astr 113 lecture 2 with prof. m. opher room 363 b science and technology 1 telephone:703-993-4571...

ASTR 113Lecture 2

with Prof. M. Opher

Room 363 B Science and Technology 1Telephone: 703-993-4571FAX: 703-993-1269E-mail: mopher@gmu.eduOffice Hours: T/R 16:30-17:30 & 16:30-1

Photosphere- look like the Sun has a definite surfaceNo! It’s a gaseous because its high temperatures!

You can see only 400km into the photosphere

What did we see last class?

• Understand how the Sun produces energy.• Be able to explain hydrostatic equilibrium and

thermal equilibrium.• Know what a stellar model is and be able to

explain the theoretical model of the Sun.• Understand how conditions in the solar interior can

be inferred from measurements of solar vibrations.• Know why scientists measure the number of

neutrinos emitted from the Sun's core and what the results imply.

The Photosphere -

the lowest of three main layers in the Sun’s atmosphere • The Sun’s atmosphere has three main layers

– the photosphere– the chromosphere– the corona

Limb darkening: …when we look near the Sun’s limb we do not see as deeply into the photosphere

High T

Low T

Absorption Lines in The Photosphere

• Photosphere nearly a perfect blackbody with T=5800 K

• *upper part of the photosphere is 4400K (cooler!)

Granulation: convection cells ~ 1000 km (600 mi)

The difference in TFrom center to edge Is ~ 300 K

-> hot gas from lower levels rises upward, cools off and plunge back Into the Sun

Form and disappear in few minutes.

Supergranules

Size: 35,000 km indiameter

The convectionmoves at 1400km/h=900mi/h

(Last a day!)

• More on… the Photosphere: • Opaque to visible light• 10-4kg/m3 (low density)

The chromosphere is characterized by spikes of rising gas

• Above the photosphere is a layer of low dense (10-4 less than the photosphere) but higher temperature gases called the chromosphere

• Emission Lines(in the red part of the

spectrum-therefore the pinkish color)

Using H filter

What about the Temperature?

Its increases from 4400 K (top of Photosphere) to 25,000 K (top of chromosphere)

Spicule last ~ 15 minutes;It roses 20km/s300,000 spiculesExist at any one time

Spicule: jet of rising gas

Located at the edgeof supergranules:(photosphere: descending gasand chromosphere: rising)

???? Answer: Magnetic Field

Spicules extend upward from the photosphere into the chromosphere along

the boundaries of supergranules

CORONA

Extends for millionof km’s.

10-6 brighter thanThe photosphere..

Streamers!

• The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density

• The solar corona blends into the solar wind at great distances from the Sun

CORONA

Emission lines In 530.3 nm(highly ionized Iron)

• The corona is actually not very hot: density is only 1011atoms/m3

While photosphere: 1023atoms/m3

Our air: 1025atoms/m3

..if we will fly a spacecraft through the corona we

Will have to worry with the heat from the photosphere!

The corona ejects mass into space to form the solar wind

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Property (1 AU) Slow Wind Fast WindFlow Speed 400 km/s 750 km/s Density 7 cm-3 3 cm-3 Variance "large", >50% Variance "small", <50%

Temperature T(proton, 1AU) ~ 200,000 K T(proton, 1 AU) ~ 50,000 K

Solar Wind: Bi-Modal Structure

Activity in the corona includes coronal mass ejections and coronal holes

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Solar Minimum-Maximum

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Quiet Sun: Granules, supergranules, spicules, solar wind

Active Sun: massive eruption; sunspots

Sunspots are low-temperature regions inthe photosphere

Sometimes they are isolated but frequently in sunspost groups

~ diameter of Earth (lasting between hours or months)

T (umbra) = 4300 KT (penumbra) = 5000 K

Stefan-Boltzman law : energy flux T4

Big group of Suspots (noteThe granulationIn the undistorbedSun)

Galileo discovered thathe could determine the Sun’srotation rate by tracking Sunspots

Carrington (1859)Showed that the Sun Does not rotate as a rigid body: equatorial regions rotate faster (27 days)Than the poles (35 days)Differential Rotation

Sunspot Cycle: 11 years cycle

Location of the sunsport vary: just after minimim the sunspots Are ~ 30 north and south of solar equator.

• Why the sunspots should exist? Why they vary?

Hale (1908) discover that the Sunspots are

associated with intense magnetic field!

Spectral lines are splitted: Zeeman Effect

Magnetic Field Affects the Motion of Ionized Matter (Plasma)!

Magnetogram: image of the Sun at two wavelengths: one just less and one just greater that the wavelength of the magnetically split spectral line. From the difference they can construct a picture Displaying the B of the sun.

Blue is B (north polarity) and Yellow: B (south polarity) (like magnet)

Sunspot group moving: the sunspots in front are called the “preceding members” the ones following behind are the “following members”.

The preceding members in one solar hemisphere: same magnetic polarity; while the preceding members in the other hemisphere have the opposite polarity.

Where the Sun has North Polarity the preceding members all have north magnetic polarity. In the opposite hemisphere, all the preceding members have south magnetic polarity.

22-Solar Cycle: the Sun reverses polarity every 11 years

The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field (H. Babcock (1960))->differential rotation and convection

More on that on page 398…

Helioseismology: insights into the Sun’s magnetic field

By comparing the speeds of sound waves in the interior->rotation rate in the Sun’s interior.

Pattern persist untilthe convection zone;In the radiative zone the Sun rotates as a rigid Body (27 days)

The suspition is that B originate between theRadiative and convectiveZone (when the twoSlide past each other)

Solar Variability Can Affect Terrestrial Climate

Given the massive economic impact of small changes in climate, we should fully understand both natural and anthropogenic causes of global change.

During the Little Ice Age, London’s Thames River froze in winter, something that no longer happens. This 19th century engraving depicts the annual Frost Fair held on the ice-bound river, this one during the winter of 1683-84.

The Sun’s magnetic field also produces other

forms of solar activity • A solar flare is a

brief eruption of hot, ionized gases from a sunspot group

• A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona

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If we want to get to Mars: we need Space Weather

Know the structure of the solar atmosphere.

Be able to describe the photosphere, its properties, and its features.

Be able to describe the chromosphere, its properties, and its features.

Be able to describe the corona, its properties, and its features.

Know what sunspots are and what the sunspot cycle is.

Be able to describe the magnetic-dynamo model of the sunspot cycle.

Understand how activity in the solar magnetic field heats the corona.

Know how solar eruptions affect us on Earth

TOPICS