Low fluence.

The question of the appropriate extrapolation to low dose has long been a subject of controversy. A linear no-threshold model is favored by regulatory bodies as the basis of RBE assignments and estimates of radiation hazards to the general population. This model is largely supported by extensive application of the linear-quadratic survival formula "fitted" statistically to a wide variety of experimental data obtained at doses typically exceeding 1 Gy, and then extrapolated to mGy for practical applications, and even to the prediction of hazards from single electrons. Such extrapolations are questionable at best, and may even prove hazardous for risk evaluations. Fluence and geometry rather than dose based data are proposed as a basis for a limiting "threshold" for a "low dose" extrapolation. The proposed threshold is one where the fluence of particles is one per square micron, where on average only 2/3 of the 1 micrometers2 pixels covering an irradiated area are traversed by one or more particles. The corresponding dose threshold is determined by the LET of the bombarding radiation. For relativistic electrons this dose is about 0.032 Gy.     

The model of turbulent plasma sheet during IMF Bz > 0

Theory of the plasma sheet with medium-scale developed turbulence gives the possibility to explain the main processes of plasma sheet bifurcation and theta-aurora formation during IMF B_z > 0. The model suggests that during IMF B_z > 0 small bulge structure in the plasma sheet center is formed. The polarization of the bulge due to dawnward electron motion and duskward ion motion decreases the large-scale electric field in the bulge region. The decrease of the large-scale field in the conditions of constant coefficient of diffusion leads to the bulge growth. The results of plasma sheet bifurcation and theta-aurora formation modelling are presented.     

Ground performance of air conditioning and water recycle system for a Space Plant Box.

Researchers from 5 Japanese universities have developed a plant growth facility (Space Plant Box) for seed to seed experiments under microgravity. The breadboard model of the Space Plant Box was fabricated by assembling subsystems developed for microgravity. The subsystems include air conditioning and water recycle system, air circulation system, water and nutrient delivery system, lighting system and plant monitoring system. The air conditioning and water recycle system is simply composed of a single heat exchanger, two fans and hydrophilic fibrous strings. The strings allow water movement from the cooler fin in the Cooling Box to root supporting materials in the Plant Growth Chamber driven by water potential deficit. Relative humidity in the Plant Growth Chamber can be changed over a wide range by controlling the ratio of latent heat exchange to sensible heat exchange on the cooling fin of the heat exchanger. The transpiration rate was successfully measured by circulating air inside the Plant Growth Chamber only. Most water was recycled and a small amount of water needed to be added from the outside. The simple, air conditioning and water recycle system for the Space Plant Box showed good performance through a barley (Hordeum vulgare L.) growth experiment.     

Yohkoh observations of the solar corona

The Yohkoh soft X-ray telescope obtains several images every 90 minutes. Data from the declining phase of the solar cycle have been used to compare the X-ray signal with other indicators of activity and to study coronal heating. X-ray emission from a north polar coronal hole is found broadly consistent with results of previous EUV observations. In diffuse emission regions, temperature rises to around 2.2 MK and levels off in the height range 1.5 – 1.9 R_O. Such emission underlies streamers and may be the source of the low-speed solar wind. X-ray signatures for Coronal Mass Ejection (CME) events which involve the detection of reduced X-ray intensities in the corona, have been developed with Yohkoh data. CME observations are described     

Imaging the solar corona in the EUV

The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Sun's corona in 4 EUV wavelengths. The He II filter at 304 Å images the chromosphere and the base of the transition region at a temperature of 5 − 8 × 10⁴ K; the Fe IX–X filter at 171 Å images the corona at a temperature of 1.3 × 10⁶ K; the Fe XII filter at 195 Å images the quiet corona outside coronal holes at a temperature of 1.6 × 10⁶ K; and the Fe XV filter at 284 Å images active regions with a temperature of 2.0 × 10⁶ K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Sun's corona, and in particular, magnetic field reconnection.     

UV photobiochemistry under space conditions

The response of spores of Bacillus subtilis, cells of Deinococcus radiodurans and conidia of Aspergillus ochraceus to actual and simulated space conditions (UV in combination with long-term exposure to extremely dry conditions, including vacuum) has been studied: The following effects have been analyzed: decrease of viability, occurrence of DNA double strand breaks, formation of DNA-protein cross-links and DNA-DNA cross-links. All organisms show an increased sensitivity to UV light in extreme dryness (dry argon or vacuum) compared to an irradiation in aqueous suspension. The UV irradiation leads in all cases to a variety of DNA lesions. Very conspicuous is the occurrence of double strand breaks. Most of these double strand breaks are produced by incomplete repair of other lesions, especially base damages. The increase in DNA lesions can be correlated to the loss in viability. The specific response of the chromosomal DNA to UV irradiation in extreme dryness, however, varies from species to species and depends on the state of dehydration. The formation of DNA double strand breaks and DNA-protein cross-links prevails in the case of B. subtilis spores. In cells of Deinococcus radiodurans DNA-DNA cross-links often predominate, in conidia of Aspergillus ochraceus double strand breaks. The results obtained by direct exposure to space conditions (EURECA mission and D2 mission) largely agree with the laboratory data.     

Physical properties of dust grains deduced by optical probing techniques

In-situ space observations of dust in the solar system are seldom possible. On the opposite, remote observations of solar light scattered by dust are relatively easy to perform from Earth- or satellite-based observatories; the evolution of the polarization of light scattered by dust particles as a function of the phase angle may provide information on the physical properties of these particles. Unfortunately, since remote observations are integrated along the line-of-sight of the observer, they can hardly be used to determine local physical properties. We have precisely developed Optical Probe techniques to forge the link between the numerous remote observations and the unique in-situ measurements. A short review of the remote observations of light scattered by cometary dust is first presented. Then, the Optical Probe concept is analyzed. Finally, the OPE instrument, which had been designed to optically probe the inner coma of comet Halley is described; its limitations and its achievements during Halley and Grigg-Skjellerup encounters are discussed.     

Evaluation of Cyanothece sp. ATCC 51142 as a candidate for inclusion in a CELSS.

Controlled ecological life support systems (CELSS) have been proposed to make long-duration manned space flights more cost-effective. Higher plants will presumably provide food and a breathable atmosphere for the crew. It has been suggested that imbalances between the CO2/O2 gas exchange ratios of the heterotrophic and autotrophic components of the system will inevitably lead to an unstable system, and the loss of O2 from the atmosphere. Ratio imbalances may be corrected by including a second autotroph with an appropriate CO2/O2 gas exchange ratio. Cyanothece sp. ATCC 51142 is a large unicellular N2-fixing cyanobacterium, exhibiting high growth rates under diverse physiological conditions. A rat-feeding study showed the biomass to be edible. Furthermore, it may have a CO2/O2 gas exchange ratio that theoretically can compensate for ratio imbalances. It is suggested that Cyanothece spp. could fulfill several roles in a CELSS: supplementing atmosphere recycling, generating fixed N from the air, providing a balanced protein supplement, and protecting a CELSS in case of catastrophic crop failure.     

INTERBALL statistical study of ion flow fluctuations in the plasma sheet

We use ion distribution measurements with CORALL instrument on-board the INTERBALL/Tail spacecraft to study plasma flows in the mid-tail (−9> X> −27 R_E) plasma sheet. Three velocity components computed every 2 minutes exhibit two types of velocity variations: Earthward bursty bulk flows (BBFs) and random flow fluctuations. Their properties are in a good agreement with the observations of the ISEE-2 spacecraft (Borovsky et al., 1997). The INTERBALL/Tail spacecraft configuration favors measurements of V_z component, in contrast to previous experiments in which only V_x and V_y were measured reliably. In the outer part of the plasma sheet V_y and V_z fluctuations were close to each other (variances σ(V_y) and σ(V_z) were about 160 and 110 km/s, respectively), but in the inner part at the dusk flank amplitude of V_y fluctuations increased and was 2 times higher than that of V_z component. This asymmetry of fluctuations should be taken into account during modern theoretical analysis and simulations.