The Sun: Earth's Star





Features of the solar atmosphere

The Active Sun



The sun is just an ordinary star

Its energy makes all life and surface processes possible

Minor changes in its energy output can have a major impact on the earth

Ice ages are an extreme possibility

Controls the climate

Differential heating of the earth - we've talked about this

Your basic garden-variety star

Not too hot or cold, too big or small

Maybe that's why we have life in our solar system

The sun is so close we can easily study how it works

Study can help us understand how the other stars work

Acts as a "benchmark" for understanding the universe

Composed of essentially 2 elements (98%)

See Table 26.3 pg. 437

Hydrogen: 92% by # of atoms; 73.4% by mass

Helium: 7.8% by # of atoms; 25% by mass

Basically, a large fusion reactor

Running at Warp 97 and out of control

Incredible amount of energy

The inner portions of the sun cannot be studied

The outer layers emit too much energy to see through them

Therefore we can only speculate as to what the inside is like

Possibly a "solid" core? We'll never know

No matter what, it's gotta be pretty hot

15 X 106 °K

DIGRESS TO: bright light


Features of the solar atmosphere

There are 4 general divisions (or layers) to the solar atmosphere

1) The Photosphere

This is the part we see (so to speak)

Not a specific surface, but a range up to 400km in depth

Like you would expect in an "atmosphere"

Pressure and density increase dramatically with depth (table 26.2, pg. 437)

But remain far less than earth atmospheric values

Temperature also increases with depth

4465°K at surface to 7610°K at 400km depth

More than 60 elements have been identified

98% is hydrogen and helium

Most is in elemental (atomic) form

Occasional molecules identified in cooler regions (sunspots)

2) The Chromosphere

Lies immediately above the photosphere

2000 to 3000 km thick

Density decreases upward

Temperature increases upward

From 4500°K at photosphere to 100,000°K at transition to corona

3) The Transition Region

Thin transition from Chromosphere to Corona: 10 to 30 km thick

Actual height above photosphere quite variable

Span "several thousand" kilometers

Very rapid increase in temperature

4) The Corona

The outermost part of the solar atmosphere

Extends for millions of miles above the Chromosphere

Thins to "a sparse wind of ions and electrons" which flows into solar system

Basically the solar wind

Very warm ­ "millions of Kelvins"

Because of the fast particle motion

But very low density

At the base of the corona there are 109 atoms/cm3

As opposed to 1016 in the upper photosphere and 1019 at sea level on earth

Therefore, the actual heat per unit volume is very low

Just not enough particles to do an adequate job of heating things up

Rapid motion doesn't completely answer why the corona (and chromosphere) are so hot

Astronomers assume the sun's magnetic field plays "a major role"

The way magnetism is converted to heat is not at all understood

Sure would be nice to know how this works

The Solar Wind

Stream of "charged particles" flowing out from the sun

Mostly protons - the "ions" mentioned above?

Moving fast - 400 km/sec at earth's orbit

Very sparse - 2 to 10 ions per cubic centimeter!

Earth is protected from these charged particles by the atmosphere

Aurora - interaction of solar wind and the earth's upper atmosphere

Solar Rotation

First identified by watching sunspots move across the surface

Same as the planets - west to east

Axis is tilted like the earth

Different angle - 7° to the plane of the ecliptic (DEFINE)

Since the sun is a gas, it need not act like a solid body

Rate of rotation varies by latitude

25 days at equator; 28 days at 40° north and south; 36 days at 80°


The Active Sun

Quite stable in a regional view

Locally, a "seething, bubbling cauldron of hot gas"

And therefore susceptible to the laws of physics which relate to the interaction of things of different temperatures

Granulation (26.9, pg 442)

Surface looks mottled and irregular - not uniformly bright

Appear to be columns of hot gas rising from below the photosphere

With the dark halos site where "cooler" gas sinks back into the sun

Like small convection cells

Not so small - 700 to 1000 km in diameter


Dark, relatively cool regions of the photosphere

Up to 1500° K cooler than the "normal" surface temperature

But still hot enough to toast some weenies and marshmallows

Short-term features - individual spots can last from hours to a few months

Have two different regions (see fig. 26.9, pg. 442)

Umbra - inner darker (and cooler) core

Penumbra - a surrounding less dark (warmer?) area

Can get quite large - up to 50,000 km in diameter

Large groups possible

Usually 2 large spots on E-W line with a cluster of smaller spots around

The occurrence of sunspots follows a definite cycle

Maximum activity at 11.1 year cycle (can vary from 8 to 16 years)

Magnetic fields of the sun

Strongly magnetic near the sunspots - >1000X the earth's

Elsewhere fairly low

When sunspots are grouped, the 2 larger spots will have opposite magnetic polarity

With all the leading spots in each hemisphere having the same polarity - but opposite to each other across the equator

And the next cycle will have the reverse polarity pattern

Results in a magnetic cycle of 22.2 years (2 times the sunspot cycle)

Prominences (page 445)

Usually associated with sunspots and areas of high magnetism

Appear as red, flame-like masses of material rising high above the surface

Can remain stable for hours to days

Very fast moving material - 700 kps to 1300 kps

Flares (page 446)

Short-term high energy emission from sun: Last 5 to 10 minutes

Can be mighty impressive

Release energy equivalent to "a million hydrogen bombs"

Apparently represents release of excess energy caught up in the magnetic field

Can have profound effects on the earth

Would be even more exciting without the atmosphere

Aurora borealis - most obvious effect

Usually occur near earth's magnetic poles

Large solar flares can result in auroras visible at lower latitudes

EX: March 1989 flare caused auroras to be seen in Arizona

Can cause fluctuations in earth's magnetics

Disrupt power lines and damage electrical equipment

EX: same 1989 flare knocked out power in Canada for 9 hours

Can play merry hell with radio, satellite, TV reception

Can actually knock satellites out of orbit and into the atmosphere

Physically (by expanding the atmosphere farther out into space)