If the Sun has an atmosphere then it has weather and understanding solar weather
is going to be fun, because it involves interesting physics.
It's going to be important because it actually effects life on earth so let's
try to understand how, in what way solar weather is very, very different from the
weather here on earth. And so the indication of solar weather
are these blemishes, these spots on the sun.
first known recording of these is in the writings of Chinese astronomer whose name
I can only pronounce as Gan De. Galileo observed the sun spots and used
them to find the rotation period around the sun,
the sun rotates about its axis moving from west to east, every 25.4 days.
This is a medieval depiction of two sun spots on the surface of the sun.
these sun spots appear as dark spots on the sun.
Here, you see a modern image of the sun with a few regions of sunspots.
They appear dark, as we said, because they're cooler than the surrounding at
the center of a sunspot, the temperature can dip as low as 4,000 Kelvin.
Because of T4, to the fourth they emit less.
And if you remember that the sun is approximately 100 times the radius of
Earth. Then the typical size of a sunspot is
about the size of Earth. And it was the observation of Wilson in
1769 that in fact, by watching the behavior of sunspots as they approach the
edge of the visible disc of the sun. That they're in fact depressions in the
solar surface. This is what a sunspot looks like when
magnified. This is an Earth like region which is
cooler than the surrounding, the center of, part of the sunspot is called the
umbra. The less dark exteriors called the
penumbra of those channels have nothing to do with it,
the words are completely borrowed. So, what are these things and hints to
the fact to the origin of these things come by observing the patterns of their
formation. It turns out that some spark number
varies with a very nice eleven year cyclicity, this famous butterfly graph
here on the right indicates, well on the bottom you see the number of,
sunspots or the total fraction of the solar disk's area covered by sunspots as
a function of years from somewhere in the eighteen hundreds all the way through the
to, the present day. And we see there's this periodic
oscillation. Lot's of sunspots there, none, and lot's
of sunspots there, none. And when you look more closely at where
along the sun they're generated that's what butterfly graph above shows us.
This is sort of a picture of The, the vertical axis is latitude along
the sun. And you see that early in a cycle
sunspots are created at mid latitudes, and then as the cycle proceeds their
formed closer and closer to the equator. Then there's a whole bunch of then at the
equator, then they all disappear. And then there's a break, and then some
more sun spots. Form at mid latitudes, and slowly they
move towards the equator. And the pattern repeats.
it's hard to see in this graph. But in fact 2010 was the end of a very
abnormally long solar minimum in which there were no sun spots.
And nobody understands exactly, as far as I can tell what the cause was for this
reason. Now.
An important a discovery is that you can associate sunspots to magnetic free
regions of strong vertical magnetic fields of the sun.
So magnetic field poking out of the surface of the sun and the way you
measure this is because it turns out that a magnetic field has an effect on the
spectrum of Atoms in the presence of a magnetic
field. You can measure the shift in the
absorption lines that we saw in the Sun's spectrum.
It's all by looking. And we can therefore measure the
magnitude of a magnetic field. And this is a magnetic field map.
Again we have the lines of latitude. And longitude along the sun.
As, as a function of time, we see that when there are lots of sun spots, there
are a lot of these regions of high magnetic field.
Notice that yellow is one polarity and blue, the opposite polarity.
So magnetic field pointing in and out of the sun.
And sunspots appear in pairs. You see regions of blue preceded by
yellow in this rotation. So there is
blue dot and then to the west of it, a yellow dot in the northern hemisphere and
the polarity is reversed in the southern hemisphere.
And this is consistent throughout a cycle and would then have in the, between
cycles the polarity reverses so that if the blue dots, we don't have two cycles
here, but if the blue dots were ahead of the yellow dots in the north in this
cycle and the yellow ahead of the blue in the north, in the south in this cycle
this would reverse from one cycle to the other.
And this is our hint. That what drives these sunspots is in
fact the magnetic field. It turns out that what makes them cool is
that, remember we talked about, when we talked about the Van Allen radiation
belts, we said that regions of strong magnetic field repel charged particles,
the [INAUDIBLE] gas coming up to the photosphere from the
interior regions is charged. Magnetic field intense magnetic fields,
like in the sunspots choke off this convection.
And for that reason, the interior of a sunspot is cooler than the surrounding
part of the sun. Down, as I said, to 4,000 Kelvin.
And. So we understand sun spots is a magnetic
phenomenon. And the fact that the particles on the in
the atmosphere are charged and therefore interact strongly with the magnetic
field. As are, by the way, all of the particles
in the interior of the sun plays an important role in understanding what the
causes of this phenomenon and some of its consequences.
So we're led to look to the solar magnetic field to try to understand that
and see how that drives the weather. the magnetic field of the sun is not
quite as that of the Earth. It's not well described.
Most times. Like a bar man in the North and the South
pole. Though at any time, there is roughly a
north and a south pole. And it is roughly aligned with the axis
of rotation. The big difference, is that in the
interior of the sun, the roiling convecting fluid that presumably
generates the magnetic field. Is not just conducting but is charged.
It's a plasma. And you recall, when we talked about the
interaction of the solar wind with the Earth's magnetic field.
That I said that because a stream of charge particles both is influenced by a
magnetic field but also creates a magnetic field.
You can imagine that we have a situation where magnetic fields and charged
particles are strongly interacting. And we can.
Model this by imagining that the magnetic field lines and the particles are tied to
each other. So if we have a weak current that we had
in our tube, the current is trapped around the magnetic field.
But if we have a stronger current, it can drag the magnetic field around, which is
what the solar wind did to Earth's magnetic field and is what the rotating.
Plasma in the interior of the Sun does to the magnetic field.
And a crucial ingredient in this is that the rotation period of the Sun is not
uniform like in, in the case of Jupiter. The Sun experiences a fluid body, that
experiences differential rotation. The sun's rotation period is 25 and a bit
days at the equator, but 29 days at the pole so that the equator rotates faster
than the pole. This has the following effect upon the
sun's magnetic field. So here's the rotating sun, it's
undergoing differential rotation. You start with sort of a dipolar magnetic
field, but the magnetic field lines are trapped by the plasma, and because the
equator rotates faster than the poles, the magnetic field lines are stretched.
So that you get first you started with a north south magnetic field, then you
ended up with an east west magnetic field.
And these field lines are stretched and these stretch field lines are kind of
like rubber bands. They contain large amounts of magnetic
energy and this releases by the field sort of reconnecting, popping out of the
sun's surface, creating all these bubbles.
And these might remind you some of the structures we saw when we looked at
Coronagraph remember that if magnetic field lines poke out of the sun then the
charged particles trapped along them will spiral along the magnetic fields and form
loops of charged particles and these are the
prominences that we saw and what happens is that this process of winding up the
magnetic field until its energetically favorable to reconnect and the sun's
field becomes a mess for a while takes about eleven years.
And so every eleven years the sun's polarity in fact reverses.
You start with the nice polar magnetic field that we started with and then you
wind it up. You produce a mess, and
the mass reinstates a polar magnetic field with the opposite North South
polarity after about eleven years and the pattern repeats itself.
This is the cause of the magnetic cycle, and of the creation of the sunspots.
So sunspots are these regions where the magnetic field lines poke out of the sun
and in and their orientation in each hemisphere is due to the direction of
rotation relative to the direction of the magnetic field and
This reverses between cycles because the polarity reverses and we get a big
vertical component where these arches poke out and at the bottom of the two
base points of each of these arches sits a sun spot.
So the most active times in the sun's magnetic history are the times when the
field is in the process of reversing. These reconnection events release the
magnetic energy stored in these magnetic fields.
I told you that you could treat them as rubber bands.
They snap back. They release a lot of energy
the local release of energy in a recollection event.
Can be on the order of ten to the 25 joules.
This leads to rather dramatic solar flares where one region of the Sun
suddenly becomes very bright, and amidst a lot of ultra-violet and x-ray light.
And these beautiful ultra-violet images, you can actually trace the outlines of
those magnetic field arches that I spoke about.
That's not because we can see the magnetic fields, but because we see the
ultraviolet emissions, from the charged particles that are trapped along the
magnetic fields. It's very pre, kind of these particles to
give such an indication of the fields. so solar flares
Are energetic events. There are far more violent events that go
on. they're called prominences or coronal
mass ejections and here is a, a collection of views from many NASA
observatories of a coronal mass ejection that occurred a few months ago.
this is an event in which an amount of hydrogen and charged particles in the
order of the mass of the earth is ejected at high speed as magnetic pressure
basically builds up and ejects these charged particles at high speed.
this one was not directed towards earth. When such events happen and are directed
towards earth this is, these periodic sudden increases in the intensity of the
solar wind that cause beautiful aurora but also
can cause disruption of communications as the electronics and communication
satellites is injured by this stream of charged particles.
And so, this is why we have so many different observatories observing the Sun
at many wave lengths, from many angles in order to be able to understand this
phenomena, if possible predict them and certainly give us the three day warning.
three days is approximately the time it takes one of these coronal mass ejections
to get from the sun to Earth. And getting the three day warning, allows
satellite operators to sort of shutdown operations in an orderly fashion and not
allow damage to happen to their satellites.
And so, we've developed a pretty decent understanding of how the sun works as a
star both in terms of the internal energetic and energy production and, in
terms of the structure of the sun itself. Our job now is going to be try to extend
that to nearby starts and the first question we're going to have to ask is,
how far are these things because anything we see needs to be interpreted
differently depending on how far it is. And so our first chore is going to be to
measure distances to stars. And it's to that, that we're going to
turn next.