ORIGIN OF 30 - 100 KEV PROTONS OBSERVED IN THE UPSTREAM REGION OF THE EARTHS BOW SHOCK

a Fermi-type acceleration model is constructed to explain the origin of energetic protons (30 ∼ 100 keV) which have been observed upstream of the bow shock. It is shown that the suprathermal protons (with energy of several keV) can be accelerated up to several tens of keV through the Fermi-type process in which the reflection at the shock front and the scattering in the upstream region are coupled. The efficiency of the scattering process is estimated by using the result of Barnes' quasilinear treatment of the wave excitation. The resultant energy spectrum and flux intensity (103 ∼ 104 protons (cm2s ster keV) in 32 ∼ 45.3keV) are consistent with the observation, and the softening of the energy spectrum observed in the dawn region can be explained by the decrease in the efficiency of the acceleration process in the dawn region due to the curvature of the bow shock and the reduction of shock strength. The spatial distribution of the flux predicted by the model is also consistent with the observation. In view of these consistencies the Fermi-type acceleration process is suggested as a possible candidate mechanism to explain the upstream protons although we do not intend to exclude other possibilities. © 1979.