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The Corona
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The corona is the outermost layer of the atmosphere of the Sun and extends to millions of
km(even larger than the diameter of the Sun itself).
The corona is best seen during an eclipse
The Corona is NOT spherical, it has “streamers” extending millions of km.
The corona is hot (2 million Kelvin degrees) and very thin (low density), it has an emission line spectrum.
Because of the high temperature (which are again due to magnetic field) particles (protons and
electrons mainly) escapes and are ejected into space forming the SOLAR WIND.
About a million tons (or a billion kg) of particles is ejected every second.
Because of its high temperature the corona is best seen in the Ultraviolet light and X-ray
Coronal holes are thinner and colder than the rest of the corona.
The high temperature of the chromospheres and the corona are due to magnetic activity.
SUNSPOTS AND MAGNETIC FIELD
Sunspots
As the name indicates, sunspot are spots on the surface of the sun.
These spots are dark regions irregular in shape about the size of the Earth.
Sunspots can last a few hours or even months, usually they last about 2 months.
Umbra and Penumbra here are not Shadows, but these regions are dark because they have a
temperature lower than the rest of the surface of the Sun.
These pictures here are taken with a thick dark filter to stop the sunlight from burning the film/detector,
but in reality the umbra is red in color and the penumbra is orange.
The temperature of the Umbra is 4,300Kelvin
The temperature of the Penumbra is 5,000Kelvin
The temperature of the rest of the surface of the Sun is 5,800Kelvin
Sometimes a group of sunspots is so large that it can be observed with the naked eye (either with a dark
filter or at sunset or sunrise when the sun will not burn your eyes).
Observing sunspots has been used long ago to determine the rotation of the Sun
In the Equator the Spin period of the sun surface is 25 days
Next to the pole the spin period of the sun surface is 35 days
 This is called differential rotation – the sun does not rotate like a solid sphere.
Rotates much faster at the equator
Sunspot Cycle
If you count the average number of sunspots on the Sun surface, it is not constant:
The number of Sunspots varies periodically with a period of 11 years. Every 11 years there is a maximum
and every 11 years there is a minimum.
The Location of the Sunspot also varies with time and it has the same period:
Just after sunspot minimum, the sunspots start appearing at a latitude of 30 degrees on both side of the
equator.
With time the sunspots are found closer to the equator. Before minimum the sunspots are close to the
equator. At minimum there basically are no sunspots.
Sunspots and Magnetic Field
When sunlight is decomposed with a prism one detects dark spectral lines in the rainbow colors
These spectral lines are due to the electrons of the atoms absorbing the photons (light) of a particular
wavelength.
When one look at the spectral lines in the vicinity of a sunspot, the spectral lines “split”:
The Splitting of the spectral lines is called the Zeeman effect and is due to the presence of a strong
magnetic field  sunspots are associated with strong magnetic fields
Because the solar atmosphere is so hot, many of its atoms are ionized (ion + electron) and these charged
particles are affected by magnetic fields (an ionized gas is called a PLASMA).
The faster charged particles move, the more they are deflected by the magnetic field.
The hotter the plasma, the faster its particles move.
 The hotter plasma is deflected by the magnetic field
 In the region of large magnetic field on the sun, the hotter plasma is deflected such that only the
cooler gas (cooler plasma) remains there  in the region of large magnetic field the gas is
colder  this explain why the sunspots are dark, because they are colder.
Magnetogram of the Sun
A Magnetogram is a color coded map of the magnetic field polarity, where one color indicates North
while the other color indicate South.
That way one can find out about the polarity (north or south) of the magnetic field inside sunspots.
Electrons will move around magnetic field lines (say North-South) in circles in a given direction.
The same Electron will move around magnetic field lines of opposite polarity (say South-North) in circles
in the opposite direction. Based on the rotational motion of the charged particles one can find the
polarity (north or south) of the magnetic field lines and therefore of the Sunspots.
Example of everyday life: approach a magnet next to an old TV or Computer screen (not the flat ones,
but the large ones with the cathodic tube).
The magnetic polarity (north-south) is inverted in the two hemispheres.
Magnetic North Polarity is in White
Magnetic South Polarity is in Black
This is more apparent in a magnetogram with a few sunspots:
In the Northern Hemisphere the magnetic south is on the right and the magnetic north in on the left
In the Southern Hemisphere the magnetic south in on the left and the magnetic north is on the right
The magnetic polarity of the sunspots is not randomly oriented but it is oriented in the direction of
rotation (East-West and West-East).
Draw magnetic field lines.
The polarity that is observed reversed itself every 11 years, such that after a full sunspot cycle the
polarity that was observed is reversed.
After another 11 years the polarity is reversed again and is therefore back to what it was 22 years
before.
 The cycle of the sunspot and magnetic field is actually 22 years, not just 11 years.
The configuration of the magnetic polarity and magnetic field lines is as follows:
The magnetic field lines over the surface where the sunspots are located are actual magnetic field lines
from deeper emerging to the surface
The Solar Dynamo
Due to the differential rotation the “normal” magnetic (dipole) inside the Sun is twisted around
Next, due to convection of gas (hot gas going upwards inside the outer envelope of the Sun), the
magnetic field lines are pushed to the surface where they emerge.
These form the sunspots seen in the photosphere of the Sun.
Make a diagram.
Due to the rotation the magnetic field lines are more twisted at a latitude of 30 degrees (maximum
sunspots).
The twisted magnetic field produces magnetic pressure which pushes the lines towards the equator
where the lines from the two hemispheres cancel each other (minimum of sunspots)
The solar rotation is like a rigid sphere inside in the volume where radiation is dominant.
In the outer volume where the convection is dominant the rotation is differential.
It is believed that the magnetic field originates in the region where the radiative layer and convective
layer meet.
The sunspot activity and the magnetic field generation (dynamo) are not fully understood, for example it
is not known why no sunspots were observed between 1645 and 1715 (little ice age) or why there was
an increased in sunspot activity around 1200AC when Earth was slightly warmer than today.
Magnetic Energy
Most of the phenomena happening in the Chromosphere and Corona are due to magnetic field, not just
the sunspots.
RECONNECTION OF MAGNETIC FIELD LINES AND RELEASE OF MAGNETIC ENERGY
The hot gas in the solar atmosphere and inside the Sun is ionized and moves following the magnetic field
lines. The ionized gas is called a plasma. When the plasma is dense it actually can move the magnetic
field lines – it is like magnetic field and plasma are attached together.
When there is convection pushing material upwards, magnetic field lines are also pushed upwards.
Some of these magnetic tubes surface and expand into the solar atmosphere to create Coronal Loops:
The material within the “arches” reaches temperatures of 2 million degrees Kelvin!
When two or more coronal magnetic loops come close to each other, the magnetic field lines interact
with each other to rearrange in a new configuration.
During this rearrangement magnetic field lines reconnect in a different way and release a tremendous
amount of energy (this is called MAGNETIC RECONNECTION).
This magnetic energy released into the solar atmosphere heats up the chromospheres and the corona.
Since the bases of the magnetic field lines are the sunspots, the most active parts of the chromosphere
and corona are above the sunspots.
The gas in the corona is heated so much (by the magnetic field reconnection) that it emits light in the
ultraviolet, extreme ultraviolet and X-ray band  back to what we learned….
Spicules and coronal heating are due to magnetic field and happen all the time.
Sunspots are also due to magnetic field but they form only when the Sun is (magnetically) “active”.
In addition there are more processes on different scales that are due to the magnetic activity of the Sun.
See next figure.
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This is a “red” picture of the chromospheres and corona at 6,560Angstroems (red light) taken
during maximal activity (sunspot maximum).
- The bright areas are called Plages and are hot regions in the chromospheres forming above
places where new sunspots are about to appear. They are due to high magnetic pressure and
high gas pressure under the surface increasing the temperature there.
- The dark “lines” are called Filaments and are cooler and denser part of the chromospheres that
extend outwards with the magnetic field lines. They are also called prominences.
Here is a ultraviolet image of the Sun showing slightly different features.
Prominence can also break free and burst into space.
In complex sunspots groups, sometimes violent eruptions quickly develop (in minutes!), with
temperature of millions of degrees and ejecting particles (electrons and nuclei) and radiation into space.
These eruptions are called Solar Flares. Solar flares can also produce prominences.
The most powerful solar activity are Coronal Mass Ejection, when the large-scale magnetic field of the
sun rapidly changes on a scale of hours only!
Solar flares and coronal mass ejection reaches the Earth within a few days, and the rain of high energy
particles (charged particles) and radiation produces aurora, can damage artificial satellites, and are
harmful to astronauts.
In terms of size and magnetic energy (from small to large) we have
- Spicules
- Coronal Loops
- Prominences
- Solar Flares
- Coronal Mass Ejections