Narrow Dust Jets in a Diffuse Gas Coma: A Natural Product of Small Active Regions on Comets

Comets often display narrow dust jets but more diffuse gas comae when their eccentric orbits bring them into the inner solar system and sunlight sublimates the ice on the nucleus. Comets are also understood to have one or more active areas covering only a fraction of the total surface active with sublimating volatile ices. Calculations of the gas and dust distribution from a small active area on a comet's nucleus show that as the gas moves out radially into the vacuum of space it expands tangentially, filling much of the hemisphere centered on the active region. The dust dragged by the gas remains more concentrated over the active area. This explains some puzzling appearances of comets having collimated dust jets but more diffuse gaseous atmospheres. Our test case is 67P/Churyumov-Gerasimenko, the Rosetta mission target comet, whose activity is dominated by a single area covering only 4% of its surface.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98552/1/0004-637X_749_1_29.pd

Modeling of Phase Separation in Uranium Mononitride

Semiempirical modeling of uranium mononitride decomposition was carried out using the laws of the chemical kinetics of heterogeneous reactions. All calculations based on results received from thermal stability tests of UN at high temperatures. Kinetic curves of UN decomposition products correspond to the self-accelerating decomposition, which follows the induction period. The experimental data of uranium nitride mass loss in the investigated range of parameters are well described obtained results. The modeling can be used to estimate the phase composition of uranium mononitride during high-temperature tests in an atmosphere that does not contain nitrogen

Стандартные образцы поглощенной дозы: расширение динамического диапазона и улучшение точности измерений

Establishment and control of metrological characteristics of measurements of absorbed ionizing radiation doses in the range of 0.01 and 200 kGy by reference materials is an urgent task due to their wide application in various industries. The most convenient means of metrological support for transferring a unit of absorbed dose rate of intense photon, electron, and beta radiation to measuring instruments in radiation technologies are reference materials with established metrological traceability to the International System of Units (SI). In the present study, a method for expanding the dynamic range of measuring the absorbed dose of high-intensity ionizing radiation by radiochromic film dosimetry systems was considered and tested. The accuracy (uncertainty) of dose measurements was estimated depending on the initial optical density of the radiation-sensitive layer of the radiochromic composition. The possibility of expanding the dose characteristics and improving the metrological characteristics of the existing reference materials of absorbed dose (in water) for use as secondary standards (Measures) of the absorbed dose of ionizing radiation reproducing and (or) storing one or more points of the selected measurement scale of the absorbed dose with increased accuracy (uncertainty) of the measured values of the absorbed dose (in water) in an extended dynamic range was shown.Установление и контроль метрологических характеристик измерений поглощенных доз ионизирующих излучений в диапазоне 0,01 и 200 кГр стандартными образцами – актуальная задача в силу их широкого применения в различных отраслях промышленности. Наиболее удобным средством метрологического обеспечения при проведении передачи единицы мощности поглощенной дозы интенсивного фотонного, электронного и бета-излучений в радиационных технологиях к средствам измерений являются стандартные образцы с установленной метрологической прослеживаемостью до международной системы единиц SI.В настоящем исследовании рассмотрен и апробирован способ расширения динамического диапазона измерения поглощенной дозы высокоинтенсивного ионизирующего излучения радиохромными пленочными дозиметрическими системами. Проведена оценка точности (неопределенности) измерений дозы в зависимости от начальной оптической плотности радиационно-чувствительного слоя радиохромной композиции. Показана возможность расширения дозных и улучшения метрологических характеристик существующих стандартных образцов поглощенной дозы (по воде) для применения в качестве вторичных эталонов (Мер) поглощенной дозы ионизирующего излучения, воспроизводящих и (или) хранящих одну или несколько точек выбранной шкалы измерений поглощенной дозы с повышенной точностью (неопределенностью) измеряемых значений поглощенной дозы (по воде) в расширенном динамическом диапазоне

Erratum: "Narrow Dust Jets in a Diffuse Gas Coma: A Natural Product of Small Active Regions on Comets" (2012, ApJ, 749, 29)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98577/1/0004-637X_758_2_144.pd

Hall Effect in the coma of 67P/Churyumov-Gerasimenko

Magnetohydrodynamics simulations have been carried out in studying the solar wind and cometary plasma interactions for decades. Various plasma boundaries have been simulated and compared well with observations for comet 1P/Halley. The Rosetta mission, which studies comet 67P/Churyumov-Gerasimenko, challenges our understanding of the solar wind and comet interactions. The Rosetta Plasma Consortium observed regions of very weak magnetic field outside the predicted diamagnetic cavity. In this paper, we simulate the inner coma with the Hall magnetohydrodynamics equations and show that the Hall effect is important in the inner coma environment. The magnetic field topology becomes complex and magnetic reconnection occurs on the dayside when the Hall effect is taken into account. The magnetic reconnection on the dayside can generate weak magnetic filed regions outside the global diamagnetic cavity, which may explain the Rosetta Plasma Consortium observations. We conclude that the substantial change in the inner coma environment is due to the fact that the ion inertial length (or gyro radius) is not much smaller than the size of the diamagnetic cavity.Comment: 23 pages, 6 figur

Numerical Studies of the Solar Energetic Particle Transport and Acceleration

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76215/1/AIAA-2005-4928-462.pd

The Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics (SHIELD) model: a self-consistent kinetic-MHD model of the outer heliosphere

Neutral hydrogen has been shown to greatly impact the plasma flow in the heliopshere and the location of the heliospheric boundaries. We present the re- sults of the Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics (SHIELD) model, a new, self-consistent, kinetic-MHD model of the outer helio- sphere within the Space Weather Modeling Framework. The charge-exchange mean free path is on order of the size of the heliosphere; therefore, the neutral atoms cannot be described as a fluid. The SHIELD model couples the MHD so- lution for a single plasma fluid to the kinetic solution from for neutral hydrogen atoms streaming through the system. The kinetic code is based on the Adaptive Mesh Particle Simulator (AMPS), a Monte Carlo method for solving the Boltz- mann equation. The SHIELD model accurately predicts the increased filtration of interstellar neutrals into the heliosphere. In order to verify the correct imple- mentation within the model, we compare the results of the SHIELD model to other, well-established kinetic-MHD models. The SHIELD model matches the neutral hydrogen solution of these studies as well as the shift in all heliospheric boundaries closer to the Sun in comparison the the multi-fluid treatment of the neutral hydrogen atoms. Overall the SHIELD model shows excellent agreement to these models and is a significant improvement to the fluid treatment of inter- stellar hydrogen.First author draf

The Study of the Properties of Uranium Dioxide Pellets Depending on the Parameters of Pressing and Sintering

The paper presents dilatometric research of uranium dioxide pellets, fabricated by compaction at different pressure values. Temperature observed at sintering start is determined and it is pointed out that this temperature doesn’t depend on compaction pressure. Keywords: uranium dioxide, compaction, sintering, dilatometry, shrink curves, geometric density, pycnometric density