At the orbit of Skylab we observed steeply falling energy spectra of nuclei with $Z>~8$ and $10\lesssim E\lesssim 40$ MeV/amu at intensities much higher than seen outside the magnetosphere. The composition is O: Ne: [Na-Si]: Fe = 100:33:13:(∼2), consistent with that of the solar corona. We suggest that heavy solar-wind ions enter the magnetosphere, are accelerated, and populate the inner radiation belt.

• Received 24 March 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.539