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

Abstract

Abstract. Magnetic ®eld measurements, taken by the magnetometer experiment (MAM) on board the German Equator-S spacecraft, have been used to identify and categorise 131 crossings of the dawn-side magnetopause at low latitude, providing unusual, long duration coverage of the adjacent magnetospheric regions and near magnetosheath. The crossings occurred on 31 orbits, providing unbiased 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 intermediate, rather than low, solar wind dynamic pressure, as it processes into the¯ank region. The apogee of the spacecraft remains close to the magnetopause for mean solar wind pressure. The occurrence of the magnetopause encounters are summarised and are found to compare well with predicted boundary location, where solar wind conditions are known. Most scale with solar wind pressure. Magnetopause shape is also documented and we ®nd that the magnetopause orientation is consistently sunward of a model boundary and is not accounted for by IMF or local magnetic shear conditions. A number of well-established crossings, particularly those at high magnetic shear, or exhibiting 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 occurrence of mirror-like signatures in the adjacent magnetosheath during a signi®cant fraction of the encounters and a high number of multiple crossings over a long time period. The latter is facilitated by the spacecraft orbit which is designed to remain in the near magnetosheath for average solar wind pressure. For most encounters, a well-ordered, tangential (draped) magnetosheath ®eld is observed and there is little evidence of large deviations in local boundary orientations. Two passes corresponding to close conjunctions of the Geotail spacecraft are analysed to con®rm boundary orientation and motion. These further show evidence of an anti-sunward moving depression on the magnetopause (which is much smaller at Equator-S). The Tsyganenko model ®eld is used routinely to assist in categorising the crossings and some comparison of models is carried out. We note that typically the T87 model ®ts the data better than the T89 model during conditions of low to intermediate u index near the magnetopause and also near the dawn-side tail current sheet in the dawnside region.

Fingerprints of collisionless reconnection at the separator, I, Ambipolar‐Hall signatures

“…[23] While deeply depressed field regions at the magnetopause have been previously reported [Sonnerup and Cahill, 1968;Aubry et al, 1971;Neugebauer et al, 1974;Gosling et al, 1990;Paschmann et al, 1986;Dunlop et al, 1999], the unprecedented suite of correlative measurements that accompany the present observations permit a new diagnosis of these structures and allow us for the first time to identify the separator with its framing separatrices, the dynamics in the layers near this separator, as well as other features and scales of the diffusion region. As discussed in the introduction, it is the detection of spatial scales that sets our identification apart from previous observations of magnetopause crossing with depressed magnetic fields in their interior.…”

Coordinated ground-based and Cluster observations of large amplitude global magnetospheric oscillations during a fast solar wind speed interval

“…11 we show the results from a minimum variance analysis (Sonnerup and Cahill, 1967) of the magnetopause crossing around 23:40 UT which shows significant shear across the boundary. The top panel shows, after Dunlop et al (1999), a scatter plot (pink crosses) of the magnetic field angles (θ and φ GSE) measured by FGM on Cluster 2 between 23:34-23:48 UT. For a tangential discontinuity (TD) crossing, the field direction through the boundary should lie nearly on a plane (since B.n = 0).…”

Cluster observes the Earth’s magnetopause: coordinated four-point magnetic field measurements

“…The IMF is southward for only brief intervals. In order to obtain an approximate, expected, magnetopause location we only use therefore pressure scaling to fit the magnetopause location (see Dunlop et al, 1999). For simplicity here, we compare magnetopause location to the geometric model of Sibeck et al (1991).…”

“…The data are from spacecraft 4 and the MVA normal is quoted on the plot. Two curves are drawn through the points (see Woodward, 1998 andDunlop et al, 1999 for a description). The green curve represents a fitted planar field geometry (where the magnetic field vector lies in different directions but on a plane) to the points and the red curve the equivalent plane defined by the MVA normal.…”