Adiabatic evolution of trapped particle pitch angle distributions during a storm main phase

Equatorial pitch angle distributions of over 200-keV ions obtained near L = 4 by Explorer 45 during a storm main phase show the development of marked minima at 90 deg pitch angle in direct association with a greater-than-a-factor-of-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.