SOLARNET Conference: The Many Scales of the Magnetic Sun

Open magnetic flux in the photosphere, especially at the poles, presents an important boundary condition for modeling the heliospheric field. The field extrapolated from polar observations is roughly 50% lower than the in-situ measurements (“open flux problem”). Here we present our efforts to characterize biases in the open flux inference and that way explain the discrepancy above. We use a state-of-the-art plage-like MURaM MHD simulation of the solar photosphere to model polarized spectra of magnetically sensitive Fe I 630 nm doublet lines, observed by the Hinode SOT/SP instrument. We mimic the realistic observation at the disk center and the pole by calculating emergent intensity, applying instrumental effects (spatial and spectral PSF, binning), and adding the photon noise. We then apply different inversion codes built upon different physical assumptions to these synthetic observations and compare results to the original simulation.

At the disk center, a significant fraction (20, down to 50%) of open flux gets lost for the telescopes with apertures smaller than 1m. At the pole, the magnetic field disambiguation becomes a critical part: the assumption of the radial field underestimates flux by around 30% while the inclusion of the telescope PSF further exacerbates the problem. The more in-depth investigation that accounts for stray light still yields significant discrepancies between the original and inferred intrinsic properties of the magnetic field. These systematic errors in field diagnostics may be reduced by high-resolution or out-of-ecliptic observations.