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.
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Solar magnetic field
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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.