General Properties

1

• General Properties
• Internal Structure
• Solar Atmosphere
• Photosphere, chromosphere, and corona
• Surface Features and Magnetic Fields
• Solar Activities
• Solar Cycle
• Sun-Earth Connection

2
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 Earth
• 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 Earths 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.

3
Chromosphere
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
regions.
4
The Sun in UV and X-Ray Corona

• The Sun in X-ray shows the structure of the
  very hot (1,000,000 K) corona
• Recall that high temperature blackbody produces
  radiation with shorter wavelength. X-ray are
  produced by blackbody with million degree
  temperature.
• We dont 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
5
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 features.
• The streamers are where slow solar wind leave the
  Sun.
• The coronal holes are where fast solar wind leave
  the Sun.

6
The Many Faces of the Sun

• By observing the Sun simultaneously at many
  different wavelengths (or colors), we can see
  different layer of the solar atmosphere, and get
  a better understanding of whats going on

7
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 photosphere.
• 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.

8

• 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

9
High-Resolution View of the Solar Surface

• This is what the surface of the Sun looks like
  with high resolutionwe see
• Sunspot
• Umbra
• Penumbra
• Solar Granulation

10
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 size

11
Sunspots
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 dont know
why there is an umbra and a penumbra in sunspots.
Neither do we know why there are such sharp
boundary between different regions
12
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.
13
Evidence of Magnetic Field in Sunspot
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 componentsand 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
14
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
  fields
• Sunspots
• Filaments and Prominences
• Coronal loops
• Without the magnetic fields, the Sun
• would be a very boring star to look at

15
Magnetic Field of the Whole Sun

• 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.

16
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 featuresfilaments!
• When the are observed outside of the solar limb,
  they appears as bright features because they
  reflect sunlight toward usprominences!
• 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
17
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.
18

• General Properties
• Internal Structure
• Solar Atmosphere
• Surface Features and Magnetic Fields
• Solar Activities
• Flares, CMEs, and Filament Eruptions
• Solar Cycle
• Sun-Earth Connection

19
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 100-megaton hydrogen
bombs exploding at the same time! Or, about a few
percent of the total energy released by the Sun
every second.
20
Filament Eruptions
Filament eruptions are usually associated with
flares and coronal mass ejection. Exactly how
they work is still under investigation...
21

• General Properties
• Internal Structure
• Solar Atmosphere
• Surface Features
• Magnetic Fields
• Solar Activities
• Solar Cycle
• Sun-Earth Connection

22
Solar Cycle---Sunspot Numbers and the Butterfly
Diagram
Solar Cycle The number of sunspots on the
surface of the Sun follows a 11-year 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 butterflytherefore it is referred to
as the butterfly diagram
23
Magnetic Field and X-Ray Variation Through one
Solar 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 photons).
• 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.

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.
24
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 Earths 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 dont 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!

25
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 rotation.
• 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 those at higher latitude.

26
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

Solid (dashed) lines represents magnetic field
lines above (under) the surface.
27
Evolution of Solar Magnetic Field During the
Solar Cycle

• Solar Minimum
• Dipole Magnetic Field
• No Sunspot

• Solar Maximum
• Toroidal Magnetic Field
• Many Sunspots

But, this is only half of the story!
The magnetic field configuration of the Sun
evolves with a 22 year cycle.
11 years later
28
The Solar Cycle Problem and the Sunspot Phenomenon

• At this point, we dont 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.

29

• General Properties
• Internal Structure
• Solar Atmosphere
• Surface Features
• Magnetic Fields
• Solar Activities
• Solar Cycle
• Sun-Earth Connection

30
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
  grids
• 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 Earths 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

31
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
  Earths 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
  latitudes?
• Create blackouts on Earth when they cause surges
  in power grids.
• http//sohowww.nascom.nasa.gov/spaceweather/lentic
  ular/

32
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!
33
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 Earths 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.
34
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 Earths magnetic
  fields can induce strong, uncontrolled electric
  current in the power lines, causing the power
  grid to overload and shutdown
• The high energy charged particles from the Sun
  are a health risk to astronauts in space orbit
  and passengers in jets flying at high altitudes
  over the north and south polar regions .
• Aurora were reported as far south as New Mexico,
  Texas, and Florida.

http//www.space.com/scienceastronomy/power_outage
_031031.html
35
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 studyNevertheless, we know that

• Solar irradiance is higher when the surface
  magnetic 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
  Earthfor example, did the Sun caused the Little
  Ice Age?

Solar constant measurements from several
satellite experiments
Sunspot Maximum
36
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
37
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 Earths upper
  atmosphere. Whats the effect of solar UV
  variations on Earths climate?

38
Solar UV Variation and Global Warming?

• 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,

39

• Like the effect of the increasing CO2 content in
  Earths 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.