We present the first observations of quiet-time interplanetary protons in the energy range 38 to 334 keV from the Apollo 16 subsatellite. These proton fluxes vary by a factor of ∼ 5 over a six-month period. Their spectrum can be represented by a power law from 38 to 334 keV during May 1972.
DOI: http://dx.doi.org/10.1103/PhysRevLett.31.1268
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