Particle-in-cell simulation of the plasma environment of a spacecraft in the solar wind

The main interactions between the plasma and the spacecraft are the wake effects, the emission of a dense photoelectron cloud and the electric charging of the surface of the spacecraft. An electrostatic particle-in-cell computer simulation model is presented, that allows the simultaneous calculation of these related effects. For different plasma properties, two-dimensional simulations yielded the steady state self-consistent potential distributions around the probe. These potentials, especially the potential barriers produced by the photoelectron cloud, have great influence on the measurements of the low energy solar wind electrons. The essential features can be verified by a comparison with selected electron distributions measured onboard the HELIOS spacecraft.     

Combined injury syndrome in space-related radiation environments.

The risk of combined injury (CI) to space travelers is a function of exposure to anomalously large surges of a broad spectrum of particulate and photon radiations, conventional trauma (T), and effects of weightlessness including decreased intravascular fluid volume, and myocardial deconditioning. CI may occur even at relatively low doses of radiation which can synergistically enhance morbidity and mortality from T. Without effective countermeasures, prolonged residence in space is expected to predispose most individuals to bone fractures as a result of calcium loss in the microgravity environment. Immune dysfunction may occur from residence in space independent of radiation exposure. Thus, wound healing would be compromised if infection were to occur. Survival of the space traveler with CI would be significantly compromised if there were delays in wound closure or in the application of simple supportive medical or surgical therapies. Particulate radiation has the potential for causing greater gastrointestinal injury than photon radiation, but bone healing should not be compromised at the expected doses of either type of radiation in space.     

An experimental S-band digital beamforming antenna

This paper describes a 12 channel S-band digital beamforming antenna aimed at radar applications. It has a 2.8-3.3 GHz agile band and the signal is sampled at IF. The design of antenna elements, calibration network, receiver modules and signal processing ASIC are discussed. Calibration procedures and experimental results, showing very good performance, are also presented. The purpose of this work is to evaluate design parameters, requirements and limitations     

Chromospheric and coronal heating mechanisms II

We review the mechanisms which are thought to provide steady heating of chromospheres and coronae. It appears now fairly well established that nonmagnetic chromospheric regions of latetype stars are heated by shock dissipation of acoustic waves which are generated in the stellar surface convection zones. In the case of late-type giants there is additional heating by shocks from pulsational waves. For slowly rotating stars, which have weak or no magnetic fields, these two are the dominant chromospheric heating mechanisms.Except for F-stars, the chromospheric heating of rapidly rotating late-type stars is dominated by magnetic heating either through MHD wave dissipation (AC mechanisms) or through magnetic field dissipation (DC mechanisms). The MHD wave and magnetic field energy comes from fluid motions in the stellar convection zones. Waves are also generated by reconnective events at chromospheric and coronal heights. The high-frequency part of the motion spectrum leads to AC heating, the low frequency part to DC heating. The coronae are almost exclusively heated by magnetic mechanisms. It is not possible to say at the moment whether AC or DC mechanisms are dominant, although presently the DC mechanisms (e.g., nanoflares) appear to be the more important. Only a more detailed study of the formation of and the dissipation in small-scale structures can answer this question.The X-ray emission in early-type stars shows the presence of coronal structures which are very different from those in late-type stars. This emission apparently arises in the hot post-shock regions of gas blobs which are accelerated in the stellar wind by the intense radiation field of these stars.     

Space technology in developing nations: an assessment

Space technology can make an immense contribution to solving the problems of the world as a whole, and the developing nations in particular. This report summarizes the use of satellites by developing countries in the fields of communications, remote sensing, disaster management and space science. India and China, in particular, have built up impressive capabilities in several of these areas, but the great majority of developing countries still do not have access to this technology. The author addresses a number of proposals to the international community for bringing the satellite revolution to the developing world.     

A VLF transmitter on the space shuttle

The feasibility of VLF wave injection using space-borne transmitters is discussed and the radiating antenna characteristics are reported.     

Models of Venus neutral upper atmosphere: Structure and composition

Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed. Tables are provided for density, temperature, composition (CO₂, O, CO, He, N, N₂, and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day- and nightsides.Estimates are made of other species, including O₂ and D. Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, O, C, and He) as well as the hot components (H, N, C, O) on the day- and nightsides.     

Analysis of solar spike events by means of symbolic dynamics methods

We have searched for interrelations of spikes emitted simultaneously at different frequencies during the impulsive phase of flare events (Fig.1). As the spikes are related to the flare energy release and are interpreted as emissions that originate at different sites having different magnetic field strengths, any relation in frequency is interpretated as a relation in space. Quantities of symbolic dynamics, such as mutual information, Shannon information and algorithmic complexity are appropriate to characterize such spatiotemporal patterns, whereas the popular estimate of fractal dimensions can be applied to low-dimensional systems only.The goal is to decide between two possible types of fragmentation depending on the energy release and emission processes, which we callglobal andlocal organization. In the global organization the whole region becomes supercritical, and the energy is released in independent, small regions. The alternative local scenario requires a trigger that spreads from initial localized events and ignites nearby regions.Mutual information which is a generalization of correlation indicates a relation in frequency beyond the bandwidth of individual spikes. The scans in the spectrograms with large mutual information also show a low level of Shannon information and algorithmic complexity, indicating that the simultaneous appearance of spikes at other frequencies is not a completely stochastic phenomenon (white noise). It may be caused by a nonlinear deterministic system or by a Markov process. By means of mutual information we find a memory over frequency intervals up to 60 MHz (Fig. 2). Shannon information and algorithmic complexity, however, describe spike events as a whole, i.e. a global source region. A global organization is also apparent in quasi-periodic changes of the Shannon information and algorithmic complexity in the range of 2–8 seconds (Fig. 3).This findings is compatible with a scenario of local organization in which the information of one spike event spreads spatially and hence triggers further spike events at different places. The region is not an ensemble of independently flashing sources, each representing a system that cascades in energy after an initial trigger. On the contrary, there is a causal connection between the sources at any time.The analysis of four spike events suggests that the simultaneous appearance of spikes is not stochastically independent but a process in which spikes at nearby locations are simultaneously triggered by a common exciter.We have shown in the case in the case of spikes that quantities from nonlinear dynamics used in this paper are helpful in detecting structural properties of complex spatio-temporal patterns. This approach seems to be promising also for several other astrophysical applications.     

The interpretations of ultraviolet observations of comets

Summary Ultraviolet observations of comets from above the Earth's atmosphere have provided excellent new results and improved older ground based observations (OH) by an order of magnitude. Satellites are especially suitable because long integration times and observations during extended time intervals are possible.The existing cometary L observations have confirmed the relatively high overall gas production rates on the order of 10³⁰ molecule s^–1. The results strongly support the concept of an icy conglomerate solid cometary nucleus. Additional observations of hydroxyl and oxygen favor water to be one of the most abundant molecules in comets. The observations are in agreement with the predominent role of water in the evaporation process of the nuclear ices but are not proof in themselves.Water did not outnumber other consitutents by orders of magnitude in comets Bennett and Kohoutek. At least in these comets, carbon-containing molecules were possibly as numerous as water. Determination of the carbon scale length is necessary for a more quantitative statement.A hydrogen velocity of 7–9 km s^–1 was observed in comet Bennett as well as in comet Kohoutek for a variety of heliocentric distances and varying production rates. Determinations of the outflow velocity from L isophotes agree with line profile observations of L and H. Hydroxyl may constitute the main source for the hydrogen atoms with v _H - 8 km s^–1. The decay process, however, leading to this particular velocity is not yet known. Possibly a large portion of the OH radicals do not decay into hydrogen atoms or at least not into slow ones. If the high velocity component of 20 km s^–1 or more comprises a larger amount (up to 50%), most of the quoted hydrogen production rates must be revised upward.The intrinsic cometary brightness is only a very crude indicator of a comet's actual gas production rate as shown by comparison of comets Bennett and TSK. Comets can be successfully used as (extra ecliptic) space probes to measure interplanetary quantities, e.g., the curvature of the extended hydrogen clouds can be used for the absolute determination of the solar emission independent of instrumental calibration. Generally time dependent hydrogen density models must be used for the interpretation. The strength of the ultraviolet L emission favors its measurement as a standard procedure for the observation of comets (possibly together with OH (3090 Å)). These observations provide the most accurate results on the total cometary gas production rate and its variation with heliocentric distance.Dedicated to Professor L. Biermann in recognition of his inspiring guidance.On leave of absence from Max-Planck-Institut für Physik und Astrophysik, Munich.     

Synchronization of QAM Signals in the Presence of ISI

A data-aided carrier reconstruction method is described for quadrature amplitude modulation (QAM) data communications over narrow-band transmission channels. Previous analyses of this problem have assumed an "infinite" bandwidth of the communication link, so that negligible intersymbol interference (ISI) resulted. On the contrary, when ISI takes place, a different structure must be used which operates on sampled data. The performance of this structure is expressed analytically as a function of 1) the statistics of the message symbols, 2) the characteristics of the channel, and 3) the parameters of the tracking system. Excellent agreement has been found between the theory and the results of a computer simulation.