Negative diffusion in planetary rings with a nearby moon

This paper analyzes a process that has been observed in simulations of numerous systems where ring material is strongly perturbed by a nearby moon. If the ring particles can be imparted with a forced eccentricity on the order of 10⁻⁵ in a single pass by the moon, particle orbits are observed to move towards regions of higher density as a result of the organized collisions that occur in the dense peaks of the satellite wake. The width of the ring can decrease by as much as 90% if the forced eccentricity is greater than 3 × 10⁻⁵ and the unperturbed geometric optical depth is greater than 0.03. The fractional change in ring width is relatively insensitive to the particle size so long as the particle radius is much less than the product of the semimajor axis and the forced eccentricity. Including a power law particle size distribution with slope of −2.8 spanning a decade in particle radius reduces the fractional width change by about 10% compared to the uniform particle-size case. Adding gravitational interactions between ring particles only has a significant effect on ring confinement if the unperturbed geometric optical depth exceeds .03, but a 40% reduction in ring width is still achieved in a self-gravitating ring of geometric optical depth 0.3 if the forced eccentricity exceeds 3 × 10⁻⁵. This process does not require the material to be in resonance with the moon, nor does it have any minimum mass constraints because particle self-gravity is not required. The collisional damping of satellite wakes therefore provides a simple mechanism by which a single moon can reduce the radial extent of any ringlet that is close to it and has sufficient optical depth for collisions to be significant.     

Response to “Comment on “Slow-mode shock candidate in the Jovian magnetosheath” by Bebesi et al.”

Hubert and Samsonov addressed our paper published in early 2010 (Bebesi et al., 2010) about a plasma event detected in the magnetosheath of Jupiter by the plasma instruments of the Cassini spacecraft. We proposed that the characteristics of the plasma parameter variations were consistent with a slow mode shock (SMS). As our title indicated, we claimed only that the event was a possible “candidate” for an SMS according to our data, which had some restrictions as discussed in the paper. As to the origin, we proposed the following: since there was a crossing of the then highly tilted Heliospheric Current Sheet in less than two days before the event, it might have played a role in initiating the shock front.We highly appreciate the opinion of the authors, but they do not point out any hard fact that would exclude the possibility of the scenario we suggested.     

Back analysis of landslide susceptibility zonation mapping for the 2005 Kashmir earthquake: an assessment of the reliability of susceptibility zoning maps

The October 2005 earthquake triggered several thousand landslides in the Lesser Himalaya of Kashmir in northern Pakistan and India. Analyses of ASTER satellite imagery from 2001 were compared with a study undertaken in 2005; the results show the extent and nature of pre- and co-/post-seismic landsliding. Within a designated study area of ~2,250 km², the number of landslides increased from 369 in 2001 to 2,252 in October 2005. Assuming a balanced baseline landsliding frequency over the 4 years, most of the new landslides were likely triggered by the 2005 earthquake and its aftershocks. These landslides mainly happened in specific geologic formations, along faults, rivers and roads, and in shrubland/grassland and agricultural land. Preliminary results from repeat photographs from 2005 and 2006 after the snowmelt season reveal that much of the ongoing landsliding occurred along rivers and roads, and the extensive earthquake-induced fissuring. Although the susceptibility zoning success rate for 2001 was low, many of the co-/post-seismic landsliding in 2005 occurred in areas that had been defined as being potentially dangerous on the 2001 map. While most of the area in 2001 was (very) highly susceptible to future landsliding, most of the area in 2005 was only moderate to low susceptible, that is, most of the landsliding in 2005 actually occurred in the potentially dangerous areas on the 2001 map. This study supports the view that although susceptibility zoning maps represent a powerful tool in natural hazard management, caution is needed when developing and using such maps.     

An acceleration scheme for the simulation of long term climate evolution

An acceleration scheme is proposed in a coupled ocean-atmosphere model for the simulation of long term climate evolutions forced by climate forcing of longer than millennia time scales. In this coordinated acceleration scheme, both the surface forcing and the deep ocean are accelerated simultaneously by the same factor. The acceleration scheme is evaluated in a 3-dimensional ocean general circulation model and in a coupled ocean-atmosphere model of intermediate complexity. For millennial climate evolution, our acceleration scheme produces reasonably good simulations with an acceleration factor of about 5. For climate evolution of even longer time scales, the acceleration factor can be increased further.     

Regional- and district-level drivers of timber harvesting in European Russia after the collapse of the Soviet Union

After the collapse of the Soviet Union, the forestry sector in Russia underwent substantial changes: the state forestry sector was decentralized, the timber industry was privatized, and timber use rights were allocated through short- and long-term leases. To date, there has been no quantitative assessment of the drivers of timber harvesting in European Russia following these changes. In this paper we estimate an econometric model of timber harvesting using remote sensing estimations of forest disturbance from 1990–2000 to 2000–2005 as our dependent variable. We aggregate forest disturbance to administrative districts – equivalent to counties in the United States – and test the impact of several biophysical and economic factors on timber harvesting. Additionally, we examine the impact that regions – equivalent to states in the United States and the main level of decentralized governance in Russia – have on timber harvesting by estimating the influence of regional-level effects on forest disturbance in our econometric model. Russian regions diverged considerably in political and economic conditions after the collapse of the Soviet Union, and the question is if these variations impacted timber harvesting after controlling for district-level biophysical and economic drivers. We find that the most important drivers of timber harvesting at the district level are road density, the percent of evergreen forest, and the total area of forest. The influence of these variables on timber harvesting changed over time and there was more harvesting closer to urban areas in 2000–2005. Even though district-level variables explain more than 70 percent of the variation in forest disturbance in our econometric model, we find that regional-level effects remain statistically significant. While we cannot identify the exact mechanism through which regional-level effects impact timber harvesting, our results suggest that sub-national differences can have a large and statistically significant impact on land-use outcomes and should be considered in policy design and evaluation.     

Geology, petrology and isotope geochemistry of massif-type anorthosites from southwest Madagascar

Four massif-type anorthosite bodies 25–100 km² in area occur within high-pressure granulite facies supracrustal gneisses in southwestern Madagascar. Two of these bodies (Ankafotia and Saririaky) appear to have been pulled apart by 40 km in a ductile shear zone, but structural features such as sub-vertical stretching lineations indicate an origin by intense west-directed flattening and pure shear. Country rocks (Graphite Series) include abundant graphite schist (some with >60% graphite), marble, quartzite, and minor amphibolite and leucogneiss. Comagmatic granitoids (e.g. charnockites) are conspicuously absent. The anorthosite bodies are dominated by coarse grained anorthosites and leuconorites (feldspars typically 3–5 cm, up to 1 m); minor norites and oxide-rich ferrogabbros occur near the margins, but ultramafic rocks are absent. Typical mineralogy of the anorthositic rocks is: plagioclase (An_41–54) + orthopyroxene (En_38–66) ± augite (Mg♯ = 32–68) ± ilmenite ± magnetite ± apatite. High-alumina (to 6.1 wt% Al₂O₃) orthopyroxene megacrysts are widespread; most have exsolutions of calcic plagioclase (An_72–85) but some contain garnet lamellae. Metamorphism has produced abundant recrystallization and sporadic coronitic garnet (Mg ♯=12–36) + clinopyroxene assemblages. Rb-Sr isotopic analyses of whole-rocks and minerals reveal no meaningful age relationships. The age of late Neoproterozoic metamorphism is best constrained at 559 ± 50 Ma by a 6-point Sm-Nd mineral isochron (whole rock, plag, pyx, ilm, apat, gar) from a Saririaky oxide-rich gabbro. The igneous crystallization age of the anorthosites is estimated at 660 ± 60 Ma by a 19-point combined whole-rock and mineral Sm-Nd isochron for samples from both the Ankafotia and Saririaky bodies. Initial isotopic ratios calculated at 0.66 Ga among 13 whole rocks are: _Nd=+2.6 to +5.2 (mean=+3.7) and I_Sr=0.70328–0.70407 (mean=0.70347), indicating derivation of the Malagasy anorthosites from a depleted mantle source, and little, if any, contamination with Archean crustal material. One anorthosite sample with _Nd=−1.4 and I_Sr=0.70344 (calculated at 0.66 Ga) probably reflects the effects of assimilation of Early to Middle Proterozoic crustal basement, but typical surrounding graphite schist (_Nd=+0.3, I_Sr=0.70636, both at 0.66 Ga; T_DM= 1131 Ma) represents only a minor potential contaminant for the anorthosite bodies. T_DM model ages of the Malagasy anorthosites (797–1280 Ma; mean of 14 samples=949 Ma), as those of most other massif-type anorthosites, are older than the true crystallization age, because of crustal contamination effects. Our isotopic data, together with recent U-Pb data from the anorthosites and surrounding country rocks, are consistent with emplacement of the Malagasy anorthosite bodies at or before the start of a protracted, high-grade metamorphic event or series of events between about 630 and 550 Ma. This period coincides with the collision between, and amalgamation of, East and West Gondwana. Received: 19 December 1997 / Accepted: 12 June 1998