R. Pellat

Convection‐driven reconnection and the stability of the near‐Earth plasma sheet

Driven convection in the near-Earth plasma sheet is investigated using 3-D, full particle simulations. The inductive response to an externally imposed, uniform convection electric field leads to the effective tailward propagation of magnetic flux, the erosion of the equatorial magnetic field, the development of an embedded thin current sheet, and the eventual formation of a neutral line at the inward edge of the plasma sheet. In addition to creating a magnetic island, the neutral line permits the growth of a kink-like mode with a characteristic scale of 1-2 R E in the east-west (y) direction. The combined plasma flows from reconnection and the kink mode are in the range of 200-400 km/s and exhibit a reversal between earthward and tailward flow as a function of y. These modifications of the plasma sheet resemble the substorm breakup changes which are observed in the current disruption region.

MAGNETIC FLOODS: A SCENARIO FOR THE VARIABILITY OF THE MICROQUASAR GRS 1915+105

We present a scenario for the variability of the microquasar GRS 1915+105. This starts from previous works, leading to the tentative identification of the Accretion-Ejection Instability as the source of the low-frequency Quasi-Periodic Oscillation of microquasars and other accreting sources. We follow the physics of this instability: its conditions (the magnetic field and geometry adapted to MHD jet models), its instability criterion, and its consequences (cooling down of the disk, heating and excitation of the corona). Comparing them to the observed properties of the source, in particular the detailed properties of its spectral states, we first derive a model for the

The kinetic response of a stochastic plasma to low frequency perturbations

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Instability of the Lembege-Pellat equilibrium under ideal magnetohydrodynamics

30 minutes cycles often exhibited by GRS 1915+105. In our model this is a limit cycle determined by the advection of poloidal magnetic flux to the inner region of the disk, and its destruction by reconnection (leading to relativistic ejections) with the magnetic flux trapped in the vicinity of the central source. We show how it leads to natural explanations for observed behaviors of GRS 1915+105, including the three basic states of Belloni et al (2000). We then discuss how this could be extrapolated further to understand the longer-term variability of this and other microquasars.

The stability of a stochastic plasma with respect to low frequency perturbations

Following suggestion that substorm breakup might be caused by an interchange or ballooning instability, several MHD and gyro-kinetic stability analysis have been performed for plasma sheet magnetic field geometries. However, the stochastic ion dynamics in the highly stressed, thin high-s near earth plasma sheet violate the locality requirements of MHD and the invarience of the magnetic moment, μ, required by gyro-kinetic theories. In this paper we develop a new linear Vlasov kinetic theory (for low frequency modes ω <ωb, ωb being the bounce frequency) which includes the dynamics of stochastic ions.

Swinging spiral waves and Alfvén turbulence in accretion disks

The magnetotail‐like equilibrium model of Lembege and Pellat [Phys. Fluids, 25, 1995 (1982)] has been used in a number of stability calculations over the past 13 years. Most of these computations examine collisionless tearing—a mode of tail instability first suggested by Coppi et al. [Phys. Rev. Lett. 16, 1207 (1966)]—or other kinetic modes and involve both analytical and computer analysis. In this Communication it is shown, analytically, that the equilibrium model of Lembege and Pellat is unstable according to ideal magnetohydrodynamics (MHD). More precisely, for the ideal MHD stability theory of Bernstein et al. [Proc. R. Soc. London Ser. A 244, 17 (1958)] it is proven that Λ=p′+Γp (v″/v′)<0.

Multipole stability revisited

Following the suggestion that substorm breakup might be caused by an interchange or ballooning instability, several magnetohydrodynamic (MHD) and gyrokinetic stability analysis have been performed for plasma sheet magnetic field geometries. However, the stochastic ion dynamics in the highly stressed, thin high‐β near earth plasma sheet violate the locality requirements of MHD and the invariance of the magnetic moment, μ, required by gyrokinetic theories. In this paper, a proper quadratic form energy principle (for low frequency modes ω<ωb, ωb being the bounce frequency) which includes the dynamics of stochastic ions is developed, and it is demonstrated that the stochastic plasma is less stable than the corresponding adiabatic plasma.

Statistical characteristics of bursty bulk flow events

We study the stability of Alfven waves in magnetized accretion disks. Balbus & Hawley have recently found that axisymmetric waves are unstable in high-β disks. It is shown here that nonaxisymmetric (spiral) waves are subject to a similar but physically different instability, present already at low β. Transient wave packets are amplified at the same time as they are sheared by differential rotation in the disk. This instability appears as a very good candidate to explain turbulent accretion, and also to drive outflows in the magnetosphere of the disk

Characteristics of ion flow in the quiet state of the inner plasma sheet

Multipole stability with respect to quasielectrostatic low frequency modes (frequencies smaller than the average ion bounce frequency) is revisited. Aside from localized modes found previously by M. N. Rosenbluth [Phys. Fluids 11, 869 (1968)], it is shown that convective type modes that are equipotentials along field lines are likely to be more unstable, when the average curvature drift is positive (v‘<0) and smaller than the local curvature drift.