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The Sun
Magnetic Fields and Charged Particles
•Charged particles are affected by magnetic fields
•They are forced to follow magnetic field lines
Magnetic fields
Plasmas
•At ordinary temperatures, atomic nuclei attract electrons to
make them neutral
•At extreme temperatures, the electrons pop loose, and the
electrons can run off freely
•This is a plasma
•Plasmas are excellent conductors
•They can generate magnetic fields
Atomic spectra
•Suppose you take an atom and bump the
electrons up (by heating it, for example).
What type of light comes out?
•Spectrum consists of emission lines
•Exactly which lines tells you the kind of
atom, like a fingerprint
Atomic spectra
•Suppose you take an atom and shine a
bright white light on it. What does the
spectrum of the light look like when it
comes out?
•Initial spectrum is continuous
•Atom absorbs certain very specific
frequencies
•The same frequencies you saw before.
Kirchoff’s Laws
•Hot thick solid, liquid, or gas – Continuous spectrum
•Thin gas – bright line spectrum
•Thin gas with hotter thick gas behind it – dark line spectrum
Kirchoff’s Laws
Continuous Spectrum:
Hot, thick solid, liquid, or gas
Bright Line Spectrum:
Hot, thin gas
Dark Line Spectrum:
Cooler gas in front of hot, thick solid, liquid, or gas
Stellar Spectra
Star A Spectrum
Hydrogen Spectrum
From this spectrum you can tell this star
A) Is made of hydrogen
B) Is made of gas
C) Is hot on inside, cooler on outside
D) All of the above
Stellar Spectra
Star B Spectrum
Hydrogen Spectrum
This star is made of:
A) Hydrogen
B) Something besides hydrogen
C) Part hydrogen, part something else
The Sun’s Spectrum
•Hundreds of lines
•Scores of Elements
Composition of a Typical Star (Sun)
•Hydrogen
•Helium
•Oxygen
•Carbon
•Iron
•Neon
•Sulfur
•Nitrogen
•Silicon
•Etc.
Know
These!
The Sun: Basic Facts
•109  Earth
diameter
•333,000 
Earth mass
•Density: a bit
higher than
water
•Rotates once
every 25 days
•Differential
rotation
March 1,
2016 from
SDO
The Sun: How We Know What We Know
Exterior
•Spectra
•Temperature
•Motion
•Chemical Composition
•Magnetic Fields
•Ground-based telescopes
•Eclipses
•Space telescopes
•Solar Wind
Interior
•Gravity
•Magnetic Fields
•Neutrinos
•Helioseismology
Solar Dedicated Spacecraft
Solar
Dynamics
Observatory
STEREO
SOHO
Ulysses
Outer Layers of the Sun
•Photosphere
•The layer we see
•Sunspots
•Chromosphere
•Where the dark lines are made
•Corona
•Prominences
•Coronal Holes
•Flares
•Solar Wind
The Photosphere
•The point at which gas is
thick enough that we can’t
see through it
•This is a little shallower
towards the edge
•Limb darkening
Here we see
shallow and
cool
Here we see
deep and hot
Granulation
•The Sun’s
surface is not
uniformly bright
•Hot regions of
upwelling gas
•Cooler regions
of falling gas
•Convection near
the surface
https://www.youtube.com/watch?v=W_Scoj4HqCQ
Granulation
•Granulation changes over time
Granulation
•Circulation of gases in outer layer
•Hot plasma rising, cool plasma falling – convection
•Underneath, evidence of circulation on a larger scale
•Supergranules larger than the Earth
Earth
Chromosphere
•A cooler layer just outside the photosphere
•Light from photosphere lines absorbed from this layer
•Bright line spectrum
•Visible during eclipses
•Red color from spectral line
of hydrogen
Spicules
•Spicules – bright small jets that reach from the
photosphere up through the chromosphere
•Last about 5 minutes
•Cause not understood
•Thousands on the Sun at any
given time
Corona
•Thin, extremely hot region
•Visible during eclipses
If you took a spectrum of
the corona, it would be
A) A bright line spectrum
B) A dark line spectrum
C) A continuous spectrum
Corona: Temperature
•High
temperature
not well
understood
•Heated by
magnetic
fields
•Like a
whip
•Low density –
has little total
heat
Sunspots
•Relatively cool regions on a
brighter, hotter background
•Large magnetic fields
What causes them:
•Mag. Field sticks out
•Plasma can’t flow
horizontally
•Prevents convection from
working
•Hot gas can’t rise
•Temperature cooler
Sunspots - Structure
Penumbra
Umbra
Sunspots often come in groups
•Pairs are
common
•North
with south
•Larger
groups are not
uncommon
The Solar Cycle
•The number of sunspots changes over time
•Approximately an eleven year cycle: Butterfly diagram
•Sunspots first appear far from the equator
•Over time, they become more numerous, and move towards
the equator
•Then they all disappear
The Solar Cycle
•Sun’s North/South magnetic pole reverses each cycle
•Not all cycles are created equal!
•1650-1700, very low level, “Maunder Minimum”
•Corresponded to “Little Ice Age” in Europe
•We just finished an unusually quiet and long Solar Minimum
•Level of sunspots/magnetic activity affects all other solar
activity
The Solar Cycle
Flares
•Explosive release of magnetic energy on surface of the Sun
•Typically reach through photosphere, chromosphere, to
corona
•Burst of energy, especially
X-rays
•Can cause coronal mass
ejections
Prominences
•Loops of magnetic fields sticking
high above the Sun’s surface
Prominences
Prominences
Prominences
•Charged
particles
following
magnetic lines
from inside the
Sun
•Show movie
Coronal Mass Ejections
•Flares can expel gas at high velocities from the Sun
•Similarly, prominences can become “disconnected”
•High temperature gas expelled at high velocity
•Up to half the speed of light
•These events are called coronal mass ejections
•They can cause satellite malfunction and communication
disruptions on Earth
•They pose a major danger to astronauts beyond the Earth’s
magnetosphere
Coronal Mass Ejections
Coronal Holes
•Most gas in the corona
is trapped by the Sun’s
magnetic fields
•“Coronal holes” allow
some to escape to space
•This creates the solar
wind
The Solar Wind
•Particles flowing outward from Sun travel to Earth and beyond
•Contains hydrogen, helium, etc.
•Magnetic fields Earth funnel them to Earth’s magnetic poles
•Causes the Aurora Borealis and Aurora Australis