Open and Closed Low Latitude Boundary Layer

We review and summarize the evidence that a closed LLBL exists, at least away from noon. We also emphasize the observation that just equatorward of the LLBL, overlapping magnetosheath and magnetospheric electrons exist on sunward convecting field lines which have no low-energy ion cutoff (i.e., closed field lines). Therefore it is inescapable to conclude that sheath plasma is introduced onto closed field lines. The dropoff of the high-energy electrons is identified not necessarily with the open/closed boundary but rather with the convection reversal boundary, since these electrons originate on the nightside and convect towards the dayside (hence they cannot exist on anti-sunward convecting field lines, even if the latter are closed). A model is discussed wherein diffusion introduces sheath plasma onto closed field lines, but merging removes these same field lines from the dayside. The competition between these two effects explains why the LLBL is thinnest at noon, where open LLBL signatures dominate (perhaps exclusively), and why the LLBL is thicker for northward IMF.