在这个免费的课程里学习目前的太阳系探索背后的科学。用物理、化学、生物和地理的法则去理解关于火星的最新的新闻，理解外太阳系，思考太阳系外的行星，寻找附近环境以及更远区域的可居住性。这个课程普遍在本科级别的数学和物理知识上讲授，但是大多数的概念和课程并不需要这些知识就能理解。小测和期末考试会考察你是否能对学习过的材料进行批判性思考，而不是简单的记忆事实性的知识。这个课程应该有些难度，但会很有收获。
Lecture 2.09: Magnetic fields
Loading...
来自 Caltech 的课程
太阳系科学
310 个评分
从本节课中
Unit 2: The insides of giant planets (week 1)
与讲师见面
Mike BrownProfessor
Planetary Astronomy
In 1955, astronomers had built
a huge radio telescope north of Washington DC in a rural area of Maryland.
This huge radio telescope,
when we think of huge radio telescopes these days,
we think of these big huge dishes that stare off into space.
This is a different kind of telescope.
It consisted of a series of antennas in a big across a field.
These antennas were designed to pick up
very low frequency radio waves that come from intergalactic space.
They had built this telescope to scan
the interstellar skies looking for low frequency radio emission.
Low frequency radio emission comes from things like
maybe electrons spinning around a magnetic field line,
electron moving like this in a magnetic field giving off radiation as it goes.
You could detect it and map it out using this large array of antennas.
Back in 1955, as these astronomers were doing this,
every once in a while, they would get a little burst of radio emission.
They looked around, and they thought,
''Maybe it's that guy who's driving home late
from his hot date that he's been on that night,
and I hope he doesn't do it again tomorrow night."
Next night, they might get another little burst.
Not every night, but many nights they would get that little burst of emission.
After months of this,
they realized that those bursts of emission were happening
four minutes earlier every night.
Something like that, that happens four minutes earlier every night is
a clear sign that what you're looking at is something periodic,
but it's not something periodic associated with the Earth.
It's something periodic associated with space.
Four minutes is how much earlier Jupiter rises every night.
So, every time Jupiter was in the beam of their giant telescope,
which happened four minutes earlier every night,
they would get a burst of this emission. Only it wasn't every time.
If it really was every time,
it might have been easier to figure out what was going on.
It was sporadic. It took
the astronomers awhile to figure out exactly why it was sporadic.
But, here is the current version of the story.
Jupiter has a giant magnetic field,
strongest magnetic field of any planet in the solar system.
In addition to having a giant magnetic field,
Jupiter has one moon,
Io, which has volcanoes on it.
This volcanoes spew off into space,
and if these materials become ionized,
that is the electrons are removed from the atoms or molecules,
those electrons will spiral down and slam into the north poles.
This is the same process that makes aurora.
On the Earth, we don't have a moon with volcanoes.
We have the solar wind coming in and slamming into the poles like this.
These spiraling in and this slamming in is what makes those radio emissions.
But that radio emission points in a certain direction like a lighthouse.
So, only when that lighthouse was looking at us,
did the astronomers see that emission.
They realized though back in 1955,
that something like this process was going on.
They didn't know about the volcanoes on Io,
but they knew that this emission from Jupiter had
to mean that it had intense magnetic field.
We now know a lot more about the magnetic fields.
We've have spacecraft fly through these regions right by Io, outside of Io,
map out the magnetic fields,
and just like on Earth,
which has aurora, Jupiter has beautiful aurora too.
Here's a picture from the Hubble Space Telescope of the North Pole of Jupiter,
and all this bright emission is the aurora
glowing in the ultraviolet. It's really fascinating.
In fact it has, these little spots,
you see little spot here,
little spot here, little spot here.
This little spot is connected by the magnetic field to Io itself.
These are, this glow is coming from those electrons coming directly down from Io.
This little spot is connected to Europa.
This little spot is connected to Ganymede.
The rest of this glow coming along on the
further towards the pole of Jupiter would be right about here.
The rest of this glow is probably caused by the solar wind
coming in just like the aurora on the Earth are caused by that solar wind.
Let me show you another one of the aurora,
just because these are spectacular.
Again, this is from Hubble Space telescope,
and now you can see both the northern aurora up here
and the southern just barely sneaking out over the southern limb through here.
That's just really amazing things to watch there.
I believe is the magnetic field line attached to Io.
Just a glance at this picture shows you
something very interesting about the magnetic field of Jupiter.
If you think of a magnetic field as north, south,
you have magnetic field lines going perfectly symmetrically about like this,
you would find that the aurora would be perfectly north,
south and symmetric around this axis,
and they definitely aren't.
The magnetic field of Jupiter is clearly tilted in this direction,
and the field lines must go like this instead.
But it's not tilted all the way in this direction over here.
It looks like it's bent like this.
Here, it's half of a rotation later
it's pointed down in this direction and more in this direction.
The rotation pole of Jupiter is straight up and down.
The magnetic field is not like this.
But the magnetic field is this.
It is referred to as a tilted dipole.
In fact, it's not even a tilted dipole,
it's an offset tilted dipole.
Instead of having the rotation axis go right to the middle of that dipole,
the rotation axis actually goes a little bit
offset from the center of that dipole more like this,
and the magnetic field of Jupiter looks something like that.
Quite an amazing amount of detail,
you can learn about the magnetic field of a planet
just by looking at things like this aurora,
by flying spacecraft through it,
measuring these magnetic fields,
and by looking at many different types of radio mission that you can see from it.
So, why does Jupiter have a magnetic field?
We talked a little bit about magnetic fields when
we were talking about Mars and its interior.
But we're going to talk in a little bit more detail about it this time.
Magnetic fields in astrophysical objects are created by a dynamo.
And there are three crucial ingredients to get a dynamo to generate a magnetic field.
First, you have to have a conducting liquid.
In the case of the earth,
that is liquid iron.
Liquid iron is conducting,
it's electrically conducting, and it's liquid.
So, that's good. In the case of Jupiter,
what are you going to guess in the case of Jupiter?
It's that liquid metallic hydrogen.
That strange state that hydrogen attains at high pressures is
critical to the fact that Jupiter has a magnetic field and a strong one at that.
What else do you need for dynamo?
You need convection.
You need to have this liquid not just sitting around doing nothing,
but you need to have it rising and falling within the planet,
making these looping motions.
How do we get convection? Well, we talked about that.
You have to have heat on the inside,
and you're trying to get rid of the heat on the outside,
and the fastest way to do that is through this convection.
Yet have one other really important thing which is rotation.
Even if you have a conducting liquid and you have all the convection in the world,
if you don't have rotation, no dynamo.
No magnetic field. Why is that the case?
The answer is really complicated.
The answer is so complicated that scientists have not actually really
solved the entire problem of how dynamos are generated,
how they stay alive.
But the basic idea you can sort of handwave at with a picture that looks like this.
This is a picture that's drawn for the Earth's magnetic field,
but you'll get the general idea.
On the earth, there is a solid inner core of iron that you see here,
and this liquid outer core from here to here,
and then the mantle. This is the mantle.
Their is the surface of the earth would be out here somewhere.
So, all this magnetic field on the Earth is generated in
this liquid outer core. How does it happen?
Well, material in the outer core rises
up because of convection and falls back down again.
But, because of the rotation,
there is a Coriolis force.
Coriolis force of course is the thing that leads
to hurricanes all spinning in nice directions.
It's a sufficiently strong force that when
ships are shooting shells many kilometers away,
they have to calculate the Coriolis force.
So, instead of pointing directly at the thing they're aiming at,
they have to point a little bit to the side because as they shoot,
it curves duo to Coriolis force.
Anything moving in a rotating frame will be subject to this Coriolis force,
and will be deflected.
So, what happens is, these rising fluids are
deflected and form these loops that look like this.
These liquid metallic, these liquid conducting loops of
material spinning around are what generate and sustain these magnetic fields,
and you get these big north-south magnetic fields moving through here like this.
The same process happens on Jupiter,
where this might be that solid core on the inside of 15 Earth masses.
This is the region of liquid metallic hydrogen,
and above here is the region of molecular hydrogen. Why does this matter?
Well, for one, it tells us there really is this liquid metallic hydrogen,
this crazy phase of hydrogen that's inside here that's causing this giant magnetic field.
Two, if we can figure out where in the planet that magnetic field is being generated,
we understand a lot more about this transition from
hydrogen molecules to this liquid metallic hydrogen.
How do we figure out where it's being generated?
Well, from far enough away,
you can't tell the difference between a field that's
just generated directly in the center as
a point and a field that's generated in an annulus somewhere further away.
This is exactly the same thing as we were
talking about with gravitational field, when I said,
if you have a spacecraft really far away,
it can't tell the difference between a point source and an oblong field here.
If you have a spacecraft measuring the magnetic field here,
it can't tell the difference between a simple dipole from the center looking like this,
and maybe something more complicated where the magnetic field is generated here,
and is a little more complicated out through here.
But by the time it gets up to here,
It's exactly the same.
What's the solution? There's always a solution.
Take your spacecraft, put it in as close as you can.
The closer you get to where these regions of the magnetic field are being generated,
the more deviation there is from this simple dipolar field of just a magnet,
and the more complicated things you see.
We already know that this is going on.
We saw that from the Aurora.
We're getting some clear indications of
a very complex field in some indication of where the magnetic field is being generated.
How did you better? Notice I drew once again the Juno spacecraft.
Juno spacecraft in orbit around Jupiter.
One of the things that's going to do is fly in very close,
measure what's going on with that magnetic field,
and understand not just what's on the very inside or if not,
there is a core of material like we talked about before.
But also understand that transition from the liquid metallic hydrogen out to
the more normal state of matter that we understand this hydrogen molecule.
So, we had two types of measurements that we could do from a spacecraft.
We could measure the gravitational field particularly as you go close,
we can measure the magnetic field again as you go close.
Both of those help us to understand the details of what's going on in the inside.
We have one more measurement that we can
make that will help us to make sense of the interior of Jupiter,
and we'll talk about that next.
