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Register a new userDependence of kinetic plasma waves on ion-to-electron mass ratio and light-to-Alfven speed ratio
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Daniel Verscharen n
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The magnetization $|Omega_{mathrm e}|/omega_{mathrm{e}}$ is an important parameter in plasma astrophysics, where $Omega_{mathrm e}$ and $omega_{mathrm{e}}$ are the electron gyro-frequency and electron plasma frequency, respectively. It only depends on the mass ratio $m_{mathrm i}/m_{mathrm e}$ and the light-to-Alfven speed ratio $c/v_{mathrm{Ai}}$, where $m_{mathrm i}$ ($m_{mathrm e}$) is the ion (electron) mass, $c$ is the speed of light, and $v_{mathrm{Ai}}$ is the ion Alfven speed. Nonlinear numerical plasma models such as particle-in-cell simulations must often assume unrealistic values for $m_{mathrm i}/m_{mathrm e}$ and for $c/v_{mathrm{Ai}}$. Because linear theory yields exact results for parametric scalings of wave properties at small amplitudes, we use linear theory to investigate the dispersion relations of Alfven/ion-cyclotron and fast-magnetosonic/whistler waves as prime examples for collective plasma behaviour depending on $m_{mathrm i}/m_{mathrm e}$ and $c/v_{mathrm{Ai}}$. We analyse their dependence on $m_{mathrm i}/m_{mathrm e}$ and $c/v_{mathrm{Ai}}$ in quasi-parallel and quasi-perpendicular directions of propagation with respect to the background magnetic field for a plasma with $beta_jsim1$, where $beta_j$ is the ratio of the thermal to magnetic pressure for species $j$. Although their dispersion relations are largely independent of $c/v_{mathrm{Ai}}$ for $c/v_{mathrm{Ai}}gtrsim 10$, the mass ratio $m_{mathrm i}/m_{mathrm e}$ has a strong effect at scales smaller than the ion inertial length. Moreover, we study the impact of relativistic electron effects on the dispersion relations. Based on our results, we recommend aiming for a more realistic value of $m_{mathrm i}/m_{mathrm e}$ than for a more realistic value of $c/v_{mathrm{Ai}}$ in non-relativistic plasma simulations if such a choice is necessary, although $dots$
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The nature of the plasma wave modes around the ion kinetic scales in highly Alfvenic slow solar wind turbulence is investigated using data from the NASAs Parker Solar Probe taken in the inner heliosphere, at 0.18 Astronomical Unit (AU) from the sun. The joint distribution of the normalized reduced magnetic helicity ${sigma}_m ({theta}_{RB}, {tau})$ is obtained, where ${theta}_{RB}$ is the angle between the local mean magnetic field and the radial direction and ${tau}$ is the temporal scale. Two populations around ion scales are identified: the first population has ${sigma}_m ({theta}_{RB}, {tau}) < 0$ for frequencies (in the spacecraft frame) ranging from 2.1 to 26 Hz for $60^{circ} < {theta}_{RB} < 130^{circ}$, corresponding to kinetic Alfven waves (KAWs), and the second population has ${sigma}_m ({theta}_{RB}, {tau}) > 0$ in the frequency range [1.4, 4.9] Hz for ${theta}_{RB} > 150^{circ}$, corresponding to Alfven ion Cyclotron Waves (ACWs). This demonstrates for the first time the co-existence of KAWs and ACWs in the slow solar wind in the inner heliosphere, which contrasts with previous observations in the slow solar wind at 1 AU. This discrepancy between 0.18 and 1 AU could be explained, either by i) a dissipation of ACWs via cyclotron resonance during their outward journey, or by ii) the high Alfvenicity of the slow solar wind at 0.18 AU that may be favorable for the excitation of ACWs.
In the present paper, we investigate the power-law behaviour of the magnetic field spectra in the Earths magnetosheath region using Cluster spacecraft data under solar minimum condition. The power spectral density of the magnetic field data and spectral slopes at various frequencies are analysed. Propagation angle and compressibility are used to test the nature of turbulent fluctuations. The magnetic field spectra have the spectral slopes between -1.5 to 0 down to spatial scales of 20 ion gyroradius and show clear evidence of a transition to steeper spectra for small scales with a second power-law, having slopes between -2.6 to -1.8. At low frequencies, f_sc<0.3f_ci(where f_ci is ion gyro-frequency), propagation angle approximately 90 degrees to the mean magnetic field, B_0, and compressibility shows a broad distribution, 0.1 < R > 0.9. On the other hand at f_sc>10f_ci, the propagation angle exhibits a broad range between 30-90 degree while R has a small variation: 0.2 < R > 0.5. We conjecture that at high frequencies, the perpendicularly propagating Alfven waves could partly explain the statistical analysis of spectra. To support our prediction of kinetic Alfven wave-dominated spectral slope behaviour at high frequency, we also present a theoretical model and simulate the magnetic field turbulence spectra due to the nonlinear evolution of kinetic Alfven waves. The present study also shows the analogy between the observational and simulated spectra.
Astrophysical shocks are often collisionless shocks. An open question about collisionless shocks is whether electrons and ions each establish their own post-shock temperature, or whether they quickly equilibrate in the shock region. Here we provide simple relations for the minimal amount of equilibration to expect. The basic assumption is that the enthalpy-flux of the electrons is conserved separately, but that all particle species should undergo the same density jump across the the shock. This assumption results in an analytic treatment of electron-ion equilibration that agrees with observations of collisionless shocks: at low Mach numbers ($<2$) the electrons and ions are close to equilibration, whereas for Mach numbers above $M sim 60$ the electron-ion temperature ratio scales with the particle masses $T_e/T_i = m_e/m_i$. In between these two extremes the electron-ion temperature ratio scales as $T_e/T_i propto 1/M_s^2$. This relation also hold if adiabatic compression of the electrons is taken into account. For magnetised plasmas the compression is governed by the magnetosonic Mach number, whereas the electron-ion temperatures are governed by the sonic Mach number. The derived equations are in agreement with observational data at low Mach numbers, but for supernova remnants the relation requires that the inferred Mach numbers for the observations are over- estimated, perhaps as a result of upstream heating in the cosmic-ray precursor. In addition to predicting a minimal electron/ion temperature ratio, we also heuristically incorporate ion-electron heat exchange at the shock, quantified with a dimensionless parameter ${xi}$. Comparing the model to existing observations in the solar system and supernova remnants suggests that the data are best described by ${xi} sim 5$ percent. (Abridged abstract.)
Large-scale two-dimensional (2D) full particle-in-cell simulations are carried out for studying the relationship between the dynamics of a perpendicular shock and microinstabilities generated at the shock foot. The structure and dynamics of collisionless shocks are generally determined by Alfven Mach number and plasma beta, while microinstabilities at the shock foot are controlled by the ratio of the upstream bulk velocity to the electron thermal velocity and the ratio of the plasma-to-cyclotron frequency. With a fixed Alfven Mach number and plasma beta, the ratio of the upstream bulk velocity to the electron thermal velocity is given as a function of the ion-to-electron mass ratio. The present 2D full PIC simulations with a relatively low Alfven Mach number (M_A ~ 6) show that the modified two-stream instability is dominant with higher ion-to-electron mass ratios. It is also confirmed that waves propagating downstream are more enhanced at the shock foot near the shock ramp as the mass ratio becomes higher. The result suggests that these waves play a role in the modification of the dynamics of collisionless shocks through the interaction with shock front ripples.
We examine the dependence of the mass-to-light (M/L) ratio of large-scale structure on cosmological parameters, in models that are constrained to match observations of the projected galaxy correlation function w(rp). For a sequence of cosmological models with a fixed P(k) shape and increasing normalization sig8, we find parameters of the galaxy halo occupation distribution (HOD) that reproduce SDSS measurements of w(rp) as a function of luminosity. Using these HOD models we calculate mean M/L ratios as a function of halo mass and populate halos of N-body simulations to compute M/L in larger scale environments, including cluster infall regions. For all cosmological models, the M/L ratio in high mass halos or high density regions is approximately independent of halo mass or smoothing scale. However, the plateau value of M/L depends on sig8 as well as Omega_m, and it represents the universal mass-to-light ratio <M/L> only for models in which the galaxy correlation function is approximately unbiased, i.e., with sig8 ~ sig8_gal. Our results for cluster mass halos follow the trend M/L = 577(Omega_m/0.3)(sig8/0.9)^{1.7} h Msun/Lsun. Combined with Carlberg et al.s (1996) mean M/L ratio of CNOC galaxy clusters, this relation implies (sig8/0.9)(Omega_m/0.3)^{0.6} = 0.75 +/- 0.06. M/L ratios of clusters from the SDSS and CAIRNS surveys yield similar results. This constraint is inconsistent with parameter values Omega_m ~ 0.3, sig8 ~ 0.9 favored by recent joint analyses of CMB measurements and other large-scale structure data. We discuss possible resolutions, none of which seems entirely satisfactory. Appendices present an improved formula for halo bias factors and an improved analytic technique for calculating the galaxy correlation function from a given cosmological model and HOD. (Abridged)
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