Cusp proton signatures and the interplanetary magnetic field

Abstract

The variation of proton average energy with latitude in the cusp has been suggested as an indicator of the means of particle entry. If magnetic merging is the principal means of particle entry, the proton average energy should fall with increasing invariant latitude; if diffusion is the principal means of particle entry, the average energy should first fall and then briefly rise as a function of latitude, showing a ‘V’ signature. In addition, the cusp protons should occur on open field lines in the former and closed field lines in the latter case. Four hundred passes of the AE‐D satellite were scanned for appropriate cusp passes during periods when IMP‐J interplanetary or magnetosheath magnetic field data were available. Sixty passes fulfilled all criteria. Of these, roughly a third showed clear or likely merging‐type energy dispersions; a third showed clear or likely V‐type energy dispersions, and a third showed unclear or no energy dispersions. The results are strongly correlated with the IMF: the merging signatures were associated with southward IMF, and the V signatures were associated with northward IMF. Unclear or no dispersion cases were associated with unsteady or weakly northward IMF or with orbits unfavorable to observe the dispersion (perpendicular to the convection direction). If one accepts the electron anisotropy boundary as an (imperfect) indicator of the last closed field line, then the cusp protons are typically on open field lines for southward IMF and on closed field lines for northward IMF. In addition, the presence of ≥3‐keV electrons in the cusp is strongly correlated with V dispersions and therefore with northward IMF.