Abstract
Abstract
Using 2D particle-in-cell plasma simulations, we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top sources in solar flares. We focus on the long-term effect of T
e,⊥ > T
e,∥ instabilities by driving the anisotropy growth during the entire simulation time through imposing a shearing or a compressing plasma velocity (T
e,⊥ and T
e,∥ are the temperatures perpendicular and parallel to the magnetic field). This magnetic growth makes T
e,⊥/T
e,∥ grow due to electron magnetic moment conservation, and amplifies the ratio ω
ce/ω
pe from ∼0.53 to ∼2 (ω
ce and ω
pe are the electron cyclotron and plasma frequencies, respectively). In the regime ω
ce/ω
pe ≲ 1.2–1.7, the instability is dominated by oblique, quasi-electrostatic modes, and the acceleration is inefficient. When ω
ce/ω
pe has grown to ω
ce/ω
pe ≳ 1.2–1.7, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z modes. After ω
ce/ω
pe reaches ∼2, the electron energy spectra show nonthermal tails that differ between the shearing and compressing cases. In the shearing case, the tail resembles a power law of index α
s
∼ 2.9 plus a high-energy bump reaching ∼300 keV. In the compressing runs, α
s
∼ 3.7 with a spectral break above ∼500 keV. This difference can be explained by the different temperature evolutions in these two types of simulations, suggesting that a critical role is played by the type of anisotropy driving, ω
ce/ω
pe, and the electron temperature in the efficiency of the acceleration.
Publisher
American Astronomical Society
Subject
Space and Planetary Science,Astronomy and Astrophysics
Cited by
2 articles.
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