ASEN 5335 Aerospace Environment -- The Sun1 Aerospace Environment ASEN-5335 Instructor: Prof. Xinlin...

ASEN 5335 Aerospace Environment -- The Sun 1

Aerospace EnvironmentASEN-5335

• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)• Contact info: e-mail: [email protected] (preferred)

phone: 2-3514, or 5-0523, fax: 2-6444,

website: http://lasp.colorado.edu/~lix• Instructor’s hours: 9:00-11:00 pm Wed at ECOT 534; Tue &

Thu, after class. • TA’s office hours: 3:15-5:15 pm Wed at ECAE 166

• Read Chapter 1 & 2.

• 1st quiz next Tuesday


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


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

• 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.


Like any star, the Sun won't last forever. There's a short window of about 1 billion years when it's possible for plants and animals to survive on Earth, and that's probably true of any planet. We're already about half way through it.

Planets like this may be quite rare in the universe, so we better take care of what we've got.

To simplify a very complicated story about the "devolving" of planet Earth, the12-billion-year lifespan is compressed to 12 hours, with the end coming at high noon.


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 E

• 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° 24° for Earth

• total luminosity 3.86 x 1026 W 1366 Wm-2 @ Earth

• escape velocity at surface 618 km s-1 11.2 km s-1

• effective blackbody temperature 5770 K


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


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

The Sun emits radiation over a range of wavelengths

ELECTROMAGNETICRADIATION

400—700 nm


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


Red: 30.4 nm He II 8x104 K

Blue: 17.1 nm Fe IX 1.3x106 K

Yellow: 28.4 nm Fe XV 2.0x106 K

Green: 19.5 nm Fe XII 1.6x106 K


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



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.



REGIONS OF THE SUN’S INTERIOR AND ATMOSPHERE

p-modes

g-modes


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 (10,420°F).


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


Granular Structure

The convective cells penetrate into the photosphere and give the appearance of a granular structure.


HOW DO WE INFER THE INTERNAL PROPERTIES OF THE SUN ?


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.


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


“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


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


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.



• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)• Contact info: e-mail: [email protected] (preferred) phone: 2-3514, or 5-0523, fax: 2-6444, website: http://lasp.colorado.edu/~lix• Instructor’s hours: 9:00-11:00 pm Wed at ECOT 534; Tue &


• Read Chapter 1 & 2, and classnotes• 1st quiz today, open book.• HW1 due Thursday


THE SUN









5. THE SOLAR CYCLE







REGIONS OF THE SUN’S INTERIOR AND ATMOSPHERE

p-modes

g-modes


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


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


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.


UV solar emission lines

and corresponding

regions and temperatures


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


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


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


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


QuickTime™ and aCinepak decompressor

are needed to see this picture.


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.


Coronal holes and Solar wind speed and density

The interplay between the inward pointing gravity and outward pointing pressure gradient force results in a rapid outward expansion of the coronal plasma along the open magnetic field lines.

At low latitudes the direction of the coronal magnetic field is far from radial. Therefore the plasma cannot leave the vicinity of the Sun along magnetic field lines. At the base of low-latitude coronal holes, however, the magnetic field direction is not far from radial, and the expansion of the hot plasma can take place along open magnetic field lines without much resistance fast solar wind.


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



• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)• Contact info: e-mail: [email protected] (preferred) phone: 2-3514, or 5-0523, fax: 2-6444, website: http://lasp.colorado.edu/~lix• Instructor’s hours: 9:00-11:00 pm Wed at ECOT 534; Tue &


• Read Chapter 2, classnotes, and handout• HW2 due today• 2nd quiz, next Thursday (2/6)


THE SUN









5. THE SOLAR CYCLE







5. THE SOLAR CYCLE

• The number of sunspots on the solar disk varies with a period of about 11 years, a phenomenon known as the solar (or sunspot) cycle, which was accidentally discovered by a German amateur named Heinrich Schwabe in 1843.

• Solar events and operational impacts also tend to follow this 11 year cycle.

• The Zurich sunspot number is defined by counting the number of spot groups and individual spots and then forming the sum: Sunspot Number (R) =k (10x Number of sunspot group + Number of individual spots), where k is a correction factor accounting for the evolution in observing technology.

• In this scheme sunspots are kept track of throughout their lifetime, and in constructing annual mean sunspot numbers a given spot is counted only once.

• The figure on the following page depicts the annual mean sunspot numbers from 1961 to 1987.

• Note the period between about 1645 and 1715 when there were no sunspots on the Sun. This period is referred to as the ‘Maunder minimum’ (Maunder, 1882).


Maunder Minimum

1Solar cycle 1: 1755-1766

Solar cycle 23:

1996-2007


Maunder Minimum


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.


Solar Minimum – 1996/7 Solar Maximum – 2000/1

eitcompare

COMPARING THE SUN IN EUVFROM 1996 TO 1999


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

cycle


Maunder Minimum

SORCE Launch (1/25/03)


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 but also energetic particles.

• 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.


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.


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 the thermal plasma in the loop directly, and indirectly by “chromospheric evaporation”. The soft or thermal x-rays seen by TRACE reflect this heating.


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)


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


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.


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



• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)• Contact info: e-mail: [email protected] (preferred)

phone: 2-3514, or 5-0523, fax: 2-6444,

website: http://lasp.colorado.edu/~lix• Instructor’s hours: 9:00-11:00 pm Wed at ECOT 534; Tue &


• Read Chapter 2, class notes, and handout

• 2nd quiz, Thursday (2/6), close book


Coronal Mass Ejection (CME)

Observations made with white-light coronagraphs in the early 1970s demonstrated that large amounts of mass (1015 and 1016 g) are sporadically ejected from the Sun into the interplanetary medium. Such transient ejections of materials are called coronal mass ejections (CMEs) .

Most CME events originate in closed magnetic field regions in the corona, where the magnetic field is strong enough to constrain the plasma from expanding outward. Although the physical origin of CMEs is not clear, there are convincing evidence and modeling results that magnetic field reconnections, due to some instabilities, precede and lead to the CMEs.

CMEs mainly produce plasma and are often accompanied by other forms of solar activity such as solar flare, radio bursts, etc.


CME models


CME models and observations


“Halo CME”


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.


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)


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


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.


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


frequency of optical solar flares during cycles 20-21


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.


The Bastille-day flare was an ‘X-class’ and accompanied by one of the largest solar energetic proton events ever recorded

c3714

Classification X-Ray Flux (ergs/cm2-sec)

C 10-3

M 10-2

X 10-1


These protons reach Earth inless than 30 minutes

Less than 1 hour afterthe initial proton arrivalthe POLAR/VIS imageris saturated and remainsso for almost a day

Vis-proton-bastille


Solar Proton Events

• Solar proton events represent an important class of flares. There very large flares eject extremely energetic protons which arrive at Earth 30 to 90 min after flare initiation. As shown in the figure below, proton events can last from 2 to 6 hours, with the lower energy tail of the proton particle distribution taking longer time to arrive at Earth.


• The position of a flare on the visible disk is one of the factors taken into account when forecasting whether or not a flare will cause a disturbance at Earth. The figure shows that flares located near the west limb of the Sun have a higher chance of interacting with Earth than east-limb flares. Historically, flares occurring near 50 degree west longitude are most likely to generate terrestrial disruptions.

ASEN 5335 Aerospace Environment -- The Sun1 Aerospace Environment ASEN-5335 Instructor: Prof. Xinlin...

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