Spatial inhomogeneity of magnetosheath proton precipitation along the dayside cusp from the Arcad Experiment

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Abstract
Soft proton (0.5–30 keV) measurements at the dayside auroral oval obtained aboard the Aureole 1 satellite are analyzed in more detail than in previous publications. Our approach in the analysis of these data is based on the distinction between soft ‘magnetosheathlike’ proton (ion) spectra (with a spectral ratio α of the differential flux at 0.5 keV to that at 5.0 keV higher than 100) and much harder ‘plasmasheetlike’ spectra (with α AE‐dependent spot (or spots) of enhanced magnetosheathlike proton precipitation observed with the highest fluxes when its position occurs close to the noon magnetic meridian. Observational data are consistent with typical spot dimensions of the order of 200 × 500 km. This spot is considered to be an ionospheric projection of the localized and possibly time‐varying plasma injection from the magnetosheath into the magnetosphere along a newly merged magnetospheric field tube. These results confirm the earlier work based on the Arcad experiment which led to the inference of a ‘patchy’ intensity pattern of soft proton precipitation in the dayside cusp; they disagree with the concept of a free penetration of magnetosheath plasma through an ever present cleftlike magnetic field branching structure extending from the dawnside to the duskside of the magnetosphere through noon, at least during quiet magnetic conditions. Independently obtained data from other satellites are briefly summarized, and general consistency is noted with the Aureole 1 results.