White Light and Magnetic Flux

Strong flares are believed to be associated with the emergence of magnetic flux through the photosphere. The figure shows a sequences of white light images (left), and corresponding magnetic inclination maps (right).

These latter images are similar to magnetograms, except that the color scale now codes the inclination of magnetic fieldlines with respect to the vertical; light green indicates a vertical magnetic field pointing towards the observer, light blue a vertical field pointing away from the observer, and orange a field parallel to the solar surface. The image is left uncolored wherever the field strength is smaller than about 200 Gauss. Magnetic neutral lines are superimposed on each set of images (in yellow on the left, black on the right). These images were constructed from data obtained with a vector magnetogram, an instrument that allows simultaneous determination of all three components of the magnetic fields at a given point on the solar surface, from the measured intensity of linear and circular polarized light. All images have been rotated so that they are viewed from a direction perpendicular to the solar surface, and tick marks are 10 Megameters (10000 km) apart in all cases.

On June 16 1992 (not shown), this active region had the appearance of a simple sunspot pair, and was hardly visible in X-rays. Five days later (top row), smaller spots and pores began to appear, resulting in a much more complex magnetic field structure in the active region as a whole, as evidenced by the magnetic inclination map and the circuitous paths traced by magnetic neutral lines. By the next day, the coalescence of smaller structures has led to the formation of numerous small sunspots and sunspot groups with well-defined penumbrae. Two days later (bottom row) the active region had grown further, and was again globally bipolar, with a global neutral line running roughly SE - NW. Examination of X-ray image sequences reveals a drastic increase in X-ray brightness on June 21, followed by nearly continuous flaring as the active region is carried over the limb in subsequent days. Note also how the magnetic field is essentially vertical in the umbra of sunspots, while in the penumbra it shows significant inclination with respect to the vertical.

Written By P. Charbonneau and O.R. White–April 18, 1995