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
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
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).
- 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.
- 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
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.
Here is a SOHO image showing the
current magnetic fields on the surface of the Sun.
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.
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.