The Magnetic
Field of the Sun

The Sun has a strong and complex magnetic field, and much solar activity appears to be directly connected with the properties of the magnetic field.

The Zeeman Effect

The magnetic field of the Sun can be probed in a rather precise and direct manner because in the presence of a magnetic field the energy levels of atoms (and ions and molecules) are split into more than one level. This causes spectral transition lines to also be split into more than one line, with the amount of splitting proportional to the strength of the magnetic field. This is called the Zeeman Effect , and the corresponding increase in the number of spectral lines is called Zeeman splitting. Thus, we can infer the presence of magnetic fields if we observe Zeeman splitting in the spectrum, and we can measure the strength of the field by measuring quantitatively the amount of Zeeman splitting.

Sunspots and Magnetic Fields

Measurement of the light from sunspots (obtained by masking off the light from parts of the Sun not in the sunspot) indicate significant Zeeman splitting of the spectral lines. Thus, sunspots are associated with strong magnetic fields. Furthermore, it is observed that
  1. When sunspots come in pairs, one tends to have a magnetic field polarity that is opposite that of the other (that is, one behaves magnetically like the north pole of a bar magnet and the other behaves magnetically like the south pole of a bar magnet).
  2. During a given sunspot cycle, the leading sunspots in groups in the northern hemisphere of the Sun all tend to have the same polarity, while the same is true of sunspots in the southern hemisphere, except that the common polarity is reversed from that of sunspots in the northern hemisphere.
  3. During the next sunspot cycle, the regularities noted in the previous point reverse themselves: the polarity of the leading spots in each hemisphere is opposite from what it was in the previous cycle.

    Solar magnetic field

    The Solar Magnetic Field

    The adjacent image shows the distribution of magnetic field on the solar surface from the Michelson Doppler Imager experiment on SOHO (January 27, 1998). 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.

    The 22 Year Magnetic Cycle

    The preceding considerations indicate that the Solar magnetic field has a 22 year cycle, exactly twice that of the sunspot cycle, because the polarity of the field returns to its original value every two sunspot cycles. Thus, the fundamental period governing solar activity is actually the 22 year magnetic cycle, and the sunspot cycle (which is exactly half that) is just a special manifestation of the magnetic cycle. As we shall see, the magnetic field plays an important role in most aspects of the active Sun (sunspots, prominences, flares, the solar wind, and the nature of the corona), so the 22 year magnetic cycle is central to the periodicity of the active Sun.

    Why are Sunspots Dark

    Well, because they are cooler than the rest of the surface. But that is only a partial explanation. Why are they cooler? The answer is the strong magnetic fields associated with the sunspots.

    Recall that in the region below the photosphere, convective cells are largely responsible for vertical motion of large packets of gas and that this bubbling activity carries heat from the interior to the solar surface (see the discussion of granules in the photosphere). Magnetic fields exert forces on charged particles, and because this solar material is highly ionized, the magnetic fields influence the convective motion.

    Detailed considerations indicate that the magnetic forces hinder the convection of heat to the surface by making it harder for the hot gases to rise. Thus, the region in sunspots having strong magnetic fields tends to be cooler than the surrounding region and thus appears darker than the surrounding regions at higher temperature.