Combining Gaia Windows: The Method and Simulations     

Pasi?NurmiEmail author 《Astrophysics and Space Science》2005,298(4):559-575

During the 5-year-observational phase of Gaia several one-dimensional observations will be obtained from the same objects at different epochs and transit angles. Combining the information from all the transits to produce a single two-dimensional image is scientifically important as this will allow to detect much fainter stars in the vicinity of the main source than otherwise possible from single observations on-board the satellite or from individual transmitted patches. Most of the emerging objects will be physical companions, but some background/foreground objects will also be found. The combination procedure further increases the image resolution in the across scan-direction. The procedure consists of combining patches that are stretched back to 2-D windows, only possible since the centroid positions of the windows are known. The initial image window from which the patches originate is not square, but elongated along the scan-direction if the Astro AF11 field of view is used in the current calculations. Transit angles can take any value between 0^∘ and 360^∘. The window combining method is a simple mapping procedure that is fast and should handle all realistic complexities occurring during the Gaia data reduction. The method is not intended to be a perfect image restoration technique, but its purpose is to fulfill the scientific requirement of detecting faint neighboring sources existing in the close vicinity of the main targets. In this study we show examples of the combined windows and illustrate how faint companions around primary targets could be detected. We investigate the performances and limitations of the method and discuss possible complexities that might occur during the final data analysis.  相似文献   

Importance of diffuse nutrient loading and lake level changes to the eutrophication of an originally oligotrophic boreal lake: a palaeolimnological diatom and chironomid analysis     

Jarmo J. Meriläinen  Juhani Hynynen  Arja Palomäki  Pasi Reinikainen  Anssi Teppo  Kaj Granberg 《Journal of Paleolimnology》2000,24(3):251-270

The recent environmental history of Lake Lappajärvi in western Finland (63°00 N, 23°30 E, area 149 km²), a humic, brown water lake with an average phosphorus content of ca. 20 g l^–1, was studied from short core sediment samples taken from the two main basins of the lake. Based on the stratigraphy of diatoms and chironomids and the sediment quality it was possible to distinguish four developmental stages during the past century: (1) a pre-industrial stage covering the time up to about 1935; (2) a stage of increasing nutrient loading (ca. 1936–1960); (3) a stage of pronounced erosion from lake level regulation and extensive ditching of the catchment area (ca. 1960–1970); and (4) a meso-eutrophic stage from ca. 1970 onwards.Acidophilous Aulacoseira distans coll. and other species typical of dystrophic, nutrient-poor lakes characterized the diatom assemblages during the first stage, and the profundal zoobenthic assemblages, characterized by Heterotrissocladius subpilosus and Micropsectra, indicated good hypolimnetic oxygen conditions and a low sedimentation of organic matter (approx. less than 50 g m^–2 a^–1). The increased loading rapidly led to changes both in diatoms and chironomids (e.g., to an early extinction of H. subpilosus in the 1950s). The process finally led to eutrophication with a successive proliferation of diatom species such as Asterionella formosa followed by Aulacoseira ambigua, Fragilaria crotonensis, and finally Melosira varians. The relative proportion of alkaliphilous species reached a maximum in the final stage and the original profundal chironomid fauna was replaced by Chironomus anthracinus gr. and C. plumosus which are typical of profundal areas suffering from temporal oxygen deficit. It is notable that the considerable decrease in waste water loading from the point sources (80–86% ) during the past two decades has not led to a recovery in the lake. This highlights the importance of diffuse loading from agriculture, forestry and other human activities even to this comparatively large lake.  相似文献   

Detection of the optical counterpart of the proposed double degenerate polar RX J1914+24     

Gavin Ramsay  Mark Cropper  Kinwah Wu  K. O. Mason  Pasi Hakala 《Monthly notices of the Royal Astronomical Society》2000,311(1):75-84

We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I -band light curve is modulated on the 9.5-min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J . The infrared colours of RX J1914+24 are not consistent with a main-sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5-min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I -band flux to the irradiated face of the donor star. The long-term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronizing mechanisms in such a close binary and other system parameters.  相似文献   

Spatial Modelling of Stream Water Quality Along an Urban–Rural Gradient          下载免费PDF全文

Maija Taka  Juha Aalto  Miska Luoto 《Geografiska Annaler: Series A, Physical Geography》2015,97(4):819-834

It is widely known that intensive land use generally decreases stream water quality, but the influence of watershed physiography is relatively poorly understood. Since management planning has to take into account the protection of water quality, the current status of stream water must be identified. The potential effects of land use and watershed physiography variables on water quality were studied in an extensive set of 83 watersheds in the Helsinki region, Finland, covering wide land‐use intensity gradient. The aims of this study were to test if the geographical information of watershed land‐use data can be used to model the stream water quality, and to examine whether the spatial water quality models are improved after including predictors of watershed physiography to the land‐use model. Water quality variables were related to watershed predictors by utilizing generalized additive models and linear mixed models, and the independent effect of the variables was investigated using a hierarchical partitioning approach. While land use turned out to be the most influential factor explaining water quality, all models improved significantly after incorporating the watershed characteristics, such as topography and soil. These results were consistent across three modelling techniques. This study, with its novel approach to examine the impacts of several watershed physiographic characteristics on urban stream water quality in northern Europe, demonstrates that spatial land use and watershed physiography data can be used as cost‐efficient predictors in stream water quality models.  相似文献   

Floodplain development in an engineered setting     

Michael Bliss Singer  Rolf Aalto 《地球表面变化过程与地形》2009,34(2):291-304

Engineered flood bypasses, or simplified conveyance floodplains, are natural laboratories in which to observe floodplain development and therefore present an opportunity to assess delivery to and sedimentation within a specific class of floodplain. The effects of floods in the Sacramento River basin were investigated by analyzing hydrograph characteristics, estimating event‐based sediment discharges and reach erosion/deposition through its bypass system and observing sedimentation patterns with field data. Sediment routing for a large, iconic flood suggests high rates of sedimentation in major bypasses, which is corroborated by data for one bypass area from sedimentation pads, floodplain cores and sediment removal reporting from a government agency. These indicate a consistent spatial pattern of high sediment accumulation both upstream and downstream of lateral flow diversions and negligible sedimentation in a ‘hydraulic shadow’ directly downstream of a diversion weir. The pads located downstream of the shadow recorded several centimeters of deposition during a moderate flood in 2006, increasing downstream to a peak of ～10 cm thick and thinning rapidly thereafter. Flood deposits in the sediment cores agree with this spatial pattern, containing discrete sedimentation layers (from preceding floods) that increase in thickness with distance downstream of the bypass entrance to several decimeters thick at the peak and then thin downstream. These patterns suggest that a quasi‐natural physical process of levee construction by advective overbank transport and deposition of sediment is operating. The results improve understanding of the evolution of bypass flood control structures, the transport and deposition of sediment within these environments and the evolution of one class of natural levee systems. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Extreme flood‐driven fluvial bank erosion and sediment loads: direct process measurements using integrated Mobile Laser Scanning (MLS) and hydro‐acoustic techniques          下载免费PDF全文

Julian Leyland  Christopher R. Hackney  Stephen E. Darby  Daniel R. Parsons  James L. Best  Andrew P. Nicholas  Rolf Aalto  Dimitri Lague 《地球表面变化过程与地形》2017,42(2):334-346

This methods paper details the first attempt at monitoring bank erosion, flow and suspended sediment at a site during flooding on the Mekong River induced by the passage of tropical cyclones. We deployed integrated mobile laser scanning (MLS) and multibeam echo sounding (MBES), alongside acoustic Doppler current profiling (aDcp), to directly measure changes in river bank and bed at high (~0.05 m) spatial resolution, in conjunction with measurements of flow and suspended sediment dynamics. We outline the methodological steps used to collect and process this complex point cloud data, and detail the procedures used to process and calibrate the aDcp flow and sediment flux data. A comparison with conventional remote sensing methods of estimating bank erosion, using aerial images and Landsat imagery, reveals that traditional techniques are error prone at the high temporal resolutions required to quantify the patterns and volumes of bank erosion induced by the passage of individual flood events. Our analysis reveals the importance of cyclone‐driven flood events in causing high rates of erosion and suspended sediment transport, with a c. twofold increase in bank erosion volumes and a fourfold increase in suspended sediment volumes in the cyclone‐affected wet season. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Beryllium‐7 wet deposition variation with storm height,synoptic classification,and tree canopy state in the mid‐Atlantic USA          下载免费PDF全文

D. L. Karwan  C. M. Siegert  D. F. Levia  J. Pizzuto  J. Marquard  R. Aalto  A. K. Aufdenkampe 《水文研究》2016,30(1):75-89

Short‐lived fallout isotopes, such as beryllium‐7 (⁷Be), are increasingly used as erosion and sediment tracers in watersheds. ⁷Be is produced in the atmosphere and delivered to the Earth's surface primarily in precipitation. However, relatively little has been published about the variation in ⁷Be wet deposition caused by storm type and vegetation cover. Our analysis of precipitation, throughfall, and sediments in two forested, headwater catchments in the mid‐Atlantic USA indicates significant variation in isotope deposition with storm type and storm height. Individual summer convective thunderstorms were associated with ⁷Be activity concentrations up to 5.0 Bq l^?1 in precipitation and 4.7 Bq l^?1 in throughfall, while single‐event wet depositional fluxes reached 168 Bq m^?2 in precipitation and 103 Bq m^?2 in throughfall. Storms originating from the continental USA were associated with lower ⁷Be activity concentrations and single‐event wet depositional fluxes for precipitation (0.7–1.2 Bq l^?1 and 15.8–65.0 Bq m^?2) and throughfall (0.1–0.3 Bq l^?1 and 13.5–98.9 Bq m^?2). Tropical systems had relatively low activity concentrations, 0.2–0.5 Bq l^?1 in precipitation and 0.2–1.0 Bq l^?1 in throughfall, but relatively high single‐event depositional fluxes due to large rainfall volumes, 32.8–67.6 Bq m^?2 in precipitation and 25.7–134 Bq m^?2 in throughfall. The largest sources of ⁷Be depositional variation were attributed to storm characteristics including precipitation amount and maximum storm height. ⁷Be activity associated with fluvial suspended sediments also exhibited the highest concentration and variability in summer (175–1450 Bq kg^?1). We conclude the dominant source of variation on event‐level ⁷Be deposition is storm type. Our results illustrate the complex relationships between ⁷Be deposition in precipitation and throughfall and demonstrate event‐scale relationships between the ⁷Be in precipitation and on suspended sediment. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Are the nearby groups of galaxies gravitationally bound objects?     

Sami-Matias Niemi  Pasi Nurmi  Pekka Heinämäki  Mauri Valtonen 《Monthly notices of the Royal Astronomical Society》2007,382(4):1864-1876

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Common temperature signal in four well‐replicated tree growth series from northern Fennoscandia     

M. Lindholm  T. Aalto  H. Grudd  D. Mccarroll  M. Ogurtsov  R. Jalkanen 《第四纪科学杂志》2012,27(8):828-834

Four Nordic temperature proxies based on tree growth at the northern timberline – ring‐width from Sweden and Finland, maximum latewood density from Sweden, and height increment from Finland – were compared. Three indexing methods were used to enhance the low (centennial and above), medium (decadal‐to‐multidecadal) and high (decadal‐to‐interannual) frequencies. The proxies are shown to have a strong temperature signal (common variance) at the interannual‐to‐multidecadal scale, while the multidecadal‐to‐centennial trends are less coherent, perhaps reflecting intra‐regional differences in growing conditions but more likely due to the more noisy regional curve standardization method used to retain the longest trends. Various methods of combining the four proxy series were explored and tested by comparison with four long temperature records from northern Fennoscandia. Only relatively high‐frequency, spline‐indexed series produced consistently positive verification statistics as a reconstruction model for summer temperature using all four proxies. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Assessing the past thermal and chemical history of fluids in crystalline rock by combining fluid inclusion and isotopic investigations of fracture calcite     

Alexander Blyth  Shaun Frape  Runar Blomqvist  Pasi Nissinen 《Applied Geochemistry》2000,15(10):144

Fluid inclusion studies combined with the isotope geochemistry of several generations of fracture calcite from the Olkiluoto research site, Finland, has been used to better understand the past thermal and fluid history in the crystalline rock environment. Typically, fracture mineral investigations use O and C isotopes from calcite and an estimate of the isotopic composition of the water that precipitated the calcite to perform δ¹⁸O geothermometry calculations to estimate past temperature conditions. By combining fluid inclusion information with calcite isotopes, one can directly measure the temperature at which the calcite formed and can better determine past fluid compositions. Isotopic, petrologic and fluid inclusion studies at the Olkiluoto research site in Finland were undertaken as part of an investigation within the Finnish nuclear waste disposal program. The study revealed that four fluids were recorded by fracture calcites. From petrologic evidence, the first fluid precipitated crystalline calcite at 151–225°C with a δ¹³C signature of −21 to −13.9‰ PDB and a δ¹⁸O signature of 12.3–13.0‰ SMOW. These closed fracture fillings were found at depths greater than 500 m and were formed from a high temperature, low salinity, Na–Cl fluid of possible meteoric water altered by exchange with wallrock or dilute basinal origin. The next fluid precipitated crystalline calcite with clay at 92–210°C with a δ¹³C signature of −2.6 to +3.8‰ PDB and a δ¹⁸O signature of 19.4–20.7‰ SMOW. These closed fracture fillings were found at depths less than 500 m and were formed from a moderate to high temperature, low to moderate salinity, Na–Cl fluid, likely of magmatic origin. The last group of calcites to form, record the presence of two distinct fluid types. The platy (a) calcite formed at 95–238°C with a δ¹³C signature of −12.2 to −3.8‰ PDB and a δ¹⁸O signature of 14.9–19.6‰ SMOW, from a high temperature, low salinity, Na–Cl fluid of possible magmatic origin. The platy (b) calcite formed at 67–98°C with a δ¹³C signature of −13.0 to −6.2‰ PDB and a δ¹⁸O signature of 15.1–20.1‰ SMOW, from a low temperature, high salinity, Ca–Na–Cl fluid of possible basinal brine origin. The two calcites are related through a mixing between the two end members. The source of the fluids for the platy grey (a) calcites could be the olivine diabase dykes and sills that cut through the site. The source of fluids for the platy (b) calcites could be the Jotnian arkosic sandstone formations in the northern part of the site. At the Olkiluoto site, δ¹⁸O geothermometry does not agree with fluid inclusion data. The original source of the water that forms the calcite has the largest effect on the isotopic signature of the calcites formed. Large isotopic shifts are seen in any water by mineral precipitation during cooling under rock–water equilibrium fractionation conditions. Different calcite isotopic signatures are produced depending on whether cooling occurred in an open or closed system. Water–rock interaction, at varying W/R ratios, between a water and a host rock can explain the isotopic shifts in many of the calcites observed. In some cases it is possible to shift the δ¹⁸O of the water by +11.5‰ (SMOW) using a realistic water–rock ratio. This process still does not explain some of the very positive δ¹⁸O values calculated using fluid inclusion data. Several other processes, such as low temperature recrystallization, boiling, kinetic effects and dissolution of calcite from fluid inclusion walls can affect isotopic signatures to varying degrees. The discrepancy between fluid inclusion data and δ¹⁸O geothermometry at the Olkiluoto site was most likely due to poor constraint on the original source of the water.  相似文献