Solar Observations

The Sun is a spherical ball of plasma. It has a radius of $696$Mm and consists of 90% Hydrogen. Energy is generated by nuclear fusion in the interior and it slowly leaks out to the surface, where it is radiated into space. In the interior the radiation is released in the form of gamma rays and is absorped and re-emmitted many, many times on its route to the surface. This increases the wavelength and at the solar surface it leaves as light. The atmosphere of the Sun may be split into 3 layers called the photosphere (the particle number density is $n \approx 10^{23}$m$^{-3}$ and the temperature $T \approx 6,000$K), the chromosphere ( $n \approx 10^{17}$m$^{-3}$ and $T \approx 10^{4}$K) and the corona ( $n \approx 10^{15}$m$^{-3}$ and $T \approx 2\times 10^{6}$K). The corona extends out to the orbits of the Earth and beyond. The Voyager missions detected the solar corona out beyond the orbit of Saturn and Neptune.

In the photosphere, we typically observe two latitudinal bands of sunspots where the magnetic field is strong ( $B \approx 3,000$G) and the plasma is cool ( $T\approx 4,000$K). The number of sunspots observed at any one time varies with an approximate 11 year period, called the Sunspot cycle. A sunspot group is immersed in an area of moderate magnetic field, called an active region. Occasionally, a violent release of energy will occur in an active region as a solar flare.

The corona is observed during eclipses as a beautiful halo of closed and open structures outlining the local magnetic field. The hot corona is heated by the magnetic field. At the base of closed arcades there are prominences (also called filaments), cool, dense sheets of plasma supported by the magnetic field.

Figure 4.1: The Sun observed in the light of Hydrogen alpha.

Figures 4.1-4.3 show the Sun on 11 July, 1991. The Hydrogen alpha ($H_{\alpha}$) image (Figure 4.1) shows up magnetic structures at chromospheric temperatures. The X-ray picture (Figure 4.2) shows hotter, denser structures at coronal temperatures. Finally, the magnetogram in Figure 4.3 shows the distribution and polarity of the magnetic field at the photosphere. Comparing the three figures, we see that the hottest coronal features are associted with bi-polar magnetic features in the photosphere.

Figure 4.2: The Sun observed in X-rays.

Figure 4.3: A magnetogram of the solar disk, showing the regions and polarity of strong magnetic fields.