Chapter 15--Our Sun - Geological Sciences

particles spiral around
magnetic field lines
stream of solar particles
from solar wind
N
S
a Interactions between Earth’s magnetic field and the magnetic field of the solar wind can
send energetic charged particles toward Earth’s poles.
Figure 15.23 Particles from the Sun cause auroras on Earth.
b Aurora along the coast of Norway.
and powerful currents induced in the ground circuits of
the Quebec hydroelectric system caused it to collapse for
more than 8 hours. The combined cost of the loss of power
in the United States and Canada exceeded $100 million. In
January 1997, AT&T lost contact with a $200-million communications
satellite, probably because of damage caused
by particles coming from another powerful solar storm.
Satellites in low-Earth orbit are particularly vulnerable
during solar maximum, when the increase in solar X rays
and energetic particles heats Earth’s upper atmosphere,
causing it to expand. The density of the gas surrounding
low-flying satellites therefore rises, exerting drag that saps
their energy and angular momentum. If this drag proceeds
unchecked, the satellites ultimately plummet back to Earth.
Satellites in low orbits, including the Hubble Space Telescope
and the Space Station, require occasional boosts to prevent
them from falling out of the sky.
Connections between solar activity and Earth’s climate
are much less clear. The period from 1645 to 1715, when
solar activity seems to have virtually ceased, was a time
of exceptionally low temperatures in Europe and North
America known as the Little Ice Age. Did the low solar activity
cause these low temperatures, or was their occurrence
a coincidence? No one knows for sure. Some researchers
have claimed that certain weather phenomena, such as
drought cycles or frequencies of storms, are correlated with
the 11- or 22-year cycle of solar activity. However, the data
supporting these correlations are weak in many cases, and
even real correlations may be coincidental.
Part of the difficulty in linking solar activity with climate
is that no one understands how the linkage might
work.Although emissions of ultraviolet light, X rays, and
high-energy particles increase substantially from solar minimum
to solar maximum, the total luminosity of the Sun
barely changes at all. (The Sun becomes only about 0.1%
brighter during solar maximum.) Thus, if solar activity
really is affecting Earth’s climate, it must be through some
very subtle mechanism. For example, perhaps the expansion
of Earth’s upper atmosphere that occurs with solar
maximum somehow causes changes in weather.
The question of how solar activity is linked to Earth’s
climate is very important, because we need to know whether
global warming is affected by solar activity in addition to
human activity. Unfortunately, for the time being at least,
we can say little about this question.
516 part V • Stellar Alchemy