Multi-point electric field observations in the high- and mid-altitude cusp

The Cluster quartet crosses the cusp region at high altitudes (8--12 R_E) in January-April and at mid altitudes (near 5--6 R_E) in August-October. At high altitudes the satellites quite frequently enter/exit the cusp through the exterior cusp whose characterisatics are much less known than those of the proper cusp. The cusp is detected as a region of enhanced low-frequency wave activity, and the wave activity extends beyond the magnetopause, into the exterior cusp region. The amplitude of the waves tends to increase with decreasing altitude. The cusp also appears as a region of increased plasma density. A typical density is several tens of electrons in cm³, but it can also often exceed 100 cm^-3. Interestingly, the high-latitude boundary layers adjacent to the cusp differ from the cusp region: the density is significantly lower and the electric fields are usually not turbulent although they are temporally variable. In this study we will concentrate on multi-point electric field observations by the Cluster electric field instrument (EFW) which lets us study both plasma drift patterns and electron density variations (from the spacecraft potential data) within the cusp region. We also utilize simultaneous ion (CIS), electron (PEACE), and magnetic field (FGM) measurements that provide us with valuable additional information about the conditions of the cusp (ion injections, field-aligned currents etc). We investigate both dynamical and structural aspects as well as the level of turbulence within the cusp region. We will put a particular emphasis on the level of coherence/non-coherence in dc electric field variation between the four points for different satellite separations between 100 and 1000 km.