ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere 1

THE SUN & EARTH’S UPPER ATMOSPHERE

9. IONOSPHERIC AND OPERATIONAL EFFECTS OF SOLAR FLARESa) radio noise including effects on GPS b) PCA eventsc) HF absorption d) sudden ionospheric disturbances

1. GENERAL CHARACTERISTICS OF THE SUN • Descriptive Data • Electromagnetic Radiation • Particle Radiation2. 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 CYCLE6. SOLAR FLARES AND CORONAL MASS EJECTIONS

• Description and Physical Processes • Classifications

7. SOLAR RADIATION AND EARTH’S NEUTRAL UPPER ATMOSPHERE• Temperature and Composition Structure • Hydrostatic equilibrium

8. SOLAR RADIATION AND EARTH’S IONOSPHERE

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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

• 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 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 36 days• inclination of sun's equator to ecliptic 7.5° 23.5° for Earth• total luminosity 3.86 x 1026 W 1366.1 Wm-2 @ Earth• escape velocity at surface 618 km s-1

• effective blackbody temperature 5778 K

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STRUCTURE OF THE SUN

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REGIONS OF THE SUN’S INTERIOR AND ATMOSPHERE

p-modes

g-modes

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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.

D = Deuteron = gamma ray = neutrinoe+ = positrone- = electron

alpha particle

Beryllium 6(unstable)

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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.

visible radiation

gammaradiation

absorption/re-emission

convection(opaque region)

CORE

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

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The photosphere is the Sun’s visible “surface”, a few hundred km thick, characterized by sunspots and granules

REGIONS OF THE SOLAR ATMOSPHERE:THE PHOTOSPHERE

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.

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)

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THE CHROMOSPHEREThe 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 an 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.

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.

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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 holes.

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

The corona is the main source of

emissions at wavelengths

< 100 nm

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The Sun emits radiation over a range of wavelengths

Emissions shown in this videocome from progressively hotterregions of the Sun, and revealdifferent features.

ELECTROMAGNETICRADIATION

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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.

radiatedenergy

insulation

In the laboratory

In a star

The radiation reacts thoroughly with the

body and is characteristic of the

body

T1

T2

T1>T2

max 1/T

radi

ated

ene

rgy

wavelengtharea T4

heating element

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Total Solar Irradiance (TSI) Composite Database compiled from many satellite TSI data 1978-present, by Claus Frohlich and Judith Lean

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T1

T2

T1>T2

max 1/T

radi

ated

ene

rgy

wavelength

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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A representative view of how the EUV portion of the solar spectrum affects the upper atmosphere. The strongest line is the He II (30.4nm), appearing near the left portion of the figure. As solar irradiance at EUV wavelengths varies on the order of 30%, the effect on thermosphere heating - and subsequent density increases - is significant. One effect is increased satellite drag; another is the significant ionization that results from increased X-ray and EUV fluxes, sometimes causing disrupted radio communications.

Upper Atmosphere Solar heating Rates

What is causing the heating ?

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Density at 200 km

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Density at 200 km