Statistical study of the electron-only mid-altitude cleft region: Cluster observations

The Cluster spacecraft have now amassed 3 years of data from crossings of the mid-altitude cleft/cusp.Comparison of particle data from the PEACE and CIS instruments shows that there is pronounceable difference in latitude between the equatorward edges of the electron and ion populations of the cleft region, and this difference increases in the dawn/dusk directions. We define the electron edge of the cleft by the first injection of the magnetosheath-like electrons and the disappearance of the trapped plasma-sheet particles.Correspondingly the ion boundary is identified by the first injection of magnetosheath-like ions. The region between these two boundaries, which we define as the 'electron-only' cleft region, contains parallel and/or antiparallel electron beams. This region is co-located with sharp enhancements of the wave activity and with transverse heating and outflow of ionospheric ions. We present a statistical study of the plasma properties of the electron-only region, including an estimation of the electron anisotropy, density and velocity. We estimate the latitudinal size of this region as a function of magnetic local time (MLT) and the interplanetary magnetic field (IMF) conditions. Moreover, using time differences between the first injection of the electrons and ions in the mid-altitude cusp, we infer a distance along the field lines to the reconnection site on the magnetopause