Measuring solar surface magnetic fields without ambiguity

The goal of this paper is to find a method to yield solar surface magnetic field measurements while inflicting minimal prior assumptions on the acceptable solutions. A small spacecraft placed at least 0.1 radian away from the Earth–Sun line can, with vector field observations from the Earth, offer unambiguous determinations of magnetic fields from the Sun. A telescope of between 10 and 40 cm aperture operating at ultraviolet (UV) wavelengths can, with a simple Stokes-V polarimeter, enhance observations made with the suite of large telescopes on the Earth. A 40 cm aperture can achieve the same diffraction limit at 400 nm that Daniel K. Innouye Solar Telescope (DKIST) might achieve at 4 microns. Even a 10 cm aperture offers higher signal to noise in Stokes V than DKIST can Q and U in magnetic fields outside sunspots, transverse to the line of sight, offering enormous benefits for understanding magnetic components transverse to the Earth–Sun line. We identify UV and infrared (IR) lines of Fe I that are useful for joint spacecraft–DKIST observations. Benefits of space observations also include line-of-sight fields measured in strong chromospheric lines in the UV that are free of seeing, such as Ca II H&K and Mg II h&k. A simple mission devoted to such space measurements will enhance our understanding of solar magnetism and hence, space weather and space climate, for decades to come.