Explorer 45 observations of ring current protons mirroring near the equator, 1--800 keV, are presented at constant first adiabatic invariant &mgr; throughout the period of the December 17, 1971, geomagnetic storm. To obtain &mgr;, we have used simultaneous magnetic field and particle observations. Particle deceleration in response to the storm time magnetic field decrease causes ring current measurements viewed at constant energy to underestimate the storm time increase in proton intensities at energies ≲200 keV. This adiabatic deceleration also accounts for the large flux decreases observed at energies >200 keV during the storm, in contradiction with previous results (S¿raas and Davis, 1968) obtained using a model for the storm time magnetic field.