Extended Data Fig. 3 |. Simplified model of the HFA.

This model includes a finite thickness of the HFA sheath (that is, shocked plasma) and shock d, which is used to determine the energy spectra of ions accelerated within the trap considering losses through the boundary. In this picture, the HFA sheath and shock fall between the two horizontal black lines, with the HFA core being a larger region below this and the solar wind being the region above this. Only ions at particular energies, pitch angles and gyrophase can remain trapped in the Fermi acceleration trap between Earth’s bow shock or magnetosheath and the HFA shock. For example, the two ion trajectories shown are at the same energy but different gyrophase when they enter the HFA sheath with enhanced magnetic field strength: the rightmost ion is reflected and can continue to be accelerated, whereas the leftmost ion escapes the trap and is lost to the solar wind.