Cluster investigation of the plasma depletion layer at the high-latitude dayside magnetopause

The Earth's magnetosheath consists of shocked solar wind plasma which is draped and diverted around the obstacle of the Earth's magnetic field. Unless magnetic reconnection at the dayside magnetopause occurs at a rate high enough to sustain the required flux and plasma transport, magnetic flux pile-up and associated plasma depletion is expected to occur as an inherent part of this process. Many details of when, how, and to what extent this happens are not well known. Here we report on magnetic flux pile-up and associated plasma depletion at the high-latitude dayside magnetopause observed with the constellation of the Cluster satellites. Observations from the first large-separation Cusp phase of the Cluster mission carried out during the first half of 2003 have been utilized. Near-radial separation of more than one Earth radius between pairs of spacecraft as they traverse the southern hemisphere, high-latitude magnetosheath and magnetopause is obtained at these times. This constellation allows us to distinguish between temporal and spatial variations in the field and plasma parameters and, hence, to determine unambiguously the characteristics of the plasma depletion layer at the high-latitude dayside magnetopause. Our results demonstrate that flux pile-up and plasma depletion occur for a variety of solar wind and interplanetary magnetic field conditions and exhibit complex dynamical features related to magnetopause dynamics.