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THE SUN
INTRODUCTION
General stellar category of type G – “a yellow runt”
Monotonously common in MWG – billions of stars like our Sun
Remarkably stable
Nothing else in the Universe (other than the Earth) is more immediately important to us
The Sun is the origin of virtually all the energy that sustains life
The Sun is the source of our weather
The Sun is the “Rosetta stone of Astrophysics”
Galileo discovered sunspots over 4 centuries ago
Surge of international interest about Sun over past 20 years
Before it was “solar dermatology;” now it’s really Astrophysics
Best resolution today shows solar structures that are 50 miles across
Nearly a dozen major space-based observatories throughout Earth – almost all launched since mid-1990s
New initiatives to understand space weather – the effects created by billions of tons of plasma that can
erupt from the Sun and cause magneto-electrical squalls throughout Solar System
Goal: end-to-end modeling – a comprehensive view of what happens all the way from the interior of the
Sun to the Earth’s upper atmosphere
SOLAR FLARES AND CMEs
Two types of explosive solar events impact Earthlings most severely:
 SOLAR FLARE – small area above the solar surface suddenly roars to tens of millions of degrees,
throwing off a surge of radiation that can cause communication blackouts, disable satellites, or kill
a spacewalking astronaut
 CME (CORONAL MASS EJECTION) – billions of tons of charged particles escape from the
sun’s halo, the corona, at millions of miles per hour. When these behemoth clouds slam into the
Earth’s protective magnetosphere, they squash magnetic field lines and dump trillions of watts of
power into the Earth’s upper atmosphere. This can overload powerlines, causing massive
blackouts, and destroy delicate instruments on anything in Earth’s orbit.
October 2003 – flare and CME occurred together (4th most powerful flare ever observed)
Back-to-back CMEs smacked the Earth. Atmosphere was so electrically charged – Northern Lights were
seen as far south as the Mediterranean. Little damage was done due to advanced warning system.
However, in 1989, a fierce CME struck the earth and blew out Hydro-Quebec’s power grid, leaving almost
7 million people without electricity, and a multi-million dollar damage bill.
SUN IS PLASMA
All of the Sun is gas
70% Hydrogen, 20% helium and 2% heavier elements
Sun is a madhouse of electromagnetic activity
On the Sun, almost everything is electrically conductive because there aren’t very many intact neutral
atoms
Overwhelming thermal and radiation energies excite electrons to the point at which they pop off their
atoms, creating a seething stew of positively charged nuclei and free negative electrons – this gaseous mix
that conducts electricity as good as copper wire is called PLASMA.
Like any electricaly charged object, plasma produces magnetic fields when it moves
As those fields shift, they induce more currents, which in turn produce more fields
This tangle of plasma and electric and magnetic effects determines the forms of nearly everything in or
above the Sun, such as the bright coronal loops and the dark areas we call sunspots. Everything we see as
solar activity is a magnetic field being acted upon by plasma and vise-versa. Both are forever in motion.
NUCLEAR FUSION IN THE CORE
Local gas and dust drifted together, drawn by gravity
Mass of sphere gets larger and gigantic pressure in center
Fusion reaction is sparked – hydrogen nuclei come together to create helium
Resulting helium nuclei are slightly less massive than the component hydrogen nuclei
The difference is converted o energy E=mc2
Most of the energy is gamma rays
It takes thousands of years for a single photon to travel ½ million miles to the surface
By that time, it has shed so much energy (due to the numerous collisions) that it emerges as visible light.
In 1920s, Sir Arthur Eddington and others were convinced that the source of the Sun’s power was some
subatomic phenomenon requiring enormous heat. This idea was ridiculed as outlandish.
By the 1950s, the fusion model was convincingly verified.
SOLAR EXPLORATION
Solar and Heliospheric Observatory (SOHO) a satellite run jointly by NASA and the European Space
Agency
Breakthroughs have been made on all solar fronts
New puzzles constantly revealed
The “ceaseless dance between plasma and magnetic fields
Everything on Sun influences everything else – and it all has a direct effect on the Earth
“The Sun is the only astronomical object that critically matters to humankind”
WHAT INTERIOR MECHANISMS PRODUCE THE SUN’S MIGHTY MAGNETIC DYNAMO?
The magnetic field drives virtually everything on the Sun
Sun has opposite north and south magnetic poles like the Earth
The Earth’s field is produced by motion of the molten iron in the outer core
The Sun’s field is produced by the internal motion of plasma
Until recently, impossible to see beneath the photosphere
Helioseismology developed – an ultrasound scan on the Sun’s interior
Global Oscillation Network Group (GONG) – worldwide array of observation stations positioned 60degrees apart to observe the Sun 24-hours a day
The idea came about in the 60s – observed that solar surface throbbed with rhythmic oscillations like the
skin of a drum
Discovery: Much faster rotation at equator (4400 mph) than at poles (545 mph)
The “shear zone” (a.k.a. – tacholine) between the regions of varying rotations is what is believed to cause
the magnetic field.
The internal motion stretches and twists the north-south magnetic field lines, wrapping them around the
Sun thus adding energy (imagine how energy is stored in a stretched rubber band)
Sometimes the powerful twisted field lines become buoyant and rise above the photosphere – these
manifest as loops, prominences and sunspots, which are observed on the surface of the Sun
WHY DO SUNSPOTS FLUCTUATE IN 11-YEAR CYCLES? WHAT EFFECT DOES THIS
HAVE ON TERRESTRIAL CLIMATE?
Sunspots can range in diameter from 1,500 miles wide to several times the width of the Earth. Sunspots are
visible because the bundled field lines impede the flow of convection
The center of the sunspot, the umbra, is 1000+ degrees cooler than the surrounding 10,000-degree
photosphere.
Chinese observed sunspots in first century
Sunspots observed by telescope in 17th century
German astronomer, Samuel Heinrich Schwabe, began a systematic count in 1826
In 1843, he reported that their number goes from minimum to maximum and back in about a decade.
1915 – American astronomer, George Ellery Hale (Mt. Wilson Observatory, California) – sunspots occur in
pairs roughly parallel to the Sun’s equator, and each half of a pair has the opposite magnetic polarity. All
spot pairs in N-Hem. Of Sun have same orientation, and all spot pairs in S-Hem. have opposite orientation.
Every 11 years, on the average, the Sun reverses its overall magnetic polarity
One complete cycle lasts an average of 22 years
No one completely understands the entire process.
The Earth’s field also reverses – the most recent reversal was 780,000 years ago.
There is evidence that sunspot cycles have direct consequences for human life
MAUNDER MINIMUM (1645-1715) no sunspots appeared on the solar face at all
Later studies done with tree rings dated from that period – significantly more Carbon-14
Higher amounts of cosmic radiation reached the earth during that time (a magnetically active sun reduces
the amt. of cosmic radiation we receive)
Although, more sunspots mean a more luminous (brighter) Sun – sunspot areas are extra bright – the
magnetism causes extra-bright areas called faculae
Investigation of another low sunspot period followed (1460-1550)
These extended minimums (1400-1850) is called the LITTLE ICE AGE
The Thames River and the lagoons of Venice regularly froze
Sunspot activity has been high over the past century (Earth’s temperatures have climbed)
The greater luminosity only accounts for ½ the global temperature increase (pre-1940) and much less than
that for the following years up to the present.
We still need to understand the structure of magnetic fields under sunspots to better predict these cycles.
HOW IS IT POSSIBLE THAT THE CORONA IS TYPICALLY 100s OF TIMES HOTTER THAN
THE EARTH’S SURFACE?
The corona is only visible during a total solar eclipse
The chromosphere is only one-millionth as dense as the photosphere
The corona is one-hundredth as dense as the chromosphere
The photosphere is 5700 degrees C; the corona is 10,000 degrees C.
Shouldn’t the layer farthest from the center of the sun be the coolest? This defies logic!
Researchers think that this is due to magnetic reconnection – or “splicing” of magnetic field lines that
causes energy to be released.
The “magnetic carpet” just underneath the surface. The tightly wound magnetic field lines come together
and release an enormous amount of energy, which accounts for the high temperature.
WHAT EXPLAINS FLARES AND THE CORONAL MASS EJECTIONS THAT ARE
RESPONSIBLE FOR ELECTRICAL TEMPESTS ON EARTH? HOW CAN THESE STORMS BE
PREDICTED?
The corona produces the most powerful particle accelerators in the solar system – flares and CMEs
Flares expel much of their energy as x-rays and are generated when electrical currents are suddenly
released as one or more magnetic field loops in the corona become strained to the breaking point and snap
into a new shape
This radiation reaches the earth in 8 minutes and can disrupt radio communications and navigation systems.
A small percentage of flares hurl out fast-moving, high-energy protons that can cripple satellites.
CMEs often follow flares
These massive eruptions of plasma can also occur on their own
CMEs take 1-3 days to reach Earth
They smash into the Earth’s magnetosphere and deform it – and if conditions right – produce a multimillion ampere ring current in the belts of charged particles that continually circle the Earth.
CMEs are even more threatening to communication satellites than flares and can take out power grids
causing power failures.
The trigger mechanism for CMEs is not known, therefore not possible to predict
With SOHO we can see these storms leaving the Sun
The magnetic polarity of a CME can change during its journey
If the polarity is opposite that of the Earth’s, it does the most damage on impact
We can predict with 80-percent accuracy if they will hit the Earth
The worst storms often come in the waning years after the solar maximum.
The most recent solar maximum ended in 2001; the strongest x-ray flare ever observed was in Nov. 2003
Sun Statistics
Mass (kg)
1.989e+30
Mass (Earth = 1)
332,830
Equatorial radius (km)
695,000
Equatorial radius (Earth = 1)
108.97
Mean density (gm/cm^3)
1.410
Rotational period (days)
25-36*
Escape velocity (km/sec)
618.02
Luminosity (ergs/sec)
3.827e33
Magnitude (Vo)
-26.8
Mean surface temperature
6,000°C
Age (billion years)
4.5
Principal chemistry
Hydrogen
92.1%
Helium
7.8%
Oxygen
0.061%
Carbon
0.030%
Nitrogen
0.0084%
Neon
0.0076%
Iron
0.0037%
Silicon
0.0031%
Magnesium
0.0024%
Sulfur
0.0015%
All others
0.0015%

The Sun's period of rotation at the surface varies from approximately 25 days at
the equator to 36 days at the poles. Deep down, below the convective zone,
everything appears to rotate with a period of 27 days.