The Coronal Plasma Density Structure over a Solar Cycle: A Comparison of Tomography and Magnetohydrodynamic Simulations

The large-scale plasma density structure of the extended solar corona is a tracer of the underlying magnetic field configuration and dictates the structure of the solar wind in interplanetary space. Accurate density maps are therefore important to probe the physics of the solar corona and the solar wind, and to improve space-weather forecasting. Density can be estimated from coronagraph observations using solar rotational tomography and is independently provided by global magnetohydrodynamic models. This study compares densities from tomography with the Magnetohydrodynamic Algorithm outside a Sphere (MAS) model across the whole corona over a solar cycle from 2007 to late 2023. The dependence of electron densities on latitude and heliocentric height is compared, with densities in better agreement at equatorial regions and lower heights. The streamer and nonstreamer regions are compared separately, as well as the average width of the streamer belt over the duration of solar cycle 24. The MAS model densities contain more fine-scale detail than those of the tomography, but there is a very good overall agreement in the structure and location of high-density features. In polar regions, where the photospheric magnetic field measurements that drive the MAS simulations present very large uncertainties, the tomographic densities are more accurate. Investigating the accuracy and reliability of these models, and understanding their limitations is crucial. Comparisons with observations highlight their advantages and disadvantages while stressing the importance of observational techniques to help constrain current and future models.