Jörg Büchner

Shaping of the magnetotail from the mantle - Global and local structuring

This paper discusses kinetic modeling of the properties of magnetotail formation from a plasma mantle source and develops a unified view of the structure of the central part of the magnetotail plasma sheet as well as the structure of its boundary layer. Trajectories of mantle protons in the presence of a uniform dawn-dusk electric field have been traced using the Tsyganenko magnetic field model for quiet periods of magnetospheric activity. The most important portion of the particle trajectories is each particles first interaction with the sharp reversal of the magnetic field in the tail midplane, because this interaction results in particle energization and chaotic scattering. The closer this interaction takes place to the X line, the larger a particles energy will become. The intensity of the chaotic scattering in the tail midplane depends also on the position in the tail at which it occurs. The energization and scattering result in a significant restructuring of the tail ion distributions, both in space and in velocity coordinates. Our model shows the evolution of the global structure of the tail with a clearly defined central plasma sheet and plasma sheet boundary layer developing from its beginnings as a plasma “nucleus” in the distant tail current sheet. This large-scale restructuring is accompanied by the creation of small-scale features in the particle distribution functions. For example, the model not only correctly reproduces the spatial distribution and velocity dispersion of the fast ion beams moving both earthward and tailward in the plasma sheet boundary layer, but also indicates that these beams should be highly structured spatially into 5-6 smaller beamlets with distinct velocities. In addition the model shows that complementary ring distribution structures should also exist in the central plasma sheet. Our analysis indicates that the ion distribution functions in the central plasma sheet should take a rather specific form in velocity space with loss regions oriented predominantly orthogonal to the magnetic field. Our results also emphasize the importance of counterstreaming populations, not only in the boundary layer, but also in the central part of the plasma sheet. Analytical calculations indicate that the properties of chaotic scattering in the magnetotail under realistic conditions (x dependence of the normal magnetic field and dawn-dusk electric field) are quite different from those predicted by earlier simple xindependent models. Finally the model results are compared with recent observations of ion distribution functions and their moments for various regions of the magnetotail, and quantitative estimates from the model are shown to be in good agreement with observations. Small-scale structuring and the presence of counterstreaming are also discussed, as well as their possible importance in explaining the observed intermittency in the plasma sheet bulk flows.

A nonlinear dynamical analogue model of geomagnetic activity

The solar wind-magnetosphere interaction is discussed within the framework of deterministic nonlinear dynamics. Linear prediction filter studies have shown that the magnetospheric response to energy transfer from the solar wind contains both directly driven and unloading components. These studies have also shown that the response is significantly nonlinear and, thus, the filter technique and other correlative techniques cannot give a complete description of that response. Phase space reconstruction studies have shown that the evolution of the nonlinear solar wind-magnetosphere system is dominated by only a few degrees of freedom; the system approaches a low-dimensional attractor on which its behavior can be described using a relatively simple nonlinear dynamical model. An earlier dripping faucet analogue model of the low-dimensional solar wind-magnetosphere system is briefly reviewed, and then a plasma physical counterpart to that model is constructed. A Faraday loop in the magnetotail is considered, and the relationship of electric potentials on the loop to changes in the magnetic flux threading the loop is developed. This approach leads to a model of geomagnetic activity which is similar to the earlier mechanical model but described in terms of the geometry and plasma contents of the magnetotail. The model is best characterized as an elementary time-dependent global convection model. The convection evolves within a magnetotail shape that varies in a prescribed manner in response to the dynamical evolution of the convection. The result is a nonlinear model capable of exhibiting a transition from regular to chaotic loading and unloading. The behavior of the model under steady loading and also some elementary forms of time-dependent loading is discussed. The model appears to properly account for all macrophysical aspects of magnetotail geomagnetic activity, it incorporates both the directly driven and the unloading components of geomagnetic activity, and it includes, in a fundamental way, the inherent nonlinearity of the solar wind-magnetosphere interaction.

Particle scattering and current sheet stability in the geomagnetic tail during the substorm growth phase

The degree of pitch angle scattering and chaotization of various particle populations in the geomagnetic tail during the substorm growth phase is studied by utilizing the Tsyganenko 1989 magnetic field model. A temporally evolving magnetic field model for the growth phase is constructed by enhancing the near-Earth currents and thinning the current sheet from the values given by the static Tsyganenko model. Changing the field geometry toward an increasingly taillike configuration leads to pitch angle scattering of particles whose Larmor radii become comparable to the field line radius of curvature. Several different cases representing substorms with varying levels of magnetic disturbance have been studied. In each case, the field development during the growth phase leads to considerable scattering of the thermal electrons relatively close to the Earth. The current sheet regions where the electron motion is chaotic are magnetically mapped to the ionosphere and compared with low-altitude measurements of electron precipitation. The chaotization of the thermal electron population occurs within a few minutes of the substorm onset, and the ionospheric mappings of the chaotic regions in the equatorial plane compare well with the region of brightening auroras. Even though the temporal evolution of the complex plasma system cannot be self-consistently described by the temporal evolution of the empirical field model, these models can provide the most accurate estimates of the field parameters for tail stability calculations.

X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue

Aims. Active galactic nuclei are known to have complex X-ray spectra that depend on both the properties of the accreting super-massive black hole (e.g. mass, accretion rate) and the distribution of obscuring material in its vicinity (i.e. the “torus”). Often however, simple and even unphysical models are adopted to represent the X-ray spectra of AGN, which do not capture the complexity and diversity of the observations. In the case of blank field surveys in particular, this should have an impact on e.g. the determination of the AGN luminosity function, the inferred accretion history of the Universe and also on our understanding of the relation between AGN and their host galaxies. Methods. We develop a Bayesian framework for model comparison and parameter estimation of X-ray spectra. We take into account uncertainties associated with both the Poisson nature of X-ray data and the determination of source redshift using photometric methods. We also demonstrate how Bayesian model comparison can be used to select among ten di erent physically motivated X-ray spectral models the one that provides a better representation of the observations. This methodology is applied to X-ray AGN in the 4 Ms Chandra Deep Field South. Results. For the 350 AGN in that field, our analysis identifies four components needed to represent the diversity of the observed X-ray spectra: (1) an intrinsic power law; (2) a cold obscurer which reprocesses the radiation due to photo-electric absorption, Compton scattering and Fe-K fluorescence; (3) an unabsorbed power law associated with Thomson scattering o ionised clouds; and (4) Compton reflection, most noticeable from a stronger-than-expected Fe-K line. Simpler models, such as a photo-electrically absorbed power law with a Thomson scattering component, are ruled out with decisive evidence (B > 100). We also find that ignoring the Thomson scattering component results in underestimation of the inferred column density, NH, of the obscurer. Regarding the geometry of the obscurer, there is strong evidence against both a completely closed (e.g. sphere), or entirely open (e.g. blob of material along the line of sight), toroidal geometry in favour of an intermediate case. Conclusions. Despite the use of low-count spectra, our methodology is able to draw strong inferences on the geometry of the torus. Simpler models are ruled out in favour of a geometrically extended structure with significant Compton scattering. We confirm the presence of a soft component, possibly associated with Thomson scattering o ionised clouds in the opening angle of the torus. The additional Compton reflection required by data over that predicted by toroidal geometry models, may be a sign of a density gradient in the torus or reflection o the accretion disk. Finally, we release a catalogue of AGN in the CDFS with estimated parameters such as the accretion luminosity in the 2 10 keV band and the column density, NH, of the obscurer.

Two spacecraft observations of a reconnection pulse during an auroral breakup

At 1130 UT on November 28, 1995, two spacecraft, Interball-Tail and Geotail, were in a favorable position to study the plasma sheet activity and an auroral breakup observed on the ground near the spacecraft ionospheric footpoints. Both spacecraft were near the neutral sheet, and they were nearly aligned along the magnetic meridian. During the auroral breakup observed at the equatorward half of the auroral oval (also registered as an AKR burst at Interball) both spacecraft simultaneously detected signatures of a reconnection pulse: The earthward plasma streaming and magnetic field dipolarization were observed at 12 R E at Interball, while the tailward energetic ion beam, then the tailward flow and the passage of a plasmoid were observed at 28 R E at Geotail. This pulse seem to proceed inside of the plasma sheet closed field lines, in the region of small (∼ 1nT) background magnetic field at the neutral sheet. At Interball position the onset of fast earthward ion flow, likely initiated by the reconnection pulse, was followed by other manifestations (dipolarization, enhancements of the magnetic turbulence and the energetic particle flux, the intensification of field-aligned currents). Auroral observations showed initial brightening delayed an approximately 1 min after the commencement of the reconnection pulse. The auroral intensification was not accompanied by a significant magnetic disturbance on the ground, and therefore the event can be classified as the pseudobreakup. We estimate magnetic flux transport characteristics and possible location of the onset region in the plasma sheet. We conclude that observations during this event are consistent with the initiation of an auroral breakup by some disturbance (e.g., Alfven wave) generated by the reconnection pulse that commenced in the neutral sheet at ∼15 R E distance.

Space Plasma Simulation

Particle-in-Cell Simulation of Plasmas- A Tutorial.- Parallel 3-D Electromagnetic Particle Code Using High Performance FORTRAN: Parallel TRISTAN.- Full Particle Electromagnetic Simulation of Collisionless Shocks.- Simulation of Electron Beam Instabilities and Nonlinear Potential Structures.- Kinetic Simulation of Inhomogeneous Plasma with a Variable Sized Grid System.- Low Noise Electrostatic and Electromagnetic Delta-f Particle-in-Cell Simulation of Plasmas.- Particle Simulation of Dusty Plasmas.- Hybrid Simulation Codes: Past, Present and Future-A Tutorial.- Hall Magnetohydrodynamics - A Tutorial.- Fluid Plasma Simulation of Coupled Systems: Ionosphere and Magnetosphere.- Global Magnetohydrodynamics - A Tutorial.- Adaptive Mesh Refinement for Global Magnetohydrodynamic Simulation.- Finite Volume TVD Schemes for Magnetohydrodynamics on Unstructered Grids.- Global Magnetohydrodynamic Simulation Using High Performance FORTRAN on Parallel Computers.- Numerical Schemes for the Analysis of Turbulence - A Tutorial.

Substorms: A global instability of the magnetosphere‐ionosphere system

Observational and numerical modeling evidence demonstrates that substorms are a global, coherent set of processes within the magnetosphere and ionosphere. This supports the view that substorms are a configurational instability of the coupled system since the entire magnetosphere changes during the expansion phase onset. It is shown that the magnetosphere progresses through a specific sequence of energy-loading and stress-developing states until the entire system collapses. This energy loading-unloading sequence is the essential basis of the Faraday Loop non-linear dynamics model which has been quite successful in describing the fundamental behavior of substorms without invoking detailed treatments of the internal substorm instability mechanism. Present-day MHD models also are seen to produce substorm-like global instabilities despite the fact that they do not treat realistically the extremely thin current sheets that play such an essential role in the near-tail dynamics prior to substorm onset. This paper discusses three-dimensional kinetic simulations that have recently shown a variety of initial plasma kinetic instability modes which all evolve quickly to a similar, globally unstable reconnection configuration. Continuing research concerning the substorm onset location and mechanisms addresses important questions of when and exactly how the substorm expansion develops. However, the loaded magnetosphere almost always progresses rapidly to the same basic reconnection configuration irrespective of the detailed localized initiation mechanism. This is likened to the catastrophic collapse of a sand dune that has reached a highly unstable configuration: Any small local perturbation can cause a complete and large-scale collapse irrespective of the perturbation details. It is concluded that the global magnetospheric substorm problem has now largely been solved and that future work should be directed toward understanding the detailed plasma physical processes that occur during substorms.

The quasi-adiabatic ion distribution in the central plasma sheet and its boundary layer

This paper derives for the first time, the consequences of quasi-adiabatic ion motion in the magnetospheric tail, which is in fact a chaotic scattering of the ions due to their essentially nonadiabatic behavior. This is caused by the nonlinearity of their equations of motion in the strongly curved tail magnetic field and results from separatrix traversals in the phase space. In this paper it is shown how the continuous violation of adiabaticity of the ion motion in the Earths magnetotail leads to a redistribution of ions between the central plasma sheet (CPS) and the plasma sheet boundary layer (PSBL). It is also shown that, due to the mechanism described in this paper, ions are accelerated to the PSBL even without any assumption of the existence of an electric field caused by reconnection with an χ line. For the CPS ion distribution this acceleration also results in the depletion of some domains in velocity space that can lead to substantial non-Maxwellian features.

Regular and chaotic particle motion in sheared magnetic field reversals

Abstract Space plasma current sheets, like those in the plasmasheet of the Earths magnetotail or in the magnetopause, create strongly curved magnetic fields. The adiabaticity of charged particles motion in them may be destroyed and deterministic chaos may appear. In this paper we investigate the influence of a magnetic shear, caused by a finite guide field component, on the chaotic particle motion. We derive the adiabaticity parameter in such fields and describe the routes to chaos.

Stellar wind regimes of close-in extrasolar planets

Close-in extrasolar planets of Sun-like stars are exposed to stellar wind conditions that differ considerably from those for planets in the solar system. Unfortunately, these stellar winds belong to the still unknown parameters of these planetary systems. On the other hand, they play a crucial role in a number of star-planet interaction processes that may lead to observable radiation events. In order to lay a foundation for the investigation of such interaction processes, we estimate stellar wind parameters on the basis of the solar wind model by Weber & Davis and study the implications of the stellar magnetic fields. Our results suggest that in contrast to the solar system planets, some close-in extrasolar planets may be obstacles in a sub- Alfvenic stellar wind flow. In this case, the stellar wind magnetic pressure is comparable to or even larger than the dynamic flow pressure. We discuss possible consequences of these findings for the wind-exoplanet interactions. Further, we derive upper limit estimates for the energies such stellar winds can deposit in the exoplanetary magnetospheres. We finally discuss the implications the sub-Alfvenic environment may have on the star-planet interaction.