Alexander Kakouris

STOCHASTIC DYNAMICS OF KEPLERIAN ACCRETION DISKS

In this work we study the growth of perturbations in Keplerian disks. Despite the asymptotic stability of the disk, a subset of optimal perturbations are found to exhibit large transient growth. The transient growth is due to the nonnormality of the underlying operator which governs the perturbation dynamics. It is shown that the amplifying perturbations produce positive momentum fluxes and a tendency of outward angular momentum expulsion during amplification. We calculate the statistical steady state that emerges under white forcing in space and time. The perturbation structure is found to be organized in coherent structures that invariably export angular momentum outward. The radial structure of the resulting angular momentum flux is in agreement with the predictions of the equilibrium theory of accretion disks. The effect of spatial localization and temporal band limiting of the forcing on the maintained momentum fluxes is investigated. We find that if the forcing is broadband and adequately distributed, accretion to the main body can be maintained by stochastic forcing.

A method for spherical harmonic analysis of Compton – Getting corrected 3-d energetic particle distributions

A method is described whereby a particle distribution measured by a number of telescopes (four in this application), observing the whole sky, mounted upon a spinning spacecraft, can be resolved into a set of spherical harmonics. The coefficients of the expansion are used to estimate the anisotropy of the particles and components of the anisotropy in different frames of reference (e.g., solar wind frame, RTN frame, spacecraft frame, etc.) for specific energy channels. For the transformation of the distribution function between frames of reference moving each other, the respective Compton–Getting correction is performed by a new geometrical approach. The respective energy change is also evaluated.

On steady shell formation in stellar atmospheres - II. Energy balance in a non-polytropic stellar envelope

The energy balance of the analytical solutions of Kakouris & Moussas ([CITE]) for a steady state of an externally heated/cooled 2-D circumstellar envelope is investigated. It is found that the required heating/cooling rates are physically realistic and can be related to specific microscopic mechanisms. We find that in the subsonic region of the wind the fluid is mechanically heated. In the supersonic stellar envelope the fluid is cooled at a rate which is consistent with radiative cooling to space. The energy balance of steady shell or blob formation in the envelopes of luminous early or late type supergiants is also investigated (Kakouris & Moussas [CITE]). We find that radiative cooling occurs in the intermediate deceleration region of the three-zone envelope. Indicative of the local thermodynamic processes is the effective polytropic index α which takes values close to the star between 1 and 4, becoming

Low Energy Energetic Particle’s Streaming as Measured by Ulysses EPAC Experiment from 1991–2000

\simeq

A Thermo-Radiatively Driven, Analytical 2-D Model for Stellar Outflows

2 at larger distances. The heated subsonic region close to the stellar surface is isothermal and becomes adiabatic at the sonic transition. We find that the polytropic index α is less than unity in the vicinity of the dense blob, indicating that the region may be dominated by convection.

Angular Momentum Transport in Keplerian Accretion Discs

The KEP/EPAC instrument, on board Ulysses spacecraft, measures directional intensities of interplanetary energetic protons and ions. We analyze Compton‐Getting corrected data received from the beginning of the year 1991 to the end of 2000 and we also perform statistical analysis for the observed angle between first‐order anisotropy of particle distribution and interplanetary magnetic field. The results show that the energetic particle’s anisotropy is mostly in the direction perpendicular to the magnetic field, indicating a diffusion across the magnetic field lines. A strong correlation is found between the perpendicular to the magnetic field streaming of protons and ions, in the energy range of MeV, and the solar wind velocity. This correlation is simply interpreted in terms cross‐field particle diffussion in the presence of an ambient plasma density gradient.

Shear dynamics of Keplerian accretion discs

Analytical, steady-state, 2-D solutions for thermally driven stellar winds were found by Kakouris & Moussas (1996). These solutions were extended to include the differential rotation of the fluid and the influence of the radiative force in an optically thin stellar atmosphere (Kakouris & Moussas 1997). The solutions use a new thermo-radiative mechanism in order to describe self-consistently outflows from early-type supergiants, for which the radiative force is important. The solutions can either describe a monotonically accelerated outflow or the existence of a deceleration region in the stellar envelope. The deceleration can lead to a density increase (blob or shell formation), depending upon the radiative force parameters (Kakouris & Moussas 1998). Fig. 1 illustrates the logarithmic density contours for our model of the wind of the hypergiant P Cygni, which was computed by using the optically thin radiative force given by Chen & Marlborough (1994) and the acceleration in the envelope determined by Lamers (1986). The results resemble the three-zone envelope of Nugis et al. (1979).