Equatorial pitch angle distributions of >200-keV ions obtained near L=4 by Explorer 45 during a storm main phase show the development of marked menima at 90¿ pitch angle in direct association with a greater than a factor 2 reduction in the equatorial magnetic field magnitude. The evolution of these pitch angle distributions can be quantitatively explained as a result of conservation of the first two adiabatic invariants provided the magnitude of the magnetic field decrease was approximately constant along field lines.