Energy-dependent evolution of ring current protons during magnetic storms

Abstract

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+. Copyright 2011 by the American Geophysical Union.