Solar flares, which often signify the departure of a CME, produce X-rays and ultraviolet radiation, as well as particles accelerated to relativistic energies. Clues to the magnetic configuration of the corona have been obtained from ground-based observations for 150 years, according to McClymont, but major advances from space observations began with the Skylab mission. These give an idea of the field geometry, which can be very complex in energetic active regions. In its simplest form, an active region exhibits the magnetic pattern of a bar magnet hidden beneath the photosphere. Under the light of a strong spectral line, the gaseous surface shows elongated features, called fibrils, which line up with the magnetic field.
Sunspots mark the positions of the poles of the magnet.
Such simple active regions, however, rarely produce flares and CMEs. An explosive energy release requires free energy. The shapes of magnetic field lines over an active region were calculated by a method that evolves an assumed potential field into a coronal force-free field. Courtesy Zoran Mikic, SAIC, San Diego. This ensures that the position at which the field is measured can be known accurately.