Solar activity

The Outer Layers of the Sun

• The Photosphere• The Chromosphere• The Corona• The Heliosphere

Plages

Bright patches surrounding sunspots. Plages are associated with concentrations of magnetic fields.

Plages

Filaments & prominences are the same thing

Prominences

Two types:

1. Quiescent – can last for months2. Active – last only for hours. Lie above

sunspots and have violent motions.

Hedgerow Prominence

Flares

Sudden violent explosions in magnetically active regions. Last from 1 to 20 minutes and release as much energy as 2.5 million 100-megaton nuclear bombs in an area about the size of Earth. This is enough energy to cause thermonuclear fusion to take place on the Sun’s surface.

Flares

The Transitional Region

A thin region (can be as thin as a few 10’s of km) above the chromosphere where the temperature rises from 10,000 K to 1 million K typical of the corona.

Corona

Outer part of the Sun’s atmosphere. About as bright as the quarter phase Moon, it is visible only when the light of the photosphere is blocked.

The corona is so thin that on Earth we would consider it a vacuum.

The Corona is surprisingly hot

The temperature of the corona is over 1 million K.

Due to sound waves from photosphere?

Discussion

At the distance of the Earth’s orbit the corona, although cooler than near the Sun, is still 0.14 million K. Why don’t astronauts burn up when they go on space walks?

Discussion

What causes the helmet shaped structures in the corona? Why doesn’t the corona spread out uniformly around the Sun?

Coronal Mass Ejection

Giant magnetic bubbles that can hurl 5 to 50 billion tons of matter at speeds of 400 km/sec.

70% of coronal mass ejections are associated with, or followed by, erupting prominences. While 40% are accompanied by solar flares that occur at about the same time and place.

The Sun-Earth connection

Coronal mass ejections and solar flares can be directed at Earth. Luckily for us, Earth has a magnetic field and an atmosphere to protect us.

Aurora

When high speed particles from the Sun collide with atoms in Earth’s upper atmosphere. The electrons are knocked into higher energy orbitals and emit light when returning to the ground state.

Solar Wind

Although the Sun’s surface gravity is much higher than the Earth’s, it is not able to contain particles with a temperature on over a million K. Thus the hot corona spews matter constantly (not just during flares, CME, and explosive prominences) into space at a rate of about 1 million tons per second.

Terrestrial Planets

Terrestrial Planets

All the terrestrial planets are more or less differentiated, i.e. the densest materials have sunk to the core and the lighter materials have floated to the surface.

The terrestrial planets were all completely molten at some time in the past.

Discussion

Why do you think all the terrestrial planets were so hot in the past? Isn’t space rather cold?

Terrestrial planets interior structure

Core – highest density material, mostly iron and nickel

Mantle – high density silicate rocks

Crust – lower density silicate rocks, granite and basalt.

Earth’s internal structure

1. Solid crust – 5 km thick under the oceans, made of basalt: silicates of aluminum, magnesium and iron with a density of about 3.5 g/cm3. Under the continents the crust is 35 to 70 km thick and is made mostly of granite: silicates of aluminum, sodium and potassium with a density of 3.0 g/cm3. The continents float on the basalt.

Discussion

What’s a silicate? Give and example of a silicate.

2. Mantle – solid, but top layer is plastic called the asthenosphere. Is about 2800 km thick and made of compounds rich in iron and magnesium. Density increases from 3.5 g/cm3 at top to 5.5 g/cm3 near the bottom.

3. Outer core – liquid. Is about 2200 km thick and is made of iron, nickel and sulfur.

4. Inner core – solid. Is about 1300 km thick and is composed of nearly pure crystalline iron with a density of 13 g/cm3.

Why is inner core solid while outer core is liquid?

Isn’t the inner core hotter than the outer core?

The melting point of substance depends on both temperature and pressure. In general, the melting point goes up with pressure. The inner core is hotter than the outer core but is under a greater pressure and thus has a higher melting temperature.

Discussion

How do you think we know what the internal structure of the Earth is when our deepest mines and wells have not made it deeper than the Earth’s crust?

Earthquakes

Earthquakes produce three types of waves that travel through the Earth.

1. Surface waves2. Primary waves, or P waves3. Secondary waves, of S waves

Discussion

Which type a wave do you think will travel better through a liquid and why?

S waves cannot travel far through a liquid. On the opposite side of the Earth from and epicenter of an earthquake, seismographs only detect P waves.

The molten core causes the P waves to bend is such a way that the density of the core can be determined.

Wave speed

The speed of seismic waves depends primarily on the density of the material through which they travel. If the density changes, the waves will be refracted, just as light is refracted in a glass lens.

Discussion

How do we know that the Earth’s inner core is solid?

Discussion

Study of seismic waves on Earth indicate that the inner solid core is rotating faster than the rest of the Earth. How do we know the core is rotating faster?

Discussion

What do you suspect would be the result of this difference in rotation rate between the solid and liquid cores?

Planetary seismic data?

We know the internal structure of the Moon because astronauts placed nuclear powered seismic stations at the Apollo landing sites. But this information is not available for any of the other planets.

Planets internal structure

Two tools without seismic data:

1) Mean density

2) Gravitational mapping – mascons

Density

Mass of the planet divided by the volume of the planet.

Higher density implies a larger percentage of high density materials, such as iron and nickel, lower density implies more silicates.

Mapping the gravitational field

By carefully tracking an orbiting space probe, concentrations of denser materials below the surface can be mapped. Space probes are accelerated more toward higher density regions.

Discussion

Which of the terrestrial worlds likely was the hottest during formation? Why?

How the planets got hot

1) Heat of accretion

2) Heat of differentiation

3) Heat from radioactive decay

2nd law of Astronomy 201

Larger planets lose heat more slowly than do smaller planets.

Heat and planets

All the terrestrial planets started out hot and have been losing heat over time by radiating it into space from their surfaces.

Discussion

Larger planets have larger surface areas, and a larger surface area should radiate more energy into space? So shouldn’t larger planets cool faster? Why do larger planets cool more slowly than smaller planets?

Volume and heat

The greater the surface area, the faster heat will be radiated. But, it is the volume that stores the heat. The greater the volume of a planet the more internal heat it can retain.

Also, more massive planets have more radiative material.

It’s geometry

The surface area increases as the square of the radius. But the volume increases as the cube of the radius. Thus, a larger sphere has less square miles to radiate the heat per cubic mile of material.

This is why people get fat!

Geologic activity

Internal heat drives geologic activity on the planets’ surfaces.

Discussion

Because heat is radiated from the surface of a planet, the surface is cooler than the interior.

How does the heat from the core of the planet get to the surface?

Plate tectonics

Lithosphere

The convective cells in the planets do not make it to the surface as on the Sun, but are stopped at the base of the lithosphere. The lithosphere includes the crust and the upper mantel region of cooler, stronger rock which does not flow as easily as the warmer, lower mantel rock.

Geologic processes

1) Impact cratering

2) Volcanism

3) Tectonics

4) Erosion

3rd law of astronomy 201

The more impact craters on a surface, the older that surface is.

Discussion

Which area on the Moon is older, the light region to the left or the dark region in the center of the picture?

Why?

All the terrestrial planets probably receive about the same number and size distribution of impacts. All the other geologic processes (volcanism, tectonics, and erosion) tend to erase impact craters on the surface.