimum they saw slightly lower velocities as of last days sometimes close to 200 kilometers a second and the average is probably less than 400.
The solar minimum is a time when the sun spots are the least active and when there are the few worst sunspots and then there's a solar maximum two years later. Yes this is the sunspots I mean 11 year 11 years sunny and solar minimum was just last year with it. So yes I see. But you were so this amounted to when just before the storm and that's right it was still was I was still fairly active. So your measurements and of of particle speeds were a little higher than they so and they have observed what and without is a particle. They were comparable. They didn't change very much. Now as far as comparing the ABS observations with theory people can have made theories which can explain the density and the ferocity of the solar wind fairly well I can explain in detail why some days it's higher than other days but the average theory does pretty well. The thing about the observed property of the solar wind it's a little hard to explain theoretically is the
temperature. The Observer's solar wind is a little too hot to be explained by just one hot sun conducting its heat away through the solar wind conducting convecting. We have to invoke another mechanism of heating this gas between the sun and the earth and probably what is happening is that this gas is so rarefied that one particle seldom collide with another in terms of mean free path. The mean free path for a particle particle collision is approximately twice the distance from the earth to the sun. I see so it could go a couple weeks without hitting another particle on the average. That's right. However the particles will collide with the magnetic with magnetic irregularities. If the magnetic field lines have a little bump in them a particle can collide with this and come away from this collision with more energy than it went in with more or less bending and the kinetics of the interaction.
So the magnetic field irregularities in the magnetic field can heat the plasma or cooling irregularities will probably tend to heat it. Or is it one way. Well you generate it. Back up a little bit. This is all a solar wind isn't travelling at the same speed. Ok what a fast bunch of particles catches up with a slower bunch of particles ahead of a magnetic disturbances or generated energy is transformed in the flowing motion into magnetic field irregularities that the field gets beyond being in mixed around this. Then we'll transfer some of this extra energy will eventually be transferred into the random motions of the particles. It's a matter of entropy in the long run. But once it's under the random motions of the particles and so I mean free path is very long and looks with just a motion you don't bump around and equalize out.
Well these magnetic irregularities are all moving in the same direction. Now that when you get into the finite there you have it there's implies MMA theory. You can shelve it. You cannot have a very and isotropic motion if you are in the system of the gas its moving away from the site you're traveling with the solar wind. The random motions of the parties would be random they wouldn't be going more to the left than to the right and more ahead than back. And because they're instabilities with a core plasma instabilities which will take over if you ever get too lopsided distribution of particle motions the fields in the particles which interact in a certain way. Again we straighten it all out interaction with us through the magnetic field of objects or particles it's right it's always a particle field interaction since the whole works is so very conductive making to be it was an integral part of it. What effect does all of this have on the behavior of the sun. If a
magnetic field of the sun is being as you say carried away along with the a small amount of the material from the surface of the sun is supplying energy to drive this whole mass eventually that energy has to come from the sun. How does a how does a wind react back on the sun. Well it probably. Doesn't react in the gross way doesn't change its mass much the whole life of the Sun is probably creased by 1 percent the energy loss isn't very great. It's much much less than the energy lost by sunlight about a tenth of a percent is lost and this energy is lost in the solar wind as in the. Sunlight. It's facts it might have. It's possible that the solar wind slows down the sun may carry away a very appreciable amount of angular momentum principally due to the magnetic field and that certainly distorts the magnetic field shape of the
sun the sun does not have the same magnetic field shape that it would have if the solo in were not going away. People have seen structures of the solar corona to be sort of drawn out into space and this is probably ionized gas the following nearly radial field was close to the sun. It's not a nice dipole away with it you know as the Earth's magnetic field. Now it's much distorted from that so this would be the major effect in the field back in the sun is the angle I can anger momentum and feel distorted by its effect of field and there's angle momentum change isn't very well known but there are some theories that have which still look reasonable which say that this is probably been an important effect. What effect does it have on the earth outside of the bow shock consummate neato sphere. Well several of the particles in the radiation belts may or may not have come from the solar wind. There's two schools of thought on this. Perhaps the out about particles originally came from the solar wind across the
Bagni dose of fear and then became slowly accelerated until there the energies that we see the particles in the radiation belt a lot more energy particles in the cellar where they could have originated. Yes it's hard for them to get in but there are some. It takes elaborate theories to get them truly. That is why you have issues I worry about the frictional effects and wave motions and things like that. Aside from the radiation belts the solar winds probably at least indirectly responsible for the Aurora are the Northern Lights indirectly. We get people the particles we see in the Aurora are more energetic than what you see in the solar wind. Some people are saying that perhaps the particles in the Aurore come from the solar wind to the Eurozone by coming up through the tail. Then they need a sphere up the geomagnetic dropped hail you were talking about a little while before all the way out there if you have thousand miles away from the Earth at some point you are a thousand if you are a thousand
away from the Earth here's a solar wind particles come back with a teardrop all the way back to the earth and that's now entered to make the right. So that's one of the very fascinating. Well what about simply the disturbed distortions of our magnetic field. Certainly when the solar wind changes if suddenly the solar wind is stronger if there's more particles traveling faster then the need o sphere will be compressed. The earth field will extend less far out into space right. This motion will cause comparable motions down in the earth's surface the compression will cause a higher field to be observed at the Earth's surface because you're taking the same number field lines and packing them in closer to you. Now the trap radiation which is in the magnetic field tracked in addition it could also be affected. That's where I think you'll get charges are accelerated. They may or may not be. Probably some are dumped into the years that Miss fear this time. Would that cause any euro. On that. Yes the Aurora occur right at the edge of the
radiation belt. That's a question whether these particles. I used to have been tracked or if they are accelerated right on this boundary and I'm sure that's been tied down. I see there's no clear indication in between the trapped particles on the rise and that need a fashion what is there and is there some sort of indication of that when there is a variation in the solar wind I so much it happens when there's a solar flare. But then there's a very it doesn't have to be a solar flare. There are these very strange regions on the Sun which are called M regions. People have been watching this for years and the geomagnetic data on their door in the face of the sun may be black that might not even be any sunspots. Whenever the masters of magnetic storms. Yes and they are all this origin of these particles on the sun in every region and ham region yes. Can you put it down to a region on the sun anyway. You just by saying the particles must have left the sun so many days ago that they probably came from a certain part as I designed the media center then how long to
take the sun to go around once on its axis relative to the earth its 27 days. I see as the earth is moving too. But so so that you could follow a particular region on the sun as it went around once every twenty seven days. This is M region storms are noticed to recur every twenty seven days sometimes a given region of the sun would result in a do you magnetic storm 40 50 times in a row. This is you use the same region with nothing obvious and the sun would cause a magnetic storm in years. And is this because of a solar wind effect. It's only because of particles. Yes yes it is. And what about sunspots are they connected with the solar wind. Well we haven't really observed any really big flares yet like you see at solar maximum but almost certainly that higher velocity and perhaps also denser and probably also hotter solar wind comes from the region of a solar flare than from the ordinary
surface of the sun. Probably it's still there some connection same Selwyn only Morse more sell more of it. I see so that there are the M regions in the solar flares and both of them have an effect on the solar wind which then affects the magnetic field of the song and its angular momentum and the magnesia a sphere in the surface of the earth in a very indirect curious electrical manner. Marcia thank you very much for joining us tonight and telling us about the solar wind that you have. This was about science with host Dr. Albert Hibbs and his guest Dr. Marcia Neugebauer. Dr. Neugebauer is a physicist at the Jet Propulsion Laboratory of the California Institute of Technology. Join us again for our next program when two more members of the Cal Tech faculty will discuss a subject of interest about science is produced by the California Institute of Technology and is originally broadcast by station KPCC Pasadena California. The programs are made available to the station by
national educational radio. This is the national educational radio network.
About science
About solar winds
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California Institute of Technology
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University of Maryland (College Park, Maryland)
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This program focuses on the scientific study of solar winds. The guest for this program is American geophysicist Marcia Neugebauer.
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Interview series on variety of science-related subjects, produced by the California Institute of Technology. Features three Cal Tech faculty members: Dr. Peter Lissaman, Dr. Albert R. Hibbs, and Dr. Robert Meghreblian.
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Guest: Neugebauer, Marcia
Host: Hibbs, Albert R.
Producing Organization: California Institute of Technology
Producing Organization: KPPC
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