Nature Of Current Systems Research Articles

Spectral Lines for Polarization Measurements of the Coronal Magnetic Field. IV. Stokes Signals in Current‐carrying Fields

We present the first theoretical, forward calculations of the Stokes profiles of several magnetic dipole ("M1") coronal emission lines produced in current-carrying magnetic structures. An idealized coronal model of Low, Fong, and Fan is used, which describes a spherically symmetric, hydrostatic background atmosphere, isothermal at a coronal temperature of 1.6 × 106 K. Embedded is a global, axisymmetric magnetic field that is everywhere potential except at a quiescent prominence, consisting of an infinitesimally thin, equatorial current sheet whose weight is supported by the outward discrete Lorentz force in the sheet. This model contains a physically nontrivial, localized magnetic structure, although the atmospheric plasma is thermally of the simplest stratification possible. The calculated M1 coronal lines show clear and distinct signatures of the presence and magnitude of this localized magnetic structure, in both linear and circular polarizations, even though the thermal structure is almost homogeneous. The morphology of maps of linear polarization is particularly sensitive to the existence and strength of the current sheets, as field lines wrap around them according to the Biot-Savart law, and the linear polarization responds to different projections of field line directions onto local radius vectors. Of the M1 lines studied, those of Fe XIII (1074.7 nm) and Si X (1430.1 nm) are especially promising because of their relatively strong linear polarization. These forward calculations provide a basis for optimism that emission-line measurements may reveal the presence and nature of current systems in the corona, and provide motivation for developing instruments capable of routinely measuring polarized light in forbidden coronal lines.

Issues in the development of virtual acoustic environments

This paper will review the nature of virtual acoustic displays, which attempt to simulate a spatial acoustic environment in real time (usually over headphones) such that a listener can dynamically interact with virtual sound sources that have apparently fixed locations in space and perceptual qualities that approximate real sound sources. They represent a new, multidisciplinary area in the study of audition, which encompasses both basic research in human perception as well as a very applied or engineering-oriented approach to system implementation. The common goal is to develop technology that is driven by human needs and requirements while resulting in useful, working systems for a variety of application areas including architectural acoustics, advanced human-computer interfaces, navigation aids for the visually impaired, telepresence and virtual reality, and as a test bed for psychoacoustical investigations of complex signals. Achieving this goal requires understanding of the nature of an acoustic object, including both spatial characteristics and nonspatial cues like the spectral and temporal properties that determine perceptual streaming. It also requires practical consideration of the means needed to realistically generate such environmental sounds in a real-time display system. Research issues in these areas will be outlined, the nature of current systems will be reviewed, and some directions for future research and development will be discussed.

Numerical Simulation Studies for Oceanic Anomalies in the North Pacific Basin: II. Seasonally Varying Motions and Structures

Abstract Seasonally varying currents and structures in the North Pacific Ocean are simulated by a baroclinic ocean model. The model has satisfactorily reproduced the gross nature of current systems and density fields as well as their seasonal variations in the North Pacific Ocean. The simulated fluctuations of oceanic transport have been found to be closely related to the imposed meridional movement of the atmospheric system. The vertically integrated transport is strong in late winter and early spring, with a maximum in the Kuroshio region of 63 × 106 m3 s−1, and weak in summer, with a minimum of about 33 × 106 m3 s−1. All three major circulation gyres, namely, the subtropic anticyclonic gyre, the subarctic and the tropic cyclonic gyres are intense in winter and broad and weak in summer. The subarctic gyre almost disappears from the North Pacific basin in July. The simulated equatorial undercurrent and countercurrent demonstrate significant seasonal changes. Both currents are strong but shallow in fall a...