Dynamics and local boundary properties of the dawn-side magnetopause underconditions observed by Equator-S

Magnetic field measurements, taken by the magnetometer experiment (MAM) on board the German Equator-S spacecraft, have been used to identify and categ orise 131 crossings of the dawn-side magnetopause at low latitude, providin g unusual, long duration coverage of the adjacent magnetospheric regions an d near magnetosheath. The crossings occurred on 31 orbits, providing unbias ed coverage over the full range of local magnetic shear from 06:00 to 10:40 LT. Apogee extent places the spacecraft in conditions associated with inte rmediate, rather than low, solar wind dynamic pressure, as it processes int o the flank region. The apogee of the spacecraft remains close to the magne topause for mean solar wind pressure. The occurrence of the magnetopause en counters are summarised and are found to compare well with predicted bounda ry location, where solar wind conditions are known. Most scale with solar w ind pressure. Magnetopause shape is also documented and we find that the ma gnetopause orientation is consistently sunward of a model boundary and is n ot accounted for by IMF or local magnetic shear conditions. A number of wel l-established crossings, particularly those at high magnetic shear, or exhi biting unusually high-pressure states, were observed and have been analysed for their boundary characteristics and some details of their boundary and near magnetosheath properties are discussed. Of particular note are the occ urrence of mirror-like signatures in the adjacent magnetosheath during a si gnificant fraction of the encounters and a high number of multiple crossing s over a long time period. The latter is facilitated by the spacecraft orbi t which is designed to remain in the near magnetosheath for average solar w ind pressure. For most encounters, a well-ordered, tangential (draped) magn etosheath field is observed and there is little evidence of large deviation s in local boundary orientations. Two passes corresponding to close conjunc tions of the Geotail spacecraft are analysed to confirm boundary orientatio n and motion. These further show evidence of an anti-sunward moving depress ion on the magnetopause (which is much smaller at Equator-S). The Tsyganenk o model field is used routinely to assist in categorising the crossings and some comparison of models is carried out. We note that typically the T87 m odel fits the data better than the T89 model during conditions of low to in termediate K-P index near the magnetopause and also near the dawn-side tail current sheet in the dawnside region.