Solar-terrestrial physics aims to understand the effects of the Sun throughout the solar system. Energy from the Sun provides the Earth with both light and heat, allowing for plants and animals, including humankind to live. The majority of the weather that we experience on Earth is due to solar energy. Space Weather is the response of our space environment to changes in the Sun. Space Weather is also a consequence of the nature of the Earth's magnetic field and magnetosphere as well as the Earth's location in the solar system. It has been postulated that the Sun can produce changes in the Earth's climate over decades or centuries.

A well documented account of a correlation between solar activity and climate was the Little Ice Age. The Little Ice Age, which was a time when things appear to have been cold over most of the world, took place between 1300 and 1850. There are records of famine and revolt taking place during these harsh times, showing the affect this severe climate had on humankind. The coldest phase of the Little Ice Age was around 1700. This period coincided with a time when sunspots were exceedingly scarce, known as the Maunder Minimum. The Maunder Minimum occurred between 1645 and 1715 and was named after the British scientist Walter Maunder who identified this period as a time of little solar activity.

Another theory concerning the cause of the ice ages was advocated by Milutin Milankovitch in the 1920s. This theory calls on the fact that the shape of the Earth's orbit changes over a period of about 100,000 years. This change in orbit alters the Sun's distance at different seasons. In addition to the change in orbit the Earth also wobbles on its axis, changing the tilt of its axis with relation to the Sun by 3 degrees and then returning to its original tilt in about 41,000 years. When one puts all of these various wobbles of the Earth together on a graph, the wiggles fit very well with the recorded ice ages. An ice age appears to have occurred when the Sun in the northern lands was too low in the sky or too far away during the summer. Since the snow would fail to melt entirely it would build up winter after winter creating an ever-thickening sheet of ice.

The premiere scientific discovery in 1958 was of the Earth's radiation belts, named after the scientist Van Allen. This discovery showed that charged atomic particles, electrons, and ions , all originating from the solar wind, were trapped in the Earth's magnetic field above the atmosphere. The Earth's magnetic field exerts electromagnetic control over the charged particles, forcing them into the Van Allen Belts. Some of these charged particles escape from the magnetosphere.

Aurora

An aurora is an event which occurs when the atmospheric molecules discussed above are excited by incoming charged particles from the solar wind. They emit energy as they fall back to their ground state. Auroras generally occur at high latitudes, near the Northern and Southern magnetic poles. The chance of seeing an aurora has been shown to rise and fall with the number of visible sunspots. These spots were first observed by Galileo and have since then been shown to increase and decrease in number over an eleven year period.

Sunspot

In essence the solar wind is the hot solar corona expanding into the interplanetary and interstellar space, flowing away from the Sun at speeds as high as millions of miles per hour. Below is a diagram illustrating the affects of the solar wind. When solar wind streams with different velocities interact with each other they create interplanetary shocks or discontinuities in density. An additional shock is created when the solar wind encounters the Earth's magnetic field. The solar wind approaching the Earth's magnetic field creates a supersonic shock wave called the bow shock. Most of the solar wind particles are heated and slowed at the bow shock and go around the Earth through what is called the magnetosheath. Some of the particles are then reflected from the bow shock into the solar wind stream in a region known as the foreshock.

As the solar wind flows out around the earth it compresses the magnetosphere on the side facing the Sun and elongates the magnetosphere on the side facing away from the Sun, creating the magnetotail. The magnetotail is believed to be several million kilometers long. Some of the solar particles are able to penetrate the barrier at the Earth's boundary and become trapped in the magnetosphere. They are then stored in the plasma sheet and radiation belts. Some of the particles rush through the openings at the poles, known as the polar cusps. Many of these particles enter the Earth's upper atmosphere to form the aurora.

Solar events can cause changes in the electrical and chemical properties of the Earth's atmosphere, in the ionosphere, and in the magnetosheath. Changes in the Earth's environment caused by solar activity can happen on time scales ranging from less than a minute to over a century. These changes cause magnetic storms, communication static, power blackouts, and navigation problems for ships and airplanes that use magnetic compasses. A solar storm will increase the density of the atmosphere, possibly damaging satellites and spacecraft or causing them to re-enter the atmosphere earlier than planned. The Sun and solar wind also appear to play a role in the long term climate changes on Earth. Our ability to predict particle outbursts and fluctuations in the plasma flowing from the Sun will become even more important as more of our science and commerce depend on the operation of vehicles in space.