Transcript The Active Sun

Module 21:
Solar Activity
& its Effects on Earth
Activity 1:
The Active Sun
Summary:
In this Activity, you will learn about
• the magnetic field of the quiet Sun
• the Zeeman Effect
• sunspots and bipolar pairs in the active Sun
• solar flares and solar cosmic rays
• quiescent and loop prominences
• the different rotation of parts of the Sun
• cycles of activity in the Sun
Click on the picture below to see part of a NASA
movie showing many of the solar features to be
discussed in this Activity.
If you have a sound card on your computer, you will
be able to hear a commentary.
Click on image to view movie
The Sun’s Magnetic Field
Just about all the objects in the sky seem to have a
magnetic field. The Earth certainly does: it is what
makes compasses point to the North.
The Sun is no exception: it has a healthy magnetic field
of its own.
If you let a magnet or compass
loose near the Sun,
it would tend to move on a
curved path like this
The Sun’s Magnetic Field
Solar astronomers can measure the Sun’s magnetic
field by using the Zeeman Effect.
Once again, astronomy becomes an amazing mix of
studying the absolutely huge - the Sun - and atoms the absolutely tiny!
Magnetic Field
When an atom is placed in a magnetic field,
the atomic energy levels each separate into
three or more sublevels, and the spectral
lines split correspondingly.
We see the result in
the light we receive
on Earth
Atoms on the surface
of the Sun are affected by
the Sun’s magnetic field
Sun
Splitting lines
The simple absorption lines in the spectrum of a gas
with no external magnetic field ...
… each may split into several finer lines.
The stronger the magnetic field, the more pronounced is
the splitting of the lines.
So by measuring the line splitting in the spectrum of a
gas, we can measure the magnetic field it is experiencing.
0.01
The Quiet Sun’s
Magnetic Field
0.009
0.008
Magnetic fields are measured in a unit
called Tesla (T), after Nikola Tesla who
did a lot of very important work on the
subject.
The average magnetic field at the
Earth’s surface is about 0.00003 T.
However the field at the surface of the
quiet sun is about 0.01 T.
That’s about three hundred times as
strong!
0.007
By the way, “quiet” means few
(if any) sunspots or flares.
0.001
0.006
0.005
0.004
0.003
0.002
0
Earth
Sun
The Sun’s magnetic field
Magnetic
fields are
usually
labelled B
The magnetic field of the
Sun varies from moment
to moment, but this
diagram gives a rough
idea of its shape.
The field lines show the
direction of the force that
would be exerted on a
compass needle.
Which is North, and which
is South?
You’ll find out later that it’s
not so simple!
rotation axis
Inside the Sun
What happens to the magnetic field lines when
they meet the Sun and go inside?
The lines only appear to penetrate about a
tenth of the way in.
There are very violent currents of charged
particles in the convective zone, just
under the photosphere and
chromosphere. Magnetic fields are set
up by electrical currents, and it is
believed that these currents are the origin
of the magnetic field.
As the currents move about, they
also mangle the magnetic field lines
rather badly!
Lines only go in to
a depth of about 10%
The Active Sun
On the surface of the Sun we see short-term, localised
effects of the incredible turmoil in the convective zone.
Active solar regions are parts of the surface of the
Sun containing
• sunspots
• flares
• prominences
and so on.
Active solar region
with sunspots
Sunspots
Here’s a photo
taken in
September 1998,
showing a pair of
sunspots in an
active region
“south” of the
“equator”.
You can also see
some activity
elsewhere.
A closer view of sunspots
Umbra:
the dark centre of
a sunspot
Penumbra:
the paler “hairy”
region outside
Inside a sunspot
Quiet region:
high temperature
low magnetic field
People have been studying the surface of
the Sun for thousands of years, and
sunspots have featured in their research.
Sunspots occur as relatively dark, cool
patches, in groups.
By cool, we mean only about 4000°K. The
rest of the photosphere is at about 5780°K.
The magnetic field near a sunspot is about
0.4 T, about 40 times that of the quiet Sun
(0.01T) and thousands of times stronger
than that of the Earth (0.00003 T).
Umbra:
low temperature
high magnetic field
Penumbra:
a bit warmer, so
not as dark
Cycles in sunspots
Over thousands of years the Sun has shown an elevenyear cycle in sunspot activity.
About 5.5 years
sunspot
minima
sunspot
maxima
About 5.5 years
Groups of sunspots
Sunspots often occur in groups.
In particular, they turn up in bipolar pairs like the two
poles of a magnet.
north pole
south pole
This particular image of
a sunspot region is taken
by an instrument which
registers the North
(called positive polarity)
and South (called
negative polarity) as
different shades.
dark  positive polarity
light  negative polarity
The life of sunspots
The groups of sunspots can contain up to 100 pairs, and
can last for months.
So what causes sunspots?
Photosphere
and chromosphere:
the Sun’s skin
As you saw earlier in this Activity, the
magnetic field lines outside the Sun get
chewed up by the time they are one-tenth
of the way inside. This is partly because
the convective layer is comprised of rising
and falling columns of hot gas.
But there is another reason …
Convection layer:
very deep, and
very turbulent
On a calm day
We’ll start with the Sun
at its quietest, when the
magnetic field lines are
nice and orderly.
Note that we’ve chosen
to draw the magnetic
field lines moving
downwards.
rotation axis
The Sun rotates, and it rotates faster near its equator.
This begins to distort the lines …
35-day rotation
B
Lines are pulled
around a bit faster
at the equator
25-day rotation
They get further and further ahead of the lines near the
poles ...
B
This part of one
B line is now
behind the Sun
Inner lines, inner peace
Eventually the lines
reconnect to form closed
loops within the Sun.
The field strength near the
surface drops almost to
zero (at least, compared to
its average value).
B
Trouble, trouble
The lines are now buried
in the upper part of the
convective layer.
And the convective layer
is full of raging currents of
gas moving to and from
the radiative region.
These currents drag the
field lines with them,
causing them to develop
kinks ...
B
And voila - sunspots!
Where the loops of field line emerge from the
surface of the Sun, you’ll see pairs of sunspots
There’ll be one where the field
comes out, and one where it
goes into the Sun again..
Second sunspot
where field enters
the Sun again
Sunspot as field
pokes out of the
Sun
B
The magnetic field loops expand, producing the B field of the
quiet sun again after 11 years.
B
… but with the
magnetic field
lines reversed.
Twenty-two years
B-spots
Although sunspot maxima occur roughly every 11 years, the entire
magnetic cycle of the Sun takes 22 years.
During this time the Sun endures two attacks of these B-spots.
0
Sunspot
activity
Lines
start
peak
to at
become
tangledLines
and go
go inside
back
Quiet sun:
The activity
outsidecan
orderly lines
be irregular,
and varies from
5.5
11
14.5
cycle to cycle
Same thing,
but B is
reversed
22 years
Modelling sunspots
Sunspots are modelled as “floating islands of
electromagnetic storms”.
Magnetic forces
push the gas
currents down.
This draws kinetic
energy away from
the surface, and so
lowers the
temperature in the
sunspot.
B
chromosphere
photosphere
convective
zone
Solar
Flares
are
another
kind of
solar
activity
we see
from
Earth.
So what are solar flares?
Solar flares are brief, violent
discharges of energy
(measured at up to 1030 J ) in
active regions of the Sun.
Electromagnetic
radiation
Solar flares emit electromagnetic radiation at all
wavelengths, and also emit solar cosmic rays, which
are charged particles accelerated by solar flares.
This 1989 H image of the
Sun shows a solar flare
which extended 300,000 km
above the photosphere.
A copious source of very
high-energy particles, the
flare lasted over an hour and
its cosmic rays would have
been fatal to any astronaut
on the Moon’s surface.
Flares this large occur only a
few times each decade, at
unpredictable times.
(H mans that the strength of
the alpha line of the Balmer
series was detected to make
the image.)
How solar flares happen
Start with a bipolar sunspot pair at the top of the convective zone: that
is, two sunspots where one is like the North pole of a magnet and the
other is like the South pole.
The magnetic field above a bipolar sunspot pair is believed to be
shaped something like this:
corona
B
chromosphere
photosphere
S
N
convective
zone
Crossed lines
Instability in the convective zone causes such havoc among the field
lines that they can even try to cross each other.
This however is not possible: you couldn’t have a compass needle
going in two directions at once!
So what you get instead is magnetic reconnection between adjacent
field lines.
corona
B
magnetic
reconnection
chromosphere
photosphere
S
N
convective
zone
Shouting about it
Now, changes in magnetic fields cause currents, so all kinds of forces
are at work when the reconnection takes place.
Particles are accelerated by these forces, with two results: their
temperature rises and, because they are accelerating, they emit
radiation.
charged particles
accelerated
B
heated plasma emits
photons of all
wavelengths
magnetic
reconnection
S
N
Firework time
A burst of plasma is flung into
space from the surface of the
Sun above a pair of sunspots.
This burst of plasma is what
we call a solar flare.
Although the sunspots are
cooler than the surrounding
gas, the flare itself is a great
deal hotter.
B
S
N
X-ray image of the Sun
This photo was composed
by taking measurements of
the X-ray emissions from
the surface of the Sun.
As solar flares and other
activities cause bursts of
radiation, including X-rays,
detecting those rays is a
handy way of checking the
sun for activity.
flare
Bipolar sunspot pair
The largest recorded solar flare to
date occurred on 4 November 2003
and was captured by X-ray detectors
onboard the SOHO satellite.
The eruption was so bright that it
actually saturated the X-ray
detector!
The flare was associated with a large group
of sunspots called 10486, which were the
cause of intense solar activity for up to three
weeks prior to November 4, including
massive coronal mass ejections that send
material at speeds of over 2000 km/s
towards the Earth.
For animations of the event, visit the SOHO website at:
http://sohowww.nascom.nasa.gov/hotshots/2003_10_28/
Prominences
Occasionally an
absolute monster of a
storm occurs, and a
solar prominence is
the result.
prominence
Types of prominences
include
• quiescent
prominences,
lasting for weeks,
and
• loop prominences,
associated with
solar flares and
lasting only an
hour or so.
This image was made by Skylab in
1973, and shows one of the largest
prominences ever recorded.
The Earth would be
about this big
Remember that the Sun is more than 100
times as wide as the Earth.
So many, many Earths would have fitted
into - or been burned to a crisp by - this
prominence.
Loop Prominences
It is thought that the plasma
(extremely hot gas, a soup of
ions and electrons) in the
chromosphere is formed into a
loop or series of loops by the
magnetic field over a sunspot
pair.
B
corona
chromosphere
photosphere
S
N
convective
zone
The Sun’s Outer layers
We can track the rotation
of the surface of the Sun
by observing its sunspots.
This is the same as the way
we know that the Moon
keeps the same face to
Earth all the time (the
surface features don’t
appear to move), and that
Jupiter rotates on its axis
(we watch the big red spot).
The inner layers
While the outer layers of the Sun rotate once in every 25
days at the equator, and once every 35 days near the
“poles”, the Sun’s inner layers probably rotate like a rigid
object.
about a 27-day period
still about a 27-day period
In Conclusion:
The activity on the surface of the Sun is mostly caused by
the following chain of events and circumstances:
• the Sun consists mostly of positively-charged ions
(largely hydrogen, H+) and negatively-charged electrons;
• the convective layer constantly sends currents of these
particles to and from the surface;
• currents create magnetic fields;
• unlike the Earth (which has a pretty solid crust), the Sun is fluid and
the equator, the poles and the innards rotate at different speeds;
• this tangles up the magnetic field just under the surface of the Sun;
• the tangles cause currents in the charged particles in the upper
layers of the Sun;
• We see these currents and explosions as sunspots, flares and
prominences, with an 11-year cycle in which the Sun swaps its North
and South magnetic poles as well.
Image Credits
Sun's Magnetic Field, Prominences, and Solar Wind
http://bang.lanl.gov/solarsys/raw/sun/sunc.avi
Sun Prominence
http://bang.lanl.gov/solarsys/raw/sun/sun.jpg
Sun - Calcium K spectral line
http://www.solar.ifa.hawaii.edu/KLine/Today/latest.jpg
Sun Spots
http://bang.lanl.gov/solarsys/raw/sun/sunspot.jpg
Solar Magnetic Fields
http://bang.lanl.gov/solarsys/raw/sun/sun2.jpg
Large solar flare and coronal mass ejection shoots tons of particles into space:
http://sohowww.nascom.nasa.gov/explore/litho/SOHOport12.html
X-Ray Sun
http://umbra.nascom.nasa.gov/images/latest_sxt.gif
The Sun has storms, hotter and cooler areas, and extending prominences
http://sohowww.nascom.nasa.gov/explore/litho/SOHOport06.html
Image Credits
Magnetic loops & prominences are often seen on the Sun
http://sohowww.nascom.nasa.gov/explore/litho/SOHOport10.html
EIT closeup of massive flare, November, 2003 - SOHO/EIT (ESA & NASA)
http://sohowww.nascom.nasa.gov/hotshots/2003_11_04/eit195cw.gif
Sun’s large CME, October, 2003 - SOHO/EIT (ESA & NASA)
http://spaceflightnow.com/news/n0310/28flare/sohoc3.jpg
Now return to the Module 21 home page, and read
more about the magnetic field and active regions of
the Sun in the Textbook Readings.
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