Underestimates of magnetic flux in coupled MHD model solar wind solutions

When validated with spacecraft observations, one enduring characteristic of global MHD solar wind models is the tendency to underestimate the interplanetary magnetic flux. This study quantifies the “missing flux” problem for models used in the coordinated Center for Integrated Space Weather Modeling study of corotating interaction regions, and identifies the model parameters most strongly related to the effect. We show that two important contributions are (1) insufficient thermal pressure in the coronal model to extract the required magnetic flux and (2) numerical diffusion in the model current sheets. Using Ulysses observations, we derive a calibration for the effective temperature in the polytropic coronal Magnetohydrodynamics Around a Sphere model that produces the expected interplanetary field at high latitudes. After recalibrating, we find that a 40% discrepancy still remains in the ecliptic plane. Moreover, the problem is 5% more severe for models of the solar cycle 23 minimum than it is for models of the cycle 22 minimum. We argue that the resolution of the heliospheric current sheet strongly affects both the general underestimate and the discrepancy between the two cycles. We also argue that improved resolution of current sheets in the low corona will further reduce the effect.

► We compare MHD model results to Ulysses and OMNI solar wind observations.
► We find that the models always underestimate the magnetic flux of the solar wind.
► Polar magnetic flux is correctable with a hotter corona of 2 MK.
► Correcting the polar magnetic flux does not fully correct the ecliptic magnetic flux.
► Poorly resolved current sheets also lead to missing flux.