PENETRATION OF THE INTERPLANETARY MAGNETIC-FIELD B-Y AND MAGNETOSHEATH PLASMA INTO THE MAGNETOSPHERE - IMPLICATIONS FOR THE PREDOMINANT MAGNETOPAUSE MERGING SITE

Magnetosheath plasma penetrates into the magnetosphere creating the pa rticle cusp, and similarly the interplanetary magnetic field (IMF) B-Y component penetrates the magnetopause. We reexamine the phenomenology of such penetration to investigate implications for the magnetopause merging site. Three models are popular: (1) the ''antiparallel'' model , in which merging occurs where the local magnetic shear is largest (u sually high magnetic latitudes); (2) a tilted merging line passing thr ough the subsolar point but extending to very high latitudes; or (3) a tilted merging line passing through the subsolar point in which most merging occurs within a few Earth radii of the equatorial plane and lo cal noon (subsolar merging). It is difficult to distinguish between th e first two models, but the third implies some very different predicti ons. We show that properties of the particle cusp imply that plasma in jection into the magnetosphere occurs most often at high magnetic lati tudes. In particular, we note the following: (1) The altitude of the m erging site inferred from midaltitude cusp ion pitch angle dispersion is typically 8-12 R(E). (2) The highest ion energy observable when mov ing poleward through the cusp drops long before the bulk of the cusp p lasma is reached, implying that ions are swimming upstream against the sheath flow shortly after merging. (3) Low-energy ions are less able to enter the winter cusp than the summer cusp. (4) The local time beha vior of the cusp as a function of B-y and B-z corroborates predictions of the high-latitude merging models. We also reconsider the penetrati on of the IMF B-y component onto closed dayside field lines. Our appro ach, in which closed field lines move to fill in flux voids created by asymmetric magnetopause flux erosion, shows that strict subsolar merg ing cannot account for the observations.