The Sun as We See It (Continued)

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. The temperature of the photosphere is 5500 degrees Centigrade, or 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. See SOHO images of C2 and C3 coronagraph http://sohowww.nascom.nasa.gov

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 (see Figure 17.24 in Chapter 17 ).

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.

A good picture of a sunspot is at http://science.msfc.nasa.gov/ssl/pad/solar/feature1.htm 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.

An interesting feature of the photosphere is a phenomenon called granulation which consists of cells of boiling gas. Observations show that hot gas is rising in the middle of these granulation cells, cooling, and then sinking back into the solar interior. The granulation shows that the physical process of convection is occurring. This proves that the interior of the Sun is even hotter than the photosphere.

Granulation >>>>>>>

The strong magnetic fields in sunspots arch way up into the solar atmosphere, producing loops. >>>>>>>>>>> http://antwrp.gsfc.nasa.gov/apod/ap001115.html and http://antwrp.gsfc.nasa.gov/apod/ap000928.html

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.

>>>>>>>>>>>> http://sohowww.nascom.nasa.gov See also Figure 17.30 of the textbook.

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. We are right now coming up to the maximum of this solar cycle.

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

The Solar Wind

The outermost layer of the Sun, the corona, flows out into space and forms a wind called the solar wind. This wind sweeps back the magnetic field of the Earth and forms a boundary between interplanetary space and the part of space dominated by the Earth.