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 electron-dominated boundary layer, named the "electron edge" of the Low-Latitude Boundary Layer (LLBL), contains soft-magnetosheath 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-parallel-directed 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 BZ- 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 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 BY- and BZ-components and the solar wind dynamic pressure, as is expected from a simple reconnection model for the origin of this region.
Highlights
The Earth’s cusps were first discussed by Chapman and Ferraro (1931) as two magnetic null points, one in each hemisphere
We present results of a statistical study of the electron edge of the Low-Latitude Boundary Layer (LLBL) observed by Cluster during mid-altitude cusp crossings: (1) The electron edge consists of low-density, fairly isotropic electrons presumably from the halo population of the solar wind
(2) This electron edge has been observed in 87% of the mid-altitude LLBL/cusp crossings with clear equatorward electron and ion boundaries by Cluster or in 35% of all events studied, which is a major difference from the previous Polar result (Topliss et al, 2001)
Summary
The Earth’s cusps were first discussed by Chapman and Ferraro (1931) as two magnetic null points, one in each hemisphere. It was suggested that these nulls would be points of solar wind plasma entry into the magnetosphere. Satellite observations revealed the existence of magnetosheathtype plasma inside the cusp region (Heikkila and Winningham, 1971). Reconnection between terrestrial magnetic field lines and the interplanetary magnetic field (IMF) is responsible for the penetration of solar wind plasma into the magnetosphere Dungey, 1961; Dungey, 1963). Under southward IMF BZ, IMF field lines may reconnect near the subsolar point and will subsequently convect poleward and antisunward, forming a broad cusp region.
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