The variable conditions in interplanetary space, usually caused by solar disturbances, which affect human health or technological systems such as satellite navigation (Global Positioning System), radio communications, and spacecraft electronics. Major geomagnetic storms caused by fast-moving coronal mass ejections (CMEs) are the principal space weather events. Other effects are caused by X-rays, radio bursts, and energetic particles generated by solar flares and CMEs. Space weather damage is caused by energetic electrons or ionized atoms penetrating spacecraft electronics or a human body, by ionospheric disturbances that affect radio propagation, and by electrical currents induced in the electrical grid or pipelines on Earth.
http://www.esa-spaceweather.net/spweather/BACKGROUND/index.html Space weather information at ESA.
http://www.swpc.noaa.gov/ Space Weather Prediction Center.