30- to 100-keV protons upstream from the Earth's bow shock

Protons of 30 to 100 keV are found upstream from the bow shock whenever interplanetary magnetic fields connect the spacecraft and bow shock. Their energy spectrum is closely power law, dJ/dE ∝ E−γ with γ usually close to 3. The spectrum is sharply cut off above 100 keV. The protons do not appear upstream of a boundary determined by the solar wind speed (the speed at which the interplanetary field is being convected) and an effective upstreaming velocity of 2.5 to 3 times the solar wind speed along the field lines. We conclude that the protons are not accelerated locally at the bow shock nor are they accelerated in a Fermi process between the bow shock and the moving interplanetary magnetic field irregularities. We believe that the lower-energy (3–4 keV) protons accelerated and reflected by the bow shock and the Alfvén waves observed upstream are in some way responsible for the origin of the 30- to 100-keV protons in very large regions upstream from the bow shock.