Besides providing light and heat, the Sun affects the Earth
through its ultraviolet radiation, the steady stream of the solar wind, and the
particle storms of great flares. The near-ultraviolet radiation from the Sun
produces the ozone layer, which in turn shields the planet from such radiation.
The soft (long-wavelength) X rays from the solar corona produce those layers of
the ionosphere that make short-wave radio communication possible. The harder
(shorter-wavelength) X-ray pulses from flares ionize the lowest ionospheric
layer, producing radio fadeouts. The Earth's rotating magnetic field is strong
enough to block the solar wind, forming the magnetosphere, around which the
solar particles and fields flow. On the side opposite to the Sun, the field
lines stretch out in a structure called the magnetotail. When shocks arrive in
the solar wind, a short, sharp increase in the field of the Earth is produced.
When the interplanetary field switches to a direction opposite the Earth's
field, or when big clouds of particles enter it, the fields in the magnetotail
reconnect and energy is released, producing the aurora borealis (northern
lights). Big flares or coronal mass ejections bring clouds of energetic
particles that form a ring current around the magnetosphere, which produces
sharp fluctuations in the Earth's field called geomagnetic storms. These
phenomena disturb radio communication and produce voltage surges in
long-distance transmission lines and other long conductors.
Perhaps the most intriguing of all terrestrial effects are
the possible effects of the Sun on the climate of the Earth. The Maunder
minimum seems well established, but there are few other clear effects. The only
other definite relationship is the temperature change associated with the
quasi-biennial oscillation of the tropical stratospheric wind. The brightness
of the Sun varies with activity; a large sunspot reduces emission by an amount
corresponding to its area. The effects of plages produce an overall increase in
the solar flux by about 0.1 percent, however, when spot activity increases.
This is a negligible effect, and so particle effects and fluctuations of
ultraviolet radiation in the stratosphere are thought to be important.
Because charged particles follow magnetic fields,
corpuscular radiation is not observed from all big flares but only from those
favourably situated in the Sun's western hemisphere. The solar rotation makes the
lines of force from the western side of the Sun (as seen from the Earth) lead
back to the Earth, guiding the flare particles there. These particles are
mostly protons because hydrogen is the dominant constituent of the Sun. Many of
the particles are trapped in a great shock front that blows out from the Sun at
1,000 kilometres per second. The flux of low-energy particles in big flares is
so intense that it endangers the lives of astronauts outside the terrestrial
magnetic field. (Encyclopedia of Britannica)