Dynamical evolution of high velocity clouds in the intergalactic medium

HI observations of high-velocity clouds (HVCs) indicate, that they are interacting with their ambient medium. Even clouds located in the very outer Galactic halo or the intergalactic space seem to interact with their ambient medium. In this paper, we investigate the dynamical evolution of high velocity neutral gas clouds moving through a hot magnetized ambient plasma by means of two-dimensional magnetohydrodynamic plasma-neutral gas simulations. This situation is representative for the fast moving dense neutral gas cloudlets in the Magellanic Stream as well as for high velocity clouds in general. The question on the dynamical and thermal stabilization of a cold dense neutral cloud in a hot thin ambient halo plasma is numerically investigated. The simulations show the formation of a comet-like head-tail structure combined with a magnetic barrier of increased field strength which exerts a stabilizing pressure on the cloud and hinders hot plasma from diffusing into the cloud. The simulations can explain both the survival times in the intergalactic medium and the existence of head-tail high velocity clouds.Comment: 11 pages, 19 figure

Interactive comment on “Influence of aquifer heterogeneity on karst hydraulics and catchment delineation employing distributive modeling approaches”

Due to their heterogeneous nature, karst aquifers pose a major challenge for hydrogeological investigations. Important procedures like the delineation of catchment areas for springs are hindered by the unknown locations and hydraulic properties of highly conductive karstic zones. In this work numerical modeling was employed as a tool in delineating catchment areas of several springs within a karst area in southwestern Germany. For this purpose, different distributive modeling approaches were implemented in the finite element simulation software Comsol Multiphysics®. The investigation focuses on the question to which degree the effect of karstification has to be taken into account for accurately simulating the hydraulic head distribution and the observed spring discharges. The results reveal that the representation of heterogeneities has a large influence on the delineation of the catchment areas. Not only the location of highly conductive elements but also their geometries play a major role for the resulting hydraulic head distribution and thus for catchment area delineation. The size distribution of the karst conduits derived from the numerical models agrees with knowledge from karst genesis. It was thus shown that numerical modeling is a useful tool for catchment delineation in karst aquifers based on results from different field observations

The Universal Piggy Bank: Designing and Implementing a System of Savings Accounts for Children

The Universal Piggy Bank: Designing and Implementing a System of Savings Accounts for Childre

In situ acceleration in the galactic center arc

For the nonthermal radio emission of the Galactic Center Arc in situ electron acceleration is imperative. The observed radio spectrum can be modeled by a transport equation for the relativistic electrons which includes particle acceleration by electric fields, momentum diffusion via scattering by magnetohydrodynamical turbulence and energy losses by synchrotron radiation. The accelerating electric fields can be regarded as a natural consequence of multiple reconnection events, caused by the interaction between a molecular cloud and the Arc region. The radio spectrum and even the recently detected 150 GHz emission, explicitely originating from the interaction regions of a molecular cloud with the magnetized Arc, can be explained in terms of quasi-monoenergetically distributed relativistic electrons with a typical energy of about 10 GeV accelerated in stochastically distributed magnetic reconnection zones

Shear-Flow Driven Current Filamentation: Two-Dimensional Magnetohydrodynamic Simulations

The process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional Magnetohydrodynamic (MHD)-simulations. This situation is typical for astrophysical systems like jets, the interstellar and intergalactic medium where the dynamics is dominated by external forces. Two different cases are studied. In one case, the system is ideal permanently and dissipative processes are excluded. In the second case, a system with a current density dependent resistivity is considered. This resistivity is switched on self-consistently in current filaments and allows for local dissipation due to magnetic reconnection. Thus one finds tearing of current filaments and, besides, merging of filaments due to coalescence instabilities. Energy input and dissipation finally balance each other and the system reaches a state of constant magnetic energy in time.Comment: 32 Pages, 13 Figures. accepted, to appear in Physics of Plasmas (049012