Isotropic and anisotropic pointing models

This paper describes an alternative approach for generating pointing models for telescopes equipped with serial kinematics, esp. equatorial or alt-az mounts. Our model construction does not exploit any assumption for the underlying physical constraints of the mount, however, one can assign various effects to the respective components of the equations. In order to recover the pointing model parameters, classical linear least squares fitting procedures can be applied. This parameterization also lacks any kind of parametric singularity. We demonstrate the efficiency of this type of model on real measurements with meter-class telescopes where the results provide a root mean square accuracy of 1.5?2 arcseconds.     

Monte-Carlo Simulations of Cosmic-ray and Internal Radiation Effects on ISOCAM on Board ISO

One of the main limitations to the sensitivity of the infrared camera ISOCAM on-board the Infrared Space Observatory (ISO) comes from responsivity variations and glitches caused by the impacts of charged particles in photo-detectors. Glitch rate measurements, glitch properties and removal methods have already been addressed during the first ISO detector workshop(Madrid, 1998) and published in a special issue of Experimental Astronomy. It appeared that glitch rate and most of glitch properties could be reproduced by Monte-Carlo simulations. This is very interesting in order to predict before launch the effect of charged particles in photo-detectors operated in space. This paper presents results of Monte-Carlo simulations of radiation effects on ISOCAM detectors. Glitch rates, spatial and energetic properties of glitches have been computed and are compared with measured values. This revised version was published online in July 2006 with corrections to the Cover Date.     

Groundwater-flow modeling in the Yucatan karstic aquifer, Mexico

The current conceptual model of the unconfined karstic aquifer in the Yucatan Peninsula, Mexico, is that a fresh-water lens floats above denser saline water that penetrates more than 40 km inland. The transmissivity of the aquifer is very high so the hydraulic gradient is very low, ranging from 7–10 mm/km through most of the northern part of the peninsula. The computer modeling program AQUIFER was used to investigate the regional groundwater flow in the aquifer. The karstified zone was modeled using the assumption that it acts hydraulically similar to a granular, porous medium. As part of the calibration, the following hypotheses were tested: (1) karstic features play an important role in the groundwater-flow system; (2) a ring or belt of sinkholes in the area is a manifestation of a zone of high transmissivity that facilitates the channeling of groundwater toward the Gulf of Mexico; and (3) the geologic features in the southern part of Yucatan influence the groundwater-flow system. The model shows that the Sierrita de Ticul fault, in the southwestern part of the study area, acts as a flow barrier and head values decline toward the northeast. The modeling also shows that the regional flow-system dynamics have not been altered despite the large number of pumping wells because the volume of water pumped is small compared with the volume of recharge, and the well-developed karst system of the region has a very high hydraulic conductivity. Electronic Publication     

Temporal variability of selected dissolved elements in the lower Orinoco River,Venezuela

Monthly analyses of pH, conductance, alkalinity, total suspended solids, dissolved major cations (Na, K, Ca and Mg), and selected dissolved trace elements (Fe, Al, Mn, Cu, Zn and Cr) were conducted on waters of the lower Orinoco River from February 2004 to May 2006. The data show strong seasonal variability. Major cations had maximum concentrations at low water, and were correlated with pH, conductance and alkalinity because of the tributaries coming from the Andean zones, where the weathering of evaporites and carbonate shales controls water chemistry. K concentrations did not show any relation with evaporite weathering, probably because large amounts of K come from the Guayana Shield rivers as a consequence of feldspar weathering. The concentrations of dissolved Fe, Mn and Cu were highest during the high water stage. Concentrations of the elements K, Cr, Zn and Cu are correlated with each other but not with dissolved Fe and Al, which probably are complexed with humic and fulvic substances. Mn concentrations did not show relationships with other variables. Inter‐annual variability of dissolved elements is explained by temporal changes in precipitation. Copyright © 2008 John Wiley & Sons, Ltd.     

Landform instability and land-use dynamics in tropical high mountains, Central Mexico

This investigation is an analysis of the influence of landform instability on the distribution of land-use dynamics in a hydrographical basin, located in the Mexican Volcanic Belt mountain range (central Mexico), currently affected by substantial changes in land use and deforestation. A landform map was produced, in addition to seven attribute maps - altimetry, drainage density, slope, relief energy, potential erosion, geology and tectonics - which were considered as factors for determining landform instability through Multi-criteria Evaluation Analysis. Likewise, the direction and rhythm of land-use dynamics were analyzed in four dates - between 1976 and 2000 - and cross tabulations were made between them, in order to analyze the trends and processes of land-use dynamics. Afterwards, the databases obtained were cross tabulated with the landform variables to derive areas, percentages and correlation indices. In the study area, high-instability landforms are associated with most ancient volcanic and sedimentary landforms, where high altitude, drainage density, slope and potential to develop gravitational and fluvial processes are the major factors favouring a land-use pattern, dominated by the conservation of extensive forest land, abandonment of human land use and regeneration of disturbed areas. In contrast, low-instability landforms correspond to alluvial plains and lava hills covered by pyroclasts, where low potential erosion to develop fluvial processes, added to water and soil availability and accessibility, have favoured a land-use pattern dominated by the expansion of agroforestry plantations and human settlements, showing a marked trend towards either intensification or permanence of the current land use and with little abandonment and regeneration.     

The magnetic properties of solar surges

High resolution on- and off-band Hα filtergrams of disk solar surges obtained with the Vacuum Tower Telescope of the Sacramento Peak Observatory have been compared to magnetic data.

1. Surges constitute clusters of very fine dark (sometimes bright) filaments where each thread connects to an Ellerman bomb brightening. If the magnetic map reveals the existence of a satellite polarity as defined by Rust (1968), the bomb(s) lies over it.
2. Although a large fraction of surges is not associated with clearly detectable satellite polarities, events are strongly favored in regions of evolving magnetic features, characterized by dimensions of about 10 000 km and significant flux change over a period of less than a day. A flux change rate of 3 × 10¹⁵ Mx s^?1 has been measured along at least three homologous bomb-surge events in a satellite of region MW 18594. Surges appear to be related to rising flux of one polarity into a region of stronger opposite flux.
3. The trajectories of surges are matched by magnetic lines of force computed in the current-free approximation.

     

An Analysis of Polar Coronal Hole Evolution: Relations to Other Solar Phenomena and Heliospheric Consequences

We use observations of the green corona low-brightness regions to construct a time series of a polar coronal hole area from 1939 to 1996, covering 5 solar cycles. We then perform a power-spectral analysis of the monthly data time series. Several persistent significant periodicities appear in the spectra, which are related with those found in solar magnetic flux emergence, geomagnetic storm sudden commencements and cosmic-ray flux at Earth. Of particular importance are the peak at around 1.6–1.8 yr recently found in cosmic-ray intensity fluctuations, and the peak at around 1 yr, also identified in coronal hole magnetic flux variations. Additional interesting features are the peaks close to 5 yr, 3 yr and the possible peak at around 30 yr, that were also found in other solar and interplanetary phenomena. Our results stress the physical connection between the solar magnetic flux emergence and the interplanetary medium dynamics, in particular the importance of coronal hole evolution in the structuring of the heliosphere.     

Evidence of space weathering in regolith breccias II: Asteroidal regolith breccias

Abstract– Space weathering products, such as agglutinates and nanophase iron‐bearing rims are easily preserved through lithification in lunar regolith breccias, thus such products, if produced, should be preserved in asteroidal regolith breccias as well. A study of representative regolith breccia meteorites, Fayetteville (H4) and Kapoeta (howardite), was undertaken to search for physical evidence of space weathering on asteroids. Amorphous or npFe⁰‐bearing rims cannot be positively identified in Fayetteville, although possible glass rims were found. Extensive friction melt was discovered in the meteorite that is difficult to differentiate from weathered materials. Several melt products, including spherules and agglutinates, as well as one irradiated rim and one possible npFe⁰‐bearing rim were identified in Kapoeta. The existence of these products suggests that lunar‐like space weathering processes are, or have been, active on asteroids.     

Reionization and the Cosmic Dawn with the Square Kilometre Array

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21 cm line from the earliest phases of star and galaxy formation in the Universe. This 21 cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.     

Heat balance for the high-temperature component of a solar flare

Shortly after the occurrence of the impulsive spikes of the two-ribbon flare of May 21, 1980, a temperature analysis of the X-ray emitting flare plasma showed the presence of a low-temperature component [n = 15 × 10¹⁰ cm^#X2212;3; T = 20 × 10⁶ K] and a high-temperature component [n = 2 × 10¹⁰ cm^#X2212;3; T = 40 × 10⁶ K]. The mean free path of an electron in the hot component is comparable to the size of the source (≈ 10⁴ km). Heat losses from the hot source can therefore not be described with classical formulae. Theoretical arguments show that most likely the electron to ion temperature ratio T _e/T_i in the hot plasma is close to unity. This implies the presence of a hot ion component (T _i ≈ 40 × 10⁶ K) as well. Under these conditions (T _e ≈ T _i) heat flux limitation by electrostatic turbulence is ineffective. However, reduction of the heat flux is still possible due to the breakdown of classical theory. It is demonstrated that only non-classical current dissipation processes can sustain a hot source against cooling by a saturated heat flux. Investigation of the collisionality as a function of position along a magnetic loop shows that the breakdown of classical theory should be expected to occur first near the base of the loop. We conclude that the newly discovered hot source is important for the energy budget of the flare, even if the heat losses are considerably reduced. It is estimated that for the May 21, 1980 flare a total of about 10³¹ ergs were necessary to maintain the hot source against heat losses over the time period that it was observed (≈ 10 min).