Magnetograms are maps of the line-of-sight component of magnetic flux at the photosphere, the sun's visible surface. The fields are measured by detecting the Zeeman shift between right-hand and left-hand circularly polarized light in a suitable magnetically sensitive absorption line. Only the line-of-sight component can be measured this way. Upper left: 10 ? Ca K line; upper right is corresponding magnetogram. Light and dark areas in image at left show where the field is large and directed out of and into the Sun, respectively.