Parallel firehose instability in electron-positron plasmas.

Area of Science:

• Plasma physics
• Astrophysical plasmas
• Space plasma physics

Background:

• Firehose instability arises in magnetized plasmas due to pressure anisotropy (P_{||}>P_{⊥}).
• Understanding this instability is crucial for magnetized plasma dynamics in space and astrophysical environments.

Purpose of the Study:

• Examine parallel firehose instability in electron-positron plasmas.
• Compare particle simulation results with linear fluid theory predictions.
• Investigate the nonlinear saturation mechanisms and stability criteria.

Main Methods:

• Utilized particle-in-cell kinetic simulations.
• Employed linear fluid theory to derive dispersion relations and instability criteria.
• Analyzed magnetic field evolution, pressure anisotropy, and wave numbers.

Main Results:

• Simulations show magnetic field growth and decay with oscillations in electron-proton plasmas.
• Nonlinear saturation state agrees with fluid theory (α=1) for smaller initial conditions.
• Kinetic resonant effects lead to saturation below α=1 for larger initial conditions.
• Dominant wave numbers (kc/ωp < 0.5) and growth rates (0.1–0.3ωc) are consistent with fluid theory.
• Both electrostatic and electromagnetic modes predicted by fluid theory were observed.

Conclusions:

• Kinetic effects significantly influence firehose instability saturation in plasmas.
• Linear fluid theory provides a good approximation for smaller scales and initial conditions.
• Discrepancies at larger scales highlight the importance of kinetic effects in plasma dynamics.