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

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)

Lecture 10

2

THE SUN

1. GENERAL CHARACTERISTICS

• Descriptive Data • Electromagnetic Radiation • Particle Radiation

2. ENERGY GENERATION AND TRANSFER

• Core Radiation Zone Convection Zone Solar Atmosphere

3. REGIONS OF THE SOLAR ATMOSPHERE

• Photosphere, Chromosphere, Corona

4. 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 b) sudden ionospheric disturbances

c) HF absorption c) PCA events

3

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.

4

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 @ Earth

• escape velocity at surface 618 km s-1

• effective blackbody temperature 5770 K

5

REGIONS OF THE SUN’S INTERIOR AND ATMOSPHERE

p-modes

g-modes

(See Fig. 5.1)

6

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.

radiatedenergy

insulation

In the laboratory

In a star

The radiation reactsthoroughly with the

body and ischaracteristic of

the body

T1

T2

T1>T2

max 1/T

rad

iate

d e

ner

gy

wavelengtharea T4

heating element

Radiation Laws

6 4

2

4 2 8 2 4

6

max

:

2 10/ / / /

exp 1

:

/ 5.67 10 /

:

2898 10

B

cB photons m s sr m

hc K T

E T W m W m K

mT

Planck's Law

Stefean - Boltzmann's Law

Wien's Displacement Law

8

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

The Sun emits radiation over a range of wavelengths

ELECTROMAGNETIC RADIATION

9

The wavelengths most significant for the space environment are X-rays, EUV andradio waves. Although these wavelengths contributeonly about 1% of the total energy radiated, energy at these wavelengths is mostvariable

10

11

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

12

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)


13

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.

visible radiation

gammaradiation

absorption/re-emission

convection(opaque region)

CORE

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.


( If radiation came straight out, it would take 2 seconds; due to all the scatterings, it takes 10 million years !)

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QuickTime™ and aPhoto decompressor

are needed to see this picture.

QuickTime™ and aCinepak decompressor



GRANULES

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HOW DO WE INFER THE INTERNAL PROPERTIES OF THE SUN ?

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HELIOSEISMOLOGY

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

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.

17

REGIONS OF THE SUN’S INTERIOR AND ATMOSPHERE

p-modes

g-modes

18

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 wherethe 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.

3. REGIONS OF THE SOLAR ATMOSPHERE:THE PHOTOSPHERE

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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 givesthe chromosphere its name (color-sphere).

In H-, a number of chromospheric features can be seen, such as bright plages around sunspots, dark filaments, and prominences above the limb.

THE CHROMOSPHERE

6563 Å

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The corona is the outermost, most tenuous region of the solar atmosphere extending to large distance and eventually becoming the solar wind.

THE CORONA

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.

21

UV solar emission lines

and corresponding

regions and temperatures

22

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.

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

TRANSITION REGION

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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).

4. FEATURES OF THE SOLAR ATMOSPHERE:SUNSPOTS

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The video below depicts regions of negative (black) qnd positive (white) magnetic polarity (like a magnet).

SUNSPOTS ARE REGIONS OF INTENSE MAGNETIC FIELDS

25

CHROMOSPHERIC FILAMENTS & PLAGES

H, 6563 Å

Filaments are the dark, ribbon-like features seen in H light against the brighter solar disk.

The material in a filament has a lowertemperature than its surroundings, andthus appears dark.

Filaments are elongated blobs of plasma supported by relatively strongmagnetic 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

26

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.

Prominences are variously describedas surges, sprays or loops.

SOLAR PROMINENCES

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28

29



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CORONAL HOLES

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.

One of the major discoveries of the Skylab mission was the observation of extended dark coronal regions in X-ray solar images.

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

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.

Maunder Minimum

32

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.

33

34

Evolution of the Sun’s X-ray emission over the 11-year solar

cycle

35

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.

36

CORONAL MASS EJECTIONS (CMEs)


37

Size of Earth Relative to Solar CME Structure

Earth

CME

• 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.

38

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.

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.

FLARE BRIGHTNESS

CATEGORIES:

F: FAINT

N: NORMAL

B: BRILLIANT

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frequency of optical solar flares during cycles 20-21

40

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 X-Ray Flux (ergs/cm2-sec)

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.

41

42



The Bastille-day flare was ‘X-class’ and accompanied by

one of the largestsolar energetic proton events

ever recorded

c3714



43

7. OPERATIONAL EFFECTS OF SOLAR FLARES

44

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.

45

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.

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.

46

Ionospheric 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.

A plasma is a gaseous mixture of electrons, ions, and neutral particles. The ionosphere is a weakly ionized plasma.

+

+

+

++

+

+

++

+

--

--

--

----

----

--

--E

The frequency of this oscillation is called the plasma frequency, = 2f = (Nee2/me)1/2,

which depends upon the properties of the particular plasma under study.

47

Radio Waves in an Ionospheric Plasma

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.

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.

MUF

LUF

48

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)

Radio Waves in an Ionospheric Plasma

49

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 X-ray emission (generally constrained to dayside).

50

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.

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.

51

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.

52

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.

53

Time Scales for Solar Flare Effects

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Miscellaneous

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REFRACTION OF ACOUSTIC WAVES IN THE SUN

Phase speed of acoustic wave

Cph

k

H

T, T = period

surface density gradient

Reflective boundaries organize wave motions into patterns by constructive and destructive interference

Increasingtemperature,

speed of soundfaster

H

Faster propagation here so waves

refract towards surface

56

“Resonant” modeshave integral # of

wavelengthsaround a

circumference

p-modes

• 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

57

These may all have similar T (~ 2-20 minutes); but, because they have different H’s, they have different Cph’s and therefore penetrate to different depths

• 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

58

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 ofHelium 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 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 bombsexploding at the same time

A SOLAR FLARE is defined as a sudden, rapid, and intense variation in brightness.


60



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 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.

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.

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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.

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

Bremsstrahlung Radiation

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)

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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 thethermal plasma component

Solar flares: Outstanding Questions

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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.

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RHESSI reveals X-rays in solar flare

This sequence of TRACE and RHESSI images shows thespectacular solar flare of April 21 2002. The green TRACEimages show material at 2 million degrees Centigrade (3.5million degrees F); the red and blue contours show soft and hardX-rays detected by RHESSI.Surprisingly, RHESSI detects X-rays well in advance of theonset 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 by Carrington Rotation

0

2

4

6

1979 1982 1985 1988 1991 1994 1997 2000

Year

CM

E R

ate

[C

ME

s/d

ay

]

CME Rate

Solwind (1979-1984)

SMM (1984-1989)

SOHO (1996-2002)

27d Average 2800MHz Solar Flux ----- (Max=254)

27d Average 2800MHz Solar Flux ----- (Max=254)

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SOHO LASCO

1996

2000

SOHO LASCO 1996 (197 CMEs)

0.00

0.05

0.10

0.15

0.20

-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Halo

Apparent Latitude [°]

Fra

ctio

n in 5

° In

terv

al

SOHO LASCO 2000 (1,534 CMEs)

0.00

0.05

0.10

0.15

0.20

-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Halo

Apparent Latitude [°]

Fra

ctio

n in 5

° In

terv

al

CME Latitude Distributions

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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.

Documents

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