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

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 <100, and an average ion energy of about 5–10 keV at noon). As was shown earlier from the data of the Arcad 1 experiment aboard the Aureole 1 satellite, the plasmasheetlike proton energy spectra are regularly observed in a continuous band along the auroral oval and constitute the so‐called proton aurora, its ions presumably drifting around the earth to the dayside from the nightside plasmasheet. The dayside polar cusp crossing is identified in the data analysis as the appearance of soft proton (ion) energy spectra with α ≥100. From this approach the data are found to be consistent with the concept of the dayside cusp as a nonstationary, nonhomogeneous, 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.