Physics of Planetary Climate Cors221: Physics in Everyday Life Fall 2010 Module 3 Lecture 5: The Sun

From Last Time Earth was hot in the Eocene 50Myr ago (+12 K or +21F), but Antarctica glaciated over 13 Myr ago and stayed that way. For the past 2.5 Myr, Earth has been in an Ice Age, characterized by semiperiodic glacial and interglacial cycles. With so much of Earth's water bound up in 2-mile-thick glaciers over North America and Siberia, global sea level was 120 meters lower. Earth’s orbital eccentricity, a measure of how far the orbit is from non- circular, changes from 0.0 to 0.05 on 100,000 year timescales (presently 0.017). Earth's obliquity, the angle of tilt of the rotation axis, changes from 22.1 to 24.5 degrees on 41,000 year timescales (presently 23.44). Day of summer solstice relative to periapsis varies on 21,000 & 26,000 year timescales. The natural variations in climate over the past 2.5 Myr, i.e. during the ice age, are well-explained by changes in insolcation due to orbit and obliquity changes. These are known as Milankovich cycles.

The Sun How is sunlight produced?

The Sun Where does the heat come from to keep the Sun's “surface” hot?

Solar Interior Tc = 1.6x10 7 K Pc = 2.34x10 11 bars

Fusion: The p-p Chain Convertes hydrogen to helium via fusion

What holds the Sun Steady: Hydrostatic Equilibrium

The Sun Through Time

Sunspots

Sunspot Counts

Sunspot Effects on Solar Brightness

Effect of Maunder Minimum

Solar Flare

Solar Prominence

The Sun Now (this morning) More at

Key Points The Sun emits light because it is surface is hot, and therefore emits blackbody radiation. It is kept hot by hydrogen fusion in its core. Over the entire life of the Sun to this point, it has gotten a little hotter. Only 70% as bright when Earth formed as it is today, leading to Faint Young Sun paradox. Sunspots are slightly cooler (and hence darker) areas on the sun caused by magnetic activity. Sunspots come in 11-year cycles during which they start at the pole and move equatorward with the Sun’s evolving magnetic field. Despite being cold parts of the Sun, the Sun emits more light when there are sunspots present because of attendant emission and X-ray activity associated with the magnetic disturbance. Sunspots = hot. No sunspots = cold. Maunder minimum from with no sunspots was cold in Europe. Check out to see what sunspots are out today, and to see the condition of the Sun and aurorae.