Ion Acceleration by Foreshock Bubbles: Magnetospheric Multiscale Observations

Foreshock bubbles (FBs) occur when interplanetary magnetic field discontinuities encounter the Earth's foreshock. These transient (similar to 1 to 5 min) features exhibit depressed densities and magnetic field strengths, enhanced temperatures, and deflected plasma flows trailed by a region of enhanced plasma density and magnetic field strength. Ions can be accelerated inside the FBs through the Fermi acceleration process. Hybrid simulations and test particle calculations predict that the maximum energies of ions accelerated by FBs reach 5.6 times the solar wind ram energy (E-sw). We identify 23 FBs from September 2015 to January 2020 Magnetospheric Multiscale spacecraft observations. Most FBs (17 of 23) occurred upstream of the dusk-side bow shock and above the ecliptic. The FBs occurred for Alfvn Mach numbers ranging from 5 to 15, with 11 FBs having an Alfvn Mach number near 10. To investigate ion energization inside the cores of the FBs we compare the proton spectra observed by the Hot Plasma Composition Analyzer and Energetic Ion Spectrometer before (upstream), during (core), and after (downstream) the FBs. The proton intensities at energies from E-sw (the solar wind ram energy, 0.5 x m x V-sw(2) ) up to about 5.6E(sw) are greater inside than outside 19 of 23 FBs, confirming that FBs can accelerate particles to these energies. The proton flux intensities at energies between E-sw and 5.6E(sw) in the core of the FBs are consistent with results from global hybrid simulations for ion energization from FBs through second-order Fermi acceleration.