Magnetosonic Mach number dependence of the efficiency of reconnection between planetary and interplanetary magnetic fields

Abstract

We present a statistical investigation into the magnetosonic Mach number dependence of the efficiency of reconnection at the Earth's dayside magnetopause. We use the transpolar voltage VPC, derived from radar observations of the ionospheric electric field, as a proxy for the dayside reconnection voltage. Our results show that the IMF clock angle dependence of VPC is closely approximated by the function f(θ) = sin2(θ/2), which we use in the derivation of a solar wind transfer function E* = ESWf(θ), wherein ESW is the solar wind electric field. We find that VPC is strongly related to E*, increasing almost linearly with small E* but saturating as E* becomes high. We also find that E* is strongly dependent on the magnetosonic Mach number, MMS, decreasing to near‐zero values as MMS approaches 12, due principally to decreasing values of the IMF strength. VPC, on the other hand, is only weakly related to MMS and, for lower, more usual values of E*, actually shows a modest increase with increasing MMS. This result has implications for the solar wind‐magnetosphere interaction at the outer planets where the Mach number is typically much higher than it is at 1 AU. Examples of SuperDARN convection maps from two high Mach number intervals are also presented, illustrating the existence of fairly typical reconnection driven flows. We thus find no evidence for a significant reduction in the magnetopause reconnection rate associated with high magnetosonic Mach numbers.