- Physics 335: Astronomy
- Instructor : M. Azad Islam
- CHAPTER 7: THE SUN - OUR
- WEB SITE: NASA Solar
- 7-1 The Solar Atmosphere
- Below the surface
- The photosphere
- The chromosphere
- The corona
- 7-2 Solar Activity
- The magnetic cycle
- Prominences and flares
- Coronal activity
- HIGHLIGHTS OF THE CHAPTER
- This chapter talks mostly about the structure and phenomena of
the observable layers of the sun.
- Atmosphere of the Sun consists of three layers, photosphere,
chromosphere and corona.
- Photosphere: Visible surface of the sun is a thin layer 500 km
of low-density gas at 6000 K from which visible photons most
easily escape. This thin layer consists of gas currents produced
from below its surface.
- Chromosphere: Easily visible during total solar eclipse. It is
a thin hot later of gas just above the photosphere. Its color is
brilliantly pink made by emission lines from gas atoms. There are
large jets called spicules extending upto the corona.
- Corona: It is the outer layer of Sun's atmosphere. It is made
up of very low density hot gas extending at least 30 times the
radius of the sun measured from the center. Its high temperature
is about 2 million degrees.
- The outer parts of the Corona merge with solar wind, a breeze
of low density ionized gas streaming away from the Sun. The earth
is frequently hit by the Solar wind.
- Sunspots that are relatively darker (because of cooler
temperature) circular patches of areas on the sun. Sunspots are
about twice the size of the earth and contain 1000 times stronger
magnetic field that its average value. Solar activity are related
to the sunspots that may pop up at any time. Solar prominences and
flares are common events as part of the solar activity. These
produce auroras and communication blackouts on earth.
- SOLAR ATMOSPHERE:
- Sun is 109 times larger in diameter than the earth. It has
300,000 times more mass than the earth. Observation of the sun is
restricted to its atmosphere only.
- The surface phenomena are directly related to what goes on in
the depth of the sun. The interior of the sun is undergoing the
Nuclear Fusion reactions that produce huge amount of energy
flowing out to the surface and beyond. Evidences of energy flow
from the interior are,
- the presence of hot and cool regions, gas motions, magnetic
fields and solar flares.
- It is the visible surface of the sun. Sun is gaseous from its
outward atmosphere right down to its center. Most of the sunlight
we receive comes from the thin layer (500 km thick) of solar gas
at a temperature of about 6000 K, called Photosphere.
- Below Photosphere the solar gas is very dense so that
radiations coming from it cannot escape outward. Such light from
under the Photosphere cannot be detected here on earth.
- The gas on top of the Photosphere is much less dense and is
unable to radiate much light.
- The thin layer of Photosphere is just dense enough (about 3000
times denser than air) to emit plenty of light. Density of gas
comparable to the air can be found below Photosphere, at a
distance of 10% of the way towards the center of the sun.
- The higher density gases in the deeper layers of Photosphere
produce continuous spectrum like liquids and solids. The lower
density gases in the upper layer of the Photosphere produce dark
Absorption lines in the background of the continuous
- Photosphere is not uniformly homogeneous, contains cell like
structures called Granulation, about the size of Texas having
lifetime of 10-20 minutes. Such a Granule is a few hundred degrees
hotter in the middle than its edge. The center of Granule rises
and its edge sinks at a speed of 1 km/sec, providing a Convection
of gas current in its structure like the movement of warm masses
of air in the earth's atmosphere and of water in the earth's
- The layer (about 10,000 km thick) on top of the Photosphere is
called CHROMOSPHERE. It is 1000 times fainter than the
Photosphere. Hence, it can be observed only during the total Solar
Eclipse. It is a sheer luck that the moon can exactly cover up the
sun upto the Photosphere during total eclipse for us to observe it
for a few seconds. When viewed, it is a thin line of pink just
above the Photosphere. The pink spectrum tells about the
conditions in the Chromosphere. It is an emission spectrum of low
density gas. The pink color results from the hue of red, blue and
violet mixture of chiefly Balmer lines emitted by hydrogen, plus
emission of other elements.
- NOTE THAT THE TEMPERATURE OF THE CHROMOSPHERE IS AT 10,000 K
TO 50,000 K, much higher than Photosphere.
- Gases in the Chromosphere is mostly transparent to the visible
light. Even so, the hydrogen atoms in it are very good absorbers
of Balmer lines. A tiny fraction of Balmer lines that can escape
from the upper atmosphere of Chromosphere, is detected as
Filtergrams recorded through the light filters at specific
wavelengths. The Filtergrams reveal flame like structure (
100-1000 km diameter) called Spicules, extending upto 10,000 km
above the Photosphere and lasting for 5-15 minutes. These Spicules
appear to be cooler region of the Chromosphere at 10,000 K, seen
as flames of burning prairie here on earth. Near center of the
solar disk, these Spicules spring up around the edge of much large
Granule structure of the Photosphere, called Supergranules.
- CORONA OR THE CROWN: Above the Chromosphere is the Corona, the
name originated from the Greek word meaning crown. During total
solar eclipse, the corona shines as milky glow (fainter than full
moon) on top of the Chromosphere. Corona is observed to be
extended upto 12 solar radii. Gas in the corona is extremely hot
at 1 million degrees K. Temperature in the Corona rises with
altitude. Density of Corona is very low (only 1-10 atoms per cm
cube) or else it will produce black body radiation. The extreme
high temperature of the Chromosphere and Corona is not well
understood by laws of physics. It is one of the mysteries to be
revealed in the nature. The outer Corona is so hot, that the sun
cannot hold it in place. The high velocity particles (mostly
protons and electrons) in it, travel out as Solar Wind. The Solar
wind at 300-800 km/sec, gusting at 1000 km/sec, travels past the
earth disrupting the communications and at times shut down the
- SOLAR ACTIVITY: Changes in sun's features over time due to
magnetic fields and powerful flow of energy
- Sunspots: Dark areas on the photosphere. Rotation of the sun
moves the sunspots. Sunspots consists of dark center umbra and
outer border penumbra. Its size is 40 sec of arc, about twice the
diameter of the earth. Usually appear in pairs or groups. Large
group may have 100 sunspots lasting about two months when in its
peak level of activity. it looks dark, because it is cooler than
rest of the photosphere. Actual color would e brilliantly orange
at temperature of 4240 K. Large difference in brightness is due to
fourth power dependence on temperature.
- Zeeman in the solar spectrum provides quite a bit of
information about the sunspot. Sunspot being cooler, do not emit
much of hydrogen Balmer lines. Instead, heavier elements requiring
less energy produce solar spectrum in sunspot. Zeeman effect
produce splitting of spectral lines into three or more lines due
to the presence of large magnetic field. From studies of splitted
spectral lines, astronomers can infer about the strength of the
magnetic field. Magnetic in sunspot is 1000 times stronger than
other regions of the photosphere. The presence of strong magnetic
field blocks any gas motion below the sunspot.
- Sunspot activity goes in 11 year cycle. The sunspots appear
near plus/minus 35 degrees latitude in the beginning of a sunspot
cycle and appear near the equator in the later period of the
cycle. The structure of its first appearance (between plus/minus
35 to 5 degrees) over the 11 year cycle when plotted in a graph,
gives the resemblance of butterfly structure. Hence, it is named
Maunder Butterfly Diagrams. It also goes through years of low
activity called Maunder minimum.
- Magnetic Cycle: Rotation of the sun near the equator is faster
than near the pole. The period of rotation is 25 days at the
equator and 27.8 days near mid latitudes. The period of rotation
also decreases as we move into the interior of the solar surface.
Such variations in time period at the surface and in the interior
of the sun, is called Differential Rotation. This differential
rotation results in changes in magnetic fields over and above the
surface. Magnetic fields are produced by the rotation of the sun
on its axis and by the flow of highly ionized (plasma) gas
(carrying huge amount of energy) from the interior to the solar
surface. This mechanism of producing magnetic field is referred to
as Dynamo Effect. The earth is believed to produce its magnetic
field in the same manner.
- Evidence from the Zeeman Effect measurements show that sun's
magnetic fields also go through a cycle of time period. Sunspots
occur in pair so that the magnetic field may have north and south
poles such as a bar magnet. A pair of sunspot would appear as a
leading and a trailing dots. In the northern part, the leading one
may become north pole and the trailing one be a south pole. This
polarity may be reversed in the south of the sun's equator.
Overall, the polarity of the sunspots reverse from cycle to next
cycle of the sun spot.
- Babcock model attempts to explain sun's magnetic cycle. This
model considers progressive variation of the tangling of the
magnetic fields to be the cause and effect reversing of the
polarity. In this model, the highly mobile electrons are trapped
(frozen) by magnetic fields. As the frozen electron gas moves, the
magnetic fields move tangling with each other. When the fields are
very strong, they create pair or groups of sunspots. The magnetic
cycle goes through a period of 22 years, twice the sunspot's
- Prominences and Flares: Large huge arch-shaped red color
protrusions around the solar disk visible during solar eclipse.
Red color is same as color of Chromosphere and comes from hydrogen
lines. These burst out around the sunspot in reaction to the
blocked flow of energy under sunspot. Prominences shoot out as
high as 500,000 km (50 times higher than spicules) in a few
- Solar flare is much smaller scale phenomenon than prominences.
It rises for a few minutes and decays in an hour or less. It emits
vast amount of x-rays through visible radiations, along with
streams of high energy electrons and protons. it releases about 10
billion megatons of TNT (a hydrogen bomb releases 100 megatons of
- Coronal Activity: The structure of the corona is small and
flattened during sunspot minimum and it is large and blazing at
sunspot maximum. Corona is not quite uniform halo. It is actually
made of streamers controlled by the solar magnetic fields. Long
thin streamers allow charged particle to escape at 2 to 3 solar
radii. In some regions of the corona, the magnetic fields do not
bend into a loop. This allows large chunk of charged particvles to
escape producing cooler regions of corona, called Coronal Holes
seen in x-ray images of the solar atmosphere. These emission of
charged particles is the cause of frequent solar winds reaching
earth's atmosphere, producing Auroras and disrupting
- Coronal holes: In some regions of solar surface, magetic
fields do not loop back, the particles freely leave the solar
surface. As a result, the cooler low density regions of corona
appear in x-ray images. These are called CORONA HOLES.
- Granulation: The surface of the photosphere is not uniform.
The surface of the photosphere is covered with a pattern
dark-edged bright cells called GRANULATION.
- Convection: When hot liquid or gas rises up at the expense of
cool liquid or gas falling down, the process is called
- Filtergram: Photographs taken by passing light through filters
that admit only one wavelength of a strong absorption line.
- Spicule: These are flame like irregular structures present on
the surface of the photosphere, 100 to 1000 km wide and extending
upto 10,000 km above the surface.
- Supergranule: Like granule, supergranule is a dark-edged cell
pattern much larger in area, about twice the diameter of the earth
and caused by heat from larger convection cells deeper below the
- Solar wind: In the outskirt of Corona region, the temperature
is so high (2,000,000 K) that charged particles such as electrons
and protons easily leave the sun in a continuous breeze.
- Helioseismology: Astronomers study the interior of the sun, by
detecting and analyzing the modes of vibrations on the surface of
sun, by a method called Helioseismology.
- Sunspot: The darker regions appearing in pairs or groups in
the sun's surface about twice the diameter of the earth moving
from higher lattitudes to lower lattitude and vanishing near the
equator of the sun in about a week or so.
- Zeeman effect: Splitting of single spectral line into two or
more components when atoms are under the influence of magnetic
- Maunder butterfly diagram: When the lattitudes wher the
sunspots begin are plotted as a function time over many years, one
finds a diagram that looks like a butterfly.
- Maunder minimum: The patterns in the diagram show periodic
activities of the sunspot that goes through a minimum. That is the
butterfly pattern is repeated.
- Differential rotation: The variation in the rotaion of the sun
with 25 days at the equator and longer near the pole.
- Dynamo effect: The dense ionized gas of the sun's interior is
a good conductor of electricity. When the sun rotates, the dense
ionized gas rotates, the outward flow of heat energy by convection
produces magnetic field by an effect similar to Dynamo effect well
known in physics. Such effect also produces magnetic field of the
- The atmosphere of the sun consists of three layers. They are
photosphere, chromosphere and corona. The innermost layer of the
three, is called the photosphere, meaning the visible surface of
the sun. It is a thin layer of low density gas belt that emits a
large amount of visible photons or light. The surface of the
photosphere is marked by granulation, a pattern produced by gas
currents rising from below the photosphere layer.
- The layer above the photosphere is called the chromosphere.
This layer is observed during the total solar eclipse when it
flashes into view for a brief period of time (a few seconds). It
is a thin hot layer of gas just above the photosphere, and its
brilliant pink color is caused by the emission lines in its
spectrum. Filtergrams of chromosphere show large jets called
spicules extending up into the corona.
- Corona is the outermost layer of sun's atmosphere. It is
composed of very low density and very hot gas extending at least
30 solar radii. It has a high temperature of 2 million degrees
Kelvin that is maintained by the magnetic field and the rotation
of the sun.The solar wind starts out from the outer part of the
corona. Solar wind is a low density ionized gas (mostly protons
and electrons) streaming away from the sun. From time to time, the
earth together with other planets in the solar system, is bathed
with the solar wind. Solar astronomers study the motion, density
and the temperature of the gases below the solar surface. This is
done by analyzing the oscillations of the solar surface.
Helioseismology is such a study. The subject requires large amount
of data and extensive computer analysis.
- The sunspots are the most convincing evidence of solar
activity. A sunspot being cooler than the rest of the photosphere
is generallty darker. The average sunspot has a size equal to
twice the size of the earth. Its magnetic field strength is more
than 1000 times stronger than the sun's average magnetic field.
The sunspots are caused by presence of highly intense magnetic
field that prevents the normal convectional flow of hot gas from
underneath the photosphere. This allows cooling of the sunspot
The average number of sunspots vary over a period of about 11
years. This phenomenon seems to be tied up with the 22 year period of
the magnetic cycle.
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- Updated 1/14/2014