97 10 25 8 v 1 2 3 O ct 1 99 7 Metal systems of the Lyman forest
An extremely careful analysis of the forest of weak metal-line systems at z ∼ 2 has allowed the separation into individual 'single-phase ionization' components with accurate parameters. The systems typically span several hundred km s −1 and within each the components show a strong coherence in their ioniza-tion properties. A general feature of the observed systems is the presence of high ionization broad components which co-exist in velocity space with more numerous, narrow, lower ionization components. From a sample of components extending to z ∼ 3.5, taken either individually or as system totals, no rapid evolution is found in the column density ratio N(Si IV)/N(C IV), contrary to previous indication of a large change near z = 3.1. Comparisons of component ion ratios including Si IV, C IV, C II and Si II from the sample at z ∼ 2 with CLOUDY derived values using several model ionizing spectra show the general dominance of the metagalactic ionizing background over local stellar radiation and give Si/C distributed 1–3 times the solar value. Indications of the evolution of structure are highlighted.
Cold Gas at High Redshift Netherlands Foundation for Research in Astronomy
We discuss the current observational and theoretical issues concerning cold gas at high redshift and present simulations showing how a number of observational issues can be resolved with planned future instrumentation.
Climatology of the cusp-related thermospheric mass density anomaly, as derived from CHAMP observations
We report on the thermospheric mass density anomaly in the vicinity of the ionospheric cusp. A systematic survey of the anomalies is presented, based on a statistical analysis of 4 years of data (2002–2005) obtained by the accelerometer onboard CHAMP. The anomalies are detected during all years and seasons in both hemispheres but with stronger signatures in the Northern Hemisphere. For the same geophysical conditions, solar flux and geomagnetic activity the anomalies in the north are larger by a factor of about 1.35. Over the course of the survey period the amplitude decreases by more than a factor of 5 while the level of solar flux reduces by a factor of 2. The anomaly strength also depends on the solar wind input. The merging electric field, E merg , is generally enhanced for about an hour before the anomaly detection. There is a quadratic response of the anomaly amplitude to E merg . For geophysical conditions of P10.7 E merg E merg , where the weight of E merg , in mV/m, is by about 50 times higher than that of the solar flux level. The solar zenith angle and the influence of particle precipitation are found to play a minor role as a controlling parameter of seasonal variation. The well-known annual variation of the thermospheric density with a minimum around June also influences the formation of the cusp anomalies. This leads to a clear hemispheric asymmetry with very weak anomalies in the south during June solstice, which is supposed to be a combined effect of the minimum in annual variation and the seasonal decrease of solar insolation in this region.