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Aurora: Sheets and waves of vibrant light caused by charged solar particles igniting gases in Earth's atmosphere. They are typically visible at northern latitudes, and sometimes (during heavy solar activity) into the mid-latitudes of the United States and elsewhere. In-depth look at the aurora

Geomagnetic storm: When unusually strong surges of solar wind (charged particles from the Sun) hit the Earth. This effect causes variations in the magnetic field which surrounds the Earth. The result are visible aurora (Northern and Southern lights).

Heliosphere: Charged particles streaming away from the Sun envelop Earth and the other planets in a magnetic bubble. "The heliosphere is the bubble this solar wind blows out into the local interstellar medium. It defines the volume of space over which our Sun's influence predominates," says Richard Marsden, the ESA's Ulysses project scientist.

The closest boundary of the heliosphere is thought to extend about 100 AU out from the Sun, which is 100 times the distance of the Earth from the Sun. The heliosphere protects us from the worst space weather -- cosmic rays. These very energetic particles are generated far away in the universe, and if not for our protective bubble, they would bombard Earth continuously and damage living cells. "Without the heliosphere, life would certainly have evolved differently—and maybe not at all," Marsden says. The heliosphere is shaped and recharged by the solar wind.
Hot plasma trapped in magnetic loops.

Magnetic Field: A field of force that is generated by electric currents. The Sun's average large-scale magnetic field, like that of the Earth, exhibits a north and a south pole linked by lines of magnetic force.

Solar flare: The Sun frequently spews plumes of energy that are more energetic than the constant solar wind. These solar flares contain gamma rays and X-rays, plus energized particles (protons and electrons). Energy can be equal to a billion megatons of TNT is released in a matter of minutes. Flare activity picks up as sunspots increase.

The magnetic explosion during a solar flare accelerates electrons and atomic nuclei to significant fractions of the speed of light. Unlike much slower atomic particles in the solar wind, which travel straight out from the Sun, the energetic particles from a flare follow curved lines of the Sun's interplanetary magnetic field. The particles slant in toward Earth from the west at about 45 degrees to the direction of the Sun.

Solar flare classification: Solar flares are classified based on their output of X-ray energy at the peak of their burst.
* C-class flares are common
* M-class flares are larger but moderately powerful
* X-class flares are the most powerful

More detail on flare classification

Solar wind: Charged particles, mostly protons and electrons, streaming out from the Sun. Earth's own protective magnetic field repels most of the solar wind. This stream of outflowing particles comes in two varieties, "fast" (about 750 km/s) and "slow" (350 to 400 km/s).

Solar wind speed varies with solar latitude and solar activity. During periods of low activity, the fast wind emanates from the poles and the slow wind from near the equator. The fast wind comes from relatively cool gaps in the corona, called coronal holes, and the slow wind from hotter coronal "streamers". The solar wind feeds the Sun's heliosphere.