The Sun (Continued)

Last time I mentioned that the three-part structure of the Sun’s atmosphere is crucial to understanding the nature of solar-terrestrial relations. The lowest part is the photosphere which comprises the opaque gases we recognize as the disk of the Sun. As mentioned last time, the temperature of the photosphere is 5800 K. Above the photosphere lie the chromosphere and corona. The corona (Latin for “crown”) is the gleaming halo seen around the Sun at times of total solar eclipse.

We expect the atmosphere of an object to drop as you go higher. This is certainly true for the Earth’s atmosphere (check the outside conditions the next time you are in a jet airliner). However, oddly enough it does not happen in the case of the solar atmosphere. The temperature in the Sun’s atmosphere rises crazily as you go up, reaching about 1 million degrees at its maximum (check Figure 14.5).

Amazingly enough, this big temperature increase in the corona is not understood. Certain aspects seem pretty well in hand, but a rigorous mathematical theory that could explain in detail the increase in temperature in the corona does not exist.

Sunspots

Sunspots are a prominent feature of the Sun. They are regions of slightly cooler temperatures (although only about 1000 degrees Kelvin), and they have very strong magnetic fields. Magnetic fields in sunspots have been measured to be thousands of the units called Gauss. By comparison, the Earth’s magnetic field is a bit under one Gauss, and a refrigerator magnetic will have a magnetic field of a couple of hundred Gauss. Pretty impressive for a region that can be larger than the Earth in extent.

>>>>>>>>>>>>>>>> Demonstration with Gaussmeter.

>>>>>>>>>>>>>>>> SOHO web page with today’s picture of the Sun

http://sohowww.nascom.nasa.gov

The magnetic fields in sunspots arch way up into the solar atmosphere, producing loops. >>>>>>>>>>> Picture of loops from SOHO web page.

These loops sometimes destabilize, and big blobs of matter are fired off into space. These blobs are called coronal mass ejections. They are actually of practical interest, because if they hit the Earth, they can cause intense auroras, problems with radio communication, etc.

>>>>>>>>>>>> SOHO pictures of recents CMEs.

The 11 Year Solar Cycle

Sunspots, CMEs, and solar explosions called solar flares are all linked together.

Interestingly enough, sunspots don’t just occur at random. They wax and wane in numbers according to an eleven year period. One can think of the Sun having a “heartbeat” with a period of 11 years. As I have mentioned ad nauseam this semester, we are right now coming up to the maximum of this solar cycle.

>>>>>> current status of sunspot counts http://www.sunspotcycle.com

Other Stars

During this semester we have learned a lot of interesting things about the solar system. A natural questions, best posed while staring up at the starry sky, is whether there are other solar systems out there with similar marvelous phenomena. An even more immediate question is whether there are other planets similar to Earth, with horrible jellyfish-like monsters staring back at us with their 7 eyestalks and planning an invasion during Riverfest next Spring.

The first step in answering this question is to first survey the nearby universe and see if there are other stars like the Sun. This brings us to the interesting astronomical discussion of solar-analog stars and solar twin stars.