Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics

We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions

The multihabitat approach of USEPA's rapid bioassessment protocols: benthic macroinvertebrates

The multihabitat approach to sampling for bioassessment is not a new concept, but has been described in detail in the USEPA Rapid Bioassessment Protocols (Barbour et al., 1999) and the AQEM project of the European Union (Hering et al., 2004). Although there are variations on this technique, the basic approach is to sample the major aquatic habitats in proportion to each representation in the stream reach. Both fish and benthic macroinvertebrates are sampled in this manner. The primary advantage of the multihabitat approach is to sample representative stream habitats that will address habitat altered systems and provide an indication of impairment from both chemical and non-chemical stressors. This technique has been shown to collect representative samples of the stream reach and to be highly precise among and within sampling crews. Four questions are addressed in this paper: (1) What are the strengths and limitations of the method for low-gradient streams; (2) What are the performance characteristics (i.e., accuracy, precision, sensitivity) of the method; (3) What is the relative ability of the method to distinguish natural variability (i.e., temporal, spatial) from human disturbance; (4) How would the method be implemented for low-gradient streams