Some comments on the limb shift of solar lines

The well-known correlation between granulation intensity and velocity fluctuations causes a shift of the average line position called the convective blue shift. It is argued that this convective blue shift is most likely reponsible for the limb effect of solar Fraunhofer lines. To explain the center-to-limb variation of this limb effect it is essential that both horizontal and vertical motions in the granulation are considered. The effects of a variation in the granulation properties across the Sun on large scale velocity pattern observations are discussed. Abnormal granulation patterns observed inactive regions and at the boundaries of supergranules could be responsible for part or all of the downflow observed there.On leave from Astronomy Department, University of Washington, Seattle, Wash., U.S.A.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 74-04129 with the National Science Foundation.     

In-situ investigation on real-time suspended sediment measurement techniques: Turbidimetry,acoustic attenuation,laser diffraction (LISST) and vibrating tube densimetry

Real-time measurements of Suspended Sediment mass Concentration(SSC) and Particle Size Distribution(PSD) are of prime importance for the investigation and management of fine-sediment related processes in surface water systems and hydraulic schemes. In a field study at the waterway of the hydropower plant(HPP) Fieschertal in the Swiss Alps, the real-time measurement performance of the following techniques and instruments were assessed based on measurements in the sediment seasons of the years 2013 and 2014:(1) turbidimetry,(2) single-frequency acoustic attenuation using a standard acoustic discharge measurement(ADM) installation,(3) laser diffraction(Laser In-Situ Scattering and Transmissometry, LISST), and(4) vibrating tube densimetry using a Coriolis Flow and Density Meter(CFDM).Reference SSCs were obtained from gravimetric analysis of 219 automatically pumped water samples.LISST additionally supplied PSD every minute. The median particle diameter, d_(50), was usually 15μm and increased occasionally to 100μm. The turbidimeter and the ADM underestimated the SSC when the transported particles were coarser than usual. Such temporary biases resulted from the poor correlation between d_(50) and SSC at this study site. The SSCs from CFDM and LISST were not or less biased by PSD variations. Mainly due to angular and flaky particle shapes, SSC from LISST needed a correction by 79% on average. With the usually prevailing silt particles, an optical path length of 5 mm and no dilution, the SSC measurement range of LISST was limited to about 1.5 g/l. The CFDM allowed measuring higher SSC than the other investigated instruments(e.g. up to 13.5 g/l). With a periodic offset correction, its relative SSC measurement uncertainty was 20% for SSC≥ 1.5 g/I. To reliably measure a wide range of SSC with temporarily variable PSD, a combination of instruments is recommended: e.g. a standard LISST, a CFDM,and an automatic water sampler for gravimetric reference measurements.     

Analysis of surface chlorophyll a associated with sea surface temperature and surface wind in the South China Sea

Ocean Dynamics - In this study, the spatial and temporal variability in surface chlorophyll a (Chl-a) in the whole South China Sea (SCS) was investigated in detail by using 8-day, 4-km, gap-free...     

Super-ensemble techniques: Application to surface drift prediction

The prediction of surface drift of floating objects is an important task, with applications such as marine transport, pollutant dispersion, and search-and-rescue activities. But forecasting even the drift of surface waters is very challenging, because it depends on complex interactions of currents driven by the wind, the wave field and the general prevailing circulation. Furthermore, although each of those can be forecasted by deterministic models, the latter all suffer from limitations, resulting in imperfect predictions. In the present study, we try and predict the drift of two buoys launched during the DART06 (Dynamics of the Adriatic sea in Real-Time 2006) and MREA07 (Maritime Rapid Environmental Assessment 2007) sea trials, using the so-called hyper-ensemble technique: different models are combined in order to minimize departure from independent observations during a training period; the obtained combination is then used in forecasting mode. We review and try out different hyper-ensemble techniques, such as the simple ensemble mean, least-squares weighted linear combinations, and techniques based on data assimilation, which dynamically update the model’s weights in the combination when new observations become available. We show that the latter methods alleviate the need of fixing the training length a priori, as older information is automatically discarded.When the forecast period is relatively short (12 h), the discussed methods lead to much smaller forecasting errors compared with individual models (at least three times smaller), with the dynamic methods leading to the best results. When many models are available, errors can be further reduced by removing colinearities between them by performing a principal component analysis. At the same time, this reduces the amount of weights to be determined.In complex environments when meso- and smaller scale eddy activity is strong, such as the Ligurian Sea, the skill of individual models may vary over time periods smaller than the forecasting period (e.g. when the latter is 36 h). In these cases, a simpler method such as a fixed linear combination or a simple ensemble mean may lead to the smallest forecast errors. In environments where surface currents have strong mean-kinetic energies (e.g. the Western Adriatic Current), dynamic methods can be particularly successful in predicting the drift of surface waters. In any case, the dynamic hyper-ensemble methods allow to estimate a characteristic time during which the model weights are more or less stable, which allows predicting how long the obtained combination will be valid in forecasting mode, and hence to choose which hyper-ensemble method one should use.     

An analysis of the error space of a high-resolution implementation of the GHER hydrodynamic model in the Mediterranean Sea

An ensemble of 250 model setups covering the Mediterranean Sea is built by perturbing various parameters: the bathymetry, the initial conditions, atmospheric forcing fields (air temperature, cloud coverage, wind), and internal model parameters (diffusion coefficients). The ensemble is then forwarded in time using the GHER hydrodynamic model, allowing to obtain information about the expected error associated with the forecast in a natural way. The evolution of this error is analyzed. In particular, we examine the time evolution and stationarity of its spatial average, and the spatial distribution of the error at different instants, by means of its first to fourth order moments, and of empirical orthogonal functions. We verify whether the a posteriori error distribution is Gaussian using the Anderson-Darling test. From these results, we are able to assess what parameters and forcing fields are most critical for the forecast. Qualitative conclusions are obtained throughout the text, in accordance with our expectations. Moreover, quantitative estimations of the expected error are also given.     

TeV Gamma Rays From the Galactic Center Direct and Indirect Links to the Massive Black Hole in Sgr A

The recent detection of TeV gamma-radiation from the direction of the Galactic Center within several arc-minutes around Sgr A^* is the first model-independent evidence of existence of high-energy particle accelerator(s) in the central 10 pc region of our Galaxy. This is an extraordinary site that harbours many remarkable objects with the compact radio source Sgr A^* – a hypothetical super-massive black hole (SMBH) – in the dynamical center of the Galaxy. Here we explore the possible direct and indirect links of the reported TeV emission to the SMBH. We show that at least three γ-ray production scenarios that take place close to the event horizon of the SMBH can explain the reported TeV fluxes. An alternative (or additional) channel of TeV radiation is related to the run-away protons accelerated in Sgr A^*. Quasi-continuous injection of relativistic protons into the surrounding dense gas environment initiates detectable high-energy gamma-ray emission. The absolute flux and the energy spectrum of this radiation component strongly depend on the history of particle injection and the character of diffusion of protons during the last 10⁵ yr. For a reasonable combination of a few model parameters, one can explain the detected gamma-ray flux solely by this diffuse component.     

Report on the solar physics-plasma physics workshop

This report summarizes the proceedings of a meeting held on 17–20 September 1974, at Stanford University. The purpose was to explore plasma physics problems which arise in the study of solar physics. Sessions were concerned with specific questions including the following: Is the solar plasma thermal or non-thermal? What spectroscopic data are required? What types of magnetic field structures exist? Do MHD instabilities occur? Do resistive or non-MHD instabilities occur? What mechanisms of particle acceleration have been proposed? What information do we have concerning shock waves? Very few questions were answered categorically but, for each question, there was discussion concerning the observational evidence, theoretical analyses, and existing or potential laboratory and numerical experiments.     

Spatial structure in lines in the 3398–3526 å region at the extreme limb: Observation,identification and interpretation

We have obtained spectrograms of high spatial and spectral resolution of the extreme solar limb, using the vacuum tower telescope of Sacramento Peak Observatory. We have identified emission lines in the range 3398–3526 Å, and classified them according to intensity, spatial structure (intensity variation), and profile. Some lines show spatial intensity variation; others do not. We show that this effect is related to the abundance of the element responsible for the line and the mean lower-level excitation potential of interlocked lines. We explain the effect in terms of radiative interlocking with other lines, as well as the characteristic size of the volume contributing to the mean intensity.Also Five College Astronomy Department, Amherst, Mass. 01003, U.S.A.Operated by the Association of Universities for Research in Astronomy Inc. under contract with the National Science Foundation.