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Sun’s size vs. other stars
 The Sun can be described as an average star, a yellow dwarf– bigger than
some, smaller than others
SOHO Project Scientist Team
Early Western science
Galileo (1600s) -- agreed with
and was one of the first scientists to
systematically observe and keep
records of the Sun and sunspots.
He correctly identified sunspots as
part of the Sun and determined
the Sun’s rotation : 25.4 days.
SOHO Project Scientist Team
More modern solar science
 Sir Isaac Newton (late 1600s) - concluded that stars were tremendously far
away and that they gave light like the Sun
 William Herschel ( 1780s) – infrared light
 Heinrich Schwabe (1843) - determined the appearance of sunspot cycles
 Robert Bunsen (1860s) - invented the spectroscope to determine the
elements found in the Sun
 George Hale (1908) - discovered the magnetic fields of sunspots
 Albert Einstein (1920s) - proposed that sunlight was made of particles. No
one believed himself until it was proven 10 years later.
Sir Isaac Newton
William Herschel
Albert Einstein
SOHO Project Scientist Team
Solar Spectrum
 Huygens (1690): light travels as waves, just likes ocean waves
 Sir Isaac Newton (1704): light from the Sun can be split into a rainbow by shining it
through a prism.
 Bunsen and Krichhoff: devised the first spectroscope to measure the color of light given off
by elements heated by a flame
 They found that each kind of atom has a special “fingerprint” I.e. a unique set of
spectral lines when the electrons are excited at high temperatures
 In 1868 during a solar eclipse Janssen observed the solar spectrum and found
“fingerprints” (spectral lines) he did not recognize.
 This new element was named HELIUM after Helios, the Greek word for Sun
SOHO Project Scientist Team
Solar Spectrum
 Ultraviolet radiation.
Chemical rays or latent light referred to what we now call ultraviolet radiation.
During the early years of the development of spectroscopy it was discovered that "invisible"
rays were emanating from the Sun. In 1800 William Herschel investigated the heating
power of rays in the solar spectrum. He found that the maximum heating effect was
located just beyond the visible red end of the solar spectrum. Initially called "heat rays“
they later became known as infrared radiation.
Soon afterwards came the discovery of ultraviolet radiation or "chemical rays" by German
scientist R.W Ritter. He found a blackening effect on silver chloride due to unseen rays
beyond the blue end of the spectrum.
SOHO Project Scientist Team
The Sun
 The Sun is 333,400 times more massive than the Earth and contains 99.86%
of the mass if the entire solar system
 It consist of 78% Hydrogen, 20% Helium and 2% of other elements
 Total energy radiated: 100 billion tons of TNT per second
 Core pressure: 340 billion times Earth’s air pressure at sea level
 Every second 700 billion tons of hydrogen are converted into helium
 4 billion tons is converted into energy each second
 The Sun will run out of fuel in 5 billion years
SOHO Project Scientist Team
The Sun’s Structure
 Core
 Where the energy is created.
 Nuclear reactions burn every second
about 700 million tons of hydrogen into
 Inside the core the particles are packed so
tightly, and the temperature is so hot, that
individual atoms ram into each other,
forming heavier helium atoms and
releasing energy
SOHO Project Scientist Team
The Sun’s Structure
 Radiation Zone
 Where energy is transported by radiation.
 Although the photons travel at the speed
of light, they bounce so many times
through the dense material that they use
about a million years to escape the Sun.
 Convection Zone
 Energy transported by convection (just
like boiling soup) where heat is
transported to the photosphere.
SOHO Project Scientist Team
The SOHO Spacecraft
Mass: 1800 Kg
Size: 4 x 9 m
The total mass of the spacecraft at launch was
1 850 kg (payload 655kg). Its overall length
along the sun-pointing axis is 4.3 metres, and
the span of the extended solar panels is 9.5
SOHO Project Scientist Team
Solar Radiation
SOHO Project Scientist Team
The Sun –
our closest star
SOHO Project Scientist Team
 Dark areas (umbra, penumbra)
 Strong magnetic fields
 Frequency varies with the 11-year
solar cycle
 Inhibit energy transport from solar
 Cooler areas, and therefore darker
Light and dark in this magnetic
scan of the Sun indicate
concentrated areas of intense
magnetic field lines.
Close-up of sunspots
SOHO Project Scientist Team
Sharpest ever pictures of the Sun – SWT LaPalma
Close-up of sunspots
SOHO Project Scientist Team
The Solar Corona
 The corona is the area just
above the surface. While
the surface is about 5,000o
Celsius, the temperature
in the corona reaches
about 2 million degrees
Celsius. What causes this
rapid increase in temperature is still one of the big
mysteries in solar physics.
5,000 o C
2,000,000 o C
The black circle divides two images.
SOHO Project Scientist Team
Total solar eclipse video
This video of the June 21, 2001 eclipse seen in Africa shows the Sun just as it is going into totality
SOHO Project Scientist Team
Credit: Serge Koutchmy, IAP, 1991
Total eclipse photos
SOHO Project Scientist Team
3 Weeks of EIT observations
Fe XII 195 Å (1.500.000 K) 17 May - 8 June 1998
SOHO Project Scientist Team
 This solar image is taken through a 10Å wide filter centered on the K line
of Calcium ( Å).
 Bright, filamentary structures, most easily seen near the limb
SOHO Project Scientist Team
 This image is taken through a filter
centered on a spectral line of
Hydrogen (H , wavelength Å) that
forms above the surface of the Sun
 Interesting new features seen on
this image are filaments, dark
string-like structures visible on the
disk, and prominences, bright
structures extending outward over
the limb
 Physically, filaments and
prominences are one and the same,
namely condensations of cooler
gas high up in the solar
SOHO Project Scientist Team
 Some filaments and prominences can reach impressive sizes, and remain visible
very far above the solar disk. This prominence was photographed in June 1946
again through a filter centered on H , and extends some 200000 km above the solar
SOHO Project Scientist Team
TRACE: “the new kid on the block”
SOHO Project Scientist Team
Rotation tangles these field lines
Solar rotation causes magnetic field lines to become twisted and stretched to
thbreaking point. These eventually break and reconnect, creating heat, intense
active regions, and solar blasts of charged particles.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
SOHO Project Scientist Team
What is the Solar Wind?
 A constant stream of particles flows from the Sun’s corona, with a temperature of
about a million degrees and with a velocity of about 450 km/s. The solar wind
reaches out beyond Pluto's orbit (about 5900 million kilometers). The drawing shows
how it pushes on
and shapes
the Earth’s
(the dotted line).
SOHO Project Scientist Team
The extended corona /solar wind
LASCO -Large Angle and Spectroscopic Coronagraph
Observe the corona from 2 - 32 Rs in
white light with overlapping fields of
SOHO Project Scientist Team
The Sun as seen with
SOHO Project Scientist Team
SOHO Captures Planet Gathering
SOHO Project Scientist Team
Comets observed with SOHO/LASCO
(over 500 comets discovered so far!)
SOHO Project Scientist Team
Comet 96P/Machholz
 Discovered in 1986, observed by SOHO in 96.
 12 times brighter in 2002 that expected
SOHO Project Scientist Team
Comet NEAT (C/2002 V1)
SOHO Project Scientist Team
Solar Activity Cycles and Climate
The Sun varies on all timescales.
• 11 year cycle (Schwabe cycle)
• 22 year cycle (magnetic cycle)
• 80-90 year cycle (Gleissberg)
•180-210 year cycle (Seuss)
 Sir William Herschel (1796): Suggested a correlation
between number of sunspots and the price of wheat
SOHO Project Scientist Team
From Solar Min towards Solar Max
SOHO Project Scientist Team
Aurora Borealis
SOHO Project Scientist Team
What is Space Weather?
 SPACE WEATHER refers to conditions on the Sun and in the solar wind, magnetosphere,
ionosphere, and thermosphere that can influence the performance and reliability of space
born and ground-based technological systems and that can affect human life or health.
“Space Weather” effects on
installations on Earth not a new
17 November 1848: Telegraph
wire between Pisa and Florence
September 1851:
Telegraph wire in New England
The following is a transcript
between Portland and Boston
Portland: “Please cut off
your battery, let us see
if we can work with the
auroral current alone”
Boston: “I have already
done so! How do you
receive my writing?”
Portland: “Very well
indeed - much better
that with batteries”
SOHO Project Scientist Team
The Biggest Sunspot Group
in 10 Years
 AR 9393 developed a complex delta-gamma
magnetic field configuration while rotating towards
the center of the Sun
 An X-flare and CME was observed on 29 March 2001
causing a severe geomagnetic storm on Earth
 Aurora was observed south in Europe
ALASKA (Zimmerman)
Nice (Benvenuto)
SOHO Project Scientist Team
The 14 July 2000 Event
SOHO Project Scientist Team
Solar Activity Cycles and Climate
During the Little Ice Age,
London’s Thames River froze in
winter in the 17th Century, a very
rare event.
"Winter Scene with Frozen Canal" by Aert van der Neer
Eddy, 1976, Science, 192, 1189
Solar activity
Friis-Christensen & Lassen, 1991, Science, 245, 698
SOHO Project Scientist Team
The Sun-Earth Connection
 How do the planet respond to solar variations?
 Disruption of technology based systems
 Harm humans in space
 Climate change
SOHO Project Scientist Team
Navigation systems - GPS
When the ionosphere between the satellites and the user
becomes turbulent and irregular, the signal may “scintillate”
and prove difficult to track
loss of signal lock on one or several satellites
Both single and dual frequency systems may be affected
 The Total Electron Content (TEC) along the path of a GPS
signal can introduce a positioning error ( up to 100 m)
 The effects on GPS could be one of the most significant space
weather effects due to the planned reliance of this system in
the future.
SOHO Project Scientist Team