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INTERNATIONAL YEAR OF ASTRONOMY
Article
THE TURBULENT SUN
Pankaj Agarwal
JIL Information Technology Ltd., Sahibabad Industrial Area, Sahibabad-201010 (U.P.)
(Photographs on inside back cover)
ABSTRACT
Sun is not quiet. The region on the solar atmosphere, where excess of magnetic flux is occurring, is referred as active
regions. Sunspots in the photosphere; prominences, plages and flares in the chromosphere tend to occur together
and collectively referred as active regions. All of these active regions may grow, change their shapes and disappear
in hours or days or weeks or months. This article highlights some features of the active regions of solar atmosphere.
1. Introduction
The sun is an average middle age star. The sun is very
important for all living beings. Life would not have sustained
on Earth without the existence of sun. Without its heat all
oceans would be frozen and without its light all plants
would die. Being a nearest star, sun is always an object
of study and can be used as a model to study the other
stars. Sun is not quiet; it has lots of turbulence in its
atmosphere. Solar atmosphere is made up of three main
layers, namely the photosphere, the chromosphere and
the corona.
Photosphere
The only visible layer of the sun is called Photosphere,
which looks like sphere of light (Fig. 1a). This is that layer
of the sun from where the light energy escapes into the
space. The photosphere is about 400 km deep.
Temperature in the photosphere is about 5600 K, and the
gas densities are quite small. Hence, the gaseous atoms
are unable to block the radiative energy flow. As the hot
atoms cool, they release their excess energy as radiation,
which streams out unrestrictedly into the space.
Chromosphere
The next layer to photosphere is the Chromosphere, which
looks as a coloured sphere (Fig. 1a). It is only momentarily
visible during a total solar eclipse as a reddish-pink strip,
which encircles the silhouette of the moon, when the moon
blocks the light from the Photosphere. It is very thick layer,
roughly about 2500 km, and its temperature rises to
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30,000K (Fig. 1b). Visually, the chromosphere is more
transparent than the photosphere.
Corona
Most mysterious layer of the sun is its upper part of the
atmosphere, which is known as Corona. Similar to the
chromosphere, it is only visible during a total solar eclipse
as a luminous white halo surrounding the glowing disk of
the sun (Fig. 1a). The corona glows during a total solar
eclipse because light from the photosphere bounces off
free electrons in the coronal plasma. The shape of the
corona is synchronized with the solar activity cycle; its
shape is just like a serrated ring around the sun, during
the peak of the solar cycle, whereas it is like delicate
plumes, during the dip of the solar cycle.
The temperature of this layer rises up to 1.7 x 106 K that
makes the corona the second hottest part of the sun, after
the central core (Fig. 1b). Solar corona is extending millions
of kilometers into space. An interesting feature of the corona
is the fact that it is much hotter than the photosphere.
The corona is much less dense than the photosphere and
hence produces less light. The possible mechanism for
the coronal heating is the induction by the sun’s magnetic
field and sonic pressure waves from below.
2. Sources of solar activity
The core of the sun is the only part of the sun that generates
energy. Though the solar energy from the core is radiated
all around uniformly, but from certain areas, such as
sunspots surrounded by plages and prominences, energy
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is not released uniformly. Changes of different descriptions
follow each other to make an endless series, are referred
as solar activates. Steady state and spherical symmetry
of the sun are continuously disturbed because of the solar
magnetic field and nonuniform solar rotation.
The sun is a gigantic ball of superheated gas; therefore it
does not rotate like a rigid body. The sun rotates faster at
equator than the poles, similarly its interior rotates faster
than the surface. Different solar rotations twist the magnetic
field lines, running from pole to pole and frozen into the
surface, i.e. the magnetic field lines and plasma behave
as if they are frozen together. Due to the shearing action
on the field, the density of magnetic field lines increase,
which is supposed to be the reason for solar activity.
3. Active regions
The region on the solar atmosphere, where excess of
magnetic flux is occurring, is referred as active regions.
Heating of local atmosphere exist due to this magnetic
flux. Sunspots in the photosphere; prominences, plages
and flares in the chromosphere tend to occur together and
collectively referred as active regions. In the formation of
these active regions magnetic disturbance plays an
important role. All of these active regions may grow, change
their shapes and disappear in hours or days or weeks or
months. It has been assumed that these all phenomena
are due to the sunspots.
Sunspots
Earlier, sunspots were considered as planetary
phenomena. In 1610, Galileo Galilei, Johann Fabricius and
Christopher Scheiner independently observed dark spots
on the face of the sun and explained sunspots as clouds
floating in the solar atmosphere. Sunspots appear as dark
blemishes on the solar disk, big spots can be visible to
the naked eye with the help of goggles. Galileo observed
the spots regularly and found that the spots appeared to
move forward across the solar disk, which suggested him
that the spots were a part of the sun and these appeared
to move forward due to the rotation of the sun. The number
of observed sunspots is treated as a measure of solar
activity.
Each sunspot has two regions umbra, i.e. dark central
region, and penumbra, i.e. less dark and filamentary
region, surrounding the umbra around the edges (Fig.2).
The diameter of penumbra is about 2.5 times of umbra.
High magnetic pressure and less temperature of the gas,
in the umbra region, decrease the density of the gas and
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make the sunspots more transparent than the
photosphere. Therefore we can see to a large depth in a
sunspot, as there is a hole in the umbra region, when
observing near the limb.
Sunspots are very bright, but they appear dark due to the
high temperature of surrounding bright solar disk. Sunspots
are cooler than photosphere as they get less energy from
the solar interior than that of photosphere. The photosphere
gets energy by convection, but in the sunspots there is
no convection to provide sufficient energy, because the
sunspots are strongly magnetized.
Generally sunspots occur in groups and the total number
of sunspots varies from no spot condition to about 400
spots. Single sunspot may appear very tiny and then grow
and split. Day to day photographs of the small sunspots
show many changes in them. Previously, a sunspot
observed as a tiny dark spot like a small sunspot’s umbra,
i.e. without penumbra, which also known as Pore. Some
of the pores become darker and larger, but without any
distinguishable internal features, and other spots split into
the sunspot groups which contain some simple spots but
with the time they increase in size, become more complex
and split into more spots and then dissolve and disappear
after some weeks. In an average, lifetimes of sunspots
are of one day, only the extraordinary big one lives few
months. Observations show that the sunspot groups start
from eastern limb of solar disc move westward and finally
vanish at the western limb.
Pair of sunspots can be considered as gaseous horseshoe
electromagnet, where hot solar gases act as electrical
conductors and carry a huge amount of electrical current
of about trillion amperes. We can consider the conducting
gases as a wire coil, which ends at a pair of spots.
Magnetic field in the umbra region is vertical, while in the
penumbra region it is approximately horizontal.
Plages
Sunspots are surrounded by bright clouds, which appear
before them. These bright clouds are known as
chromosphere faculae or Plages, i.e. beaches. Presence
of plages is also a symbol for solar magnetic activities
and used to define the degree of active region. Plages
appear as a brighter area than the surrounding
chromosphere (Fig.3). Plages are the regions of gas of
higher density than the surrounding atmosphere, which
are covered by hot corona and glow with heat. Sunspots
are darker and plages are brighter than their surrounding
mediums, because sunspots have stronger magnetic field
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while plages do not, in other words plages get large
convection energy from the solar interior. Therefore the
plage fields increase the relatively smaller flow of
mechanical energy, which originates as noise in the
convection zone and dissipate in the solar atmosphere.
Solar flares
After the sunspots, another significant and most dramatic
phenomenon of solar activity is solar flares, which can be
defined as a sudden, rapid, and intense variation in
brightness, which influence the earth strongly. On Sept.1,
1859, Richard Carrington and Richard Hodgson, were
observing sunspots, independently, and they suddenly
viewed a pair of brilliant white ribbons flash, only for 15
minutes, across the umbra and mentioned that as solar
flares. Solar flare is the final major out-burst of solar activity
and has importance in solar physics and interplanetary
physics. Solar flares release a huge amount of plasma
and radiate it in a very short interval of time because of
annihilation of magnetic energy, which is very high near
the sunspots, into the kinetic energy. Flares are highly
concentrated release of energy in the solar corona and
observed as a small, star-like bright spots emerging
suddenly within the plages area, i.e. near a large sunspot
or a group of sunspots (Fig. 4). These bright spots expand
explosively for few minutes and cool down quickly.
Brightening of such spots rapidly spreads along preferred
direction and converse a large area and gradually decays
in about 1/3 to 3 hours. Generally, flares are very small in
comparison with all other structures of an active region.
Flares can be classified on the basis of their brightness,
area, and the intensity.
Large flares or great flares are of much importance as
they cover most of the active region with lifetime of very
few hours. These great flares extend into ribbons, which
bend along the boundaries of opposite polarity regions and
light up like a flash only for one or two minutes. This duration
is referred as flash phase or impulsive phase. After this
flash phase, a great flare may expand and some times
covers up one percent area of the solar surface within few
minutes and then the flare shrinks gradually and finally
disappears within few hours. This phase is referred as
gradual phase. Due to the rapid changes and
disappearance of the flares within few minutes, observation
of flare is a big problem.
Prominences
During the peak of solar activity, most of the plasma enters
the corona through chromosphere, this ejected plasma
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condenses into the regions of higher density and lower
temperature, and flow gradually downward towards the
chromosphere along the magnetic field lines, in a very
graceful shape and this activity is referred as Prominence
and appears as dark filaments on the solar disc (Fig.4).
This high-density material is supported against the solar
gravitational attraction by the magnetic field lines, where
these lines are parallel to the surface. Prominences can
be classified as highly dense and cool gas suspended
above the surface, in the corona. It is considered that the
different linear and filamentary structures of the
prominences have strong electric current running within
their gases. The temperature of the prominences is less
than the chromosphere and the shape of prominences
indicates that they are well insulated against the
surrounding heat and coronal radiation. Reason for this
insulation may be due to magnetic and electric field within
the prominences.
Prominences have a large variety of shapes, sizes, and
life times due to the magnetic field structures and the
sources of condensed matter. Types of prominences in
which upward motion take place are spicules, surges,
sprays and eruptive filaments (all are short-lived with lifetime
of few minutes to few hours). Types of prominences in
which downward motion take place are coronal rains, loop
prominences (both are short-lived with life time of about
an hour or so) and long-lived filaments. Long-lived
prominences, also known as quiescent prominences, are
important for the coronal structure. These quiescent
prominences are gradually changing prominences and
always seen away from active regions with sunspots. The
lifetimes of such type of prominences are up to 6 to 10
months and remain after the disappearing of other activities
of an active region.
Coronal holes
In x-ray photographs, distinct regions of low x-ray emission
are referred as coronal holes (Fig. 5). One of the largest
coronal holes, extended from north to south is known as
coronal hole number one, which looks like the map of Italy.
Coronal holes are visible in the regions without magnetic
field. Because of the presence of very low-density gases
in coronal holes there is lack of radiation and this may be
the reason for their dark appearance. It has been suggested
that the magnetic field lines are open over coronal holes,
which emerge from photosphere to somewhere in
interplanetary space, while magnetic field lines are closed
over active regions e.g. loops. Open field lines of coronal
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holes permit the gases to flow from the sun towards the
interplanetary space. Hence, the coronal holes are empty.
Solar wind
Resulting out going gas, from the coronal holes is referred
as solar wind. Solar wind arrived at earth at least three or
four days later the coronal hole faced the earth because
of the wind transits from the sun to the earth at least in
three or four days.
4. Why to study the sun
Study the sun is important, as it drives the weather on the
Earth. Any variations of the sun’s output affect the Earth’s
climate. The winds and circulation of ocean patterns are
all affected by the sun’s energy output. The differential
heating of the Earth generates the winds and major ocean
currents as well as causing different seasons. However,
the solar cycle has a deep impact on our climate. Studies
also show a close correlation between Earth’s ice ages
and the solar activities.
In our technology-based society we depend extensively
on satellites and various forms of high frequency
communication systems. We know that most of the solar
activities are driven from the dominant magnetic force in
the solar atmosphere. In contrast to the dense plasmas
in the sun’s lower regions, the plasmas of the solar
atmosphere are very dilute and unable to restrain the
gigantic magnetic field of the sun, which may result in
geomagnetic storms. Geomagnetic storms have a direct
effect on the Earth’s atmosphere, impacts satellites,
perturbing their orbits, scoring their surfaces, and disrupting
communication system. Geomagnetic storms consist of
very high-energy protons, which ionize the Earth’s upper
atmosphere and expand the ionosphere and disrupt long
distance radio signals. Satellites’ orbit around the Earth
can be disturbed by the enhanced drag on the satellites
from the expanded atmosphere. Energetic protons escape
into interplanetary space are dangerous for the electronic
instruments in space as well as for the electronic
components of satellites. Furthermore, after one or two
days, the Earth is slammed with a magnetic shock wave
travelling at more than 1000 kilometers per second that
may totally disrupt power grids everywhere on Earth.
and b) studies of the inside of the sun. Studies of the solar
interior reveal for the motions and thickness of the various
internal zones as predicted by various models of stellar
interiors such as the nuclear core, the radiative zone, and
the convective zone. The interface between the radiative
and convective zones is just like a shell where the sun
generates the magnetic fields ultimately seen on the
surface in sunspots and other structures associated with
the 11 year solar cycle. Therefore, understanding of the
interior of the sun is important to understand solar activities
due to the effects of intense magnetic fields. Studies of
solar atmosphere also provide an opportunity to study
some natural physical processes, e.g., study of solar flares
is similar to the study of controlled thermonuclear fusion
in a laboratory. Still, there are mysteries about the structure
of the sun. There are so many unsolved problems about
the sun, as how the sun’s surface and corona interact,
how energy is transported from the solar surface into the
outer atmosphere?
Author is thankful to the learned referee for his invaluable
suggestions to improve the manuscript. He is also thankful
to Dr. Rakesh Sharma for his help in modifying the
manuscript. However all images are edited but by courtesy
of NASA.
BIBLIOGRAPHY
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Athey R.G., The Solar Chromosphere and Corona:
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Dwivedi B.N., Current Science, 75, 10, 1006-1011,
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Lang K.R., The Cambridge Encyclopedia of the Sun,
Cambridge University Press, Cambridge, 2001.
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Stix M., The Sun: An Introduction, Springer-Verlag,
Berlin, 1991.
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Sturrock P.A., Holzer T.E., Mihalas D.M., Ulrich R.K.
(eds), Physics of the Sun, D. Reidel Publishing
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Taylor R.J., The Sun as a Star, Cambridge University
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Wentzel D., The Restless Sun, Smithsonian Institute
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Zirin H., Astrophysics of the Sun, Cambridge University
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5. Areas of solar research
Main thrust areas of research in solar physics are: a)
studies of the outer solar atmosphere and its variation,
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PHOTOGRAPHS ACCOMPANING THE ARTICLE “THE TURBULENT SUN”
Fig. 1. (a) Layers of solar atmosphere, where white halo surrounding the photosphere is the corona, (b) Variation in solar
temperature above the photosphere.
Fig. 2. Sunspots, along with illustrated magnetic lines for a
pair of sunspots, in left side box, and zoomed view of a large
sunspot, in right side box, in which central dark region is
umbra and surrounding less dark region is penumbra.
Fig. 3. CaK image of the sun shows the filaments and plages
on the sun.
Fig. 4. X-ray image shows the prominences and flares on
the sun, whereas in side boxes zoomed views of the two
features have also been shown.
Fig. 5. X-ray image of the sun shows the coronal holes along
with the illustrated magnetic lines for coronal holes, in side
box.
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