MHD modelling of open flux evolution around solar maximum by coronal model COCONUT

Abstract: To evaluate impact of temporal evolution and commonly used harmonic filtering of magnetograms, and the empirically defined oversimplified heating source terms on open-field distributions, we use a series of hourly-updated magnetograms, preprocessed by the 10th- and 50th-order filtered PF solvers, to drive COCONUT, configured with different heating prescriptions, to mimic coronal evolutions during CRs 2282 and 2283. We evaluate the simulated open magnetic flux at 1.01~$R_s$, 3~$R_s$, and 0.1~AU, and compare them with interplanetary observations. The results show that the simulated unsigned open flux evaluated near the solar surface can be comparable to that derived from interplanetary in situ observations. However, in low corona, numerous small-scale closed-field magnetic structures introduce magnetic polarity inversion interfaces within the open field, cancelling part of the open field near these interfaces during the volume-integration procedure of the finite-volume method. Consequently, the simulated unsigned open flux can be reduced by up to 45% at 0.1~AU and decreases more rapidly in the low corona. The results also indicate that moderate adjustments to the heating source term can effectively regulate the magnitude of the unsigned open magnetic flux. Preprocessing the initial magnetogram by a PF solver with limited spherical harmonics can reduce the open flux in the low corona and alter the distribution of open-field regions, but has little effect on the total unsigned open flux at larger heliocentric distances. The ratio of the maximum to minimum open unsigned magnetic flux can reach 1.4 within a single solar maximum CR. These findings highlight the necessity of considering finer grid resolution around magnetic polarity inversion interfaces, more realistic heating mechanisms, and the time-evolving regime of MHD coronal modelling when further addressing the ``open flux problem".