The sun affects everything on Earth. Its magnetic fields and solar wind interact with Earth's own field and can have strong effects. There are various causes of solar wind activity from the sun. One of the major effects is geomagnetically induced currents. The sun's activities can also interfere with communications and satellites. There are also biological effects for life in space (humans in a space shuttle, for example). We cannot escape the influence of the sun, nor would we want to, for the sun provides the necessary elements of life as we know it.
The sun's magnetic field interacts with the Earth's field in many ways. The Earth's field deflects the particles of solar wind and protects Earth's inhabitants from their harmful effects (see biological effects below). The magnetopause is the region about 10 Earth radii above the surface where most harmful particles from the sun are stopped. Because of the solar wind, the Earth's magnetic field has a mangetotail, much like a comet's tail, which stretches away from the sun. The sun's magnetic field actually stretches well into the planetary solar system, and it can join with the Earth's field in magnetic reconnection, a coupling which happens most easily when the two fields are anti-parallel.
When particles from solar wind enter Earth's atmosphere, they can corkscrew down and form a huge electric generator which can exceed 10^12 watts! Currents generated in the upper atmosphere are responsible for the aurora borealis and its southern partner, the aurora australis. An electromotive force of about 100,000 volts is generated by upper-atmosphere currents, which can create geomagnetically induced currents (see below).
The bulk of the adverse effects the sun causes on Earth are directly related to three types of events: erupting flares, disappearing filaments, and coronal holes facing Earth. Flares can last for minutes or hours. They are major sources of excess radiation in the form of Bremsstrahlung x-rays (see page on Ways of Seeing the Sun). When filaments disappear, they either evaporate like clouds on Earth or rise up and become giant prominences. Prominences are major sources of mass ejections from the sun, because they can break away from the sun and hurl their plasma into space and towards Earth. Coronal holes are another source of ejection. Because of their open magnetic field structure, they act as conduits for steady streams of plasma from the sun.
When the solar wind is especially intense, a geomagnetic storm strikes Earth. The Earth's magnetic field is compressed on the side facing the sun, and the many charged particles which reach Earth cause rapidly changing currents to flow through the upper atmosphere. These upper-atmosphere currents cause geomagnetically induced currents to flow through any and all conductors on the ground. The voltage induced can be as high as 10 volts per mile. The currents usually race through conductive materials in the ground, but the igneous rock found at high latitudes has very low conductivity. That means that the currents tend to flow through more conductive objects made my humans. This can cause severe problems. For example, geomagnetically induced currents overloaded transformers and capacitor banks in the Hydro-Quebec power system in 1989. This caused the system to shut down, and produced the temporary loss of 9450 megawatts of electrical power.
Communications can be greatly affected by the sun's activity. This was not noticed until World War II, when radio became a major form of communication. Many systems of communication rely on the ionosphere to bounce the radio signals from source to target, but these communications are affected when the ionosphere is distorted due to geomagnetic storms. When the sun goes through periods of increased radio output, it can jam signals on the Earth, causing ships, airplanes, and satellites to lose communication.
Many of the satellites put into orbit are in an orbit just high enough to avoid the drag of Earth's atmosphere. During periods of high solar activity, however, an increase in UV output and auroral energy input causes the Earth's atmosphere to heat up and expand. This can cause the low satellites to experience more frictional drag than usual, which can cause orbital decay. Satellites without propulsion, or which run out of fuel, prematurely fall to Earth. This is how Skylab was lost in 1979. Nearly the opposite happens to satellites in high orbit. They are out of reach of the atmosphere, and are protected from dangerous solar wind by the magnetosphere. During high solar activity, the magnetosphere is pushed in far enough for solar wind to hit the craft with many charged particles, which can cause damaging electrical discharges.
The Earth's magnetic field protects us from most harmful biological effects of the sun. It may be possible, however, for people in areas of low protection, such as in high orbit or near the poles of the Earth, to be exposed to dangerous levels of radiation. Any interplanetary travelers would have to be very well protected in their craft to avoid excess radiation. The sun has other effects too. Some animals, such as homing pigeons, use the Earth's magnetic field to navigate. If the sun disturbs the Earth's field, these animals may become confused and lost. Some studies also show correlations between high solar activity and death rates for near-death patients, and increased industrial and traffic accidents.
Life could not exist on Earth without the sun. The sun is by far the major source of energy for the Earth. Not only does radiated heat from the sun maintain the Earth's temperature and give light, but the sun's rays also feed billions of plants and algae through photosynthesis. Since these creatures are at the bottom of the food chain, the sun in this way indirectly provides sustenance for nearly everything on the planet. We have also been able to harness the sun's energy by building solar cells and by using concave surfaces and windows to bend and trap the sun's light. Besides all of its other effects, the sun is necessary for life as we know it.