arXiv subjects

Alvaro Osorio

Publications and source records attributed to Alvaro Osorio.

2 recordsLinked to original sources

arXiv2021

Stochastic Electron Acceleration by Temperature Anisotropy Instabilities Under Solar Flare Plasma Conditions

Using 2D particle-in-cell (PIC) plasma simulations we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top (ALT) sources in solar flares. We focus on the long-term effect of $T_{e,\perp} > T_{e,\parallel}$ instabilities by driving the anisotropy growth during the entire simulation time, through imposing a shearing or a compressing plasma velocity ($T_{e,\perp}$ and $T_{e,\parallel}$ are the temperatures perpendicular and parallel to the magnetic field). This magnetic growth makes $T_{e,\perp}/T_{e,\parallel}$ grow due to electron magnetic moment conservation, and amplifies the ratio $\omega_{ce}/\omega_{pe}$ from $\sim 0.53$ to $\sim 2$ ($\omega_{ce}$ and $\omega_{pe}$ are the electron cyclotron and plasma frequencies, respectively). In the regime $\omega_{ce}/\omega_{pe}\lesssim 1.2-1.7$ the instability is dominated by oblique, quasi-electrostatic (OQES) modes, and the acceleration is inefficient. When $\omega_{ce}/\omega_{pe}$ has grown to $\omega_{ce}/\omega_{pe}\gtrsim 1.2-1.7$, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z (PEMZ) modes. After $\omega_{ce}/\omega_{pe}$ reaches $\sim 2$, 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 $\alpha_s \sim$ 2.9 plus a high-energy bump reaching $\sim 300$ keV. In the compressing runs, $\alpha_s \sim$ 3.7 with a spectral break above $\sim 500$ keV. This difference can be explained by the different temperature evolutions in these two types of simulations, suggesting a critical role played by the type of anisotropy driving, $\omega_{ce}/\omega_{pe}$ and the electron temperature in the efficiency of the acceleration.

astro-ph.SR↗

arXiv2017

Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field

We use 2D particle-in-cell (PIC) simulations to study the effect of the saturated whistler instability on the viscous heating and nonthermal acceleration of electrons in a shearing, collisionless plasma with a growing magnetic field, \textbf{B}. In this setup, an electron pressure anisotropy with $p_{\perp,e} > p_{||,e}$ naturally arises due to the adiabatic invariance of the electron magnetic moment ($p_{||,e}$ and $p_{\perp,e}$ are the pressures parallel and perpendicular to \textbf{B}). If the anisotropy is large enough, the whistler instability arises, efficiently scattering the electrons and limiting $\Delta p_e$ ($\equiv p_{\perp,e}-p_{||,e}$). In this context, $\Delta p_e$ taps into the plasma velocity shear, producing electron heating by the so called anisotropic viscosity. In our simulations, we permanently drive the growth of $|\textbf{B}|$ by externally imposing a plasma shear, allowing us to self-consistently capture the long-term, saturated whistler instability evolution. We find that besides the viscous heating, the scattering by whistler modes can stochastically accelerate electrons to nonthermal energies. This acceleration is most prominent when initially $\beta_e\sim 1$, gradually decreasing its efficiency for larger values of $\beta_e$ ($\equiv 8\pi p_e/|\textbf{B}|^2$). If initially $\beta_e \sim 1$, the final electron energy distribution can be approximately described by a thermal component, plus a power-law tail with spectral index $\sim 3.7$. In these cases, the nonthermal tail accounts for $\sim 5\%$ of the electrons, and for $\sim 15\%$ of their kinetic energy. We discuss the implications of our results for electron heating and acceleration in low-collisionality astrophysical environments, such as low-luminosity accretion flows.

astro-ph.HE↗