The Sun is a very active star. Solar eruptions called flares and coronal mass ejections, also known as CMEs, frequently take place, often without warning.
The number of eruptive events varies with an approximately 11-year cycle. When activity levels are at their highest, the number of explosive events is about 10 times higher than during periods of low activity. These explosive events are the main source of space weather disturbances near the Earth i.e. they are the main drivers of space weather.
The active Sun in January 2000, as seen by the EIT instrument onboard SOHO. Note the large numbers of bright features and eruptions taking place.