Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind

Abstract

[1] Previous works have emphasized the significant influence of the solar wind Alfvén Mach number (M A ) on magnetospheric dynamics. Here we report statistical, observational results that pertain to changes in the magnetosheath flow distribution and magnetopause shape as a function of solar wind M A and interplanetary magnetic field (IMF) clock angle orientation. We use all Cluster 1 data in the magnetosheath during the period 2001-2010, using an appropriate spatial superposition procedure, to produce magnetosheath flow distributions as a function of location in the magnetosheath relative to the IMF and other parameters. The results demonstrate that enhanced flows in the magnetosheath are expected at locations quasi-perpendicular to the IMF direction in the plane perpendicular to the Sun-Earth line; in other words, for the special case of a northward IMF, enhanced flows are observed on the dawn and dusk flanks of the magnetosphere, while much lower flows are observed above the poles. The largest flows are adjacent to the magnetopause. Using appropriate magnetopause crossing lists (for both high and low M A ), we also investigate the changes in magnetopause shape as a function of solar wind M A and IMF orientation. Comparing observed magnetopause crossings with predicted positions from an axisymmetric semi-empirical model, we statistically show that the magnetopause is generally circular during high M A , while is it elongated (albeit with moderate statistical significance) along the direction of the IMF during low M A . These findings are consistent with enhanced magnetic forces that prevail in the magnetosheath during low M A . The component of the magnetic forces parallel to the magnetopause produces the enhanced flows along and adjacent to the magnetopause, while the component normal to the magnetopause exerts an asymmetric pressure on the magnetopause that deforms it into an elongated shape.

The Physical Processes of CME/ICME Evolution

“…The analyses of MCs in many studies (e.g., Chané et al 2005;Dasso et al 2006Dasso et al , 2007Ruffenach et al 2012Ruffenach et al , 2015Lavraud et al 2013Lavraud et al , 2014 have revealed the predicted features of flux rope erosion. Dasso et al (2006) examined the 21 January 2005 ICME and found a large imbalance in B z that they attribute to magnetic reconnection.…”

A model of the magnetosheath magnetic field during magnetic clouds

“…Observations indicate the presence of dawn-dusk asymmetries in the magnetosheath speed (Longmore et al, 2005;Walsh et al, 2012). The properties of the magnetosheath during low Alfvén Mach number (M A ) solar wind, corresponding to ICMEs and MCs conditions, have been investigated both observationally and numerically in Lavraud and Borovsky (2008) and Lavraud et al (2013). Their results show that the solar-windmagnetosphere coupling is altered in low M A conditions, and phenomena such as asymmetric and enhanced flows or asymmetric magnetopause compression are observed.…”

Observational aspects of IMF draping-related magnetosheath accelerations for northward IMF

“…These accelerated flows are not due to reconnection but are rather the result of the draping of the interplanetary magnetic field (IMF) around the magnetosphere (e.g., Chen et al, 1993;Lavraud et al, 2007Lavraud et al, , 2013Rosenqvist et al, 2007;Petrinec at al., 1997;Lavraud and Borovsky, 2008;Harris, 2011). As explained below, a characteristic feature of these events, which is useful in their identification, is that the flow tends to be perpendicular to the local magnetic field.…”

“…We also carry out a statistical analysis to determine global features of these accelerations, with focus mainly on magnetosheath speeds that exceed the speed of the solar wind. A statistical survey of global magnetosheath speeds for speeds smaller than the solar wind is undertaken by Lavraud et al (2013).…”