On the variations of protons during the magnetospheric substorm at Earth and Mercury in the near-tail: A comparative study

This study carries out a comparative study of the variations of plasma sheet protons during the magnetospheric substorm at Earth and Mercury with the measurements of THEMIS and MESSENGER. The proton distributions in both magnetospheres were interpreted by Kappa distribution, which is constituted from a Maxwellian in low energy portion and a power law in high energy portion. The number densities of protons are found to be around an order of magnitude higher at Mercury than at Earth. The temperatures are several times smaller and the κ values are broader at Mercury than at Earth. Protons, in both magnetospheres, become denser and cooler during the substorm growth phase, and more tenuous and hot after the dipolarizations. While the changes of κ during the substorms are several times higher at Mercury (> 60%) than at Earth (< 20%). The small changes of κ at Earth indicate that spectrum-preserving processes, like adiabatic betatron acceleration, play an important role in the acceleration of protons, while the large variations of κ at Mercury indicate the importance of spectrum-altering processes, such as acceleration due to serpentine cross-tail motions and wave-particle interactions, in the behavior of protons. We suggest this different character of protons could closely relate to the distinct properties of the two magnetospheres. Most importantly, the scale of Mercury's magnetosphere is much smaller than that of Earth's magnetosphere. The relatively thin plasma sheet and small radius of curvature of the magnetic field lines in Mercury's magnetotail would make spectrum altering processes important.