The plasma regime at comet Giacobini-Zinner

This review of the plasma regime sampled by the encounter of the International Cometary Explorer spacecraft (ICE) with the comet Giacobini-Zinner, discusses the shock, or bow wave, ion pickup, ionization mechanisms, and the cometary plasma tail.

The observations are consistent with the existence of a weak shock, which may be pulsating, but do not exclude the suggestion by Wallis and Dryer that the shock, though present around the sub-solar point, is in process of decaying to a wave on the flanks.

Pickup of cometary ions provokes, by means of several mechanisms, ion cyclotron, mirror, beam and electrostatic instabilities which cause strong turbulence in the inner coma, as indicated in the power spectra of the magnetic field in the coma and the surrounding volume. Heavy mass loading and consequent slowing down of the solar wind is observed. Acceleration of ions by a stochastic mechanism is indicated.

Ionization of cometary neutrals occurs principally by photoionization and charge exchange. Alfvens critical velocity mechanism, likely operates only in the inner coma not visited by ICE. A steep increase of nearly two orders of magnitude in electron density occurs in the tail, where electron velocity distributions show evidence of entry of electrons from the solar wind. The turbulence there is damped by the high ion density and low temperature.

In general, the vicinity of the comet is filled with plasma phenomena and a rich variety of corresponding atomic and molecular processes can be studied there. Comparison between the ICE, Giotto, and Vega observations forms a most valuable future study.     

Optimal Design Strategy of Switching Converters Employing Current in jected Control

This paper analyzes a buck/boost regulator employing current-injected control (CIC). It reveals the complex interactions between the dc loop and the current-injected loop and underlines the fundamental principle that governs the loop gain determination. Three commonly used compensation techniques are compared. The integral and lead/lag compensation are shown to be most desirable for performance optimization and stability.     

Comet Halley: Nucleus and jets (results of the VEGA mission)

The VEGA-1 and VEGA-2 spacecraft made their closest approach to Comet Halley on 6 and 9 March, respectively. In this paper those results of the onboard imaging experiment which were obtained around closest approach are discussed. The nucleus of the comet was clearly identifiable as an irregularly shaped object, with overall dimensions of (16±1)×(8±1)×(8±1) km. The nucleus rotates in the prograde sense about an axis nearly perpendicular to the orbital plane with a period of 53±2 hours. Its albedo is only $\text{0.04±}\text{0.02}\text{0.01}$ Many of the jet features observed during the second fly-by have been spatially reconstructed. Their sources form a quasi-linear structure on the surface. The dust above the surface is shown to be generally optically thin with the exception of certain specific dust jets. Brightness features on the surface are clearly seen. Correlating our data with other measurements, we conclude that the dirty snow-ball model will probably need to be revised.     

What is climate?

The present meaning of the word $\text{climate}$ evolved simultaneously with the formulation of research priorities which are regarded as necessary to explore the causes and, consequences of observed environmental changes. Climate can be understood in its widest sense as the totallity of influences to which the biosphere is exposed. Of primary interest are changes of these environmental conditions which are caused by the variability of the $\text{climate system}$. This expression is used in the sense of a physical system in which energy conversions take place, and which is forced, e.g. by the solar input, geologic events and man's activity. The climate system is composed of five major subsystems: atmosphere, hydrosphere, land surfaces, cryosphere and biosphere. These subsystems interact in a complex non-linear manner in different time scales. Major climate determining processes are outlined and the research priorities are discussed, which are regarded essential to investigate them and their interactions.     

Modeling international cooperation in human space exploration for the twenty-first century.

The policy process of international cooperation in space exploration. including optimistic and pessimistic scenarios for the twenty-first century, is modeled and examined in this study. In the optimistic scenario, international cooperation involves a balanced and interdependent distribution of capabilities between states, their respective national space agencies and communities of space scientists and space engineers. Cooperation is characterized by interstate participation in critical path components and joint research and development. In the pessimistic scenario, international cooperation is structured and dominated politically and economically by powerful states vis-a-vis weaker states. Cooperation is limited to coordination of separate nationally approved projects and augmentation of capabilities in noncritical path components. On the basis of these two scenarios, policy predictions and implications relevant to exploration missions in the twenty-first century, such as a human-tended lunar base and human missions to Mars, are presented and discussed.     

Multimodule parallel series-loaded resonant converters

The design and implementation of a multimodule parallel series-loaded resonant (SLR) converter system is presented. The SLR converter to be paralleled is operated in the n=2 discontinuous mode (DCM). Its dc analysis and dynamic modeling are made. In parallel operation, an average control technique is proposed to compensate the mismatch in current control characteristics of each parallel converter. Good dynamic and static current sharing characteristics are obtained. In addition, to obtain good output voltage regulating control performance, a design procedure is presented to find the parameters of feedback voltage controller according to the prescribed specifications     

Low-harmonic three-phase inverters with nonpulsating terminalcurrents

A basic class of three-phase inverters with nonpulsating terminal currents, namely, the four-topology four-inductor four-capacitor class is derived and studied. The expressions of duty-ratios required for low-harmonic dc-to-three-phase-ac conversion, ranges of output voltage variation, and component stresses for given specifications of inverters are derived. The characteristics of inverters include low-harmonic power conversion, low component stresses, variable-frequency variable-amplitude output voltages of values both below and above the dc input value, and the use of minimum number of inductors to achieve nonpulsating terminal currents     

CFAR detection in clutter with unknown correlation properties

We develop a constant false-alarm rate (CFAR) approach for detecting a random N-dimensional complex vector in the presence of clutter or interference modeled as a zero mean complex Gaussian vector whose correlation properties are not known to the receiver. It is assumed that estimates of the correlation properties of the clutter/interference may be obtained independently by processing the received vectors from a set of reference cells. We characterize the detection performance of this algorithm when the signal to be detected is modeled as a zero-mean complex Gaussian random vector with unknown correlation matrix. Results show that for a prescribed false alarm probability and a given signal-to-clutter ratio (to be defined in the text), the detectability of Gaussian random signals depends on the eigenvalues of the matrix R_c^-1R_s. The nonsingular matrix R_c and the matrix R_s are the correlation matrices of clutter-plus-noise and signal vectors respectively. It is shown that the “effective” fluctuation statistics of the signal to be detected is determined completely by the eigenvalues of the matrix R_c^-1R_s. For example the signal to be detected has an effective Swerling II fluctuation statistics when all eigenvalues of the above matrix are equal. Swerling I fluctuation statistics results effectively when all eigenvalues except one are equal to zero. Eigenvalue distributions between these two limiting cases correspond to fluctuation statistics that lie between Swerling I and II models     

New atmospheric temperature results from the pioneer Venus entry probes

New Venus temperature profiles, derived from engineering measurements of the Small Probe Net Flux Radiometer (SNFR) instruments generally confirm to high accuracy the vertical structure and horizontal temperature contrast results of Seiff et al. /1/.