Giotto magnetic-field results on the boundaries of the pile-up region and the magnetic cavity

Abstract

Data from Giotto's magnetometer collected during the encounter at Comet Halley on March 13/14, 1986, were used to investigate the most conspicuous boundaries in the inner plasma region surronding Comet Halley, i.e., the pile-up boundary and the ionopause. The normal vectors at the boundary crossings were determined using the minimum variance technique. The magnetic pile-up boundary was modeled by a paraboloid with the axes of both surfaces aligned with the aberrated solar wind, yielding the values of the stand-off distance towards the aberrated solar wind to be equal to 215,000 km. For no flaring, i.e., a half-sphere cylinder shape, the stand-off distance was estimated to be 143,000 km. The results on the ionopause surface were inconsistent with a boundary with simple symmetry properties. The data indicate that the surface of the ionopause must be characterized either by a smooth surface of radius 4600 km displaced from the origin by 1500 km or strong ripples aligned with the aberrated solar wind direction, or a combination of both.