The Evolution of Trailing Plumes from Active Regions

We have studied the evolution of several high-latitude flux `plumes', i.e., unipolar regions, trailing from active regions which emerged near sunspot maximum in cycle 23. The observed patterns are compared with simulations using a simple flux transport equation based on the observed flux for an earlier Carrington rotation. In addition to the long recognized poleward migration and diffusion of flux from active regions, it is found that the evolution of the trailing plumes may be influenced by flux which emerges above latitude 35° over areas of all scales. We describe two cases in which the emerging flux appears in the form of bipolar flux patterns which are not obviously related to sunspots. Further, we find instances in which the observed surface flux decreases or spreads at rates which cannot be explained solely in terms of diffusion using the normally accepted rates. Thus in several cases the poleward migration of flux cannot be described in terms of passive transport by advection and diffusion as considered here, and further investigation of the processes that contribute to the evolution of the polar fields is required.     

Near-infrared spectra of the leading and trailing hemispheres of Enceladus

We present individual spectra 0.8-2.5 μm of the leading and trailing hemispheres of Enceladus obtained with the CorMASS spectrograph on the 1.8 m Vatican Advanced Technology Telescope (VATT) at the Mount Graham International Observatory. While the absorption bands of water ice dominate the spectrum of both hemispheres, most of these bands are stronger on the leading hemisphere than the trailing hemisphere. In addition, longward of 1 μm, the continuum slope is greater on the leading hemisphere than the trailing hemisphere. These differences could be produced by the presence of particles on the trailing side that are smaller and/or microstructurally more complex than those on the leading side, consistent with the preferential erosion or structural degradation of regolith particle grains on the trailing side by magnetospheric sweeping. We also explore compositional differences between the two hemispheres by applying Hapke spectrophotometric mixture models to the spectra whose components include water ice and ammonia hydrate (1% NH₃⋅H₂O). We find that spectral models which include as much as 25% by weight ammonia hydrate intimately mixed with water ice and covering 80% of the illuminated area of the satellite fit the observed spectrum of both the leading and trailing hemispheres. Areal (checkerboard) mixing models of ammonia hydrate and water ice fit the leading hemisphere with 15% of the surface comprised of ammonia hydrate and the trailing hemisphere with 10% ammonia hydrate. Therefore, while these spectral data do not contain an unambiguous detection of ammonia hydrate on Enceladus, our spectral models do not preclude the presence of a modest amount of 1% NH₃⋅H₂O on both hemispheres. We examine spectral differences and similarities between both hemispheres and the tenuous E ring within which Enceladus orbits. The spectral resolution (R=λ/Δλ) of these CorMASS data (R∼300) is comparable to but nevertheless higher than that of the Visual-Infrared Mapping Spectrometer (VIMS) (R=225) onboard the Cassini spacecraft.     

Correlation of Winds and Geographic Features with Production of Certain Infrasonic Signals in the Atmosphere

Summary Of the waves which propagate in the atmosphere at acoustic velocity in the period range from 10 to 100 s, one type has been classified by triangulation as arising principally from mountainous regions. These signals were first described as 'northwesters' or '310 ers' by the NBS Geoacoustics Group under R. K. Cook at Washington, D.C., from the predominant direction of arrival. Subsequent operation of an observatory at Boulder, Colorado by Vernon Goerke gave a source region by triangulation in the Pacific Northwest, primarily in Montana and Alberta. Installations of observatories at College, Alaska (Wilson) and Pullman, Washington-Moscow, Idaho (Craine and Thomas) enlarged the data base available, and triangulation showed the principal source areas to be along the coast of British Columbia and in the inland Rocky Mountains of the British Columbia-Alberta border. This paper discusses the presently known characteristics of this class of infrasonic waves, locates the triangulation areas, reviews selected events, and suggests that certain of these waves are produced as aerodynamic sound. The paper shows a correlation between the 500 mb jet stream velocity and direction in these mountainous regions, and the detection of these atmospheric pressure waves.     

Trans-auroral zone auroral infrasonic wave observations

The auroral infrasonic wave (AIW) substorm morphologies are compared for two trans-auroral zone stations, Inuvik, N.W.T. Canada (70°·4 dip lat) and College, Alaska (64°·6 dip lat), that lie along the same magnetic meridian with a north-south separation of 738 km. Statistical studies of the number of AIW received at College over a 5 yr period and at Inuvik over a 2 yr period as well as studies of individual auroral substorms observed at both stations have shown that in the morning sector many more AIW are observed at College than at Inuvik. This difference is related to the changing location of the westward auroral electroject with local time (Weins and Rostoker, 1973). The distribution of frequency of occurrence of AIW horizontal trace velocity V_η is presented for College data together with a discussion of the effects on the distribution of (1) source speed, (2) wind shear, (3) geometry of the AIW mach cone with respect to the observing station, (4) the filtering of AIW with high ray path apogees and (5) the decrease in AIW amplitude with increasing mach number.     

Mesozoic plutons of the Yidun Arc, SW China: U/Pb geochronology and Hf isotopic signature

The Yidun Arc is a Triassic volcanic arc located between the Songpan Garzê Fold Belt and the Qiangtang Block, southwest China. To constrain the age of a number of the major granitic plutons from the Yidun Arc, laser ablation ICP-MS U/Pb analysis of zircon was conducted. Hafnium isotope data was also acquired through laser-ablation multicollector ICPMS analysis of zircon, with the aim of gaining insight into the age and nature of the source region of the plutons. Three age groups have been identified from seven granite samples: Early–Middle Triassic ( 245 to 229 Ma), Late Triassic ( 219 to 216 Ma) and Cretaceous ( 105 to 95 Ma). Hafnium analysis shows the Triassic granites to have negative and variable ε_Hf values and Mesoproterozoic ( 1.6 Ga) depleted-mantle model ages, which is interpreted to reflect derivation from an isotopically heterogeneous, largely crustal source. The Cretaceous granite shows higher and less variable ε_Hf values and slightly younger model ages ( 1.3 Ga), and is interpreted to be derived from melting of a more homogeneous crustal source. A depleted-mantle model age of  1.5 Ga is calculated from the pooled Triassic and Cretaceous samples. The source region for these magmas may be tentatively correlated with Mesoproterozoic material of the Yangtze Craton, which has been suggested to underlie the Yidun Arc; however, further work is necessary to demonstrate this suggestion.     

Field Study of Vertical Screening Distance Criteria for Vapor Intrusion of Ethylene Dibromide

Several regulatory agencies recommend screening petroleum vapor intrusion (PVI) sites based on vertical screening distance between a petroleum hydrocarbon source in soil or groundwater and a building foundation. U.S. Environmental Protection Agency (U.S. EPA) indicate the risk of PVI is minimal at buildings that are separated by more than 6 feet (1.8 m) from a dissolved-phase source and 15 feet (4.6 m) from a light nonaqueous phase liquid (LNAPL) source. This vertical screening distance method is not, however, recommended at sites with leaded gasoline sources containing ethylene dibromide (EDB) because of a lack of field data to document EDB attenuation in the vadose zone. To help address this gap, depth-discrete soil-gas samples were collected at a leaded gasoline release site in Sobieski, Minnesota (USA). The maximum concentration of EDB in groundwater (175 μg/L) at the site was high relative to those observed at other leaded gasoline release sites. Soil gas was analyzed for EDB using a modification of U.S. EPA Method TO-14A that achieved analytical detection limits below the U.S. EPA Vapor Intrusion Screening Level (VISL) for EDB based on a 10⁻⁶ cancer risk (<0.16 μg/m³). Concentrations of EDB in soil gas above LNAPL reached as high as 960 μg/m³ and decreased below the VISL within a source-separation distance of 7 feet. This result coupled with BioVapor model predictions of EDB concentrations indicate that vertical screening distances recommended by regulatory agencies at PVI sites are generally applicable for EDB over the range of anticipated source concentrations and soil types at most sites.     

Visibility in airborne volcanic ash: considerations for surface transportation using a laboratory-based method

All modes of surface transportation can be disrupted by visibility degradation caused by airborne volcanic ash. Despite much qualitative evidence of low visibility on roads following historical eruptions worldwide, there have been few detailed studies that have attempted to quantify relationships between visibility conditions and observed impacts on network functionality and safety. In the absence of detailed field observations, such gaps in knowledge can be filled by developing empirical datasets through laboratory investigations. Here, we use historical eruption data to estimate a plausible range of ash-settling rates and ash particle characteristics for Auckland city, New Zealand. We propose and implement a new experimental set-up in controlled laboratory conditions, which incorporates a dual-pass transmissometer and solid aerosol generator, to reproduce these ash-settling rates and calculate visual ranges through the associated airborne volcanic ash. Our findings demonstrate that visibility is most impaired for high ash-settling rates (i.e. > 500 g m^?2 h^?1) and particle size is deemed the most influential ash characteristic for visual range. For the samples tested (all < 320 μm particle diameter), visibility was restricted to ~ 1–2 m when ash settling was replicated for very high rates (i.e. ~ 4000 g m^?2 h^?1) and was especially low when ash particles were fine-grained, more irregular in shape and lighter in colour. Finally, we consider potential implications for disruption to surface transportation in Auckland through comparisons with existing research which investigates the consequences of visual range reduction for other atmospheric hazards such as fog. This includes discussing how our approach might be utilised in emergency and transport management planning. Finally, we summarise strategies available for the mitigation of visibility degradation in environments contaminated with volcanic ash.