Optical and particle signatures of magnetospheric boundary layers near magnetic noon: Satellite and ground-based observations

In this paper we present a set of satellite and ground‐based observations suggesting that energetic magnetospheric electrons cannot be used as an unambiguous discriminator between open and closed field lines on the dayside. Using two data sets from the Defense Meteorological Satellite Program (DMSP) F13 and NOAA 12 satellites flying through dayside Type 1 cusp aurora (both close in time and space), we reach two apparently incompatible conclusions. Cusp/mantle precipitation, stepped cusp signatures, and antisunward convection in the DMSP F13 data set strongly suggest open magnetic field lines. On the other hand, NOAA 12 observed a mixture of magnetosheath and isotropic energetic particles. Trapped energetic electrons are traditionally regarded as being on closed flux. However, in addition to earlier proposed trapping on open field lines, we suggest that transmission lines connecting merging sites near the cusp in the Southern Hemisphere with the northern auroral ionosphere can be several tens of RE long. Alfvén wave transit times of several minutes may make it impossible to determine from satellite measurements in the ionosphere whether magnetic field lines threading low‐latitude boundary layer (LLBL) plasmas are open or closed. New research tools will be needed to unify understanding of complementary particle measurements from the DMSP and NOAA satellites.