A comparison of bow shock models with Cluster observations during low Alfvén Mach number magnetic clouds

Abstract

Abstract. Magnetic clouds (MCs) are very geoeffective solar wind structures. Their properties in the interplanetary medium have been extensively studied, yet little is known about their characteristics in the Earth's magnetosheath. The Cluster spacecraft offer the opportunity to observe MCs in the magnetosheath, but before MCs reach the magnetosphere, their structure is altered when they interact with the terrestrial bow shock (BS). The physics taking place at the BS strongly depends on ΘBn, the angle between the shock normal and the interplanetary magnetic field. However, in situ observations of the BS during an MC's crossing are seldom available. In order to relate magnetosheath observations to solar wind conditions, we need to rely on a model to determine the shock's position and normal direction. Yet during MCs, the models tend to be less accurate, because the Alfvén Mach number (MA) is often significantly lower than in regular solar wind. On the contrary, the models are generally optimised for high MA conditions. In this study, we compare the predictions of four widely used models available in the literature (Wu et al., 2000; Chapman and Cairns, 2003; Jeřáb et al., 2005; Měrka et al., 2005b) to Cluster's dayside BS crossings observed during five MC events. Our analysis shows that the ΘBn angle is well predicted by all four models. On the other hand, the Jeřáb et al. (2005) model yields the best estimates of the BS position during low MA MCs. The other models locate the BS either too far from or too close to Earth. The results of this paper can be directly used to estimate the BS parameters in all studies of MC interaction with Earth's magnetosphere.