1. AOS 101 Weather and Climate Lisha M. Roubert University of Wisconsin-Madison Department of Atmospheric & Oceanic Sciences

2. The Sun is the star at the center of the Solar System. It is our source of energy and heat, which sustains life on Earth, and controls our climate and weather. It generates energy by fusing hydrogen nuclei into Helium. While it looks small, a hundred earths could fit inside it. The moon looks larger, because it’s only 400,000 km from the earth, as opposed to the sun, which is 150,000,000 km away. Due to this distance, the sun looks relatively serene and constant. However, it is nothing of the sort!

3. Observing the sun closely is difficult because it is so bright. Two methods are pinhole telescopes, and filters. A common filter used to study the solar surface is an H-Alpha filter. It produces an image like the one to the right. This can show convective cells and plasma filaments.

4. Another way to look at the sun is to photograph the x-rays coming from it. The image at the left shows this. X-rays tend to show the outer layers of the sun, and the complex magnetic fields.

5. From these observations, it became clear that the sun isn’t constant. The first observations of this were done by the ancient Chinese, who looked at the sun’s reflection in mud puddles. They could see spots come and go on the sun. These became called sunspots, although they are actually massive arches of plasma following magnetic fields. sunspots

6. Over time, scientists began to describe a “solar cycle”. The solar cycle is a process during which the magnetic field of the sun flips polarity. In other words, the Sun's north pole becomes its south pole. The changing magnetic field can cause a great amount of solar activity, increasing the number of sunspots, solar flares and coronal mass ejections. ▫ At the solar maximum sunspots and solar flares occur regularly ▫ 1/2 way through the cycle solar activity decreases During the last solar cycle, there were 805 days with no sunspots, which is the longest time without since serious solar study began in 1755. http://www.windows2universe.org/sun/ac tivity/sun_mag_field_rotate_tangle.html

7. Differential Rotation ▫The sun rotates faster towards the equator because it is not a solid body. This difference in rotation speed causes the magnetic field to wrap around the sun.

8. Sunspots are temporary phenomena on the surface of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. Sunspots expand and contract as they move across the surface of the sun and can be as large as 80,000 kilometers in diameter, making the larger ones visible from Earth without the aid of a telescope.

9. The number of sunspots correlates with the intensity of solar radiation, thus affecting the amount of solar irradiance we receive on Earth. The surrounding margins of sunspots are brighter than the average, and so are hotter; overall, more sunspots increase the sun's solar constant or brightness. However, the variation caused by the sunspot cycle to solar output is relatively small, on the order of 0.1% of the solar constant. The effects of this on Earth’s temperature is relatively un-measurable.

10. Homework: Find two current solar study programs and describe in a paragraph for each one how they are studying the sun. ▫ 1 space-based ▫ 1 ground based We will have a discussion next time based on this.