Geo‐effectiveness and radial dependence of magnetic cloud erosion by magnetic reconnection

Abstract

Magnetic flux erosion by magnetic reconnection occurs at the front of at least some magnetic clouds (MCs). We first investigate how erosion influences the geo‐effectiveness of MCs in a general sense and using a south‐north magnetic polarity MC observed on 18–20 October 1995. Although the magnetic shear at its front may not be known during propagation, measurements at 1 AU show signatures of local reconnection. Using a standard MC model, an empirical model of the geomagnetic response ( Dst ), and an observational estimate of the magnetic flux erosion, we find that the strength of the observed ensuing storm was ~30% lower than if no erosion had occurred. We then discuss the interplay between adiabatic compression and magnetic erosion at the front of MCs. We conclude that the most geo‐effective configuration for a south‐north polarity MC is to be preceded by a solar wind with southward IMF. This stems not only from the formation of a geo‐effective sheath ahead of it but also from the adiabatic compression and reduced (or lack thereof) magnetic erosion which constructively conspire for the structure to be more geo‐effective. Finally, assuming simple semiempirical and theoretical Alfvén speed profiles expected from expansion to 1 AU, we provide first‐order estimates of the erosion process radial evolution. We find that the expected reconnection rates during propagation allow for significant erosion, on the order of those reported. Calculations also suggest that most of the erosion should occur in the inner heliosphere, and up to ~50% may yet occur beyond Mercury's orbit.