Electrons and Protons Accelerated in Mercury's Magnetic Field

Abstract

Fluxes of protons with energies of ∼ 550 kev and electrons with energies of ∼ 300 kev which exceed approximately 10⁴ and 10⁵ cm^-2 sec^-1, respectively, have been discovered in the magnetosphere of Mercury. Electron fluxes > 10³ cm^-2 sec^-1 also are observed in the outbound pass of the Mariner 10 spacecraft throuigh the magnetosheath. The intensity versus time profiles of the particle fluxes in the magnetosphere appear with sudden onsets of ∼ 10⁴ cm^-2 sec^-1 beginning at interplanetary background levels and persisting for times equivalent to their being distriblited spatially over regions having a scale size comparable to the planetary radius. For a spectral form dJ/dE α E-γ, where J is the differential particle intensity and E is the kinetic energy, the typical values of γ are γ_p = 5.5 for protons above 500 kev and γ^e ≥ 9 for electrons above 170 kev. Large coherent electron intensity oscillations (variations of factors of 10 to 100) have been discovered with characteristic periods of ∼ 6 seconds and with higher frequency components. In some cases proton bursts are found in phase with these oscillations. On the basis of the experimental evidence and a knowledge of the general magnetic field intensities and directions along the trajectory of Mariner 10 provided by the magnetic field observations, it is shown that the radiation events observed in the magnetosphere and magnetosheath are transient and are not interpretable in terms of stable trapped particle populations. Furthermtiore, experimental evidence strongly supports the view that the particles are impulsively accelerated and that the acceleration source is not more distant from the point of observation along lines of force than ∼ 8 x 10³ to 16 x 10³ kilometers (3 to 6.5 units of Mercury's radius). Candidates for the regions most likely to be sources of particle acceleration are discussed, namely, the magnetotail and the magnetosheath. It is pointed out that the phenomena discovered at Mercury will place more stringent conditions on allowed mnodels for electron and proton acceleration than have heretofore been possible in studies within the earth's magnetosphere.