The image of the visible solar "surface" - the photosphere - showing several sunspot groups, observed on June 22nd, 2000. The enlarged image in the inset shows the dark umbra and the structured penumbra of a sunspot. The "quiet" regions are composed of the solar granulation, a convective flow pattern that brings hot matter and energy from inner layers to the surface. The full solar disk is recorded in the blue spectral range by Precision Solar Photospheric Telescope (RISE/PSPT) in Rome and the detail view was obtained by the Vacuum Tower Telescope (VTT) on Tenerife operated by the Kiepenheuer Institut für Sonnensphysik, Freiburg im Breisgau.

Solar Activity: Sunspots, flares and other forms of the solar activity, forming the 11-year solar cycle, are various manifestations of the evolving solar magnetic field. The solar irradiation at the Earth ("solar constant") shows the same periodicity.

The "solar constant" observed by satellites outside the atmosphere. (Courtesy of Greg Kopp, University of Colorado)

The variations of the so-called Sunspot Relative Number since the beginning of the XVIII century, when systematic observations started (data compilation and image preparation by SIDC, Brussels, click on imag for newest version).

A solar flare is a complex process of an abrupt release of the energy stored in the coronal magnetic field structures. The coronal plasma is heated within a few minutes to several tens of millions degrees, causing also flaring of the chromosphere. The solar radio and X-ray emission can become more than ten thousand times stronger than normal. Large flares usually happen in the course of huge coronal eruptions, so-called coronal mass ejections (CMEs). During the eruption, the unstable coronal magnetic structure is accelerated to speeds on the order of 1000 km/s, usually arriving to the Earth in 2-4 days.

Sequence of images taken in the Hα spectral line on Apr. 11th, 2013, showing a so-called chromospheric flare. The field of view is ca. 290.000 x 290.000 km².

The structure of the solar corona is governed by the global solar magnetic field and the sunspot-related magnetic fields. In previous times the corona was visible on the Earth only during total solar eclipses. (Image taken by A. Sudy during the total eclipse of March 29th, 2006, Turkey). Nowadays, we can permanently observe the structure and dynamics of the corona by coronagraphs and from satellites.