Evolution of spectral index of energetic protons in the magnetopause crossing at the subsolar point

Abstract

The evolution of the spectral index of the omnidirectional differential flux of protons (within a range of 40 to 600 keV) in the magnetopause crossing near the subsolar point was analyzed. The work is based on the measurement data of the THEMIS international project as of July 18, 2007. A specific feature of the event under study is the possibility of determining the spectral index at energies >40 keV with a time resolution of 3 s. It is shown that the ion distribution functions, both outside and inside the magnetopause, can be approximated by kappa-like distributions with power low high-energy tail in the analyzed crossing. A high level of fluctuations of the spectral index of energetic ions was shown to have been observed near the magnetopause. The fluctuation level substantially reduces inside the magnetosphere. In this case, the spectral index has a value of ~6 on average and remains constant up to inner regions of the magnetosphere.