Showing papers by "Ulrich Geppert published in 2016"

Gravitational radiation from neutron stars deformed by crustal Hall drift

Abstract: Aprecondition for the radio emission of pulsars is the existence of strong, small-scalemagnetic field structures (‘magnetic spots’) in the polar cap region. Their creation can proceed via crustal Hall drift out of two qualitatively and quantitatively different initial magnetic field configurations: a field confined completely to the crust and another which penetrates the whole star. The aim of this study is to explore whether these magnetic structures in the crust can deform the star sufficiently to make it an observable source of gravitational waves. We model the evolution of these field configurations, which can develop, within ∼104–105 yr, magnetic spotswith local surface field strengths∼1014 Gmaintained over�106 yr. Deformations caused by the magnetic forces are calculated.We show that, under favourable initial conditions, a star undergoing crustal Hall drift can have ellipticity � ∼ 10−6, even with sub-magnetar polar field strengths, after ∼105 yr. A pulsar rotating at ∼102 Hz with such � is a promising gravitational wave source candidate. Since such large deformations can be caused only by a particular magnetic field configuration that penetrates the whole star and whose maximum magnetic energy is concentrated in the outer core region, gravitational wave emission observed from radio pulsars can thus inform us about the internal field structures of young neutron stars.

Metallic Surfaces under Interplanetary Medium - Degradation Mechanisms and Protection Possibilities

Abstract: Thin metallic films are commonly used in space industry. Important applications are e.g. multilayer insulation blankets (MLI) or solar sail membranes. In a sufficiently large distance from the Earth atmosphere, the solar wind and electromagnetic radiation are the dominating factors for material degradation. The solar protons while penetrating the metals recombine to neutral hydrogen atoms and then form molecular hydrogen bubbles. Their high concentration within the metals’ lattice has a direct influence on their physical properties. Up to now, no material that was exposed to the interplanetary space conditions has been returned to Earth. Therefore, studies both theoretical and experimental carried out in the terrestrial laboratories are necessary to predict the changes in the mechanical and thermo-optical properties of the metals. The aim is to present the basics behind formation processes of molecular hydrogen bubbles as well as their influence on physical properties of exposed metals to the solar wind. The other degradation mechanisms are discussed as well, e.g. the delamination processes caused by corpuscular radiation. Ideas of protection possibilities of the metals are drawn. The experimental studies were performed by use of the Complex Irradiation Facility (CIF) of the DLR’s Institute of Space Systems in Bremen, Germany. The CIF’s linear proton accelerator allows to irradiate specimens to well defined proton fluxes. A series of experiments have been made to simulate the growth of the bubbles at different physical conditions which represent those at the interplanetary space. The post-degradation studies of the exposed metallic films have been performed by use of an electron microscope and number of image-processing methods to analyze the morphology of the specimens. It has been proven that at the early stage of the bubble growth the average bubble radius R evolves according to R~t^1/3 law. One of the consequences of the molecular hydrogen bubbles formation is a significant increase of the metallic surface roughness or changes of the samples’ thermo-optical properties. However, the bubble formation depends strictly on physical conditions at which the samples are exposed to the solar wind protons. Dependency on bubble formation mechanism due to temperature as well as proton kinetic energy is given.

Molecular Hydrogen Bubbles Formation on Thin Vacuum Deposited Aluminum Layers After Proton Irradiation

Abstract: Metals are the most common materials used in space technology. Metal structures, while used in space, are subjected to the full spectrum of the electromagnetic Radiation together with particle irradiation. Hence, they undergo degradation. Future space missions are planned to proceed in the interplanetary space, where the protons of the solar wind play a very destructive role on metallic surfaces.Unfortunately, their real degradation behavior is to a great extent unknown. Our aim is to predict materials’ behavior in such a destructive environment. Therefore both, theoretical and experimental studies are performed at the German Aerospace Center (DLR) in Bremen, Germany. Here, we report the theoretical results of those studies. We examine the process of H2-bubble formation on metallic surfaces. H2-bubbles are metal caps filled with Hydrogen molecular gas resulting from recombination processes of the metal free electrons and the solar protons. A thermodynamic model of the bubble growth is presented. Our model predicts e.g. the velocity of that growth and the reflectivity of foils populated by bubbles. Formation of bubbles irreversibly changes the surface quality of irradiated metals. Thin metallic films are especially sensitive for such degradation processes. They are used e.g. in the solar sail propulsion technology. The Efficiency of that technology depends on the thermo-optical properties of the sail materials. Therefore, bubble formation processes have to be taken into account for the planning of long-term solar sail missions.