The effects of viewing angle on the inference of magnetic shear in preflare active regions

The magnetic shear at a point within an active region field configuration can be defined (Hagyard et al., 1984b) as the difference in angle between the observed photospheric transverse field and that of a reference potential field calculated using the observed line-of-sight field as a boundary condition. Using analytic models for non-potential (but force-free) fields representative of preflaring active regions, we calculate the degree of magnetic shear along the magnetic neutral line that such fields would exhibit, as a function of the location and orientation of the active region on the solar disk. We find that, except for regions close to disk center, the position of the inferred neutral line (zero line-of-sight field) is significantly different from the actual neutral line (zero radial field), and that the calculated reference potential field also varies significantly with the position of the region. Thus the inferred degree of shear can vary significantly with the position and orientation of the region, due to (a) straightforward geometric projection effects, (b) the shift of the inferred neutral line relative to its true position, and (c) variations in the reference potential field. The significance of these results for flare prediction is considered.Presidential Young Investigator.     

Regional Advection Perturbations in an Irrigated Desert (RAPID) experiment

Summary The RAPID field experiment took place in August–September 1999 at a site 25km south of Twin Falls, Idaho, USA. The experiment concerned micrometeorological observations over extensive, well-irrigated fields covered with the fast-growing crop alfalfa. During daytime, on a number of days the sensible heat flux was negative and the latent heat flux exceeded net radiation. The energy required for the latent heat flux to be larger than net radiation has to be advected from elsewhere. As the fields were large, we refer to this process as regional advection. Local advection, on the other hand, refers to advection effects, where the wet to dry transition is on a field scale. Evidence is presented that the RAPID data are subject to regional advection conditions.A simple model, based on Penman-Monteith, is derived that describes the regional-advection case rather well. The influence of wind speed under those conditions is illustrated using data and the model. The correlation coefficients between temperature and horizontal wind component appear to be good indicators for advection.     

Importance of the vadose zone in analyses of unconfined aquifer tests

Analytical models commonly used to interpret unconfined aquifer tests have been based on upper-boundary (water table) conditions that do not adequately address effects of time-varying drainage from the vadose zone. As a result, measured and simulated drawdown data may not agree and hydraulic parameters may be inaccurately estimated. A 72-hour aquifer test conducted in Cape Cod, Massachusetts, in a slightly heterogeneous, coarse-grained, glacial outwash deposit was found to be a good candidate for testing models with different upper-boundary conditions. In general, under the commonly invoked assumption of instantaneous drainage, measured and simulated drawdowns were found to agree with one another only at late time and early time. In the intermediate-time range, because of delayed drainage, measured drawdowns always exceeded simulated values, most noticeably in piezometers located near the water table. To reduce these discrepancies, an analytical model was developed that can fully account for time-varying drainage given that the aquifer is not strongly heterogeneous. The approach is flexible as the model, which makes use of empirical relations, does not constrain drainage to follow any particular functional relation. By this approach, measured and simulated drawdowns agree over the complete time range, and the estimated parameters are consistent with prior studies and with what is known about the aquifer geometry, stratigraphy, and composition. By properly accounting for vadose zone drainage, it was found that realistic estimates of all hydraulic parameters, including specific yield, could be obtained with or without the use of late-time data.     

Hepatic ethoxyresorufin O-deethylase (EROD) activity in flounder (Platichthys flesus) from contaminant impacted estuaries of the United Kingdom: continued monitoring 1999-2001

The determination of hepatic ethoxyresorufin O-deethylase (EROD) has been used to assess the induction of the mixed function oxygenase system (MFO) of flounder (Platichthys flesus) in UK estuaries. Induction of the MFO system denotes possible exposure to certain organic contaminants (e.g. polycyclic aromatic hydrocarbons, polychlorinated biphenyls) and its measure has been incorporated in national monitoring programmes. This study presents EROD monitoring data from 5 UK estuaries taken between 1999 and 2001 and builds on data from previous years. The results reveal that for all sampled estuaries EROD values have been significantly (p < 0.05) elevated on the majority of occasions in comparison with the reference estuary, the Alde in Suffolk, UK. However, the limited temporal scale of the reported monitoring does not allow any conclusions to be drawn with respect to trends in the data. Possible factors influencing the data (size, gender, seasonality, reproductive status, etc.) are discussed and recommendations for continued monitoring are made.     

Potential field extrapolation using three components of a solar vector magnetogram with a finite field of view

The potential magnetic field from a finite planar boundary is extrapolated into the upper hemisphere using information from all three magnetic field components. The method determines, first, the transverse field associated with the observed normal magnetic intensity. Then by subtraction, the method determines the associated transverse magnetic field observed in the interior (i.e., in the field of view) of the magnetogram which is due to the normal flux exterior to the field of view of the magnetogram. Inverting this information gives an approximation to the exterior normal flux. The combination of the observed normal flux of the interior and the approximation of the exterior normal flux is employed to calculate the potential field. The formulation of the problem results in an ill-posed integral inversion problem in which a regularized solution is obtained using the singular value decomposition (SVD) technique in conjunction with an appropriate Tikhonov-Phillips filter. The technique can be applied to correcting potential field calculations which are influenced by out-of-view fluxes, e.g., for a high spatial resolution vector magnetogram with a small field of view in which there is no supporting exterior data. The problem studied is also important in providing a regularized solution of the Cauchy potential problem. The method provides a much larger range of convergence than the method of Gary and Musielak (1992), and, in fact, is stable in the total upper hemisphere.The U.S. Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.     

Vivianite auto-oxidation

Vivianite, Fe₃(PO₄)₂×8?H₂O, (010) surfaces cleaved in an N₂ gas atmosphere are examined using X-ray photoelectron spectroscopy (XPS). Quantitative evaluation of Fe(2p ₃₂) and O(1s) spectra show cleaved surfaces are partly oxidized. Ferric hydroxide is identified as an oxidation product. An auto-reduction-oxidation mechanism involving rupture of hydrogen bonds between the H₂O ligands which hold together the sheet structure of vivianite is proposed.     

Simulation of steady-state flow in three-dimensional fracture networks using the boundary-element method

An efficient method for simulating steady-state flow in three-dimensional fracture networks is formulated with the use of the boundary-element method. The host rock is considered to be impervious, and the fractures can be of any orientation and areal extent. The fractures are treated as surfaces where fluid movement is essentially two-dimensional. Fracture intersections are regarded as one-dimensional fluid conduits. Hence, the three-dimensional geometric characteristics of the fracture geometry is retained in solutions of coupled sets of one- and two-dimentional equations. Use of the boundary-element method to evaluate the fluid responses in the fractures precludes the need to internally discretize the areal extent of the fractures.