Buoyancy Instabilities in Weakly Magnetized Low Collisionality Plasmas

I calculate the linear stability of a stratified low collisionality plasma in the presence of a weak magnetic field. Heat is assumed to flow only along magnetic field lines. In the absence of a heat flux in the background plasma, Balbus (2000) demonstrated that plasmas in which the temperature increases in the direction of gravity are buoyantly unstable to convective-like motions (the ``magnetothermal instability''). I show that in the presence of a background heat flux, an analogous instability is present when the temperature decreases in the direction of gravity. The instability is driven by the background heat flux and the fastest growing mode has a growth time of order the local dynamical time. Thus, independent of the sign of the temperature gradient, weakly magnetized low collisionality plasmas are unstable on a dynamical time to magnetically-mediated buoyancy instabilities. The instability described in this paper is predicted to be present in clusters of galaxies at radii from ~ 0.1-100 kpc, where the observed temperature increases outwards. Possible consequences for the origin of cluster magnetic fields, ``cooling flows,'' and the thermodynamics of the intercluster medium are briefly discussed.Comment: 5 pages; 1 cartoon; ApJ in pres

Angular Momentum Transport in Particle and Fluid Disks

We examine the angular momentum transport properties of disks composed of macroscopic particles whose velocity dispersions are externally enhanced (``stirred''). Our simple Boltzmann equation model serves as an analogy for unmagnetized fluid disks in which turbulence may be driven by thermal convection. We show that interparticle collisions in particle disks play the same role as fluctuating pressure forces and viscous dissipation in turbulent disks: both transfer energy in random motions associated with one direction to those associated with another, and convert kinetic energy into heat. The direction of angular momentum transport in stirred particle and fluid disks is determined by the direction of external stirring and by the properties of the collision term in the Boltzmann equation (or its analogue in the fluid problem). In particular, our model problem yields inward transport for vertically or radially stirred disks, provided collisions are suitably inelastic; the transport is outwards in the elastic limit. Numerical simulations of hydrodynamic turbulence driven by thermal convection find inward transport; this requires that fluctuating pressure forces do little to no work, and is analogous to an externally stirred particle disk in which collisions are highly inelastic.Comment: 15 pages; final version accepted by ApJ; minor changes, some clarificatio

A Kinetic Alfven wave cascade subject to collisionless damping cannot reach electron scales in the solar wind at 1 AU

(Abridged) Turbulence in the solar wind is believed to generate an energy cascade that is supported primarily by Alfv\'en waves or Alfv\'enic fluctuations at MHD scales and by kinetic Alfv\'en waves (KAWs) at kinetic scales $k_\perp \rho_i\gtrsim 1$. Linear Landau damping of KAWs increases with increasing wavenumber and at some point the damping becomes so strong that the energy cascade is completely dissipated. A model of the energy cascade process that includes the effects of linear collisionless damping of KAWs and the associated compounding of this damping throughout the cascade process is used to determine the wavenumber where the energy cascade terminates. It is found that this wavenumber occurs approximately when $|\gamma/\omega|\simeq 0.25$, where $\omega(k)$ and $\gamma(k)$ are, respectively, the real frequency and damping rate of KAWs and the ratio $\gamma/\omega$ is evaluated in the limit as the propagation angle approaches 90 degrees relative to the direction of the mean magnetic field.Comment: Submitted to Ap

Turbulence and Particle Heating in Advection-Dominated Accretion Flows

We extend and reconcile recent work on turbulence and particle heating in advection-dominated accretion flows. For approximately equipartition magnetic fields, the turbulence primarily heats the electrons. For weaker magnetic fields, the protons are primarily heated. The division between electron and proton heating occurs between $\beta \sim 5$ and $\beta \sim 100$ (where $\beta$ is the ratio of the gas to the magnetic pressure), depending on unknown details of how Alfv\'en waves are converted into whistlers on scales of the proton Larmor radius. We also discuss the possibility that magnetic reconnection could be a significant source of electron heating.Comment: 17 pages (Latex), incl. 2 Figures; submitted to Ap

Magnetic fluctuation power near proton temperature anisotropy instability thresholds in the solar wind

The proton temperature anisotropy in the solar wind is known to be constrained by the theoretical thresholds for pressure anisotropy-driven instabilities. Here we use approximately 1 million independent measurements of gyroscale magnetic fluctuations in the solar wind to show for the first time that these fluctuations are enhanced along the temperature anisotropy thresholds of the mirror, proton oblique firehose, and ion cyclotron instabilities. In addition, the measured magnetic compressibility is enhanced at high plasma beta ($\beta_\parallel \gtrsim 1$) along the mirror instability threshold but small elsewhere, consistent with expectations of the mirror mode. The power in this frequency (the 'dissipation') range is often considered to be driven by the solar wind turbulent cascade, an interpretation which should be qualified in light of the present results. In particular, we show that the short wavelength magnetic fluctuation power is a strong function of collisionality, which relaxes the temperature anisotropy away from the instability conditions and reduces correspondingly the fluctuation power.Comment: 4 pages, 4 figure

Self-similar structure of a hot magnetized flow with thermal conduction

We have explored the structure of hot magnetized accretion flow with thermal conduction. The importance of thermal conduction in hot accretion flows has been confirmed by observations of the hot gas surrounding Sgr $A^*$ and a few other nearby galactic nuclei. For a steady state structure of such accretion flows a set of self similar solutions are presented. In this paper, we have actually tried to re-check the solution presented by Abbassi et al. (2008) using a physical constrain. In this study we find that Eq 29 places a new constrain that limits answers presented by Abbassi et al. 2008. In that paper the parameter space in which it is established in the new constrain was plotted. However, the new requirement makes up only a small parameter space with physically acceptable solutions. And now in this manuscript we have followed the idea with more effort, and tried to find out how thermal conduction influences the structur of the disks in a physical parameter space. We have found out that the existence of thermal conduction will lead to reduction of accretion and radial and azimuthal velocities as well as the vertical thickness of the disk, which is slightly reduced. Moreover, the surface density of the disk will increase when the thermal conduction becomes important in the hot magnetized flow.Comment: Accepted for publication, AP

The Origin of the Young Stars in the Nucleus of M31

The triple nucleus of M31 consists of a population of old red stars in an eccentric disk (P1 and P2) and another population of younger A stars in a circular disk (P3) around M31's central supermassive black hole (SMBH). We argue that P1 and P2 determine the maximal radial extent of the younger A star population and provide the gas that fueled the starburst that generated P3. The eccentric stellar disk creates an $m=1$ non-axisymmetric perturbation to the potential. This perturbed potential drives gas into the inner parsec around the SMBH, if the pattern speed of the eccentric stellar disk is $\Omega_p \lesssim 3-10 {\rm km s^{-1} pc^{-1}}$. We show that stellar mass loss from P1 and P2 is sufficient to create a gravitationally unstable gaseous disk of \sim 10^5\Msun every $0.1-1$ Gyrs, consistent with the 200 Myr age of P3. Similar processes may act in other systems to produce very compact nuclear starbursts.Comment: 10 pages, 7 figures, accepted by ApJ, changes made from referee suggestion