Search

The scientific return from an extreme-ultraviolet spectrometer depends on the accuracy and precision of its radiometric calibration. For the Coronal Diagnostic Spectrometer on SOHO, radiometric calibration started pre-launch in the laboratory and continued after launch by making comparison measurements of the same area of the Sun with calibrated sounding rocket payloads and also by intercalibration with the SUMER instrument on SOHO. The present work uses the measurement of line ratios to monitor and validate the calibration over the first six years of observation. As well as using branching ratios and line ratios independent of the electron temperature and density, line ratios dependent on electron temperature or density have also been used successfully to validate and monitor the calibration. The results indicate that, within the uncertainties, the radiometric calibration has been validated and maintained over the first six years of observations apart from three specific wavelengths, 338.98 Å, 315.0 Å, and 311.8 Å. Problems with lines at 608.4 Å, 303.4 Å (seen in second order), 335.4 Å, and 360.7 Å are attributed to difficulties with the burn-in correction.

We present a study of the differential emission measure (DEM) of a “quiet Sun” area observed in the extreme ultraviolet at normal incidence by the Coronal Diagnostic Spectrometer (CDS) on the SOHO spacecraft. The data used for this work were taken using the NISAT_S observing sequence. This takes the full wavelength ranges from both the NIS channels (308–381 Å and 513–633 Å) with the 2 arcsec by 240 arcsec slit, which is the narrowest slit available, yielding the best spectral resolution. In this work we contrast the DEM from subregions of 2 $\times$80 arcsec2with that obtained from the mean spectrum of the whole raster (20 $\times$240 arcsec2). We find that the DEM maintains essentially the same shape in the subregions, differing by a constant factor between 0.5 and 2 from the mean DEM, except in areas were the electron density is below $2 \times 10^7$cm-3and downflow velocities of 50 km s-1are found in the transition region. Such areas are likely to contain plasma departing from ionisation equilibrium, violating the basic assumptions underlying the DEM method. The comparison between lines of Li-like and Be-like ions may provide further evidence of departure from ionisation equilibrium. We find also that line intensities tend to be lower where velocities of the order of 30 km s-1or higher are measured in transition region lines. The DEM analysis is also exploited to improve the line identification performed by [CITE] and to investigate possible elemental abundance variations from region to region. We find that the plasma has composition close to photospheric in all the subregions examined.

We present results from an analysis of a small two-ribbon flare which occurred above emerging flux in solar active region NOAA 8218 on 1998, May 13th and which was observed by the Swedish Vacuum Solar Telescope (SVST) on the island of La Palma, Spain. The relatively simple magnetic morphology and small size of the flare together with the high quality of the SVST observations allow us to examine the essential properties of flares in emerging flux regions in greater detail than before. 
In this paper we compare and contrast the flaring emerging flux region simultaneously with a non-flaring emerging flux region within the same field of view. Unusual magnetic footpoint motions are observed in the flaring region, coincident with the Hαkernels, which result in a high level of shearing of the magnetic neutral line between opposite polarities. The Hαimages show dark filament structures which form an inverted S-like shape immediately prior to the flare and then separate after the energy release disrupts the magnetic field. We interpret the motions and structures as strong evidence for the emergence of a twisted magnetic flux rope which developed a sheared configuration with the overlying magnetic field. In contrast the companion region shows separating footpoints, with apparent arch-like filament connections in the Hαimages, consistent with the expected appearance of emerging flux. The observations imply that the attachment of the inverted S-shaped structure may be an observational consequence of the magnetic reconnection or untwisting of the field which triggered the flare. We also find some evidence that the increase in magnetic flux is faster in the flaring region.
Finally, we propose a simple schematic model of the emergence of a twisted magnetic flux rope and attached branch which can account for the observed footpoint motions and Hαstructures of the flaring region. Such a model can, in principle, induce partial magnetic reconnection in the overlying coronal field and we found some evidence of coronal loop footpoint brightenings which support our conclusions. Our high resolution study supports the results of previous authors that even a small twisted structure in an emerging flux tube can be important for flare production.

The differential emission measure (DEM) of a solar active region is derived from SERTS-89 rocket data between 170 and 450 Å (Thomas & Neupert [CITE]). The integral inversion to infer the DEM distribution from spectral line intensities is performed by the data adaptive smoothingapproach (Thompson [CITE], [CITE]). Our analysis takes into account the density dependence of both ionisation fractions and excitation coefficients according to the collisional-radiative theoryas implemented in ADAS, the Atomic Data and Analysis Structure (McWhirter & Summers [CITE]; Summers [CITE]; Summers [CITE]). Our strategy aims at checking, using observational data, the validity and limitations of the DEM method used for analysing solar EUV spectra. We investigate what information it is possible to extract, within defined limitations, and how the method can assist in a number of cases, e.g. abundance determination, spectral line identification, intensity predictions, and validation of atomic cross-sections. Using the above data and theory, it is shown that a spurious multiple peak in the DEM distribution between $\log(T_{\rm e})=6.1$and 6.7, where Teis the electron temperature, may derive from an inaccurate treatment of the population densities of the excited levels and ionisation fractions or from using an integral inversion technique with arbitrary smoothing. Therefore, complex DEM structures, like those proposed for solar and stellar coronae by several authors, must be considered with caution. We address also the issue of systematic differences between iso-electronic sequences and show that these cannot be unambiguously detected in the coronal lines observed by SERTS. Our results indicate that a substantial improvement is required in the atomic modelling of the complex element Fe. The elemental abundance ratio Si/Ne is found to be close to its photospheric value. The same result may be true for the Fe/Ne abundance, but this latter result is uncertain because of the problems found with Fe.

The paper presents an integrated view of the population structure and its role in establishing the ionization state of light elements in dynamic, finite density, laboratory and astrophysical plasmas. There are four main issues, the generalized collisional-radiative picture for metastables in dynamic plasmas with Maxwellian free electrons and its particularizing to light elements, the methods of bundling and projection for manipulating the population equations, the systematic production/use of state selective fundamental collision data in the metastable resolved picture to all levels for collisonal-radiative modelling and the delivery of appropriate derived coefficients for experiment analysis. The ions of carbon, oxygen and neon are used in illustration. The practical implementation of the methods described here is part of the ADAS Project.

Powdery mildew is believed to be the most important disease attacking barley over a large part of the world. Recent experiments with fungicides have indicated that losses due to the disease arc generally greater than had been suspected. The discovery of a systemic fungicide, which can be applied to the crop as a seed-dressing, a granule or as a spray, promises greatly improved control. The adoption of cultural measures and the use of resistant varieties offer an integrated approach.

IN 1933, Samuel and Garrett1published their observation that ascospores are violently ejected from the perithecia of the fungus Ophiobolus graminis (Sacc.) Sacc. and thus become air-borne; it had previously been supposed that these spores were merely washed down into the soil by rain. Samuel and Garrett, therefore, suggested that the association between outbreaks of the take-all disease of wheat in its whiteheads form and seasons of good spring rainfall in South Australia might be connected with the occurrence of suitable conditions for liberation and dispersal of ascospores, and for subsequent infection of wheat roots by them. Later, however, Garrett2reported that he had been unable to secure infection of wheat seedling roots by inoculation with ascospores when the seedlings were growing in unsterilized soil or sand. Infection was readily obtained, however, by inoculation of seedlings growing in completely sterile soil or sand. Garrett explained his results by postulating that the food reserves of an individual ascospore were insufficient for establishment of a progressive root infection, unless supplemented by the microbial nutrients liberated by soil sterilization, or by those provided by root excretions, which would be wholly available to the germinating ascospores in sterile, but not in unsterile, sand.