As a dazzling effect of space weather -- conditions in space that are produced by the Sun and can affect Earth's magnetic field -- auroras are Earth's natural light show. When the gases in Earth's atmosphere interact with the perpetual stream of particles flowing from the Sun, luminous colors and shapes can be seen in the dark skies near Earth's magnetic poles. Although auroras are always present in some form, they vary in intensity, and often they are either too faint to be seen or are obscured by or clouds or daylight.

Weather conditions in space are determined by the Sun, which goes through an approximately eleven-year cycle of high and low activity. The strength of the solar wind -- the constant stream of charged particles flowing from the Sun's corona -- depends on the level of the Sun's activity. During times of high activity, the number of sunspots increases. These cooler, darker regions on the Sun's surface are readily observable and coincide with less visible but energetically significant intense bursts of radiation called solar flares. Other major weather events that occur with greater frequency during times of sunspot maxima include coronal holes, which release high-velocity particles, and coronal mass ejections (CME), which eject huge amounts of high-speed solar material. Events such as solar flares, coronal holes, and CMEs increase the outflow of protons and electrons in the solar wind.

The solar wind carries this stream of particles through the solar system, dragging the interplanetary magnetic field (IMF) with it. The IMF is the magnetic field produced by the Sun, similar to Earth's magnetosphere but much larger. Typically, the magnetosphere surrounds and protects the Earth by deflecting most of the solar wind. However, when the IMF and the magnetosphere interact, some solar particles are able to leak in along the magnetic field lines at the North and South Poles, causing auroras.

When high-energy particles from the solar wind collide with gases in Earth's upper atmosphere, the gases are excited. Through the same process that makes a neon sign glow, excited gas particles temporarily gain energy. When they drop back to their regular energy levels, the released energy can be seen as light in the aurora.

Particularly brilliant auroras are produced when the Sun is active or when the IMF lines up with Earth's magnetic axis, as it does around the time of the spring and fall equinoxes. However, other phenomena also result from this increased interaction between the solar wind and Earth's magnetosphere. The increase of solar particles also causes significant disturbances in the magnetosphere in the form of geomagnetic storms. Such storms disrupt radio communications and navigation systems, and are also hazardous to satellites, electrical equipment, and pipelines.

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