Energy‐dependent evolution of ring current protons during magnetic storms

The storm time evolution of equatorially mirroring H + ions in the inner magnetosphere has been statistically examined by using data from the Polar satellite. We focused on two energy ranges of H + observed by Polar; 31–80 keV and 125–173 keV, which are referred to as low‐ and high‐energy components, respectively. The following two phases were defined; the developing phase (pre‐storm time to near the most disturbed time) and the declining phase (near the most disturbed time to post‐storm time), which is 3 days before (after) the equatorial crossing of Polar during the storm time. We obtained the following results: (1) Low‐energy H + tends to increase during the developing, and to decrease during the declining at all magnetic local times (MLTs) except for the pre‐noon sector. (2) The low‐energy H + is anti‐correlated with the magnetic field, probably indicating that the low‐energy H + reduces the equatorial magnetic field due to a diamagnetic effect. (3) High‐energy H + tends to increase on the dayside during the declining phase. (4) The high‐energy H + is poorly correlated with the magnetic field. High‐energy H + behaves significantly different from the low‐energy H + , and that some process other than betatron acceleration, diffusion and substorm‐associated injection could have been responsible for the variation of the high‐energy H + .