A.J. Tylka, P.R. Boberg, J.H. Adams, Jr., T. Kleis, R. Beaujean
Galactic cosmic rays are generally believed to be bare nuclei, fully stripped of all orbital electrons during passage through several g/cm^2 of interstellar matter. Over the years there have been numerous reports of so-called ``below-cutoff'' Fe-group cosmic rays, with momentum below the minimum required for a fully-ionized particle to reach Earth's inner magnetosphere. These reports comprised just a few tens of ions, but stimulated much speculation about partially-ionized Galactic cosmic rays, suggesting a nearby source of Galactic cosmic rays. In this Letter we compare these previous reports to new observations of below-cutoff Fe-group ions from passive track detectors exposed for nearly 6 years in low-Earth orbit aboard NASA's Long Duration Exposure Facility (LDEF) satellite. The LDEF fluxes are only a few percent of the previously reported fluxes. This comparison suggests that the flux of below-cutoff ions must be out of phase with the observed solar-cycle variation of other (non-solar) cosmic ray sources and/or must strongly increase with decreasing altitude in low-Earth orbit. Both of these features are inconsistent with the notion of partially-ionized Galactic cosmic rays as the source of the observed below-cutoff ions.