Solar flares: The impulsive phase

Only during the previous solar cycle have systematic observations begun to be made with the sensitivity and time resolution, and the continuous coverage required to catch the impulsive phase and measure the rapid variations present in many wavelength ranges. Observations in X-rays, gamma-rays, UV, H, and radio wavelengths all reveal rapid variations during the impulsive phase and have contributed to our understanding of the different phenomena involved. Results have been obtained from several spacecraft, from rocket and balloon flights, and from ground-based observations. These are reviewed in the context of a simple single loop flare model with a view to showing what results are consistent with this model and what the major problems are in our understanding of the impulsive phase. New instrumentation planned for observations during the present Cycle 22 will provide a concerted attack on the impulsive phase as part of the Max '91 program.     

Sediment Organic Carbon in Todos Santos Bay, Baja California, Mexico

Sediment grain size and organic carbon (OC) data collected over the past 50 years, together with δ¹³C values of OC in recently collected samples, were analyzed to improve understanding of sediment OC distribution and abundance in Todos Santos Bay. Sediments in the submarine canyon at the mouth of the bay and in quiet-water locations along the shore are fine grained, high in OC, and have generally low δ¹³C values; sediments in high-energy environments are low in OC and have high δ¹³C values. A bivariate isotopic mixing model indicates that none of the sediments contain >50% terrigenous OC (average ~30%), and that the terrigenous OC content of the sediments is a small proportion of the OC content of local soils. Sediment OC composition is apparently controlled by energy-related sorting and deposition, oxidation of much of the original terrigenous OC, and replacement of some terrigenous OC by marine OC.     

The relationship between zooplankton,conductivity and lake-water ionic composition in 111 lakes from the Interior Plateau of British Columbia,Canada

Zooplankton collected from vertical net tows were related to the environmental variables from 98 lakes from the Interior Plateau of British Columbia. Canonical correspondence analysis showed that both salinity and ionic composition (pH and Mg) of the lake-water made major and significant contributions to the first two ordination axes (=0.42 and 0.11 respectively,P<0.05). BothArtemia franciscana andMoina hutchinsoni had their highest relative abundance in meso-hypersaline waters. However,Artemia franciscana preferred waters that were higher in Mg and Ca, whileMoina hutchinsoni was found in waters that were lower in Mg and Ca. Similarly, at intermediate salinities,Daphnia pulex and the calanoid copepods preferred waters slightly lower in Mg and Ca, whereasCeriodaphnia laticaudata andSimocephalus spp. were relatively more common in waters higher in Mg and Ca. Because the freshest lakes studied varied much less in ionic composition, the zooplankton in these lakes did not show a preference to ionic composition. As expected, multi-generic groups, such as the calanoid copepods, cyclopoid copepods and nauplii, had wider tolerances to conductivity than groups identified to lower taxonomic levels. Significant weighted-averaging regression and calibration models of conductivity were developed based on zooplankton species composition from the study lakes (r ²=0.56,P<0.05). Samples composed largely of multi-generic taxa yielded the worst estimates of salinity in the reconstruction model. This study suggests that zooplankton community composition may be developed into a useful proxy for paleosalinity reconstruction.     

Monitoring changes of clouds

An analysis of the spatial and temporal scales of cloud variability and their coupling provided by the results from existing cloud observing systems allows us to reach the following conclusions about the necessary attributes of a cloud monitoring system. (1) Complete global coverage with uniform density is necessary to obtain an unbiased estimate of cloud change and an estimate of the reliability with which that change can be determined. (2) A spatial sampling interval of less than 50 km is required so that cloud cover distributions will generally be homogeneous, or statistically homogeneous, within a sample. (3) A sampling frequency of at least six times a day ensures not only that the diurnal and semi-diurnal cycles are not aliased into long term mean values, but also that changes in them can be monitored. (4) Since estimated climate changes are only evident on a decadal time-scale, unless cloud monitoring is continuous with a record length greater than 10 years and has very high precision ( 1%) instrument calibration with overlapping observations between each pair of instruments, it will not be possible either to detect or to diagnose the effects of cloud changes on the climate.     

The solubility of [4H]Si defects in α-quartz and their role in the formation of molecular water and related weakening on heating

The nature of the solubility of water as [4H]_Si defects in quartz, and their role in providing a source of molecular water on heating, is investigated. Existing ab inito energy calculations on the incorporation of water in quartz are used to show that energetically 4H for Si substitution is likely to constitute the most prevalent mode of water uptake on the atomic scale in quartz under equilibrium conditions, and that the planar defects previously observed by a number of different authors by electron microscopy in wet quartz are likely to be planar rafts of aggregated [4H]_Si defects which are formed on supersaturation. These new conclusions call into question the previous identification of the planar defects as high pressure water clusters and require that their role in the production of molecular water in the context of recent theories of hydrolytic weakening be re-assessed. Accordingly the existing ab initio results have been used to establish the characteristics of the phase diagram for the system quartz-water in the temperature and pressure range of interest in hydrolytic weakening. Additional electron-optical experiments on wet quartz show that, on annealing at temperature in the electron microscope, similar planar defects develop in wet quartz by a diffusion process. In the context of existing theories of hydrolytic weakening it is now proposed that the conversion of [4H]_Si defects to molecular water, where this is dictated by the equilibrium phase diagram, leads to a relatively large increase in volume and to the appearance of the bubbles of free water and the nucleation of associated prismatic dislocation loops of Burgers vector b=1/3 a $\langle 11\bar 20\rangle $ as previously observed. Ultimately the development of these loops leads to dislocation-induced plasticity.     

Geochronology and geochemistry of eclogites from the Mariánské Lázně Complex,Czech Republic: Implications for Variscan orogenesis

The Mariánské Lázn complex (MLC) is located in the Bohemian Massif along the north-western margin of the Teplá-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Teplá-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated _Nd values and initial ⁸⁷Sr/⁸⁶Sr ratios. Five of the eight eclogite samples have high _Nd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower _Nd values (+ 5.4 to –0.8) and widely variable ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7096). Such low _Nd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Viséan) thrusting of the Teplá terrane over Saxothuringian rocks and Late Viséan extension.     

Analysis of COMPTEL gamma-ray burst locations and spectra

In its first three years of operation, the COMPTEL instrument on theCompton Gamma-Ray Observatory has measured the locations (mean accuracy 1°) and spectra (0.75-30 MeV) of 18 gamma-ray bursts and continues to observe new events at a rate of 1/month. With good angular resolution and sensitivity at MeV energies, the growing COMPTEL burst catalog is an important new piece of evidence in the on-going GRB mystery. The COMPTEL burst locations are consistent with an isotropic distribution of sources, yet the spatial coincidence of two of the bursts indicates the possibility of repetition. The COMPTEL burst spectra are in most cases consistent with a single power law model with spectral index in the range 2–3. However, two bursts show evidence of a spectral break in the MeV range. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby (within the Galaxy) or the gamma-ray emission is relativistically beamed. We present an overview of analysis results obtained from the COMPTEL burst catalog concentrating on the search for burst repetition and the implications of highly variable MeV emission.     

Radar observations of asteroid 1580 Betulia

Radar observations of the asteroid 1580 Betulia, made at a wavelength of 12.6 cm, show a mean radar cross section of 2.2 ± 0.8 km² and a total spectral bandwidth of 26.5 ± 1.5 Hz. Combining our bandwidth measurements with the optically determined rotation period sets a lower limit to the asteroid's radius of 2.9 ± 0.2 km.     

Spatial and temporal evolution of soft and hard X-ray emission in a solar flare

We study the spatial and temporal characteristics of the 3.5 to 30.0 keV emission in a solar flare on April 10, 1980. The data were obtained by the Hard X-ray Imaging Spectrometer aboard the Solar Maximum Mission Satellite. It is complemented in our analysis with data from other instruments on the same spacecraft, in particular that of the Hard X-ray Burst Spectrometer.Key results of our investigation are: (a) Continuous energy release is needed to substain the increase of the emission through the rising phase of the flare, before and after the impulsive phase in hard X-rays. The energy release is characterized by the production of hot (5 × 10⁷ T 1.5 × 10⁸ K) thermal regions within the flare loop structures. (b) The observational parameters characterizing the impulsive burst show that it is most likely associated with non-thermal processes (particle acceleration). (c) The continuous energy release is associated with strong chromospheric evaporation, as evidenced in the spectral line behavior determined from the Bent Crystal Spectrometer data. Both processes seem to stop just before flare maximum, and the subsequent evolution is most likely governed by the radiative cooling of the flare plasma.