The Sun's Magnetic Field

The Sun has a strong and complex magnetic field that is on average several times stronger than the Earth's field, and much solar activity appears to be directly connected with the properties of the magnetic field.

Magnetograms

The adjacent image shows the distribution of magnetic field on the solar surface for a day in August 1999. Such an image is called a magnetogram. In this magnetogram, black denotes a negative polarity (magnetic field pointing into the Sun) while white denotes a positive polarity (magnetic field pointing out of the Sun). Large concentrations of both polarities are found near active regions and sunspots. Here is an image showing the current magnetic fields on the surface of the Sun.

Mathematical Models

The adjacent image illustrates the complex magnetic fields associated with active regions on the solar surface. The top two panels display observations and the bottom two show fields calculated within a mathematical model.

Panel (a) is the corona imaged by the Yohkoh Soft X-ray Telescope on January 4th, 1994. The hot coronal loops connect two active regions on opposite sides of the Sun's equator. Panel (b) illustrates corresponding magnetic field measurements of the same region, with positive fields in white and negative fields in black. Panel (c) shows a top view of some three-dimensional magnetic field lines obtained from a mathematical model that extrapolates the surface fields. These lines are in reasonable agreement with the structure observed in panel (a). Panel (d) shows these same field lines as viewed from the side.

Magnetic Fields for other Stars
An extremely interesting question is whether other stars exhibit magnetic activity and cycles similar to that of the Sun. We cannot check this as directly as for the Sun because of their large distances from us, but there is strong indirect evidence that other stars exhibit similar effects.
For example, it is observed that for the Sun there is a correlation between solar magnetic activity and the strength of some strong calcium lines in the spectrum. These lines can be detected for other Sun-like stars, and it is found that some of them exhibit periodic behavior in the strength of these lines, as would be expected if they were going through active and quiet magnetic periods. As a second example, there is some indication of "starspots" on other stars, as inferred by studying their detailed light output as a function of time. Finally, there is some evidence from broadening of spectral lines by the Zeeman effect for magnetic fields comparable in strength to that of the Sun for some other stars.