Every 11 years our Sun reaches a peak of activity that makes its influence felt across the Solar System, but what causes it?
Even as you read this, enormous explosions are wracking our local star, 150 million kilometres (93 million miles) from Earth. Twisted lines of magnetic field are channelling hot gas far above the surface of the Sun, occasionally 'shortcircuiting' to release incredible amounts of energy that fires out particles across space at up to 80 per cent the speed of light. 2013 marks the latest 'solar maximum', the centrepiece of an 11-year period of activity in the Sun's upper layers that is officially known as Solar Cycle 24. At its peak, the Sun's activity is expected to release epic levels of energy inbursts that will make themselves felt across the Solar System. While astronomers on Earth will watch eagerly for spectacular displays of the polar auroras (northern and southern lights), engineers will be on the alert for trouble. During the 1989 maximum, a solar storm triggered a blackout across Canada and north-east USA as it overloaded electricity grids, while a decade before, a similar event sent NASA's Skylab space station plunging back to Earth in a premature, uncontrolled descent. Already in 2013, solar flares have briefly disrupted GPS satellite navigation signals and radio communications through their effects on the ionosphere - an upper region of our planet's atmosphere that is often used to send long distance radio signals. The solar maximum changes the Sun in a variety of ways. The number and size of sunspots on its visible surface is at its greatest, turning the bright orb into a mottled, blotchy disc of light when it is projected through safe viewing equipment. Seen via special filters, the disc of the Sun reveals bright loops around its edges, known as prominences, and dark streaks across its face, called filaments. Prominences and filaments are, in fact, the same phenomenon, appearing bright against the background of space but dark against the hotter solar surface. Bright spots above the solar disc mark the locations of huge solar flares, and seen from space or during an eclipse, the Sun's outer atmosphere, or corona, is much bigger and brighter than normal. Although for years we believed that the Sun's overall energy output changed very little throughout the cycle, in 2009 astronomers revealed evidence for significant changes in its emission of ultraviolet radiation, which some think has the potential to affect Earth's climate. Yet the solar maximum is just one phase in the overall solar cycle. At other times, the Sun is relatively quiet and free of sunspots and flares. The cycle was first documented in 1843 by German astronomer Samuel Heinrich Schwabe, after he spent 17 years recording the number and distribution of sunspots. Soon after, Swiss astronomer Johann Rudolf Wolf had traced the cycle as far back as 1745 using historical records, and introduced the numbering system that is still used to refer to different cycles. Today the number and distribution of sunspots is often represented by the so-called 'butterfly diagram': the cycle starts off at solar minimum, with very few sunspots visible at fairly high latitudes on either side of the Sun's equator. Each individual spot may last a few days or weeks, but the pattern of distribution changes with time. Over several years, sunspots occur at lower latitudes and their number, size and intensity increase, reaching a peak at solar maximum. Over the next few years, as the spots draw closer to the equator, their numbers and intensity decline again.
(from "How it Works?")