Bi-dimensional observation of waves near the mesopause at auroral latitudes

During a campaign of optical observations at high latitude, a bi-dimensional study of the wave structure of the OH layer has been performed in December 1981 from Sodankyla (Finland). This site is one of the three stations of the EISCAT ionosphere sounding system. It has been found that a wave field covering an area of 1 million km² may extend to latitudes as high as 70°N. The OH wave structure shows many similarities with noctilucent clouds. The fairly large horizontal wavelength, of the order of 40 km cannot easily be explained by a wave motion at an interface. The observed wave structure seems to be a result of the propagation of an internal gravity wave in the 80–100 km region. This wave structure was often recorded during the same time as an active aurora was present. As a result, it appears that the perturbation might be correlated with particle precipitations at auroral latitudes.     

Methods of Hierarchical Control of Water Quality Taking into Account Manipulations of the Center and Countermoves of Enterprises

Methods of hierarchical control including manipulations of a Center and the countermove of enterprises are considered using as an example a dynamic two-level model of river water quality control in the presence of point pollution sources and in the absence of nonpoint sources.     

The Preboreal climate reversal and a subsequent solar‐forced climate shift

Accurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric ¹⁴C; (ii) a temporary decline of atmospheric CO₂; and (iii) an increase in the GRIP ¹⁰Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.     

Geochronological evidence for late‐Grenvillian magmatic and metamorphic events in central Taimyr,northern Siberia*

U–Th–Pb analyses of zircons from six granites and one metasediment collected in the accretionary Central belt of Taimyr, Arctic Siberia, demonstrate that Neoproterozoic (c. 900 Ma) granites intrude late Mesoproterozoic/early Neoproterozoic amphibolite facies metamorphic rocks. This is the first time in the Mamont–Shrenk region that Neoproterozoic ages have been recognized for these lithologies, previously thought to be Archaean/Palaeoproterozoic in age. The Mamont–Shrenk Terrane (MST) represents a Grenvillian age (micro?) continent intercalated with younger Neoproterozoic ophiolites during thrusting and accreted to the northern margin of the Siberian craton sometime before the late Vendian. Basement to the MST may have been derived from the Grenvillian belt of east Greenland. Viable tectonic reconstructions must allow for an active margin along northern Siberia (modern day coordinates) in the middle Neoproterozoic.     

A model for the constant-head pumping test conducted in vertically fractured media

An analytical model for the constant-head pumping test is developed for a partially penetrating well that has a finite thickness skin, and intersects a single vertical fracture. In the model, the fracture is fully confined and flow occurs only in the fracture. The model is developed using Laplace transform and finite Fourier transform methods. The model is to be used for analysing well test data from vertically fractured media and for verifying numerical models. Dimensionless curves are used to study the effects of a finite thickness skin and a partially penetrating wellbore. In the presence of a finite thickness skin, or a partially penetrating wellbore, a typical flow response for the constant-head pumping test has three distinct periods of flow corresponding to small-, intermediate- and large-time. Small- and large-time approximations are presented for the model. For tests where the wellbore is fully penetrating, or the partial penetration ratio is known, these approximations can be used to analyse field data.     

Simultaneous five color (UBVRI) polarimetry of VV Puppis

Observations of linear and circular polarization in five colour bands during a highly active state of VV Puppis in January-86 are reported. A strong linear polarization pulse with the maximum in the blue, P_B22 %, is observed at the end of the bright phase when the active pole is at the limb and a weaker secondary pulse, P_B7 %, is seen in the beginning of the bright phase, when the active pole reappears. Strong positive circular polarization is also observed in the blue and the ultraviolet, P_UP_B18 %, P_V10 % during the bright phase. The circular polarization reverses the sign in the B and V bands during the faint phase and a negative polarization hump is seen when the active pole crosses the limb. The circular polarization in the V band reaches the value P_V–10% at the hump, after which it remains near P_V–5% during the faint phase. This is probably due to radiation coming from the second, less active pole and accretion thus takes place onto both poles. The wavelength dependences of the positive and negative parts of the circular polarization curve are different and no polarization reversal is seen in the U band. The position angle of the linear polarization is well determined during a large portion of the cycle, especially in the V band, thanks to the activity from both poles. A best fit to the position angle curve, taking into account also the duration of the positive circular polarization phase interval =0.40 (in the V band), yields the values of orbital inclination i=78°±2° and the colatitude of the active magnetic pole =146°±2°. The relatively good fit to the position angle data indicates that the simple dipole model is nearly correct in the case of VV Puppis. Some wavelength dependence is, however, seen in the position angle curves, especially in the I band where the slope / at the main pulse is considerably smaller than in the other bands. The shape of the position angle curves changes also in the blue and the ultraviolet around the middle of the bright phase. This is probably due to optical thickness effects as the side of the accretion column which is toward the observer changes near this phase.Based on observations made at the European Southern Observatory, La Silla, Chile.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.On leave from University of Helsinki Observatory     

Atomic data relevant to line formation in strongly magnetized white dwarf stars

The discovery of magnetic field strengths in the range of 500 million Gauss in the objects Grw+70°8247 and PG1031+224 has given enormous impetus to investigations of magnetic white dwarf stars. We have determined intensities of Balmer transitions for B>10⁸ Gauss as a function of field and find strong oscillations of the transitions strengths, which are of importance to the quantitative analysis of the observed spectra and the interpretation of polarimetric measurements in these objects.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.