Unlike the Earth's magnetic field which changes slowly, the Sun's magnetic field can change quite rapidly. Small bits of field can change over a few minutes. During solar flares a whole section of the Sun's magnetic field can be disrupted in minutes or hours. For example, on April 14, 1994, Yohkoh observed such a large-scale eruptive event which resulted in a cloud of solar material weighing 1 billion tons moving towards the Earth at a speed of over 1 million miles per hour (the event can be viewed from our flare page).

The most rapid changes to the Sun's magnetic field occur locally, in restricted regions of the magnetic field. However, the entire structure of the Sun's global magnetic field changes on an 11 year cycle. Every 11 years, the Sun moves through a period of fewer, smaller sunspots, prominences, and flares - called a "solar minimum" - and a period of more, larger sunspots, prominences and flares - called a "solar maximum." A maximum and a minimum, taken together, make up one solar cycle. During the 11 years, the strongest magnetic fields (in sunspots) slowly migrate towards the Sun's equator from locations about midway to the Sun's poles. After 11 years, when the next cycle starts, the magnetic field poles are reversed.

Click to enlarge

Click to enlarge

The last solar maximum was in 1989-90. The next will be in 2000-01. The picture shown here is a collage of Yohkoh images taken every 120 days for four years. The difference between the active Sun (1991, on the left) and the quiet Sun (1995, on the right) is dramatically apparent.

The image below shows the 11 year magnetic cycle on the Sun from about 1600 to the present day. In the late 1600's (A.D. 1645-1715) the cycle ceased briefly in what is known as the Maunder Minimum. It coincided with a period of colder-than-average temperatures in northern Europe called the Little Ice Age. Neither the Maunder Minimum nor the 11 year cycle is fully understood.