Polarization-mode separation and the emission geometry of pulsar 0823 + 26: A new pattern of pulsar emission?

We explore the detailed polarization behaviour of pulsar 0823 + 26 using the technique of constructing partial ‘mode-separated’ profiles corresponding to the primary and secondary polarization modes. The characteristics of the two polarization modes in this pulsar are particularly interesting, both because they are anything but orthogonal and because the secondary mode exhibits a structure seen neither in the primary mode nor in the total profile. The new leading and trailing features in the secondary mode, which appear to represent a conal component pair, are interpreted geometrically on the basis of their width and the associated polarization-angle traverse as an outer cone. If the secondary-mode features are, indeed, an outer cone, then questions about the significance of the pulsar’s postcursor component become more pressing. It seems that 0823 + 26 has a very nearly equatorial geometry, in that both magnetic poles and the sightline all fall close to the rotational equator of the star. We thus associate the postcursor component with emission along those bundles of field lines which are also equatorial and which continue to have a tangent in the direction of our sight line for a significant portion of the star’s rotation cycle. It seems that in all pulsars with postcursor components, this emission follows the core component, and all may thus have equatorial emission geometries. No pulsars with precursors in this sense — including the Crab pulsar — are known. The distribution of power between the primary and secondary modes is very similar at both 430 and 1400 MHz. Our analysis shows that in this pulsar considerable depolarization must be occurring on time scales that are short compared to the time resolution of our observations, which is here some 0.5–1.0 milliseconds. One of the most interesting features of the modeseparated partial profiles is a phase offset between the primary and secondary modes. The secondary-mode ‘main pulse’ arrives some 1.5 ± 0.1‡ before the primary-mode one at 430 MHz and some 1.3 +0.1 ‡ at 21 cm. Given that the polar cap has an angular diameter of 3.36‡, we consider whether this is a geometric effect or an effect of differential propagation of the two modes in the inner magnetosphere of the pulsar.     

Amazonian northern mid-latitude glaciation on Mars: A proposed climate scenario

Recent geological observations in the northern mid-latitudes of Mars show evidence for past glacial activity during the late Amazonian, similar to the integrated glacial landsystems in the Dry Valleys of Antarctica. The large accumulation of ice (many hundreds of meters) required to create the observed glacial deposits points to significant atmospheric precipitation, snow and ice accumulation, and glacial flow. In order to understand the climate scenario required for these conditions, we used the LMD (Laboratoire de Météorologie Dynamique) Mars GCM (General Circulation Model), which is able to reproduce the present-day water cycle, and to predict past deposition of ice consistent with geological observations in many cases. Prior to this analysis, however, significant mid-latitude glaciation had not been simulated by the model, run under a range of parameters.In this analysis, we studied the response of the GCM to a wider range of orbital configurations and water ice reservoirs, and show that during periods of moderate obliquity (? = 25-35°) and high dust opacity (τ_dust = 1.5-2.5), broad-scale glaciation in the northern mid-latitudes occurs if water ice deposited on the flanks of the Tharsis volcanoes at higher obliquity is available for sublimation. We find that high dust contents of the atmosphere increase its water vapor holding capacity, thereby moving the saturation region to the northern mid-latitudes. Precipitation events are then controlled by topographic forcing of stationary planetary waves and transient weather systems, producing surface ice distribution and amounts that are consistent with the geological record. Ice accumulation rates of ∼10 mm yr⁻¹ lead to the formation of a 500-1000 m thick regional ice sheet that will produce glacial flow patterns consistent with the geological observations.     

The topology and polarization of subbeams associated with the 'drifting' subpulse emission of pulsar B0943+10 – V. A new look at the low-frequency burst-mode emission

This paper reports new observations of pulsar B0943+10 carried out at the Pushchino Radio Astronomy Observatory (PRAO) at the low radio frequencies of 42, 62 and 112 MHz. B0943+10 is well known for its exquisitely regular burst-mode (B-mode) drifting subpulses as well as its weaker and chaotic quiescent mode. Earlier Arecibo investigations at 327 MHz have identified remarkable, continuous changes in its B-mode subpulse drift rate and integrated-profile shape with durations of several hours. These PRAO observations reveal that the changes in profile shape during the B-mode lifetime are strongly frequency dependent – namely the measured changes in the component amplitude ratio are more dramatic at 327 and 112 MHz as compared with those at 62 and 42 MHz. The differences, however, are most marked during the first several tens of minutes after B-mode onset; after an hour or so the profile shape changes tend to be more similar at all four frequencies. We also have found that the linear polarization of the integrated profile increases continuously throughout the lifetime of the B mode, going from hardly 10 per cent just after onset to some 40–50 per cent after several hours. Pulsar B0943+10's B mode thus provides a unique new opportunity to investigate continuous systematic changes in the plasma flow within the polar flux tube. While refraction in the pulsar's magnetosphere may well play some role, we find that the various frequency-dependent effects, both between and within the two modes, can largely be understood geometrically. If the modes and B-mode decay reflect systematic variations in the carousel-'spark' radius and emission height then a specific set of profile and linear polarization changes would be expected.     

Chemical Evolution of Intercumulus Liquid, as Recorded in Plagioclase Overgrowth Rims from the Skaergaard Intrusion

The intercumulus liquid of a crystal mush fills pore spaces,and typically solidifies to form overgrowths on cumulus grainsand poikilitic post-cumulus minerals. If the liquid is immobile,solidification produces zoned intercumulus minerals, as a resultof progressive fractionation of the residual liquid. Convectionwithin the mush results in buffering of the liquid composition,and thus limits mineral zonation. For fully solidified cumulates,‘fossil’ changes in liquid composition or porosityare difficult to identify. However, detailed study of immobileminor components of plagioclase overgrowth rims can provideinformation about the progressive solidification of intercumulusmaterial. Ti contents of plagioclase overgrowths, in samplesfrom the lowermost parts of the Skaergaard Intrusion, show strongvariations with anorthite content. With decreasing X_An, Ti concentrationsfirst rise and then fall, consistent with changing TiO₂ contentsof the intercumulus liquid during solidification. TiO₂ in plagioclasedecreases sharply at An₅₅, reflecting local saturation of Fe–Tioxides. Ti in clinopyroxene oikocrysts also falls rimward, butzoning in faster diffusing species (Fe, Mg) is limited. Otherthan slight reverse zones that may occur on the plagioclasemargins, X_An falls continuously during crystallization. Thereverse zoning is interpreted as the result of compaction-drivendissolution and reprecipitation of plagioclase. The continualdecrease in X_An is exploited, together with back-scattered electronimages of the cumulates, to produce calibrated images showingregions of progressive crystallization. This allows the regionscrystallizing at each stage of solidification to be visualized.These images show that the final remnants of interstitial meltwere present in triangular pockets and as thin grain-boundarymelt films. This approach can provide information about theprogressive reduction of porosity during cumulate solidification. KEY WORDS: residual liquid; cumulate; plagioclase; porosity; Skaergaard     

Occurrence and distribution of hydrocarbons in the surface microlayer and subsurface water from the urban coastal marine area off Marseilles, Northwestern Mediterranean Sea

Aliphatic (AHs) and polycyclic aromatic hydrocarbons (PAHs) were analyzed in dissolved and particulate material from surface microlayer (SML) and subsurface water (SSW) sampled at nearshore observation stations, sewage effluents and harbour sites from Marseilles coastal area (Northwestern Mediterranean) in 2009 and 2010. Dissolved and particulate AH concentrations ranged 0.05–0.41 and 0.04–4.3 μg l⁻¹ in the SSW, peaking up to 38 and 1366 μg l⁻¹ in the SML, respectively. Dissolved and particulate PAHs ranged 1.9–98 and 1.9–21 ng l⁻¹ in the SSW, amounting up 217 and 1597 ng l⁻¹ in the SML, respectively. In harbours, hydrocarbons were concentrated in the SML, with enrichment factors reaching 1138 for particulate AHs. Besides episodic dominance of biogenic and pyrogenic inputs, a moderate anthropisation from petrogenic sources dominated suggesting the impact of shipping traffic and surface runoffs on this urbanised area. Rainfalls increased hydrocarbon concentrations by a factor 1.9–11.5 in the dissolved phase.     

A gap in the North American Central Plains conductivity anomaly

The palaeotectonic models of the North American continent have been deeply influenced by the concept of a North American Central Plains conductivity anomaly (NACPCA) which was postulated as a result of large area, wide-spaced, array studies, using the Magnetic Deep Sounding (MDS) method.

A recent closely spaced magnetotelluric (MT) survey across this proposed anomaly near the Canadian-U.S. boundary by Jones and Savage (1986) failed to detect the NACPCA at the location suggested by the MDS. However, this survey did verify a conductivity anomaly discovered by Rankin and Kao (1979) and a new anomaly 75 km east of NACPCA. Our present study verifies this latter anomaly and quantitative results are presented. The striking isotropy in apparent resistivity does not prevent the interpretation of anomalous resistivity distributions. This is discussed in the text.

The failure of the MDS array studies to detect our anomaly is readily explainable by the wide station spacing; however, the failure of the MT survey to detect the NACPCA leads us to the conclusion that it may not be a continuous feature.