magnetograms

Magnetogram is an image or map that shows the distribution of magnetic fields on the surface of a magnetized object, most commonly the Sun. Solar magnetograms can depict either the line-of-sight component of the photospheric magnetic field or the full vector field. The line-of-sight signal is typically derived from the circular polarization of magnetically sensitive spectral lines through the Zeeman effect, producing a B_LOS map that is often corrected for viewing angle. Vector magnetograms involve measuring the full set of Stokes parameters and inverting them to obtain magnetic field strength, inclination, and azimuth, with a subsequent resolution of the 180-degree azimuth ambiguity in the transverse field.

Instruments and data: Solar magnetograms are produced by ground- and space-based spectropolarimeters and magnetographs, delivering full-disk or regional maps. Notable solar instruments include the Michelson Doppler Imager on SOHO, the Helioseismic and Magnetic Imager on SDO, Hinode’s Solar Optical Telescope spectro-polarimeter, and SOLIS/VSM. Magnetogram time series enable tracking of magnetic flux emergence and cancellation and the evolution of active regions. They are used to identify polarity inversion lines, monitor sunspots, and provide boundary conditions for coronal magnetic field models via extrapolations such as potential-field or nonlinear force-free fields.

Limitations: Measurements are typically taken in the photosphere, so the coronal field must be inferred. Projection effects near the solar limb, instrumental noise, and, for transverse fields, the 180-degree ambiguity pose challenges. Cadence and spatial resolution vary by instrument, with modern space-based instruments offering high-cadence, high-resolution full-disk maps.