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Lapenta G.,Center for Mathematical Plasma Astrophysics | Ashour-Abdalla M.,University of California at Los Angeles | Walker R.J.,University of California at Los Angeles | El Alaoui M.,University of California at Los Angeles
Geophysical Research Letters | Year: 2016

Ion heating during a substorm on 15 February 2008, starting at 0348UT, is studied with a new approach recently described in Ashour-Abdalla et al. (2015). The general conditions of the magnetotail are obtained from a global magnetohydrodynamic (MHD) model and are used to drive a full kinetic particle-in-cell (PIC) simulation of a 3-D region of the tail. Within the kinetic box, the ions, the electrons, and the fields evolve self-consistently. The large scales are captured by the MHD model and the small scales by the PIC model based on the MHD state. This approach is used to study ion heating. Different heating mechanisms were analyzed by examining the velocity distributions at different locations. In the x direction heating occurs as the reconnection-generated ion jet interacts with the environment in which it propagates. The heating is found mostly in the separatrices and increases downstream of the reconnection region. In the y direction the heating is less intense and is found near the dipolarization fronts. It occurs as ions become magnetized and gyrotropize the distribution function. In addition, ions can be heated in the y direction by the reconnection electric field near the reconnection site. In the z direction the ions are heated by the formation of beams moving along z between the separatrices. ©2015. American Geophysical Union. Source


Liang H.,University of California at Los Angeles | Ashour-Abdalla M.,University of California at Los Angeles | Lapenta G.,Center for Mathematical Plasma Astrophysics | Walker R.J.,University of California at Los Angeles
Journal of Geophysical Research A: Space Physics | Year: 2016

Spacecraft observations near a magnetotail X line show that oxygen (O+) ions are minor species during nonstorm substorms, but they can become major species during some of the storm time substorms. Dipolarization fronts (DFs), which are characterized by a sharp increase northward magnetic field in the magnetotail, are commonly observed during magnetospheric substorms. In this study, we investigated the O+ effects on DFs and the reconnection rate during magnetotail reconnection. We used a 2.5-D implicit particle-in-cell simulation in a 2-D Harris current sheet in the presence of H+ and O+ ions. Simulation runs with equal number densities of O+ and H+ (O+ run) and with pure H+ ion species (H+ run) were performed. Comparing the two different runs, we found that both the reconnection rate and the DF speed in the O+ run were much less than those in the H+ run. By studying the force balance and plasma composition at the DF, we found that the outflow magnetic flux and DF propagation were encumbered by the current sheet O+ inertia, which reduced the DF speed and delayed the reconnection rate in the O+ run. We also found an ambipolar electric field in the O+ run due to the different inflow and outflow speeds of O+ and electrons in the O+ diffusion region. As a result, this ambipolar electric field induced O+ drag on the convective magnetic field in the O+ diffusion region. The small reconnection rate determined in the O+ run can be attributed to the current sheet inertia and the O+ drag on the convective magnetic flux. ©2016. American Geophysical Union. Source


Felten T.,Ruhr University Bochum | Schlickeiser R.,Ruhr University Bochum | Yoon P.H.,University of Maryland University College | Yoon P.H.,Kyung Hee University | Lazar M.,Center for Mathematical Plasma Astrophysics
Physics of Plasmas | Year: 2013

General expressions for the electromagnetic fluctuation spectra in unmagnetized plasmas are derived using fully relativistic dispersion functions and form factors for the important class of isotropic plasma particle distribution functions including in particular relativistic Maxwellian distributions. In order to obtain fluctuation spectra valid in the entire complex frequency plane, the proper analytical continuations of the unmagnetized form factors and dispersion functions are presented. The results are illustrated for the important special case of isotropic Maxwellian particle distribution functions providing in particular the thermal fluctuations of aperiodic modes. No restriction to the plasma temperature value is made, and the electromagnetic fluctuation spectra of ultrarelativistic thermal plasmas are calculated. The fully relativistic calculations also provide more general results in the limit of nonrelativistic plasma temperatures being valid in the entire complex frequency plane. They complement our earlier results in paper I and III of this series for negative values of the imaginary part of the frequency. A new collective, transverse, damped aperiodic mode with the damping rate γ ∝ - k - 5 / 3 is discovered in an isotropic thermal electron-proton plasma with nonrelativistic temperatures. © 2013 AIP Publishing LLC. Source


Lazar M.,Ruhr University Bochum | Lazar M.,Center for Mathematical Plasma Astrophysics
Astronomy and Astrophysics | Year: 2012

Context. Observations regularly show low-frequency fluctuations of the interplanetary magnetic field (IMF), which are attributed to the electromagnetic ion-cyclotron (EMIC) waves generated either locally and self-consistently by the kinetic anisotropies of ions, or closer to the Sun (through a nonlinear cascade from long to short wavelengths), and transported by the super-Alfvenic solar wind. As a back reaction, ions can be pitch-angle scattered and accelerated, leading to the observed suprathermal populations, which are invariably anisotropic and are well described by the generalized Kappa models. Aims. A refined analysis is proposed for the EMIC wave instability as one of the most plausible constraints for the proton temperature anisotropy T p, < T p, where and denote directions relative to the stationary IMF. In the context of a strong, but not clear competition with the mirror instability that can develop in the same conditions, an advanced Kappa model is expected to provide the first realistic insights into the EMIC instability conditions in the solar wind. Methods. Because the solar wind is a poor-collisional plasma, the dispersion/stability formalism is based on the fundamental kinetic Vlasov-Maxwell equations for an nonthermal bi-Kappa distributed plasma. EMIC solutions are derived exactly numerically, providing accurate physical correlations between the maximum growth rates and the instability threshold conditions, which are here derived for the full range of values of the plasma beta, including the solar wind and magnetospheric plasma conditions. Results. The lowest thresholds (close to the marginal stability), which are the most relevant for the instability conditions, decrease with the increase in density of suprathermal populations. This is contrary to what was found before in a less general model, but it is fully predicted by the enhanced fluctuations of this instability for sufficiently low temperature anisotropies. These results furthermore support a fast and efficient EMIC instability involving the relaxation of kinetic anisotropies and (re)heating plasma particles. © 2012 ESO. Source


Eliasson B.,University of Strathclyde | Lazar M.,Center for Mathematical Plasma Astrophysics | Lazar M.,Ruhr University Bochum
Physics of Plasmas | Year: 2015

This paper presents a numerical study of the linear and nonlinear evolution of the electromagnetic electron-cyclotron (EMEC) instability in a bi-Kappa distributed plasma. Distributions with high energy tails described by the Kappa power-laws are often observed in collision-less plasmas (e.g., solar wind and accelerators), where wave-particle interactions control the plasma thermodynamics and keep the particle distributions out of Maxwellian equilibrium. Under certain conditions, the anisotropic bi-Kappa distribution gives rise to plasma instabilities creating low-frequency EMEC waves in the whistler branch. The instability saturates nonlinearly by reducing the temperature anisotropy until marginal stability is reached. Numerical simulations of the Vlasov-Maxwell system of equations show excellent agreement with the growth-rate and real frequency of the unstable modes predicted by linear theory. The wave-amplitude of the EMEC waves at nonlinear saturation is consistent with magnetic trapping of the electrons. © 2015 Author(s). Source

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