Neutral gas plasma interactions in space plasma

A sounding rocket experiment, CRIT-II, involving the injection of shaped-charge barium in ionospheric plasma was conducted on May 7, 1989, to investigate Alfven\u27s critical ionization velocity (CIV) hypothesis in space. The CRIT-II main payload was instrumented to make in situ measurements within the neutral barium beam. Among the detectors, UNH provided three energetic particle detectors and two photometers. The data from these detectors are presented. The typical features of the CIV effect were observed including plasma density enhancement, energy and momentum loss of a fast ion beam, excitation of plasma waves, and electron heating. It was found by optical observations that about 4% of the neutral barium was ionized. We believe that about one half of these barium ions were created by electron impact ionization--a CIV mechanism. The cross section for collisions between the barium atoms and the ionospheric oxygen ions was also calculated, assuming that the other half of ionizing barium ions were mainly generated by charge exchange, and found to be in the range from 1 \times \ 10\sp{-17} cm\sp{-2} at a velocity of 4 km/s to 1 \times \ 10\sp{-15} cm\sp{-2} at a velocity of 20 km/s. We also confirmed that the early observed ions were originally from the collisionally accelerated neutral oxygen which charge exchanges with the local oxygen ions. The early stage of electron heating was confirmed to be the result of lower hybrid instabilities excited by the precursor ion beam, using our quasi-linear model calculation. However, the wave spectrum during the passage of main streaming barium was found to be inconsistent with the lower hybrid instabilities proposed by current CIV theories. This could be the main reason for a relatively low ionization yield that one otherwise would expect from CRIT-II. A multi-fluid model of the wave dispersion relation for an unmagnetized beam with finite width in a magnetized plasma was also derived. We found that the nonuniform beam density effect could be the main driver which altered the plasma wave spectrum from the typical lower hybrid waves. A quasi-DC electric field model based on the momentum coupling between an ionizing barium beam and an ionospheric plasma was developed. We found that CIV is a self-limiting ionization process in a conical type of neutral beam, which may have caused the low ionization yield in most of the shaped-charge CIV experiments

Global magnetohydrodynamic simulation of the 15 March 2013 coronal mass ejection event-Interpretation of the 30-80 MeV proton flux

The coronal mass ejection (CME) event on 15 March 2013 is one of the few solar events in Cycle 24 that produced a large solar energetic particle (SEP) event and severe geomagnetic activity. Observations of SEP from the ACE spacecraft show a complex time-intensity SEP profile that is not easily understood with current empirical SEP models. In this study, we employ a global three-dimensional (3-D) magnetohydrodynamic (MHD) simulation to help interpret the observations. The simulation is based on the H3DMHD code and incorporates extrapolations of photospheric magnetic field as the inner boundary condition at a solar radial distance (r) of 2.5 solar radii. A Gaussian-shaped velocity pulse is imposed at the inner boundary as a proxy for the complex physical conditions that initiated the CME. It is found that the time-intensity profile of the high-energy (>10 MeV) SEPs can be explained by the evolution of the CME-driven shock and its interaction with the heliospheric current sheet and the nonuniform solar wind. We also demonstrate in more detail that the simulated fast-mode shock Mach number at the magnetically connected shock location is well correlated (r_(cc) ≥ 0.7) with the concurrent 30–80 MeV proton flux. A better correlation occurs when the 30–80 MeV proton flux is scaled by r^(−1.4)(r_(cc) = 0.87). When scaled by r^(−2.8), the correlation for 10–30 MeV proton flux improves significantly from r_(cc) = 0.12 to r_(cc) = 0.73, with 1 h delay. The present study suggests that (1) sector boundary can act as an obstacle to the propagation of SEPs; (2) the background solar wind is an important factor in the variation of IP shock strength and thus plays an important role in manipulation of SEP flux; (3) at least 50% of the variance in SEP flux can be explained by the fast-mode shock Mach number. This study demonstrates that global MHD simulation, despite the limitation implied by its physics-based ideal fluid continuum assumption, can be a viable tool for SEP data analysis

The linear dependence of polar cap index on its controlling factors in solar wind and magnetotail

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94909/1/jgra21722.pd

Accuracy comparison of implant impression techniques: A systematic review

Background Several studies link the seamless fit of implant-supported prosthesis with the accuracy of the dental impression technique obtained during acquisition. In addition, factors such as implant angulation and coping shape contribute to implant misfit. Purpose To identify the most accurate impression technique and factors affecting the impression accuracy. Material and Methods A systematic review of peer-reviewed literature was conducted analyzing articles published between 2009 and 2013. The following search terms were used: implant impression, impression accuracy, and implant misfit. A total of 417 articles was identified, 32 were selected for review. Results All 32 selected studies refer to in vitro studies. Fourteen articles compare open and closed impression technique, 8 advocate the open technique and 6 report similar results. Other 14 articles evaluate splinted and non-splinted techniques; all advocating the splinted technique. Polyether material usage was reported in 9; 6 studies tested vinyl polysiloane and 1 study used irreversible hydrocolloid. Eight studies evaluated different copings designs. Intra-oral optical devices were compared in 4 studies. Conclusions The most accurate results were achieved with two configurations: (1) the optical intra-oral system with powder; and (2) the open technique with splinted squared transfer copings, using polyether as impression material.FCT – Fundação para a Ciência e Tecnologia in the scope of the Ph.D. grant SFRH/BD/68270/2010, project EXPL/BBB-BMD/2146/2013 and within the Project Scope UID/CEC/00319/201

Long‐term variation of driven and unloading effects on polar cap dynamics

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95078/1/jgra21680.pd

Microstructure and thermal stability of Fe, Ti and Ag implanted Yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) was implanted with 15 keV Fe or Ti ions up to a dose of 8×1016 at cm−2. The resulting “dopant” concentrations exceeded the concentrations corresponding to the equilibrium solid solubility of Fe2O3 or TiO2 in YSZ. During oxidation in air at 400° C, the Fe and Ti concentration in the outermost surface layer increased even further until a surface layer was formed of mainly Fe2O3 and TiO2, as shown by XPS and ISS measurements. From the time dependence of the Fe and Ti depth profiles during anneal treatments, diffusion coefficients were calculated. From those values it was estimated that the maximum temperature at which the Fe- and Ti-implanted layers can be operated without changes in the dopant concentration profiles was 700 and 800° C, respectively. The high-dose implanted layer was completely amorphous even after annealing up to 1100° C, as shown by scanning transmission electron microscopy. Preliminary measurements on 50 keV Ag implanted YSZ indicate that in this case the amorphous layer recrystallizes into fine grained cubic YSZ at a temperature of about 1000° C. The average grain diameter was estimated at 20 nm, whereas the original grain size of YSZ before implantation was 400 nm. This result implies that the grain size in the surface of a ceramic material can be decreased by ion beam amorphisation and subsequent recrystallisation at elevated temperatures

Hemispheric asymmetry of the dayside aurora due to imbalanced solar insolation

Abstract Unlike the nightside aurora, which is controlled mainly by magnetic field reconnection in the magnetotail, the dayside aurora is closely associated with magnetic field merging at the dayside magnetopause. About two decades ago, it was discovered that the aurora is also controlled by solar insolation. Because the finding was based on data acquired mainly in the Northern Hemisphere, an outstanding question is if the auroral solar insolation effect also exists in the Southern Hemisphere. The present study addresses this question by studying dayside auroras from both hemispheres. We analyze 6 years’ worth of Earth disk emissions at far ultraviolet wavelengths acquired by the Global UltraViolet Imager on-board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite from 2002 to 2007. It is found that the solar insolation effect also exists in the Southern Hemisphere. In essence, the energy flux deposited as electron precipitation, is larger when the polar hemisphere is sunlit and is smaller when the polar hemisphere is dark. Because auroras are produced mainly by electron precipitation and because electrons are the main current carrier, this north–south asymmetry is consistent with the previous finding that larger (smaller) field-aligned currents are flowing out of the sunlit (dark) hemisphere. This trend is independent of the solar wind driving, suggesting that it is an effect associated with solar insolation. A small north–south asymmetry in the dayside auroral energy flux was identified. We discuss the asymmetry in the context of magnetospheric current and voltage generators