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• General Properties
• Internal Structure
• Solar Atmosphere
– Photosphere, chromosphere, and corona
Surface Features and Magnetic Fields
Solar Activities
Solar Cycle
Sun-Earth Connection
The Surface of the Sun…
In images of the Sun, we see a sharp edge, which we perceive as the surface
of the Sun. However, like the surfaces of the Jovian planets (the Gas
Giants), it is not a firm, solid, thick surface that we can stand on like on the
Density of the atmosphere on the surface of the Earth is
– 1.3 kg/m3, or about
– 1  1025 N2 molecules per cubic meter, and
Density of the atmosphere on the surface of the Sun is
– 1  1023 particles per cubic meter, or about
only 1% the density of the Earth’s atmosphere.
Density of the solar atmosphere just a few
thousand kilometer above the ‘surface’,
or in the solar corona
– 1  1014 particles per cubic meter
The rapid decrease of the density within a
short distance is the reason that we see a
sharp edge…
The surface layer is where sunlight are
generated. It is referred to as the Photosphere.
The Chromosphere is a thin, irregular layer above the photosphere in which the
temperature rises up from 5,800 K to about 20,000 K. This layer is usually
observed in the red wavelength of the Hydrogen absorption line. It is therefore
termed Chromosphere, meaning color-sphere,
The Sun in Calcium absorption line in
blue (393 nm) wavelength.
The Sun in Hydrogen absorption line in
red (656 nm) wavelength.
Bright patches (the Plages) and dark spots (sunspots) are related to higher magnetic field
The Sun in UV and X-Ray –
The Sun in X-ray shows the structure of the very hot (1,000,000 K)
• Recall that high temperature blackbody produces radiation with
shorter wavelength. X-ray are produced by blackbody with million
degree temperature.
• We don’t know why the coronal temperature is so high…
• The Sun in one of the emission spectra of Helium in the UV (30.4 nm) shows
the structure of a cool regions of the corona.
X-ray image of the Sun.
UV image of the Sun in He II 30.4
The Solar Corona in ‘White Light’
This is an image of total
solar eclipse.
• The radiation are
reflection of sunlight
by the electrons in
the corona.
• A radial gradient has
been removed from
the image to
enhance the coronal
• The streamers are
where slow solar
wind leave the Sun.
• The coronal holes
are where fast solar
The Many Faces of the Sun
01-May-2005 20:48 UT
01-May-2005 20:48 UT
01-May-2005 20:48 UT
EIT FeX 195 A
01-May-2005 20:48 UT
EIT FeXV 284 A
01-May-2005 20:48 UT
01-May-2005 20:48 UT
By observing the
at many different
wavelengths (or
colors), we can
see different
layer of the solar
atmosphere, and
get a better
understanding of
what’s going
The Coronal Heating Problems
• The temperature of the Sun is the highest in its core,
about 15 million degrees.
• The temperature decreases as we move outward toward
the surface, dropping to 6,000 K at the photosphere.
• The temperature then rises to about 20,000 K in the
chromosphere, just a few thousand km above the
• The temperature than rises rapidly to 1to 2 million
degrees in the corona.
• We do not understand how the corona is heated, and
this is one of the important unresolved questions of
solar physics.
General Properties
Internal Structure
Solar Atmosphere
Surface Features and Magnetic Fields
– Sunspots, Granulation, Filaments and
Prominences, Coronal Loops…
• Solar Activities
• Solar Cycle
• Sun-Earth Connection
High-Resolution View of the Solar
This is what the surface
of the Sun looks like
with high
resolution…we see
• Sunspot
– Umbra
– Penumbra
• Solar Granulation
Solar Granulation
On the surface of the Sun, we can see the action of convection…
Image of solar granulation. The
bright center of the cells are where
hot gas rise to the surface. The
narrow dark lanes are where cold
gas sink to the ‘bottom’.
• Each cell is about 1,000 km in
Sunspots are dark features on the surface
of the Sun. Sunspots are strongly
magnetized region on the surface of the
Sun. They appear dark because the
presence of very strong magnetic fields
helps the plasma inside the sunspot to
balance the pressure of the plasma outside of
the sunspot. Therefore, the thermal pressure
(related to the temperature of the plasma) of
the plasma inside the sunspot is lower,
leading to lower temperature, and lower
intensity (darker compared with the
surrounding area).
We still don’t know why there is an umbra and a
penumbra in sunspots. Neither do we know why
there are such sharp boundary between different
Early Clues of Sunspot Magnetic Field
Sunspot group seen in H
(Hydrogen absorption line)
Sunspots are strongly magnetized
region on the surface of the Sun.
The brightness structure of a
sunspot seen in the absorption line
of hydrogen resemble the
magnetic field lines surrounding a
bar magnet.
Bar Magnet The pattern formed
by the small magnetized iron bars
shows the magnetic field lines.
Evidence of Magnetic Field in
Spectra of magnetic field
sensitive absorption lines
from a slice (the dark
vertical line at the center
of the image on the left) of
a sunspot.
The presence of a magnetic field in the solar
atmosphere can be seen in the Zeeman Effect of the
spectral line on the right. Some spectral lines have
three components…and magnetic field can change the
energy level of two of them. Thus, the spectral line
will be split into three lines when there is a strong
magnetic field.
The separation between the
lines measures the strength
of the magnetic fields
The Sun as a Magnetic Star
Today, we know that almost all the solar surface and coronal
features (except for solar granulation, which is generated by
convection) we talked about so fare are related to magnetic
• Sunspots
• Filaments and Prominences
• Coronal loops
Without the magnetic fields, the Sun
would be a very boring star to look at…
Magnetic Field of the Whole
• A magnetogram shows the magnetic
field on the surface (the photosphere)
of the Sun. The black and white
patches show where the magnetic
fields are strong.
– White indicates magnetic field
pointing toward us.
– Black indicates magnetic fields
pointed away from us.
– The large patches of black and
white are due to sunspot and
active regions with strong
magnetic fields.
• The pepper-and-salt patterns outside of
the active regions indicates that there
are magnetic fields everywhere on the
surface of the Sun.
Filaments and Prominences
Filaments and prominences are cool and dense gas suspended high in the
solar atmosphere, and embedded in the very hot solar corona.
• When they are observed on the solar surface, they appear as dark
absorption features…filaments!
• When the are observed outside of the solar limb, they appears as bright
features because they reflect sunlight toward us…prominences!
• How they can survive in the million-degree temperature corona, and stay
high up against gravity is still a mystery. We know magnetic field plays an
important role, but the details is not well understood.
The Grand Daddy Prominence
A huge solar prominence observed in 1946
Coronal Loops
We believe that the loops we see in the
solar corona trace the magnetic field
lines. However, the magnetic fields are
everywhere in the corona. we are not
quite sure why only some of the field
lines are bright…
High resolution image of the coronal
obtained in the UV wavelength
obtained by TRACE satellite.
General Properties
Internal Structure
Solar Atmosphere
Surface Features and Magnetic
• Solar Activities
– Flares, CMEs, and Filament
• Solar Cycle
• Sun-Earth Connection
Solar Activities---Flares
A flare is defined as a sudden, rapid,
and intense variation in brightness.
A solar flare occurs when magnetic
energy that has built up in the solar
atmosphere is suddenly released.
Radiation is emitted across virtually
the entire electromagnetic spectrum,
from radio waves at the long
wavelength end, through optical
emission to x-rays and gamma rays
at the short wavelength end. The
amount of energy released is the
equivalent of millions of 100megaton hydrogen bombs exploding
at the same time! Or, about a few
percent of the total energy released
by the Sun every second.
Filament Eruptions
Filament eruptions are
usually associated
with flares and
coronal mass
ejection. Exactly
how they work is
still under
General Properties
Internal Structure
Solar Atmosphere
Surface Features
Magnetic Fields
Solar Activities
Solar Cycle
Sun-Earth Connection
Solar Cycle---Sunspot Numbers and
the Butterfly Diagram
Solar Cycle
The number of sunspots on the
surface of the Sun follows a 11year cycle.
Butterfly diagram
Sunspots appear at higher latitude
at the beginning of the solar cycle,
and migrate toward the equator as
the cycle evolve. So, when we plot
the latitude of the sunspots as a
function of time, the patterns looks
like a series of butterfly…therefore
it is referred to as the butterfly
Magnetic Field and X-Ray Variation
Through one Solar Cycle
The black-and-white patterns show the
surface magnetic field variation through one
sunspot cycle (11 years). Notice the reversal
of the ordering at the beginning and the end
of the cycle.
The temperature of the solar
corona a few million degrees (no
explanation yet).
The high temperature causes it to
emit photons mostly in the UV and
X-ray wavelengths (high energy
The activities in the solar corona
also follow the solar cycle.
In fact, the level of almost every
aspect of solar activities (flares,
coronal mass ejections, etc.)
follows the solar cycle.
How Does Solar Cycle Work?
The magnetic field of the Sun is postulated to be generated at the bottom of the
convection zone. This magnetic field then rises up to the surface and expand into
the corona, to produce the magnetic features we see.
• Since the magnetic field of the Sun reverse its orientation every 11 years, the
solar cycle is really a 22-year magnetic cycle. In comparison, the Earth’s
magnetic field direction is stable.
– The number of sunspot only depends on the strength of the solar
magnetic activities, but not the orientation of the magnetic fields.
Therefore, sunspot number cycle is half that of the magnetic field cycle.
• How does the Sun changes its magnetic field orientation every 22 years?
We don’t have a complete theory yet. However, there are a few clues. For
example, we believe that the differential rotation of the Sun must play a role
in changing the magnetic field configuration from that of a dipole (like a bar
magnet) to that of a torus (shaped like a doughnut).
The exact mechanism of the solar cycle is still unknown!
Differential Rotation of the Sun
The Sun does not rotate like a solid
body. It rotates every 25 days at the
equator and takes progressively
longer to rotate one revolution at
higher latitudes, up to 35 days at the
poles. This is known as differential
• You can pick a few sunspots
located at different latitude from
the movie on the right, and trace
them as they rotate across the
solar disk. Using the time
information at the lower-left-hand
corner of the images, you can
calculate the rate of rotation of the
Sun at different latitudes.
• You should find that sunspots near
the equator rotate faster than
Effects of Differential Rotation
At the surface of the Sun, and deeper
in the interior, when we move the
solar plasma, the magnetic fields
embedded in the plasma will move
with the plasma. This is referred to
as the frozen-in magnetic fields.
• So, the effect of differential
rotation is the stretching of the
magnetic fields that was
originally in the north-south
direction to make them runs
along the east-west direction…
• Sometimes a small section of
the magnetic field would pop up
through the surface. This will
make a sunspot, and it
happens more frequently during
solar maximum…
(dashed) lines
magnetic field
lines above
(under) the
Evolution of Solar Magnetic Field
During the Solar Cycle
Solar Minimum
Solar Maximum
Dipole Magnetic Field
No Sunspot
~5 years
22 years
Toroidal Magnetic Field
Many Sunspots
But, this is
only half
of the
The magnetic
of the Sun
evolves with a
11 years 22 year cycle.
The Solar Cycle Problem and the
Sunspot Phenomenon
• At this point, we don’t have a satisfactory theory for the
solar cycle. Neither do we have a complete
understanding of the sunspot phenomenon. These are
two more important problems of solar physics that needs
to be solved.
General Properties
Internal Structure
Solar Atmosphere
Surface Features
Magnetic Fields
Solar Activities
Solar Cycle
Sun-Earth Connection
Sun-Earth Connection
How do Solar Activities Affect Earth?
In short time scale (compared with the lifetime of the Sun)
• Space Weather and Geomagnetic Storm
Flares and Coronal Mass Ejections (CME) bombard the Earth with high
energy charged particles, causing interruption to communications, and power
• Solar Irradiance Variations and Possible climate change
The solar energy input determines the temperature on the surface of the Earth.
If the Sun is to increase its luminosity by 1%, it will have significant effect on
Earth’s temperature. It will increase by about 0.75 K.
In long time scale
• The Sun will eventually evolve into a red giant star, increases its energy
output and its physical size. Earth may eventually be engulfed by this
enlarged Sun, and life will be extinguished on Earth  Next chapter…
Space Weather
Space Weather (from NASA SoHO Space Weather Website)
Space weather happens with a solar storm from the Sun
travels through space and impacts the Earth’s
magnetosphere. Studying space weather is important to our
national economy because solar storms can affect the
advanced technology we have become so dependent upon
in our everyday lives. Energy and radiation from solar flares
and coronal mass ejections can
• Harm astronauts in space
• Damage sensitive electronics on orbiting spacecraft?
• Cause colorful auroras, often seen in the higher
• Create blackouts on Earth when they cause surges in
power grids.
From Coronal Mass Ejections…
Space weather starts with Coronal
Mass Ejection on the Sun…
Coronal mass ejections and flares are
due to the changes in the magnetic
field structures in the solar corona.
However, details mechanism is still
not clear, and we cannot predict
when flares and CME are going to
occur yet!
To Geomagnetic Storm…
Flares and coronal mass ejection send high energy charged particles (electrons,
protons) into space. If the direction and speed of these particles are just right,
they can reach the Earth. These high energy particles are harmful to life on
Earth. They can also cause damages to satellites operating in space, as well as
power grids.
• Charged particles travel along the magnetic field lines
• We are protected by Earth’s magnetic field, which directs the majority of
the high energy charged particles toward the north and south poles to
produce the aurora borealis and aurora Australis.
Charged particles
spiral around the
magnetic field lines.
Effects of Geomagnetic Storm
• In October 31, 2003, a series of strong
geomagnetic storms damaged two
satellites, caused the power grid in
Sweden to shutdown, cutting power to
20,000 customers, disrupted radio
communication and broadcast
systems, and forced the airlines to
change flight plans.
– The large variation of the Earth’s magnetic
fields can induce strong, uncontrolled electric
current in the power lines, causing the power
grid to overload and shutdown…
Solar Irradiance Variations
Modern measurements showed that the solar constant is really not a constant.
The energy output of the Sun is modulated by the magnetic activity. But
details of how this happens is still under study…Nevertheless, we know that
• Solar irradiance is higher
Solar constant measurements from several
when the surface magnetic
satellite experiments
field is stronger (when ther
are more sunspots)…
• The amplitude of the solar
irradiance variation is about 2
W/m2, or about 0.1%.
• This variation is too weak to
cause climate change.
• But, if solar magnetic
activities was significantly
reduced or enhanced for a
long period of time, it can
change the climate of the
Earth…for example, did the
Sun caused the Little Ice Age?
Sunspot Maximum
Little Ice Age (1650-1700)
• During a period that lasted
approximately 50 years from the mid
1650s to the early 1700, the
temperatures in northern Europe had
their lowest values for the past
millennium, with winter temperatures
being on average 1 to 2 degrees colder
than in later periods.
• This period has been called the Little
Ice Age. During this period, access to
Greenland was largely cut off by ice
from 1410 to the 1720s. At the same
time, canals in Holland routinely froze
solid, glaciers advanced in the Alps,
and sea-ice increased so much that no
open water was present in any
direction around Iceland in 1695.
Aert van der Neer, Dutch, 1603-1688
Winter Scene with Frozen Canal
Was The Sun Responsible for
the Little Ice Age?
Maunder Minimum, the period with reduced sunspot number around 1,650 AD, was
coincident with the little ice age of western Europe.
• Does the reduced sunspot number imply reduced solar energy output, causing the
temperature on Earth to drop?
• Given the small amplitude of the total solar irradiance variation (0.1%), it is unlikely
that the total solar irradiance variation is responsible for the global warming trend
we have seen in the last 100 years. But the amplitude of the UV irradiance variation
is much larger…
• Solar UV radiation interact with Earth’s upper atmosphere. What’s the effect of
solar UV variations on Earth’s climate?
Solar UV Variation and Global
From Haberreiter et al., Advances in Space Research 35 (2005) 365-369…
1. Introduction
It is known that the variability of the solar UV irradiance has a considerable
effect on the terrestrial atmosphere. Recently, Egorova et al. (2004) have
shown that the introduction of solar UV flux into a spectral Global
Circulation Model (GCM) with a chemistry transport model leads to an
intensification of the polar vortex and a statistically significant warming of
up to 1.2 K over North America and Siberia. Due to a missing longterm
record of the solar UV irradiance with a sufficient temporal resolution,
• Like the effect of the increasing CO2
content in Earth’s atmosphere is still not
clear, we do not understand the
mechanisms that are causing the
variations of the solar irradiance, nor do
we understanding the details of how solar
input affect the global climate.