Page 99 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
rotates on its axis, which we now know is a rotation rate which differs
with solar latitudes. As it turns out, this differing rotation rate represents
part of the process which produces sunspots and solar flares.

As mentioned earlier, magnetic fields passing through a plasma become
trapped and move with the plasma. The differing rotation rate of the sun
causes these trapped magnetic field lines to “wind up” and increase their
field strength to the point where they produce a magnetic pressure on the
plasma. In these areas, the gas pressure is lower, which lowers the gas
temperature. As we have seen, the sun “looks” yellow due to the temper-
ature at which its surface is radiating. Sunspots are darker-looking patch-
es of lower temperature due to these areas of increased magnetic field
strength at the surface. Magnetic field strengths of 1,500 G in these loca-
tions are not uncommon.

Solar Flares. The actual cause of solar flares is still not completely under-
stood. One explanation states that the magnetic field lines continue to
wind up to a strength where they pop out from the surface of the sun, car-
rying solar plasma into space. The escaping material may be accelerated
by the field above the surface to velocities approaching relativistic speeds
up to 106 dsec. Intense electromagnetic energy in ail wavelengths, from
X-rays through radio, is also released by solar flares. As we shall see
shortly, arrival of these high-energy particles and intense radiation to the
vicinity of the earth creates a situation which can affect the operation of
ground systems as well as spacecraft.

Solar Cycle. Records have been kept on the number of sunspots visible on
the sun’s surface since the mid-l700s, and review of these records indi-
cates that the number of sunspots changes over time with some regulari-
ty. At times, no sunspots at all may be visible, while at other times over
100 sunspots may exist simultaneously. If one describes the period of lit-
tle or no sunspot activity as solar minimum and the period of highest
activity as solar maximum, the average time from one minimum or max-
imum period to the next can be defined as the solar cycle. Though the
solar cycle varies between approximately seven to thirteen years, the aver-
age cycle has been found to be about eleven years. Also, on average, it
seems to take less time to rise to solar maximum, around four years, than
to settle back to solar minimum, around seven years. The next solar max-
imum is expected to occur around the year 2000.