Particle data obtained from the Defense Meteorological Satellite Program spacecraft for periods of northward interplanetary magnetic field show that ion spectra above the polar cap arcs usually display the same character as those in the midnight auroral oval. Evidently, this means that polar cap arcs and the midnight oval arcs have the same source. At the same time, ion spectra in spikes of particle precipitation in the discrete oval vary with MLT in a regular manner, such that the flux of ions in the energy range 0.1 < Ei < 1 keV increases from midnight toward noon. This feature of ion spectra makes it possible to identify the boundary of the polar cap from the precipitation patterns when the interplanetary magnetic field is northward. The structural zones such as the cusp proper and the poleward and equatorward edges of the cusp can be separated from the daytime oval region using specific features of the ion spectra. The ion population in all these zones is composed of three parts, namely, the mantle, the cusp, and the low-latitude boundary layer plasmas. Densities of all plasma fractions are maximal in the cusp proper and decrease when moving toward the cusp periphery.
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