Space weather refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and that can affect human life or health.

The Sun is the main driver of space weather near Earth. Other phenomena such as galactic cosmic rays also contribute to space weather.

Solar Eruptions

The Sun is a very active star. Solar eruptions called flares and coronal mass ejections, also known as CMEs, frequently take place, often without warning.
The number of eruptive events varies with an approximately 11-year cycle. When activity levels are at their highest, the number of explosive events is about 10 times higher than during periods of low activity. These explosive events are the main source of space weather disturbances near the Earth i.e. they are the main drivers of space weather.

Protuberances

Protuberances are big tubular archs, filled with relatively cool gas, rising over the surface of the Sun far into the corona. There, they often stay for many days and weeks stable, almost not moving, obviously shielded magnetically against the surrounding hot corona. Sometimes they suddenly burst and drive CMEs into the space. Here there is another unsolved problem in understanding space weather: The forecast of such eruptions and CMEs, before they occur!

Similar to the interaction of terrestrial high and low-pressure areas, also space weather changes with time because of the galactic cosmic rays pouring in through the heliosphere as well as the solar wind and the interplanetary magnetic field.

Close to the Earth, the intensity of the galactic cosmic rays is in average 20% lower during solar maximum compared to the intensity during solar minimum. Galactic cosmic rays are measured with ground based and space based instruments.

Should manned space missions take place during solar maximum conditions (because of lower galactic cosmic ray intensity)? No, this would be only half of the story. Actually, the risk increases during solar maximum condition because high energetic particles from the Sun (sometimes referred to as solar cosmic rays) excited by solar eruptions occur more often because the number eruptions increases!

The galactic and solar cosmic rays contribute to the radiation exposure of flight attendants. This is the reason why the European Council published a guideline to protect flight attendants (guideline 96/29/Euratom).

Magnetosphere

The region in the near Earth space which is affected by the Earth magnetic field, is called magnetosphere. Near the Earth, the magnetic field is a dipole, similar to the field of a bar magnet. The Earth's radiation belts ("Van Allen belt") is located in this region. Because of the interaction between the solar wind and the Earth magnetic field there are deviations from the dipole shape in longer distance from the Earth: the magnetic field is compressed on the dayside and on the nightside, the magnetic field is stretched to a tail. This tail reaches several hundred thousand kilometers into the space.

Ionosphere

The ionosphere is the ionised part of the Earth's atmosphere. Most of the atmospheric gas molecules in the ionosphere are ionised by the ultraviolet radiation coming from the Sun. Additionally, charged particles from the magnetosphere reach the atmosphere and contribute to the ionosphere.The ionosphere forms several layers ranging in altitude from about 60 km to more than 1000 km. The ionosphere is electrically conducting. Therefore, it reflects radio waves at frequencies less than about 30MHz like a plate or a mirror. Higher frequency radio waves - up to 10GHz - can pass through the ionosphere, but undergo modification. Electric currents flowing at these altitudes modify the geomagnetic field, induce voltage effects at the ground and warm up the ionosphere like an electric heater.

In analogy to terrestrial weather, the ionosphere is highly dynamic and variable. The most important variation is due to the diurnal variation of solar radiation. This causes a peak in ionisation shortly after noon and a minimum just before sunrise. Compare this to the temperature variation at the Earth's surface which varies in a similar way. Since solar illumination also depends on the season, and the geographic location, the same variation is seen in ionospheric density.Long-term modulation of the ionospheric plasma is closely related to the solar cycle. At altitudes above 200 km, the plasma motion is mainly controlled by the geomagnetic field lines.

Thermosphere

Space weather influences the atmosphere in many ways. Most affected it the upper atmosphere (thermosphere). The most impressive aspect are polar lights (formerly known as "aurora borealis" in the northern hemisphere and "aurora australis" in the southern hemisphere). But also other effects are relevant: cosmic radiation causes particle precipitation in the thermosphere, electric currents can heat the thermosphere causing an expansion of the atmosphere. This affects the orbit hights of low earth orbit satellites.

The colour of the polar lights depends on the chemical nature of the gas particles and the energy of the precipitating electrons. Ozon atoms emit green and red light while nitrogen ions emit blue and violet colors.

Our society is becoming increasingly dependent on technology that can be affected by space weather. Electronics, mobile phone communications, electricity supplies, navigation systems can all experience space weather related problems. At the AGU 2011-Space Weather Workshop, Louis Lanzerotti from the New Jersey Institute of Technology in Newark, N.J., explained what happens. "At their most benign, such space weather events trigger beautiful aurora in the night sky as incoming particles collide with Earth's atmosphere and produce light. But space weather can also adversely impact our modern technological infrastructure. Even in the mid-1800s, telegraph operators noticed that auroras in the night sky coincided with disruptions to telegraph operation – and today such disruptions can affect a much wider array of technologies that have developed over the last century. Such space weather-produced effects include loss of radio contact for airplanes on transpolar flights, astronauts imperiled by radiation, damage to electric grids, and disruption of cell phone service and underwater telecommunication cables, and destruction of satellite electronics. The effects of the sun can be complex, subtle, and some times quite surprising, and we need accurate models and accurate forecasts to protect modern technology."

Several worldwide activities related to space weather are currently carried out. For example in Germany, Europe and USA, several space weather centres do research, provide service, participate in related commercial and technological applications.
The Space Environment Center in Boulder, ESA Space Weather Network and the COST action 724 are carrying out international space weather activities. Further international space weather activities - for instance in Australia and Japan - have been developed in the recent years as well. 
In Germany space weather related activities are carried out by DLR (ionosphere weather and prediction), by GFZ Potsdam (Champ satellite), IAP Kühlungsborn (space weather and atmosphere) and by the University of Greifswald/Institute of Physics/Space Weather Observatory (space weather reports and real time observation of plasma clouds by means of the MuSTAnG telescope).