Evidence for newly closed magnetosheath field lines at the dayside magnetopause under northward IMF

Abstract

[1] We analyze the structure of the high-latitude magnetopause under steady interplanetary magnetic field (IMF). We use 56 magnetopause encounters of Cluster spacecraft from 2001 to 2003 to explore the statistical properties of the magnetosheath electron boundary layer, observed outside the high-latitude dayside magnetopause. We focus on the occurrence of low absolute parallel heat flux in this layer and its dependence on the magnetic field clock angle simultaneously measured by Cluster. The low absolute parallel heat fluxes result from the presence of bidirectional heated electrons in the magnetosheath electron boundary layer and are primarily observed when the local magnetic field is northward. The bidirectional heated electrons are interpreted as the signature of newly closed magnetosheath field lines that have reconnected at the highlatitude magnetopause, tailward of the cusp, in both hemispheres. This study strongly suggests that double high-latitude reconnection is a tenable mechanism for the formation of the low-latitude boundary layer and potentially of the cold, dense plasma sheet under northward IMF. Although the efficiency (in terms of mass and energy transfer) of this mechanism is still to be investigated, it is an obvious way of capturing solar wind plasma under northward IMF.

Formation of dayside low‐latitude boundary layer under northward interplanetary magnetic field

“…The newly closed field lines shorten as the kink of the field line propagates sunward from the X line, as predicted by Song and Russell [1992]. [17] Therefore, our simulation indicates that the LLBL can form by high-latitude reconnection, as suggested by observations [e.g., Lavraud et al, 2006]. While our simulation model is focused mainly on the dayside, the nightside extension is limited and the X lines are close to the cusps and the tailward boundary.…”

“…Under a strongly northward IMF, the magnetopause can be divided into sublayers separated by sharp boundaries. It has been suggested that high-latitude magnetic reconnection [Gosling et al, 1991;Le et al, 1996;Lavraud et al, 2004Lavraud et al, , 2006 occurring in both northern and southern hemispheres, tailward of the cusps, results in the formation of newly closed field lines, and thus the capture of magnetosheath particles on these field lines [Song and Russell, 1992;Onsager et al, 2001;Lavraud et al, 2006]. The proposed process [Song and Russell, 1992] for such formation of the LLBL under northward IMF includes the shortening and sinking of the newly closed magnetic flux tubes and the azimuthal expansion of the flux tube and associated density along the magnetopause via interchange instability.…”

Geotail observations of two-component protons in the midnight plasma sheet

“…However, the cold proton component on the dayside has a perpendicular anisotropy (Nishino et al, 2007), which disagrees with the parallel anisotropy observed in the midnight region. Therefore, double high-latitude reconnection, although it may actually take place (Onsager et al, 2001;Lavraud et al, 2005Lavraud et al, , 2006, does not seem to directly contribute to refilling of the midnight plasma sheet.…”

Statistics of counter-streaming solar wind suprathermal electrons at solar minimum: STEREO observations

“…Subject to less controversy, counter-streaming strahls are also observed within closed solar wind field lines with both ends attached to the Sun, such as within interplanetary coronal mass ejections (ICMEs) (e.g., Zwickl et al, 1983;Pilipp et al, 1987b;Gosling et al, 1987, 2002, Gosling, 1990 and newly closed field regions following from magnetic reconnection in the solar wind at the heliospheric current sheet (HCS) Lavraud et al, 2009) (see also Onsager et al, 2001, andLavraud et al, 2006, for a similar concept applied to the magnetosphere).…”