What is space weather?

The European Space Agency (ESA) uses the following definition for space weather:

Space weather refers to the environmental conditions in Earth's magnetosphere, ionosphere and thermosphere due to the Sun and the solar wind that can influence the functioning and reliability of spaceborne and ground-based systems and services or endanger property or human health.

What does it mean?
The Earth has a magnetic field, which can be shown with a compass. Outside the Earth, the magnetic field has the shape of a bullet, called the magnetosphere.

The shape of Earth's magnetosphere. (Image courtesy of ESA/NASA)

This shape is influenced by the magnetic field lines of the Sun and the solar wind (a stream of charged particles, mainly protons, expelled by the Sun). This makes the Sun the most important source of space weather. Other sources are the radiation belts (discovered by Van Allen in 1958) and cosmic particles.
As long as we stay on the ground, the magnetic field of the Earth protects us sufficiently against the radiation caused by these charged particles. But not so for astronauts in space or our modern technology, even on the ground.
Some of the effects are:

• Spacecraft anomalies (e.g. unexpected reboots)
• GPS signal disruption
• Ground induced currents (GIC) in electrical transmission lines, pipelines, ...

In 1859, a large solar storm (known as the Carrington event) caused a shutdown of the telegraph systems between Europe and North America.

But the most well-known effect of space weather is the aurora or northern lights.