Far-infrared detection limits – II. Probing confusion including source confusion

We present a comprehensive analysis for the determination of the confusion levels for the current and the next generation of far-infrared surveys assuming three different cosmological evolutionary scenarios. We include an extensive model for diffuse emission from infrared cirrus in order to derive absolute sensitivity levels taking into account the source confusion noise due to point sources, the sky confusion noise due to the diffuse emission, and instrumental noise. We use our derived sensitivities to suggest best survey strategies for the current and the future far-infrared space missions Spitzer , AKARI ( ASTRO-F ), Herschel and SPICA . We discuss whether the theoretical estimates are realistic and the competing necessities of reliability and completeness. We find the best estimator for the representation of the source confusion and produce predictions for the source confusion using far-infrared source count models. From these confusion limits considering both source and sky confusions, we obtain the optimal, confusion limited redshift distribution for each mission. Finally, we predict the cosmic far-infrared background (CFIRB), which includes information about the number and distribution of the contributing sources.     

Far-infrared detection limits – I. Sky confusion due to Galactic cirrus

Fluctuations in the brightness of the background radiation can lead to confusion with real point sources. This type of confusion with background emission is relevant when making infrared (IR) observations with relatively large beam sizes, since the amount of fluctuation tends to increase with the angular scale. To quantitively assess the effect of the background emission on the detection of point sources for current and future far-IR observations by space-borne missions such as Spitzer , ASTRO-F , Herschel and Space Infrared Telescope for Cosmology and Astrophysics ( SPICA ), we have extended the Galactic emission map to a higher level of angular resolution than that of the currently available data. Using this high-resolution map, we estimate the sky confusion noise owing to the emission from interstellar dust clouds or cirrus, based on fluctuation analysis and detailed photometry over realistically simulated images. We find that the confusion noise derived by simple fluctuation analysis agrees well with the results from realistic simulations. Although sky confusion noise becomes dominant in long wavelength bands  (>100 μm)  with 60–90 cm aperture missions, it is expected to be two orders of magnitude lower for the next generation of space missions (with larger aperture sizes) such as Herschel and SPICA .     

Solar source of the interplanetary planar magnetic structures

Planar magnetic structure (PMS) is an interplanetary magnetic structure in which magnetic field vectors are all parallel to a plane but highly variable in both magnitude and direction in that plane. This magnetic structure corresponds to re-entrant loops of magnetic field lines in the photosphere that emanate into interplanetary space. To find information on the generation site, occurrence properties of PMSs are investigated by using the interplanetary magnetic field data obtained by Sakigake and ISEE-3 spacecraft. No significant correlation is found between PMS occurrence and the solar wind velocity gradient which would suggest interplanetary formation of PMSs. No significant correlation is found between the PMS events and flares or filaments, either. Instead, a half of the PMSs were projected to the vicinity of the sector boundary in the source surface magnetic field, although there are exceptions when PMS appeared in the center of a sector. The PMS planes were not parallel to the current sheet at the sector boundary. Sometimes PMSs were observed recurrently at the same heliospheric longitude in successive rotations of the Sun, suggesting persistence of the source of PMS on the Sun. The orientation of the PMS planes were not conserved in the recurrent PMSs.     

Decision-theoretic sensitivity analysis for reservoir development under uncertainty using multilevel quasi-Monte Carlo methods

At various stages of petroleum reservoir development, we encounter a large degree of geological uncertainty under which a rational decision has to be made. In order to identify which parameter or group of parameters significantly affects the output of a decision model, we investigate decision-theoretic sensitivity analysis and its computational issues in this paper. In particular, we employ the so-called expected value of partial perfect information (EVPPI) as a sensitivity index and apply multilevel Monte Carlo (MLMC) methods to efficient estimation of EVPPI. In a recent paper by Giles and Goda, an antithetic MLMC estimator for EVPPI is proposed and its variance analysis is conducted under some assumptions on a decision model. In this paper, for an improvement on the performance of the MLMC estimator, we incorporate randomized quasi-Monte Carlo methods within the inner sampling, which results in an multilevel quasi-Monte Carlo (MLQMC) estimator. We apply both the antithetic MLMC and MLQMC estimators to a simple waterflooding decision problem under uncertainty on absolute permeability and relative permeability curves. Through numerical experiments, we compare the performances of the MLMC and MLQMC estimators and confirm a significant advantage of the MLQMC estimator.     

The formation of floor-fractured craters in Xanthe Terra

Floor-fractured craters (FFC) are a peculiar form of degradation of impact craters defined by the presence of crevice networks and mesas affecting crater floors. They are preferentially distributed near chaotic terrains and outflow channels. The scope of this paper is to present a detailed systematic analysis of FFC at Xanthe Terra. FFC morphologies in this region are classified into five types making a picture of different stages of the same degradation process. FFC are geographically intermixed with un-fractured normal craters (non-FFC). Young craters are less prone to show this type of degradation, as suggested by fresh ejecta layer with preserved crater floor. Size distributions of FFC and non-FFC indicate that larger craters are preferentially fractured. Careful examinations of the crater floor elevations reveal that the crevices often extend deeper than the original crater cavity. Furthermore, an onset depth for the formation of FFC is evidenced from the difference of spatial distributions between FFC and non-FFC. Roof-collapsed depressions observed in the same region have been also documented and their characteristics suggest the removal of subsurface material at depth from about 1200 to 4000 m. These observations taken together suggest a subsurface zone of volume deficit at depth from 1 to 2 km down to several kilometers responsible for FFC formation. Then a scenario of FFC formations is presented in the context of groundwater discharge events at the late Hesperian. This scenario involves two key processes, Earth fissuring and piping erosion, known to occur with rapid groundwater migrations on Earth.     

Numerical model and flume experiments of single- and two-layered hillslope flow related to slope failure

A hillslope flow model is developed considering 3D saturated and unsaturated flow of water during rainfall events. A finite difference-based numerical model of hillslope flow processes is developed. Four different experiments are done to see the effects of a single- and double-layered soil in pore-water pressure dynamics and slope failure. Results from the numerical model are verified with experimental results. The numerical and experimental values of the pore-water pressure and moisture contents are in good agreement. The results show that the hillslope heterogeneity caused by multiple layers of soil has greater influence on hillslope pore-pressure dynamics and slope failure patterns. The depth of slope failure shows high dependency on layering characteristics of the soil slope and pattern of rainfall. The proposed model provides a perspective on failure mechanism of a single- or double-layered slope under rainfall infiltration.     

20th century seasonal moisture balance in Southeast Asian montane forests from tree cellulose δ18O

The seasonally varying moisture balance in a montane forest of Southeast Asia is reconstructed for the 20th century from the oxygen isotopic composition (δ¹⁸O) of subannual tree cellulose samples of Pinus kesiya growing at 1,500?m elevation on Doi Chiang Dao in northern Thailand. The cellulose δ¹⁸O values exhibit a distinctive annual cycle with amplitude of up to 12?‰, which we interpret to represent primarily the seasonal cycle of precipitation δ¹⁸O. The annual mean δ¹⁸O values correlate significantly with the amount of summer monsoon precipitation, and suggest a temporal weakening relationship between the South Asian monsoon and El Ni?o-Southern Oscillation over the late 20th century. The cellulose δ¹⁸O annual maxima values, which reflect the dry season moisture status, have declined progressively over the 20th century by about 3.5?‰. We interpret this to indicate a change in the contribution of the isotopically distinct fog water to the dry season soil moisture in response to rising temperature as well as deforestation.