This week's image is provided by Zoran Mikic , SAIC, San Diego. 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. 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.