Ion Temperature Perturbations Research Articles

Acoustic Disturbances in Galaxy Clusters

Abstract Galaxy cluster cores are pervaded by hot gas which radiates at far too high a rate to maintain any semblance of a steady state; this is referred to as the cooling flow problem. Of the many heating mechanisms that have been proposed to balance radiative cooling, one of the most attractive is the dissipation of acoustic waves generated by active galactic nuclei. Fabian et al. showed that if the waves are nearly adiabatic, wave damping due to heat conduction and viscosity must be well below standard Coulomb rates in order to allow the waves to propagate throughout the core. Because of the importance of this result, we have revisited wave dissipation under galaxy cluster conditions in a way that accounts for the self-limiting nature of dissipation by electron thermal conduction, allows the electron and ion temperature perturbations in the waves to evolve separately, and estimates kinetic effects by comparing to a semicollisionless theory. While these effects considerably enlarge the toolkit for analyzing observations of wavelike structures and developing a quantitative theory for wave heating, the drastic reduction of transport coefficients proposed in Fabian et al. remains the most viable path to acoustic wave heating of galaxy cluster cores.

Temperature dynamics and velocity scaling laws for interchange driven, warm ion plasma filaments

The influence of electron and ion temperature dynamics on the radial convection of isolated structures in magnetically confined plasmas is investigated by means of numerical simulations. It is demonstrated that the maximum radial velocity of these plasma blobs roughly follows the inertial velocity scaling, which is proportional to the ion acoustic speed times the square root of the filament particle density times the sum of the electron and ion temperature perturbations. Only for small blobs the cross field convection does not follow this scaling. The influence of finite Larmor radius effects on the cross-field blob convection is shown not to depend strongly on the dynamical ion temperature field. The blob dynamics of constant finite and dynamical ion temperature blobs is similar. When the blob size is on the order of 10 times the ion Larmor radius the blobs stay coherent and decelerate slowly compared to larger blobs which dissipate faster due to fragmentation and turbulent mixing.

Stabilization of magnetic curvature-driven Rayleigh–Taylor instabilities

AbstractThe finite ion Larmor radius (FLR) stabilization of the magnetic curvature-driven Rayleigh–Taylor (MCD RT) instability in a low beta plasma with nonzero ion temperature gradient is investigated. Finite electron temperature effects and ion temperature perturbations are incorporated. A new set of nonlinear equations for flute waves with arbitrary wavelengths as compared with the ion Larmor radius in a plasma with curved magnetic field lines is derived. Particular attention is paid to the waves with spatial scales of the order of the ion Larmor radius. In the linear limit, a Fourier transform of these equations yields an improved dispersion relation for flute waves. The dependence of the MCD RT instability growth rate on the equilibrium plasma parameters and the wavelengths is studied. The condition for which the instability cannot be stabilized by the FLR effects is found.

Thermospheric temperatures above Poker Flat, Alaska, during the stratospheric warming event of January and February 2009

We report observations of neutral and ion temperatures at approximately 240 km altitude above Poker Flat, Alaska, taken during the stratospheric warming event of January and February 2009. Neutral temperatures were recorded by a single‐etalon Fabry‐Perot spectrometer observing the nighttime airglow and aurora at 630 nm wavelength, whereas ion temperatures were obtained with the Poker Flat Incoherent Scatter Radar, mostly during daylight. Neutral and ion temperatures generally tracked each other closely; both data sets exhibited a similar overall seasonal trend during January to April 2009 and even showed similar short‐term day‐to‐day fluctuations. Further, the daily fluctuations correlated highly with geomagnetic activity. Both the short‐term temperature fluctuations and their overall seasonal trend matched expectations based on the MSIS model evaluated for this period at hourly intervals. A cooling response was seen in neutral temperature over a 2–3 week period during the main phase of the stratospheric warming. The magnitude of this cooling was around 50 K relative to the seasonal trend. A corresponding signature was also discernible in the ion temperatures, although it was less prominent than the neutral response. This was largely because the cooling was masked by ion temperature perturbations driven by magnetic activity. The strength of the response in ion temperature was observed to depend on the local time of the observations, which is consistent with results presented previously.

Role of anomalous transport in onset and evolution of neoclassical tearing modes

A key role in the evolution of the neoclassical tearing modes (NTMs) belongs to the radial profiles of the perturbed plasma flow, temperature and density which are determined by the conjunction of the longitudinal and cross-field transport arising from thermal conduction, particle diffusion and viscosity. In a tokamak, the perpendicular transport of particles, heat and momentum is typically anomalous. In this paper the results of theoretical studies on the influence of anomalous perpendicular heat transport and anomalous ion perpendicular viscosity on early stages of NTM evolution are presented. Several parallel transport mechanisms competitive with anomalous cross-island heat transport in the formation of the perturbed electron and ion temperature profiles within the island are considered. The perturbed electron temperature profile is established in competition between anomalous perpendicular electron heat conductivity and parallel electron heat convection. The formation of the ion perturbed temperature profile was found to be dependent on the island rotation frequency. The perpendicular ion heat conductivity is balanced by the parallel transport associated with the ion inertia for an island rotating with subsonic frequency or with island rotation with respect to the plasma for supersonic islands. The partial contributions from the plasma electron and ion temperature perturbations in the bootstrap drive of the mode and magnetic curvature effect were taken into account in construction of a generalized transport threshold model of NTMs. This model gives more favourable predictions for NTM stability and qualitatively modifies the scaling law for βonset. The anomalous perpendicular ion viscosity is shown to modify the collisionality dependence of the polarization current effect, reducing it to the low collisionality limit. In its turn a viscous contribution to the bootstrap drive of NTMs is found to be of the same order as a conventional bootstrap drive for the islands of width close to the characteristic one of the transport threshold model. A viscous contribution to the perturbed bootstrap current is destabilizing for the island rotating in the ion diamagnetic drift direction. In this case, an alternative threshold mechanism should be considered.

Suppression of the Neoclassical Tearing Modes in Tokamaks under Anomalous Transverse Transport Conditions when the Magnetic Well Effect Predominates over the Bootstrap Drive

A study is made of the suppression of neoclassical tearing modes in tokamaks under anomalous transverse transport conditions when the magnetic well effect predominates over the bootstrap drive. It is stressed that the corresponding effect, which is called the compound suppression effect, depends strongly on the profiles of the electron and ion temperature perturbations. Account is taken of the fact that the temperature profile can be established as a result of the competition between anomalous transverse heat transport, on the one hand, and longitudinal collisional heat transport, longitudinal heat convection, longitudinal inertial transport, and transport due to the rotation of magnetic islands, on the other hand. The role of geodesic effects is discussed. The cases of competition just mentioned are described by the model sets of reduced transport equations, which are called, respectively, collisional, convective, inertial, and rotational plasmophysical models. The magnetic well is calculated with allowance for geodesic effects. It is shown that, for strong anomalous heat transport conditions, the contribution of the magnetic well to the generalized Rutherford equation for the island width W is independent of W not only in the collisional model (which has been investigated earlier) but also in the convective and inertial models and depends very weakly (logarithmically) on W in the rotational model. It is this weak dependence that gives rise to the compound effect, which is the subject of the present study. A criterion for the stabilization of neoclassical tearing modes by the compound effect at an arbitrary level of the transverse heat transport by electrons and ions is derived and is analyzed for two cases: when the electron heat transport and ion heat transport are both strong, and when the electron heat transport is strong and the ion heat transport is weak.

Electrostatic Ion-Temperature-Gradient Modes and Anomalous Transports in a Collision-Dominated Plasma

By employing a two fluid model, a set of equations for ion pressure gradient driven low-frequency (in comparison with the ion-gyrofrequency) electrostatic waves in a collisional magnetoplasma has been derived. The equations exhibit a coupling between the electrostatic potential, the parallel ion velocity and ion-temperature perturbations. In the linear limit, a new dispersion relation has been derived and analyzed numerically to demonstrate the existence of ion-temperature-gradient driven drift-dissipative modes. Since in a highly collisional plasma there is a departure from the Boltzmann electron response, there appears anomalous particle and ion-energy transports due to nonthermal fluctuations. Taking some typical tokamak parameters, the transport coefficients are numerically depicted for maximally growing drift wave fluctuations. The results of our investigation are useful for the understanding of fluctuation driven transports in tokamak edges in which collisions are dominant.

Thermal transport in collision dominated edge plasmas

Effects of ion temperature gradients on strong ballooning drift resistive edge modes are considered. The ion temperature perturbations are found to have a significant influence on the stability properties. In particular, the ion temperature gradient leads to a finite-Larmor radius stabilization of the strongest mode with an ideal growth rate for lower heating power than that required for stabilization due to the poloidal sheared rotation.

Distributions of the intensity of ion temperature perturbations in the thermosphere

The results are presented from the study of vertical, seasonal, and latitudinal variations of ion temperature perturbations in the altitude region 100–400 km using measurements at the network of IS radars over a multiyear period. The intensities of ion temperature perturbations having periods less than 6 hours are obtained using the least squares fit filtration of tides from the data. The intensities of the perturbations are maximized in the altitude region 150–180 km at low latitudes. At high latitudes the maxima become much smaller. Seasonal and latitudinal variations of the intensity have different structure below and above 150–180 km. At altitudes below 120–130 km the intensity of the ion temperature perturbations has a maximum in January and a minimum in July at high latitudes, and two maxima (in summer and winter) at low latitudes. Above 150–180 km the intensity of ion temperature perturbations has maxima at high and low latitudes in all seasons. The high‐latitude maxima can be attributed to the variations of electric fields and particle precipitation and to the atmospheric waves propagated from below and generated in the auroral region. The reasons for the low‐latitude maxima of the intensity are not clear enough. A possible reason may be the increased generation of internal gravity waves by tides in the equatorial upper atmosphere.

ODRIC — A one-dimensional linear resistive MHD code in cylindrical geometry

The primitive resistive MHD equations in cylindrical geometry are time-advanced. Fourier analysis in the poloidal and toroidal directions and subsequent linerization are performed, resulting in a 1-D (radial) system for the perturbed quantities. Separate equations for the electron and ion temperature perturbations are solved. Hall terms and the thermal force vector are included in Ohm's law. Anisotropic thermal conductivity and viscosity are included in the model.

Linear studies of resistive interchange modes in a cylindrical reversed field pinch

Resistive interchange modes in a cylindrical reversed field pinch are studied using a one-dimensional, linear, compressible initial value code. Separate equations for the electron and ion temperature perturbations are solved. Hall terms and the thermal force vector are included in Ohm’s law. Anisotropic thermal conductivity and viscosity are included in the code model. Calculations are carried out for various values of poloidal and toroidal mode number, Lundquist number, Suydam parameter, Hall parameter, thermal conductivity, viscosity, etc., with respect to uniform density equilibria known to be stable to tearing modes. It is shown that in the cold ion limit sufficiently large Hall terms cause all modes that are tested to become stable. However for Ti=Te and ignoring the effects of viscosity and thermal conductivity, there is a critical value of the ratio of Alfvén to ion cyclotron frequency above which the ‘‘even’’ mode not only dominates the ‘‘odd’’ mode but is likely to have a growth rate significantly larger than that of the odd mode in the absence of Hall terms. Inclusion of a classical tensor thermal conductivity, while having little effect on the odd mode in the absence of Hall terms, does stabilize the even mode for sufficiently large Hall parameter. Inclusion of a classical tensor viscosity reduces the growth rate of (but does not necessarily stabilize) the odd mode. Inclusion of Hall and thermal force terms, tensor thermal conductivity and tensor viscosity causes all modes that are tested to stabilize. Results are compared to other contemporary studies.