High-latitude observations from the Polar spacecraft on 21 January 1998 show a region of closed magnetic field lines containing several distinct solar wind ion populations in the energy range from 200 keV e-1. Precipitating ion fluxes in this region are consistent with the low-latitude boundary layer (LLBL). A global MHD simulation of this event (using input from the Wind spacecraft upstream from the Earth's bow shock) reproduces Geotail and Interball/Tail spacecraft observations in the outer magnetosphere and magnetosheath. These results demonstrate that the simulation faithfully reproduces the global magnetic field configuration of the magnetosheath and magnetosphere and provides confidence for the interpretation of the LLBL observations from Polar. Results from the simulation show that a single evolutionary process of plasma entry into the cusp and evolution to the LLBL can account for the development of a multi-energy ion population of solar wind origin on closed magnetic field lines in the magnetosphere. Sources with direct access to the ultimately closed LLBL field lines observed by Polar are the dayside magnetosheath (low-energy ions below 3 keV e-1), the quasi-parallel bow shock (higher-energy ions between 3 and 100 keV e-1), and possibly the duskside magnetopause (highest-energy ions above 100 keV e-1).