The Influence of Solar Wind-Magnetosphere Coupling Functions on the estimation of Dst Index

In this paper we investigate the role of different solar wind magnetosphere coupling functions on the Dst index calculated by the low order nonlinear dynamical WINDMI model. Previously we have shown that the two phase decay in the Dst index can be accounted for by including geotail current dynamics into the calculation. During that investigation we used the rectified solar wind electric field v_{xB_z} as a baseline for the simulations and analysis. Here we include an evaluation of four other coupling functions, that together with the rectified vB_s, fall into three categories. In the first category only v_x and B_z are considered geoeffective. In the second category v_x, B_z and B_y and solar wind dynamic pressure are considered geoeffective. In the third category we have the Borovsky coupling function in which only the solar wind dynamic pressure is considered to control reconnection rate and therefore determines geoeffectiveness. Our results indicate that for a majority of cases, at most only v_x, B_y and B_z are needed to sufficiently account for the supply of energy to the ring current and geotail current components that contribute to the Dst index. The more complex coupling functions sometimes perform extremely well on storm datasets, but at other times do not reproduce the Dst index faithfully. The solar wind dynamic pressure contribution appears to be correctly accounted for through the calculation of the Dmp formula of tet{burton1975}. The degree to which the B_y component affects the Dst index is not entirely clear from our results, but in most cases it's inclusion slightly over-emphasizes the ring current contribution, and slightly under-emphasizes the geotail current contribution.