Statistical study of the location and size of the electron edge of the Low-Latitude Boundary Layer as observed by Cluster at mid-altitudes

Abstract

Abstract. The nature of particle precipitations at dayside mid-altitudes can be interpreted in terms of the evolution of reconnected field lines. Due to the difference between electron and ion parallel velocities, two distinct boundary layers should be observed at mid-altitudes between the boundary between open and closed field lines and the injections in the cusp proper. At lowest latitudes, the electrondominated boundary layer, named the "electron edge" of the Low-Latitude Boundary Layer (LLBL), contains softmagnetosheath electrons but only high-energy ions of plasma sheet origin. A second layer, the LLBL proper, is a mixture of both ions and electrons with characteristic magnetosheath energies. The Cluster spacecraft frequently observe these two boundary layers. We present an illustrative example of a Cluster mid-altitude cusp crossing with an extended electron edge of the LLBL. This electron edge contains 10-200 eV, low-density, isotropic electrons, presumably originating from the solar wind halo population. These are occasionally observed with bursts of parallel and/or anti-paralleldirected electron beams with higher fluxes, which are possibly accelerated near the magnetopause X-line. We then use 3 years of data from mid-altitude cusp crossings (327 events) to carry out a statistical study of the location and size of the electron edge of the LLBL. We find that the equatorward boundary of the LLBL electron edge is observed at 10:00-17:00 magnetic local time (MLT) and is located typically between 68 • and 80 • invariant latitude (ILAT). The location of the electron edge shows a weak, but significant, dependence on some of the external parameters (solar wind pressure, and IMF B Z -component), in agreement with expectations from previous studies of the cusp location. The latitudinal extent of the electron edge has been estimated using new multi-spacecraft techniques. The Cluster tetrahedron crosses the electron and ion boundaries of the LLBL/cusp Correspondence to: Y. V. Bogdanova (jb@mssl.ucl.ac.uk) with time delays of 1-40 min between spacecraft. We reconstruct the motion of the electron boundary between observations by different spacecraft to improve the accuracy of the estimation of the boundary layer size. In our study, the LLBL electron edge is distinctly observed in 87% of mid-altitude LLBL/cusp crossings with clear electron and ion equatorward boundaries equivalent to 35% of all LLBL/cusp crossings by Cluster. The size of this region varied between 0 • -2 • ILAT with a median value of 0.2 • ILAT. Generally, the size of the LLBL electron edge depends on the combination of many parameters. However, we find an anti-correlation between the size of this region and the strength of the IMF, the absolute values of the IMF B Y -and B Z -components and the solar wind dynamic pressure, as is expected from a simple reconnection model for the origin of this region.

Cusp observations during a sequence of fast IMF <I>B<sub>Z</sub></I> reversals

“…At the poleward edge of the cusp, CIS detected downwardmoving H + ions, with V Z ∼ −150 km/s in the GSM coordinate system. Inside the cusp, CIS observed a number of ion injections, displaying a hint of reversed ion dispersion, whereas PEACE observed relatively steady electron fluxes, typical of cusp populations (Lavraud et al, 2004;Bogdanova et al, 2006). Plasma convection remained weakly tailward, with V X ∼ −30 km/s, throughout the initial period of northward IMF.…”

Separatrix regions of magnetic reconnection at the magnetopause

“…The ion edges are defined in a similar way (see also Gosling et al, 1990). In some studies a difference is made between the ion edge and the fluid boundary of the reconnection jet (Bogdanova et al, 2006). In particular, this is possible to do at lower altitudes.…”

“…The separatrix region on the magnetospheric side of dayside magnetopause has been extensively studied at low altitudes (Lockwood et al, 1994(Lockwood et al, , 1996Sandholt et al, 1998Sandholt et al, , 2002 and mid (5-7 R E ) altitudes (Topliss et al, 2001;Bogdanova et al, 2004Bogdanova et al, , 2006, as well as close to the magnetopause (Gosling et al, 1990;Khotyaintsev et al, 2006;Retinò et al, 2006). In some studies the separatrix region is referred to as the electron edge of the low latitude boundary layer (e.g.…”

“…In some studies the separatrix region is referred to as the electron edge of the low latitude boundary layer (e.g. Bogdanova et al, 2006). On the basis of 8 events Bogdanova et al (2004) discuss plasma and wave signatures characteristic for the separatrix regions (ULF waves, electron beams and outflow of ionospheric ions).…”

The Alfvén edge in asymmetric reconnection

“…Taking into account kinetic effects, well defined ion and electron edges should form during magnetic reconnection as suggested by Gosling et al (1990). These boundaries have been identified in space data and have been studied in detail (e.g., Bogdanova et al, 2006).…”