Interpretation of magnetic field perturbations in the earth's magnetopause boundary layers

Studies of the boundary layers in the vicinity of the Earth's dayside magnetopause are important in determining the nature of the processes which couple the magnetosphere to the flowing solar wind, thereby driving magnetospheric convection. In this paper we examine theoretically the magnetic field and plasma properties expected in the boundary regions for various models involving either diffusion or reconnection at the boundary. For diffusion models the transport of magnetosheath momentum across the magnetopause will result in field shears on either side of the boundary, the field rotations being in opposite senses on either side relative to the undisturbed fields. The directions of these rotations depend upon location at the magnetopause relative to the momentum transfer region and to the noon meridian. In reconnection models the effect of the tension of the open boundary layer field lines must be taken into account in addition to the magnetosheath flow, but on the super-Alfvénic flanks of the magnetosphere the latter still dominates, so that qualitatively similar effects will occur in the two models. More detailed, quantitative or statistical studies are then required to distinguish the two models in this regime. In the sub-Alfvénic dayside region, however, open field tension effects will dominate in reconnection models such that boundary layer field and plasma properties will then be determined mainly by the magnetosheath magnetic field configuration. In particular the East-West flow in the magnetospheric boundary layer will be controlled largely by the East-West field in the magnetosheath, leading to flow reversals across the magnetopause in some quadrants of the magnetopause. This behaviour is directly related to the Svalgaard-Mansurov effect and is a signature unique to reconnection models. The boundary layer fields are also expected to tilt towards the field on the opposite side of the boundary in these models on the dayside. “Toward” tilting can also occur in this regime in diffusion models, but “away” tilting, a signature unique to dayside diffusion, should also occur equally frequently. Finally, we briefly discuss previously published high-resolution ISEE 1 and 2 data from the boundary regions in the light of our results. We find that “toward” tilting generally occurs in boundary region crossings previously identified as being reconnection-associated and we present some examples in which the above unique reconnection signature has been observed. During impulsive FTE-like events, however, the field may tilt in either direction, possibly as a result of field line twists, thus complicating our simple picture in this case. We also show that the “reverse draping” observations presented by Hones et al. (1982) approximately satisfy the open magnetopause stress balance conditions.