Investigation of the occurrence of significant deviations in the magnetopause location: Solar wind and foreshock effects

Abstract. The dynamic motion of the magnetopause, the boundary between the Earth's magnetic field and the interplanetary magnetic field, is mainly driven by pressure variations and changes in the interplanetary magnetic field. Common magnetopause models can predict the location of the magnetopause in response to upstream conditions from different sets of input parameters, including pressure and the interplanetary magnetic field. However, recent studies have shown that some effects of upstream conditions may still be poorly understood, as deviations between model and in situ observations beyond the expected scatter due to constant magnetopause motion are quite common. Using data from the three most recent multi-spacecraft missions to near Earth space (Cluster, THEMIS and MMS), we investigate the occurrence of these large deviations in observed magnetopause crossings from common empirical models. By comparing the results from different models, we find that the occurrence of these events appears to be model independent, suggesting that some physical processes may be missing from the models. To find these processes, we test whether the deviant magnetopause crossings are statistically associated with foreshocks and/or different solar wind types and show that in at least 50 % of cases the foreshock can be responsible for the large deviations in the magnetopause's location. In the case where the foreshock is unlikely to be responsible, two distinct classes of solar wind are found to occur most frequently in association with the occurrence of magnetopause deviations: the "fast" solar wind and the solar wind plasma associated with transients such as interplanetary coronal mass ejections. Therefore, the plasma conditions associated with these solar wind classes could be responsible for the occurrence of deviant magnetopause observations. Our results may help to develop new and more accurate models of the magnetopause, which will be needed, for example, to accurately interpret the results of the upcoming SMILE mission.