Absence of upstream energetic ions under turbulent radial interplanetary magnetic field

Two cases of observations on board the IMP 8 spacecraft of upstream energetic ions were used to test the viability of the Fermi mechanism as an efficient accelerator of particles with E not less than 50 keV at the earth's bow shock, under conditions which are more constrained than those applied in all previously published detailed analyses of upstream ion events. In addition to the nearly radial IMF, the following particle and field conditions were present: (1) in situ cyclotron-resonant wave activity, (2) a seed energetic particle population, and (3) small (not above 25 deg) theta(Bn) at the points of connection of the spacecraft to the bow shock. The analysis of data from days 67, 1979 and 303, 1980 showed that, despite these conditions, no ion enhancements attributable to the Fermi process could be detected.