Meteor head echo radar data: Mass-velocity selection effects

High-power, large-aperture (HPLA) radars detect the plasma that forms in the vicinity of a meteoroid and moves approximately at its velocity; reflections from these plasmas are called head echoes. For over a decade, HPLA radars have been detecting head echoes with peak velocity distributions >50 km/s. These results have created some controversy within the field of meteor physics because previous data, including spacecraft impact cratering studies, optical and specular meteor data, indicate that the peak of the velocity distribution to a set limiting mass should be <20 km/s [Love, S.G., Brownlee, D.E., 1993. Science 262, 550-553]. Thus the question of whether HPLA radars are preferentially detecting high-velocity meteors arises. In this paper we attempt to address this question by examining both modeled and measured head echo data using the ALTAIR radar, collected during the Leonid 1998 and 1999 showers. These data comprise meteors originating primarily from the North Apex sporadic meteor source. First, we use our scattering theory to convert measured radar-cross-section (RCS) to electron line density and mass, as well as to convert modeled electron line density and mass to RCS. We subsequently compare the dependence between mass, velocity, mean-free-path, RCS and line density using both the measured and modeled data by performing a multiple, linear regression fit. We find a strong correlation between derived mass and velocity and show that line density is approximately proportional to mass times velocity^3.1. Next, we determine the cumulative mass index using subsets of our data and use this mass index, along with the results of our regression fit, to weight the velocity distribution. Our results show that while there does indeed exist a bias in the measured head echo velocity distribution, it is smaller than those calculated using traditional specular trail data due to the different scattering mechanism, and also includes a bias against the low-mass, very high-velocity meteoroids.     

Evolution of the dark matter distribution with three-dimensional weak lensing

We present a direct detection of the growth of large-scale structure, using weak gravitational lensing and photometric redshift data from the COMBO-17 survey. We use deep R -band imaging of two  0.5 × 0.5 deg²  fields, affording shear estimates for over 52 000 galaxies; we combine these with photometric redshift estimates from our 17-band survey, in order to obtain a 3D shear field. We find theoretical models for evolving matter power spectra and correlation functions, and fit the corresponding shear correlation functions to the data as a function of redshift. We detect the evolution of the power at the 4.7σ level given reasonable priors, and measure the rate of evolution for  0 < z < 1  . We also fit correlation functions to our 3D data as a function of cosmological parameters σ₈ and  Ω_Λ  . We find joint constraints on  Ω_Λ  and σ₈, demonstrating an improvement in accuracy by ≃40 per cent over that available from 2D weak lensing for the same area.     

Geochemical weathering at the bed of Haut Glacier d'Arolla,Switzerland—a new model

Waters were sampled from 17 boreholes at Haut Glacier d'Arolla during the 1993 and 1994 ablation seasons. Three types of concentrated subglacial water were identified, based on the relative proportions of Ca²⁺, HCO₃^? and SO₄^2? to Si. Type A waters are the most solute rich and have the lowest relative proportion of Si. They are believed to form in hydrologically inefficient areas of a distributed drainage system. Most solute is obtained from coupled sulphide oxidation and carbonate dissolution (SO–CD). It is possible that there is a subglacial source of O₂, perhaps from gas bubbles released during regelation, because the high SO₄^2? levels found (up to 1200 µeq/L) are greater than could be achieved if sulphides are oxidized by oxygen in saturated water at 0 °C (c.414 µeq/L). A more likely alternative is that sulphide is oxidized by Fe³⁺ in anoxic environments. If this is the case, exchange reactions involving Fe^III and Fe^II from silicates are possible. These have the potential to generate relatively high concentrations of HCO₃^? with respect to SO₄^2?. Formation of secondary weathering products, such as clays, may explain the low Si concentrations of Type A waters. Type B waters were the most frequently sampled subglacial water. They are believed to be representative of waters flowing in more efficient parts of a distributed drainage system. Residence time and reaction kinetics help determine the solute composition of these waters. The initial water–rock reactions are carbonate and silicate hydrolysis, and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites. Hydrolysis is followed by SO–CD. The SO₄^2? concentrations usually are <414 µeq/L, although some range up to 580 µeq/L, which suggests that elements of the distributed drainage system may become anoxic. Type C waters were the most dilute, yet they were very turbid. Their chemical composition is characterized by low SO₄^2? : HCO₃^? ratios and high pH. Type C waters were usually artefacts of the borehole chemical weathering environment. True Type C waters are believed to flow through sulphide‐poor basal debris, particularly in the channel marginal zone. The composition of bulk runoff was most similar to diluted Type B waters at high discharge, and was similar to a mixture of Type B and C waters at lower discharge. These observations suggest that some supraglacial meltwaters input to the bed are stored temporarily in the channel marginal zone during rising discharge and are released during declining flow. Little of the subglacial chemical weathering we infer is associated with the sequestration of atmospheric CO₂. The progression of reactions is from carbonate and silicate hydrolysis, through sulphide oxidation by first oxygen and then Fe^III, which drives further carbonate and silicate weathering. A crude estimate of the ratio of carbonate to silicate weathering following hydrolysis is 4 : 1. We speculate that microbial oxidation of organic carbon also may occur. Both sulphide oxidation and microbial oxidation of organic carbon are likely to drive the bed towards suboxic conditions. Hence, we believe that subglacial chemical weathering does not sequester significant quantities of atmospheric CO₂ and that one of the key controls on the rate and magnitude of solute acquisition is microbial activity, which catalyses the reduction of Fe^III and the oxidation of FeS₂. Copyright © 2002 John Wiley & Sons, Ltd.     

The effect of atomic polarization on early estimates of prominence magnetic field strengths

In this paper we present our results of the application of the magnetograph formula to synthetic Stokes V profiles in prominences. We investigate both the Zeeman and atomic-polarization signatures within the V profile and question why previous attempts to determine magnetic field strength, on average, were correct, even if the magnetograph formula does not apply in general.The National Center for Atmospheric Research is sponsored by the National Science FoundationSponsored by the Summer Undergraduate Program of the High Altitude Observatory     

Economic commodity or environmental crisis? An interdisciplinary approach to analysing the bushmeat trade in central and west Africa

Bushmeat is a large but largely invisible contributor to the economies of west and central African countries. Yet the trade is currently unsustainable. Hunting is reducing wildlife populations, driving more vulnerable species to local and regional extinction, and threatening biodiversity. This paper uses a commodity chain approach to explore the bushmeat trade and to demonstrate why an interdisciplinary approach is required if the trade is to be sustainable in the future.     

A large cloud outburst at Titan's south pole

Images of Titan acquired over five nights in October 2004 using the adaptive optics system at the Keck Observatory show dramatic increases in tropospheric cloud activity at the south pole compared with all other images of Titan clouds to date. During this time, Titan's south polar clouds brightened to more than 18 times their typical values. The Cassini Ta flyby of Titan occurred as this storm was rapidly dissipating. We find that the brightness of this cloud outburst is consistent with the dramatic transient brightening of Titan observed in atmospheric windows on two nights in 1995 by Griffith et al. [Griffith, C.A., Owen, T., Miller, G.A., Geballe, T., 1998. Nature 395 (6702) 575-578] if we scale the brightness of the cloud by projecting it onto the equator. While apparently infrequent, the fact that large cloud events have been observed in different seasons of Titan's year indicates that these large storms might be a year-round phenomenon on Titan. We propose possible mechanisms to explain these occasional short-term increases in Titan's cloud activity.     

Layering and double-diffusion style convection in Europa's ocean

We assess the effects of fluid composition and depth-dependent hydrostatic pressure on dynamics in a europan ocean primarily composed of aqueous Na₂SO₄ and MgSO₄, for both a saturated and a dilute ocean. We observe a salinity- and pressure-dependent check on buoyancy in putative upwellings, which could act as a mechanism for storing heat in the ocean's base. Uptake of salt to a warmed parcel of water from the seafloor environment causes upwellings to lose buoyancy before reaching the base of the overlying ice, implying a tendency toward ocean stratification. We make an analogy to double-diffusive convecting systems observed in the Red Sea and elsewhere in Earth's waters. Using currently understood parameterizations for onset and stability, we estimate a range of temperatures and salinities for which double-diffusive convection may occur in Europa's ocean. In the Red Sea, boundary layers separating convecting zones have been observed to move upward as the lower layer acquires heat and salt. We examine the possibility of stratification and double-diffusive convection as mechanisms for heat storage in Europa's ocean, and possible correlation with recently inferred changes in ice surface alteration style over the last 30-80 Myr.     

HIGH SPATIAL AND TEMPORAL RESOLUTION OPTICAL SEARCH FOR EVIDENCE OF METEOROID FRAGMENTATION

A digital image intensified CCD camera with an electronically gated image intensifier was used to produce very short duration images of meteors. The observational system employed a 0.40 m F/4.5 Newtonian telescope to obtain high spatial resolution. A second intensified CCD camera was used to yield height information using parallax. At a typical meteor height one pixel (for the vertically oriented system) corresponded to about 1.1 m. A sampling of 59 mainly sporadic meteors was analyzed. There is clear variability from meteor to meteor, with many meteors (nearly 50%) showing only a small amount of wake, while some meteors (approximately 20%) have the off segments completely filled in.