Here I intend to describe what it's on the Sun that influences the Earth to make an aurora develop.
Sunspots are dark spots in the solar photosphere (Sun's visible surface), which can have a diameter of 2,000-100,000 km. The sunspot consists of a dark interior, the umbra with a temperature 2000 - 2500 degrees lower than the photosphere's 5800 °K. The umbra is sometimes surrounded by a lighter area known as the penumbra and its temperature is about 500 degrees lower than the photosphere's. A sunspot suddenly appears as a dark poor and grows in size until it covers a larger or smaller part of the photosphere, it can divide into smaller groups of spots or disappear after a couple of weeks or months. Very large sunspots, or groups of sunspots, often have longer duration The sunspots vary periodically in numbers with a cycle of eleven years. This variation is called the solar cycle. The latest maximum occurred in April 2000. The long awaited solar cycle 24 arrived early January 2008. Solar cycle max is expected around the year 2011 or 2012.
Take a look at the sharpest
picture ever made of a sunspot. Picture made with the Swedish solar
telescope at La Palma.
The most violent events on the Sun are the solar flares, which occur near the sunspots. When a solar flare is discharged, a sudden release of energy appearing as electromagnetic radiation on all wavelengths is spread out. Solar flares are very intense radio wave transmitters, and on the Earth we receive radiation many times stronger than from any other cosmical radio source. A plasma cloud consisting of electrons and protons, a CME (Coronal Mass Ejection), is eventually thrown out into the solar wind with a speed up to 2000 km per s. The solar wind which normally has a speed of 300 - 400 km per s increases in intensity and when it reaches the Earth, after a couple of days, magnetic disturbances and aurora occur on the Earth.
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