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- Title
Local Acceleration of Protons to 100 keV in a Quasi‐Parallel Bow Shock.
- Authors
Stasiewicz, Krzysztof; Eliasson, Bengt; Cohen, Ian J.; Turner, Drew L.; Ergun, Robert E.
- Abstract
Recent observations in the quasi‐parallel bow shock by the Magnetospheric Multiscale spacecraft show rapid heating and acceleration of ions up to an energy of about 100 keV. It is demonstrated that a prominent acceleration mechanism is the nonlinear interaction with a spectrum of waves produced by gradient‐driven instabilities, including the lower hybrid drift (LHD) instability, modified two‐stream (MTS) instability, and electron cyclotron drift (ECD) instability. Test‐particle simulations show that the observed spectrum of waves can rapidly accelerate protons up to a few hundreds of keV by the ExB mechanism. The ExB wave mechanism is related to the surfatron mechanism at shocks, but through the coupling with the stochastic heating condition, it produces significant acceleration on much shorter temporal and spatial scales by the interaction with bursts of waves within a cyclotron period. The results of this study are built on the heritage of four‐point measurement techniques developed for the Cluster mission and imply that the concepts of Fermi acceleration, diffusive shock acceleration, and shock drift acceleration are not needed to explain proton acceleration to hundreds of keV at the Earth's bow shock. Plain Language Summary: The acceleration, or energization, of particles is a common and fundamental process throughout the universe. In particular, particle acceleration is a common occurrence at collisionless shocks that occur in plasmas in our solar system and beyond. This study presents new observations of the acceleration of protons to high energies (hundreds of keV) by waves at the bow shock upstream of the Earth, where the solar wind first encounters Earth's magnetic field. The observations by NASA's Magnetospheric Multiscale (MMS) mission show a specific event where energized protons are measured, along with wave activity that is consistent with theoretical expectations for a certain type of interaction between the waves and particles that allows the waves to transfer energy to the protons. Numerical simulations are also performed and the results agree with what was observed in real life by the spacecraft. The results are important because they provide insight into potential processes that can create high‐energy particles both near the Earth and at other astrophysical systems. Key Points: At quasi‐parallel shocks, ions are rapidly accelerated from 10 eV to 100 keV by the ExB mechanism of electrostatic wavesThe waves are produced by current‐driven instabilities with frequencies between the proton and the electron cyclotron frequenciesThe acceleration occurs locally within a few gyroperiods
- Subjects
ELECTRON cyclotron resonance sources; ION sources; MAGNETOSPHERE; SOLAR magnetism; GEOMAGNETISM
- Publication
Journal of Geophysical Research. Space Physics, 2021, Vol 126, Issue 7, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2021JA029477