The magnetic influence of the storm time ring current on high-energy particles is demonstrated by using a simulation of the ring current incorporating self-consistent magnetic and electric fields. Observations by the Polar satellite show that the magnetic field is occasionally depressed by 50% or more near the equatorial plane at &lt
6 RE. We call them equatorially magnetic depression events (EMDEs) and focus on the most intense EMDE observed during an intense storm on 22 October 1999. The simulation predicts that under a strong convection electric field, the magnetic field strength is highly depressed around L = 5 by newly injected ions of energy 80 keV or less. The depressed magnetic field causes a significant adiabatic decrease in the high-energy ion flux at pitch angles near 90° to conserve the first adiabatic invariant. A more tail-like (shortened) magnetic field line causes an enhancement of the flux at pitch angles near 0° and 180° to conserve the second adiabatic invariant. Consequently, a butterfly-like pitch angle distribution (PAD) is formed, which agrees with the Polar observation. We propose that the adiabatic process could have acted not only on the high-energy component of the protons but also on relativistic electrons in the outer radiation belt. This notion is supported by simultaneous Polar observation of relativistic electron fluxes that show a decrease at pitch angles near 90° and a slight increase at pitch angles near 0° and 180°. PADs of protons and electrons can be used to distinguish nonadiabatic processes acting selectively on electrons from adiabatic ones. Copyright 2008 by the American Geophysical Union.