Hill-Siscoe and LFM transpolar potentials compared

We investigate the similarities and differences in the behavior of the transpolar potential calculated according to the Hill-Siscoe model and inferred from the LFM global MHD model in a wide range of idealized solar wind and ionospheric parameters. Essentially, the parameter space \{\mathrm Esw,\mathrm Pdyn,Σ\}, where \mathrm Esw is the interplanetary electric field, \mathrm Pdyn is the solar wind ram pressure, and Σ is the ionospheric conductance, is populated with the Hill-Siscoe and MHD transpolar potentials. In particular, the feature of the Hill-Siscoe model is verified, whereby an increase in the ram pressure leads to an increase of the threshold IEF, the value at which the potential starts to saturate. This tendency indicates that at least for some values of the ionospheric conductance the Hill-Siscoe saturation domain in the \{\mathrm Esw,\mathrm Pdyn\}-plane overlaps with the low Mach number region. To verify this conjecture the global MHD model is modified to simulate solar wind flows that are just slightly super- magnetosonic.