Ion Acceleration by Foreshock Bubbles

Foreshock bubbles (FBs) form as a result of the interaction between solar wind discontinuities and backstreaming ion beams in the foreshock. They are carried by the solar wind in the anti‐sunward direction and are associated with high energy ions. We employ electromagnetic hybrid (kinetic ions, flui...

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Bibliographic Details
Published in:Journal of geophysical research. Space physics 2021-05, Vol.126 (5), p.n/a
Main Authors: Omidi, N., Lee, S. H., Sibeck, D. G.
Format: Article
Language:English
Subjects:
Acceleration
Bubbles
Cavitons
collisionless shocks
Energy distribution
Energy spectra
foreshock bubbles
foreshock cavitons
Ion acceleration
Ion beams
Ions
Mathematical analysis
particle energization
second‐order Fermi
Solar energy
Solar wind
Solar wind velocity
Topology
Wind speed
Wind velocities
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Summary:Foreshock bubbles (FBs) form as a result of the interaction between solar wind discontinuities and backstreaming ion beams in the foreshock. They are carried by the solar wind in the anti‐sunward direction and are associated with high energy ions. We employ electromagnetic hybrid (kinetic ions, fluid electrons) simulations and test particle calculations to investigate ion acceleration by FBs. Simulation results show that the maximum energies which the ions attain are not sensitive to the topology of the FBs, which varies from spherical to planar limits. However, the solar wind velocity does control the maximum energies reached by FB acceleration. Test particle calculations show that ion energization takes place through reflection of sunward moving ions from FBs through one or more collisions consistent with second‐order Fermi acceleration. The results also indicate that energy gain does not reach that expected for elastic reflections. Examination of ion energy spectra within different FBs shows a self‐similar pattern consisting of distribution peaked at the solar wind ram energy Esw with a distribution tail that reaches energies of ∼5.6 Esw. This behavior is consistent with second‐order Fermi acceleration and the inherent limitations on energy gain in this process. We also discuss the role of nonlinear structures called foreshock cavitons in the acceleration process. Key Points Ions are accelerated by foreshock bubbles through second‐order Fermi process Maximum energies attained are determined by solar wind velocity Ion energy spectra within foreshock bubbles exhibit self‐similarity
ISSN:2169-9380
2169-9402
DOI:10.1029/2020JA028924