THE SUN

1
THE SUN
Lecture 10

• This set of slides was compiled by Prof. Jeff
  Forbes of the Aerospace Engineering Department,
  University of Colorado, Boulder
• (It is used here with his permission, which I
  received at CDG Airport, Paris, France, on
  4/12/03)

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THE SUN

• GENERAL CHARACTERISTICS
• Descriptive Data Electromagnetic
  Radiation Particle Radiation
• ENERGY GENERATION AND TRANSFER
• Core ? Radiation Zone ? Convection Zone ?
  Solar Atmosphere
• REGIONS OF THE SOLAR ATMOSPHERE
• Photosphere, Chromosphere, Corona
• FEATURES OF THE SOLAR ATMOSPHERE
• Coronal Holes, Flares, Sunspots, Plages,
  Filaments Prominences
• THE SOLAR CYCLE
• 6 . SOLAR FLARES AND CORONAL MASS EJECTIONS
• Description and Physical Processes
  Classifications
• 7. OPERATIONAL EFFECTS OF SOLAR FLARES
• a) radio noise b) sudden ionospheric
  disturbances
• c) HF absorption c) PCA events

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Our Sun

• Our Sun is a massive ball of gas held together
  and compressed under its own gravitational
  attraction.
• Our Sun is located in a spiral arm of our Galaxy,
  in the so-called Orions arm, some 30,000
  light-years from the center.
• Our Sun orbits the center of the Milky Way in
  about 225 million years. Thus, the solar system
  has a velocity of 220 km/s
• Our galaxy consists of about 2 billion other
  stars and there are about 100 billion other
  galaxies
• Our Sun is 333,000 times more massive than the
  Earth .
• It consists of 90 Hydrogen, 9 Helium and 1 of
  other elements
• Total energy radiated equivalent to 100 billion
  tons of TNT per second, or the U.S. energy needs
  for 90,000 years - 3.86x1026 W
• Is 5 billions years old another 5 billion to go
• Takes 8 minutes for light to travel to Earth
• The Sun has inspired mythology in many cultures
  including the ancient Egyptians, the Aztecs, the
  Native Americans, and the Chinese.

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OTHER SUN FACTS

• radius 6.96 x 105 Km 109 RE
• mean distance from earth (1 AU) 1.49 x
  108 Km 215 RS
• mass 1.99 x 1030 Kg 330,000 ME
• mean density 1.4 x 103 Kg m-3 1/4 rE
• surface pressure 200 mb 1/5 psE
• mass loss rate 109 Kg s-1
• surface gravity 274 ms-2 28 gE
• equatorial rotation period 26 days
• near poles 37 days
• inclination of sun's equator to ecliptic
  7 23.5 for Earth
• total luminosity 3.86 x 1026 W 1368 Wm-2 _at_
  Earth
• escape velocity at surface 618 km s-1
• effective blackbody temperature 5770 K

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REGIONS OF THE SUNS INTERIOR AND ATMOSPHERE
p-modes
g-modes
(See Fig. 5.1)
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The Sun radiates at a blackbody temperature of
5770 K
A blackbody is a perfect radiator in that the
radiated energy depends only on temperature of
the body, resulting in a characteristic emission
spectrum.
insulation
radiated energy
?max ? 1/T
In a star
T2
heating element
The radiation reacts thoroughly with the body and
is characteristic of the body
T1gtT2
radiated energy
T1
In the laboratory
area a T4
wavelength
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Radiation Laws
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ELECTROMAGNETIC RADIATION
The Sun emits radiation over a range of
wavelengths
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The wavelengths most significant for the space
environment are X-rays, EUV and radio waves.
Although these wavelengths contribute only
about 1 of the total energy radiated, energy
at these wavelengths is most variable
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PARTICLE RADIATION

• The Sun is constantly emitting streams of
  charged particles, the solar wind, in all outward
  directions.
• Solar wind particles, primarily protons and
  electrons, travel at an average speed of 400km/s,
  with a density of 5 particles per cubic
  centimeter.
• The speed and density of the solar wind increase
  markedly during periods of solar activity, and
  this causes some of the most significant
  operational impacts

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2. ENERGY GENERATION AND TRANSFER

• The core of the Sun is a very efficient
  fusion reactor burning hydrogen fuel at
  temperatures 1.5 x 107 K and producing He
  nuclei
• 4 H1 ? He4 26.73 MeV
• This 26.73 MeV is the equivalent of the mass
  difference between four hydrogen nuclei and a
  helium nucleus. It is this energy that fuels the
  Sun, sustains life, and drives most physical
  processes in the solar system. (See eqs 5.1 to
  5.5 for details)

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Between the radiation zone and the surface,
temperature decreases sufficiently that electrons
can be trapped into some atomic band states,
increasing opacity convection then assumes main
role as energy transfer mechanism.
( If radiation came straight out, it would take 2
seconds due to all the scatterings, it takes 10
million years !)
Near the surface, in the photosphere, radiation
can escape into space and again becomes the
primary energy transport mechanism. The
photosphere emits like a black body _at_ 5770 K.
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GRANULES
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HOW DO WE INFER THE INTERNAL PROPERTIES OF THE
SUN ?
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HELIOSEISMOLOGY

is the study of the interior of the
Sun from observations of the vibrations of
its surface.
In the same way that seismologists use
earthquakes and explosions to explore Earths
crust, helioseismologists use acoustic waves,
thought to be excited by turbulence in the
convection zone, to infer composition,
temperature and motions within the Sun.
By subtracting two images of the Suns surface
taken minutes apart, the effects of solar
oscillations are made apparent by alternating
patches in brightness that result from heating
and cooling in response to acoustic vibrations of
the interior.

• Another way of inferring the corresponding
  upward and downward motions of the surface
• is by measuring the Doppler shifts of spectral
  lines.

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REGIONS OF THE SUNS INTERIOR AND ATMOSPHERE
p-modes
g-modes
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3. REGIONS OF THE SOLAR ATMOSPHERE THE
PHOTOSPHERE
The photosphere is the Suns visible surface,
a few hundred km thick, characterized by
sunspots and granules The solar surface is
defined as the location where the optical depth
of a ? 5,000 Å photon is 1 (the probability of
escaping from the surface is 1/e)
The photosphere is the lowest region of the solar
atmosphere extending from the surface to the
temperature minimum at around 500 km. 99 of the
Suns light and heat comes out of this narrow
layer.
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THE CHROMOSPHERE
The chromosphere is the 2000 km layer above
the photosphere where the temperature rises from
6000 K to about 20,000 K. At these higher
temperatures hydrogen emits light that gives off
a reddish color (H-alpha emission) that can be
seen in eruptions (prominences) that project
above the limb of the sun during total solar
eclipses.
When viewed through a H-alpha filter, the sun
appears red. This is what gives the chromosphere
its name (color-sphere). In H-a, a number of
chromospheric features can be seen, such as
bright plages around sunspots, dark filaments,
and prominences above the limb.
6563 Å
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THE CORONA
The corona is the outermost, most
tenuous region of the solar atmosphere extending
to large distance and eventually becoming the
solar wind.
The most common coronal structure seen on eclipse
photographs is the coronal streamer, bright
elongated structures, which are fairly wide near
the solar surface, but taper off to a long,
narrow spike.
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UV solar emission lines and corresponding regions
and temperatures
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The corona is characterized by very high
temperature (a few million degrees) and by the
presence of a low density, fully ionized plasma.
Here closed field lines trap plasma and keep
densities high, and open field lines allow plasma
to escape, allowing much lower density regions to
exist called coronal hoes.
TRANSITION REGION
At the top of the chromosphere the temperature
rapidly increases from about 104 K to over 106 K.
This sharp increase takes place within a narrow
region, called the transition region. The
heating mechanism is not understood and remains
one of the outstanding questions of solar physics
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4. FEATURES OF THE SOLAR ATMOSPHERESUNSPOTS
Sunspots are areas of intense magnetic fields.
Viewed at the surface of the sun, they appear
darker as they are cooler than the surrounding
solar surface - about 4000oC compared to the
surface (6000oC).
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SUNSPOTS ARE REGIONS OF INTENSE MAGNETIC FIELDS
The video below depicts regions of negative
(black) qnd positive (white) magnetic polarity
(like a magnet).
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CHROMOSPHERIC FILAMENTS PLAGES
Filaments are the dark, ribbon-like features
seen in Ha? light against the brighter solar
disk. The material in a filament has a
lower temperature than its surroundings, and thus
appears dark. Filaments are elongated blobs of
plasma supported by relatively strong magnetic
fields. Plages are hot, bright regions of the
chromosphere, often over sunspot regions, and
are often sources of enhanced 2800 MHz (10.7 cm)
radio flux
Ha, 6563 Å
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SOLAR PROMINENCES
Prominences are variously described as surges,
sprays or loops.
Filaments are referred to as prominences when
they are present on the limb of the Sun, and
appear as bright structures against the
darkness of space.
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CORONAL HOLES
One of the major discoveries of the Skylab
mission was the observation of extended dark
coronal regions in X-ray solar images.
Coronal holes are characterized by low density
cold plasma (about half a million degrees colder
than in the bright coronal regions) and unipolar
magnetic fields (connected to the magnetic field
lines extending to the distant interplanetary
space, or open field lines). Near solar minimum
coronal holes cover about 20 of the solar
surface.
The polar coronal holes are essentially permanent
features, whereas the lower latitude holes only
last for several solar rotations.
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5. THE SOLAR CYCLE
Maunder Minimum
The number of sunspots (Zurich or Wolf
sunspot number -- see Intro) on the solar disk
varies with a period of about 11 years, a
phenomenon known as the solar (or sunspot) cycle.
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Sunspot latitude drift
The remarkably regular 11-year variation of
sunspot numbers is accompanied by a similarly
regular variation in the latitude distribution of
sunspots drifts toward the equator as the solar
cycle progresses from minimum to maximum.
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Evolution of the Suns X-ray emission over the
11-year solar cycle
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6. CMEs SOLAR FLARES

• Flares and CMEs are different aspects of solar
  activity that are not necessarily related.
• Flares produce energetic photons and particles.
  
• CMEs mainly produce low-energy plasma.
• CMEs and flares are very important sources of
  dynamical phenomena in the space environment.
• The triggering mechanisms for CMEs and flares,
  and the particle acceleration mechanisms, are
  not understood beyond a rudimentary level.
  However, this knowledge is essential for
  development of predictive capabilities.

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CORONAL MASS EJECTIONS (CMEs)
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Size of Earth Relative to Solar CME Structure

• The Earth is small compared to the size of the
  plasma blob from a Coronal Mass Ejection (CME).
• The image shows the size of a CME region shortly
  after lift off from the solar corona.
• The CME continues to expand, as it propagates
  away from the Sun, until its internal pressure is
  just balanced by the magnetic and plasma pressure
  of the surrounding medium.

CME
Earth
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Optical Classification of Flares
The optical (as seen in Hydrogen-alpha light)
classification of a flare is made using a
two-character designation. For example, a 1B
designation indicates a brilliant intensity
flare covering a corrected area between 100 and
249 millionths of the solar hemisphere.
FLARE BRIGHTNESS CATEGORIES F FAINT N
NORMAL B BRILLIANT
The most common optical flare intensity or
brilliance classification is based on the
doppler shift of the H-alpha line. This doppler
shift is a measure of the ejected gas particle
velocity and is used by observers to make a
subjective estimate of flare intensity.
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frequency of optical solar flares during cycles
20-21
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X-Ray Classification of Flares
The most common x-ray index is based on the peak
energy flux of the flare in the 1 to 8 Å soft
x-ray band measured by geosynchronous
satellites. These measurements must be made from
space, since the Earths atmosphere absorbs all
solar x-rays before they reach the Earths
surface.
The left categories are broken down into nine
subcategories based on the first digit of the
actual peak flux. For example, a peak flux of 5.7
x 10-2 ergs/cm2-sec is reported as a M5 soft
x-ray flare.
Classification X-Ray Flux (ergs/cm2-sec)
C 10-3 M 10-2 X 10-1
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The Bastille-day flare was X-class and
accompanied by one of the largest solar energetic
proton events ever recorded
c3714
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7. OPERATIONAL EFFECTS OF SOLAR FLARES
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Solar Effects on Radio Wave Reception

• Radio Noise Storms. Sometimes an active region on
  the Sun can produce increased noise levels,
  primarily at frequencies below 400 MHz. This
  noise may persist for days, occasionally
  interfering with communication systems using an
  affected frequency.

• Solar Radio Bursts. Radio wavelength energy is
  constantly emitted from the Sun however, the
  amount of radio energy may increase significantly
  during a solar flare. These bursts may interfere
  with radar, HF (3 30 MHz) and VHF (30 300
  MHz) radio, or satellite communication systems.
  Radio burst data are also important in helping to
  predict whether we will experience the delayed
  effects of solar particle emissions.

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Solar Effects on Radio Wave Reception
Systems in the VHF through SHF range (30 MHz to
30 GHz) are susceptible to interference from
solar radio noise.
If the Sun is in the reception field of the
receiving antenna, solar radio bursts may cause
Radio Frequency Interference (RFI) in the
receiver, as depicted here.
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Ionospheric Plasma
A plasma is a gaseous mixture of electrons, ions,
and neutral particles. The ionosphere is a
weakly ionized plasma.
--

--
If, by some mechanism, electrons are displaced
from ions in a plasma the resulting separation of
charge sets up an electric field which attempts
to restore equilibrium. Due to their momentum,
the electrons will overshoot the equilibrium
point, and accelerate back. This sets up an
oscillation.

--

--

--


--
--



--
--
E

The frequency of this oscillation is called the
plasma frequency, ? 2?f (Nee2/me)1/2, which
depends upon the properties of the particular
plasma under study.
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Radio Waves in an Ionospheric Plasma
A radio wave consists of oscillating electric and
magnetic fields. When a low-frequency radio wave
(i.e., frequency less than the plasma frequency)
impinges upon a plasma, the local charged
particles have sufficient time to rearrange
themselves so as to cancel out the oscillating
electric field and thereby screen the rest of
the plasma from the oscillating E-field.
This radio wave (low frequency) cannot penetrate
the plasma, and is reflected. For a high
frequency wave (i.e., frequency greater than the
plasma frequency), the particles do not have time
to adjust themselves to produce this screening
effect, and the wave passes through.
MUF
LUF
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Radio Waves in an Ionospheric Plasma
The critical frequency of the ionosphere (foF2)
represents the minimum radio frequency capable of
passing completely through the ionosphere.
N(cm-3)1.24x104 f2 (MHz)
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Ionospheric Disturbances
Ionospheric disturbances occur when the Earths
ionosphere (50 500 km) experiences a temporary
fluctuation in degree of ionization. This
variation can result from geomagnetic activity
(and the influences of the neutral atmosphere),
or it can be the direct result of X-rays and EUV
produced by a solar flare. A Sudden Ionospheric
Distrurbance (SID) is a disturbance that occurs
almost simultaneously with a flares X-ray
emission (generally constrained to dayside).
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• When collisions between oscillating electrons and
  ions and neutral particles becomes sufficiently
  frequent (as in the D-region, 60 90 km), these
  collisions absorb energy from the radio wave
  leading to what is called radio wave absorption.

Short Wave Fade (SWF) is a particular type of SID
that can severely hamper HF radio users (up to 20
30 MHz) by causing increased ionization and
signal absorption which may last for up to 1-2
hours.
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Solar Particle Events and Polar Cap Absorption
Part of the energy released in solar flares are
in the form of accelerating particles (mostly
proton and electrons) to high energies and
released into space. PCA events occur when high
energy protons spiral along the Earths magnetic
field lines towards the polar ionospheres
D-region (50 90 km altitude).
These particles cause significant increased
ionization levels, resulting in severe absorption
of HF radio waves used for communication and some
radar systems. This phenomenon, sometimes
referred to as polar cap blackout, is often
accompanied by widespread geomagnetic and
ionospheric disturbances.
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In addition, LF and VLF systems may experience
phase advances when operating in or through the
polar cap during a PCA event due to changes in
the Earth-ionosphere waveguide.
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Time Scales for Solar Flare Effects
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Miscellaneous
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REFRACTION OF ACOUSTIC WAVES IN THE SUN
Reflective boundaries organize wave motions into
patterns by constructive and destructive
interference
Phase speed of acoustic wave
surface density gradient
?H
Increasing temperature, speed of sound faster
Faster propagation here so waves refract towards
surface
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• These acoustic waves (where pressure is the
  restoring force)
• are called p-modes
• Internal gravity waves and surface waves also
  exist these
• are called g-modes and f-modes, respectively

Resonant modes have integral
of wavelengths around a circumference
p-modes
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• The frequency of an acoustic mode, and the
  spatial distance and the length of time it takes
  to re-appear at the surface after being refracted
  lower down, are sensitive to the properties of
  the intervening region.
• Seismic studies of Earths interior are
  performed by measuring the propagation of waves
  from a point source (i.e., explosion or
  earthquake epicenter)
• On the Sun, helioseismic
• studies are based on statistical
• correlations between various
• points on the Sun

These may all have similar T ( 2-20 minutes)
but, because they have different lHs, they have
different Cphs and therefore penetrate to
different depths
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SOME CONTRIBUTIONS OF HELIOSEISMOLOGY

• Convection zone deeper (R0.71) than previously
  thought.
• Opacity used in models was too low.
• Limits set on the abundance of
• Helium in convection zone.
• Rotation rate of the convection zone
• is similar to that of surface.
• Near the convection zone base,
• rotation rate near the equator
• decreases with depth, and rotation
• rate at high latitudes increases
• with depth, so that the outer
• radiation zone is rotating at a
• constant intermediate rate.
• The shear between the outer radiation zone and
  inner convection zone may hold the key to the
  11-year cycle.

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A SOLAR FLARE is defined as a sudden, rapid, and
intense variation in brightness.
A solar flare occurs when magnetic energy that
has been built up in the solar atmosphere is
suddenly released. Radiation is emitted across
the spectrum -- radio, visible, x-ray,
gamma-rays The amount of energy released is
equivalent to millions of 100-megaton hydrogen
bombs exploding at the same time
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In solar flares, electrons are both heated to
high temperatures, and accelerated
The electrons are thought to be accelerated by
the collapse of stretched magnetic field lines
high above the solar surface (magnetic
reconnection'').
The accelerated electrons heat up th thermal
plasma in the loop directly, and indirectly by
chromospheric evaporation. The soft or thermal
x-rays seen by TRACE reflect this heating.
The hard X-rays from the base of the active
region are bremsstrahlung'', or braking
radiation'', caused by electrons slamming into
the dense gases at the bottom of the corona. This
heated chromospheric gas rises up (chromospheric
evaporation) and also heats the thermal plasma
in the loop.
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Bremsstrahlung Radiation
High-energy electrons are decelerated through
attraction by positively-charged low-energy
ions. When electrons are decelerated, they give
off radiation called bremsstrahlung (or
braking) radiation, usually in the form of
hard x-rays, i.e., energies of order 10-100 keV
The type of radiation given off by the heated
thermal (10-30 million K) plasma is different,
consisting of soft x-rays (typically 1-10 keV),
and spectral lines from the elements in the hot
plasma, and some thermal bremsstrahlung from
very hot thermal plasma (gt 30 million K)
There are typically three stages to a solar flare
(each lasting from seconds to 1 hour).
precursor stage release of magnetic energy is
triggered. Soft x-ray emissions. impulsive
stage protons and electrons are accelerated to
energies exceeding 1 Mev radio waves, hard
x-rays, and gamma rays are emitted. decay
stage gradual build up and decay of soft x-rays.
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Solar flares Outstanding Questions

• What fraction of the energy released in flares
  goes into accelerating electrons and what
  fraction goes directly into heating electrons?
• Where does this heating and acceleration occur?
• What is the relationship between heating and
  acceleration?
• How are electrons accelerated to these high
  energies and heated to these high temperatures?
• We don't know the answers to any of these
  questions. The most direct tracer of
• these electrons is the x-ray emission they
  produce.
• Observations of hard x-rays (10-100 keV) allow
  us to study the
• accelerated electrons and the hottest plasma in
  flares
• Observations of soft x-rays (1-10 keV) allow us
  to study the
• thermal plasma component

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The first x-ray images gt 30 keV have been
obtained with the hard X-ray Telescope on the
Yohkoh satellite.
The relationship between the nonthermal
(accelerated) electrons and the hottest thermal
electrons can be studied by observing the time
evolution of both components during a flare.
Likewise, the relationship between these
energetic components and somewhat cooler thermal
plasma can be studied by comparing the hard x-ray
observations with the evolution of the soft x-ray
emission.
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RHESSI reveals X-rays in solar flare
This sequence of TRACE and RHESSI images shows
the spectacular solar flare of April 21 2002. The
green TRACE images show material at 2 million
degrees Centigrade (3.5 million degrees F) the
red and blue contours show soft and hard X-rays
detected by RHESSI. Surprisingly, RHESSI detects
X-rays well in advance of the onset of the flare
in the TRACE sequence.
Images of both hard and soft x-rays are crucial
for determining where the flare energy is
released and sorting out the relationships among
the thermal and non-thermal components
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CME Rate
Solwind (1979-1984)
SMM (1984-1989)
SOHO (1996-2002)
27d Average 2800MHz Solar Flux ----- (Max254)
27d Average 2800MHz Solar Flux ----- (Max254)
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SOHO LASCO
CME Latitude Distributions
1996
2000
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How are flares and CME's
related? Both involve the eruption of a magnetic
neutral line (but the spatial and temporal scales
are different!)

• The need to release built-up magnetic field
  energy leads to both flares and CMEs.
• There is good association between CMEs and
  Long-Duration-Event (LDE) soft X-ray flares.