Download THE SUN - University of Mass Lowell, Space Science Laboratory

Lecture 10
THE SUN
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)
THE SUN
1.
2.
3.
4.
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
5. THE SOLAR CYCLE
6 . SOLAR FLARES AND CORONAL MASS EJECTIONS
• Description and Physical Processes • Classifications
7. OPERATIONAL EFFECTS OF SOLAR FLARES
a) radio noise
c) HF absorption
b) sudden ionospheric disturbances
c) PCA events
2
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.
3
OTHER SUN FACTS
•
•
•
•
•
•
•
•
•
•
•
•
•
radius
mean distance from earth (1 AU) =
mass
mean density
surface pressure
mass loss rate
surface gravity
equatorial rotation period
near poles
inclination of sun's equator to ecliptic
total luminosity
escape velocity at surface
effective blackbody temperature
6.96 x 105 Km 109 RE
1.49 x 108 Km 215 RS
1.99 x 1030 Kg 330,000 ME
1.4 x 103 Kg m-3 1/4 rE
200 mb
1/5 psE
109 Kg s-1
274 ms-2
28 gE
26 days
37 days
7°
23.5° for Earth
3.86 x 1026 W
1368 Wm-2 @ Earth
618 km s-1
5770 K
4
REGIONS OF THE SUN’S
INTERIOR AND ATMOSPHERE
p-modes
g-modes
(See Fig. 5.1)
5
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
heating element
In the laboratory
T2
The radiation reacts
thoroughly with the
body and is
characteristic of
the body
T1
area
 T4
T1>T2
wavelength
6
Radiation Laws
Planck's Law:
2c 106  4
 photons / m 2 / s / sr / m 
B 
exp  hc  K BT   1
Stefean - Boltzmann's Law:
E   T 4 W / m 2   5.67 108 W / m 2  K 4
Wien's Displacement Law:
max
2898  106
m

T
ELECTROMAGNETIC RADIATION
The Sun emits radiation over a
range of wavelengths
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
8
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
9
10
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
11
2. ENERGY GENERATION AND TRANSFER
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
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)
12
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.
absorption/
re-emission
CORE
gamma
radiation
visible radiation
( If radiation came
straight out, it would
take 2 seconds; due to
all the scatterings, it
takes 10 million years !)
convection
(opaque region)
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 @ 5770 K.
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
13
GRANULES
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
QuickTime™ and a
Photo decompressor
are needed to see this picture.
14
HOW DO WE INFER THE
INTERNAL PROPERTIES OF THE SUN ?
15
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 Earth’s
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
Sun’s 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.
16
REGIONS OF THE SUN’S
INTERIOR AND ATMOSPHERE
p-modes
g-modes
17
3. REGIONS OF THE SOLAR ATMOSPHERE:
THE PHOTOSPHERE
The photosphere is the Sun’s 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 Sun’s light and heat
comes out of this narrow layer.
18
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).
6563 Å
In H-, a number of chromospheric features can
be seen, such as bright plages around sunspots,
dark filaments, and prominences above the limb.
19
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.
20
UV solar
emission lines
and
corresponding
regions and
temperatures
21
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
22
4. FEATURES OF THE SOLAR ATMOSPHERE:
SUNSPOTS
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).
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
23
SUNSPOTS ARE REGIONS OF INTENSE
MAGNETIC FIELDS
The video below depicts regions of
negative (black) qnd positive (white)
magnetic polarity (like a magnet).
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
24
CHROMOSPHERIC FILAMENTS
& PLAGES
Filaments are the dark, ribbon-like
features seen in H light against the
brighter solar disk.
H, 6563 Å
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
25
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.
26
27
28
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
29
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.
30
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.
31
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.
32
33
Evolution of the Sun’s
X-ray emission over
the 11-year solar
cycle
34
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.
35
CORONAL MASS EJECTIONS (CMEs)
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
36
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
37
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.
38
frequency of optical solar flares during cycles 20-21
39
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
Earth’s atmosphere absorbs all solar x-rays before they reach the Earth’s
surface.
Classification
(ergs/cm2-sec)
X-Ray Flux
C
10-3
M
10-2
X
10-1
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.
40
41
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
The Bastille-day flare was ‘Xclass’ and accompanied by
one of the largest
solar energetic proton events
ever recorded
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
c3714
42
7. OPERATIONAL EFFECTS OF SOLAR FLARES
43
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.
44
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.
45
Ionospheric Plasma
A plasma is a gaseous mixture of electrons,
ions, and neutral particles. The ionosphere is
a weakly ionized plasma.
--
+
+ --
+
+
+ -+
--
+ +
E
--
+
+
--
----
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.
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.
46
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
47
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)
48
Ionospheric Disturbances
Ionospheric disturbances occur when
the Earth’s 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 flare’s Xray emission (generally constrained to
dayside).
49
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.
50
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 Earth’s magnetic field
lines towards the polar ionosphere’s 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.
51
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
Earthionosphere
waveguide.
52
Time Scales for Solar Flare Effects
53
Miscellaneous
54
REFRACTION OF ACOUSTIC WAVES IN THE SUN
Reflective boundaries organize wave motions into
patterns by constructive and destructive
interference
Phase speed of acoustic wave
C
ph


k


H
T
, T = period
surface density gradient
H
Increasing
temperature,
speed of sound
faster
Faster propagation
here so waves
refract towards surface
55
• 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
56
• 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 Earth’s 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 (~ 220 minutes); but, because they
have different H’s, they have
different Cph’s and therefore
penetrate to different depths
57
SOME CONTRIBUTIONS OF HELIOSEISMOLOGY
•
Convection zone deeper (R=0.71) 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.
58
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
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
The amount of energy
released is equivalent to
millions of 100-megaton
hydrogen bombs
exploding at the same time
59
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 xrays seen by TRACE reflect this
heating.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
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.
60
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 (> 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.
61
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
62
The first x-ray images > 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.
63
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
64
CME Rate
CME Rate by Carrington Rotation
CME Rate [CMEs/day]
6
Solwind
(1979-1984)
SMM (19841989)
SOHO
(1996-2002)
4
2
0
1979
1982
1985
1988
1991
Year
1994
1997
2000
27d Average 2800MHz Solar Flux
----- (Max=254)
27d Average 2800MHz Solar Flux ----- (Max=254)
65
CME Latitude Distributions
SOHO LASCO
1996
SOHO LASCO 1996 (197 CMEs)
Fraction in 5° Interval
0.20
0.15
0.10
0.05
0.00
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
40
50
60
70
80
90
Halo
Apparent Latitude [°]
2000
SOHO LASCO 2000 (1,534 CMEs)
Fraction in 5° Interval
0.20
0.15
0.10
0.05
0.00
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
80
90
Halo
Apparent Latitude [°]
66
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 LongDuration-Event (LDE) soft X-ray
flares.
67