Fermi acceleration of electrons inside foreshock transient cores. Issue 9 (5th September 2017)

• Record Type: Journal Article
• Authors: Liu, Terry Z.; Lu, San; Angelopoulos, Vassilis; Hietala, Heli; Wilson, Lynn B.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 9(2017)
• Page Start: 9248
• Page End: 9263
• Publication Date: 2017-09-05
• Subjects: ion foreshock -- particle acceleration -- shock processes -- kinetic phenomenon; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2017JA024480
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: Foreshock transients upstream of Earth's bow shock have been recently observed to accelerate electrons to many times their thermal energy. How such acceleration occurs is unknown, however. Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) case studies, we examine a subset of acceleration events (31 of 247 events) in foreshock transients with cores that exhibit gradual electron energy increases accompanied by low background magnetic field strength and large‐amplitude magnetic fluctuations. Using the evolution of electron distributions and the energy increase rates at multiple spacecraft, we suggest that Fermi acceleration between a converging foreshock transient's compressional boundary and the bow shock is responsible for the observed electron acceleration. We then show that a one‐dimensional test particle simulation of an ideal Fermi acceleration model in fluctuating fields prescribed by the observations can reproduce the observed evolution of electron distributions, energy increase rate, and pitch angle isotropy, providing further support for our hypothesis. Thus, Fermi acceleration is likely the principal electron acceleration mechanism in at least this subset of foreshock transient cores. Key Points: In the observations, the evolution of electron distributions in foreshock transients is well explained by a simple Fermi acceleration model We apply 1‐D test particle simulations further confirming Fermi acceleration model Our ideal Fermi model which requires low background field and strong fluctuations can explain a subset of electron acceleration events