The proton energy density distribution near the equatorial plane in the inner magnetosphere was statistically investigated by utilizing data from the Polar/MICS instrument. Results show that the proton energy density depends on the solar wind velocity and IMF Bz, and that during a storm main phase the energy density increases at midnight while the energy density decreases slightly or remains constant at noon. Three possible hypotheses are discussed to find a primary source of the energy density variation observed. After comparing with results of a numerical simulation, on the average, the convection hypothesis is thought to be reasonable to account for the storm-time enhancement of the energy density as well as the local time asymmetry of it. The local time asymmetry during the storm main phase is hardly produced during the storm that substorm-associated transport or diffusive transport dominates the convective transport. The diffusive or substorm-associated transport is likely needed to account for the "background" population which consists of high energy protons (more than hundreds of keV), but is expected to be minor in the storm-time enhancement of the energy density because on the average, the "background" population tends to remain rigidly throughout a storm. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.