Seasonal and interannual dynamics of free-living bacterioplankton and microbially labile organic carbon along the salinity gradient of the Potomac River

Samples were collected at stations located in the mesohaline, oligohaline, and tidal fresh regions of the Potomac River, Maryland, between April 1998 and December 1999 to evaluate the seasonal distribution of bacterioplankton and microbially labile organic carbon (MLOC) in relation to hydrodynamic parameters (dissolved oxygen, salinity, and temperature). Bacterioplankton abundance (BA) averaged 13 × 10⁶ cells ml⁻¹ at all stations, a value that is higher than the average observed in many other temperate estuaries around the world, and were almost exclusively free-living. During the summer of 1998, BA often exceeded 30 × 10⁶ cells ml⁻¹ in the mesohaline region during periods of anoxia in subpycnocline waters. Dissolved MLOC typically accounted for 40% of total MLOC and on some occasions during summer and autumn accounted for 80%. A significant positive relationship between dissolved MLOC and BA was evident in the mesohaline Potomac River, the region where anoxia occurs each summer, but the regressions of particulate MLOC and chla on BA were not significant at this location. In the mesohaline Potomac River, BA regressed negatively and significantly on dissolved oxygen (r²=0.50, p<0.001). BA may be an important indicator of ecosystem health in this and other eutrophied estuaries, because of the relationships between BA, dissolved MLOC, and dissolved oxygen in the salinity stratified Potomac River and because free-living BA was elevated along the length of the river.     

Distribution,sediment magnetism and geochemistry of the Saksunarvatn (10 180 ± 60 cal. yr BP) tephra in marine,lake, and terrestrial sediments,northwest Iceland

In 1997, seismic surveys in the troughs off northwest and north Iceland indicated the presence of a major, regional sub‐bottom reflector that can be traced over large areas of the shelf. Cores taken in 1997, and later in 1999 on the IMAGES V cruise, penetrated through the reflector. In core MD99‐2269 in Húnaflóaáll, this reflector is shown to be represented by a basaltic tephra with a geochemical signature and radiocarbon age correlative with the North Atlantic‐wide Saksunarvatn tephra. We trace this tephra throughout northwest Iceland in a series of marine and lake cores, as well as in terrestrial sediments; it forms a layer 1 to 25 cm thick of fine‐ to medium‐grained basaltic volcanic shards. The base of the tephra unit is always sharp but visual inspection and other measurements (carbonate and total organic carbon weight %) indicate a more diffuse upper boundary associated with bioturbation and with sediment reworking. Off northwest Iceland the Saksunarvatn tephra has distinct sediment magnetic properties. This is evident as a dramatic reduction in magnetic susceptibility, an increase in the frequency dependant magnetic susceptibility and ‘hard’ magnetisation in a −0.1T IRM backfield. Geochemical analyses from 11 sites indicate a tholeiitic basalt composition, similar to the geochemistry of a tephra found in the Greenland ice‐core that dates to 10 180 ± 60 cal. yr BP, and which was correlated with the 9000 ¹⁴C yr BP Saksunarvatn tephra. We present accelerator mass spectrometry ¹⁴C dates from the marine sites, which indicate that the ocean reservoir correction is close to ca. 400 yr at 9000 ¹⁴C yr BP off northwest Iceland. Copyright © 2002 John Wiley & Sons, Ltd.     

Relation of CME Speed and Magnetic Helicity in CME Source Regions on the Sun during the Early Phase of Solar Cycles 23 and 24

To investigate the relations between coronal mass ejection (CME) speed and magnetic field properties measured in the photospheric surface of CME source regions, we selected 22 disk CMEs in the rising and early maximum phases of the current Solar Cycle 24. For the CME speed, we used two-dimensional (2D) projected speed observed by the Large Angle and Spectroscopic Coronagraph onboard the Solar and Heliospheric Observatory (SOHO/LASCO), as well as a 3D speed calculated from the triangulation method using multi-point observations. Two magnetic parameters of CME source regions were considered: the average of magnetic helicity injection rate and the total unsigned magnetic flux. We then classified the selected CMEs into two groups, showing: i) a monotonically increasing pattern with one sign of helicity (group A: 16 CMEs) and ii) a pattern of significant helicity injection followed by its sign reversal (group B: 6 CMEs). We found that: 1) 3D speed generally shows better correlations with the magnetic parameters than the 2D speed for 22 CME events in Solar Cycle 24; 2) 2D speed and the magnetic parameters of 22 CME events in this solar cycle have lower values than those of 47 CME events in Solar Cycle 23; 3) all events of group B in Solar Cycle 24 occur only after the beginning of the maximum phase, a trend well consistent with that shown in Solar Cycle 23; 4) the 2D speed and the helicity parameter of group B events continue to increase in the declining phase of Solar Cycle 23, while those of group A events abruptly decrease in the same period. Our results indicate that the two CME groups have a different tendency in the solar cycle variations of CME speed and the helicity parameters. Active regions that show a complex helicity evolution pattern tend to appear in the maximum and declining phases, while active regions with a relatively simple helicity evolution pattern appear throughout the whole solar cycle.     

Diagnostics of Coronal Bright Points using IRIS,AIA, and HMI Observations

We perform the detailed imaging and spectroscopic analysis of two coronal bright points (CBPs). These CBPs are dominated by bright dots or elongated bright features. Their rapid temporal variations lead to a continuous change in their overall morphology at chromospheric and transition-region (TR) temperatures. A 3D potential magnetic field extrapolation predicts the dominance of magnetic loops in the extent of both CBPs, which are clearly visible at the Si iv 1393.75 Å line formation temperature. Short, low-lying magnetic loops or loop segments are the integral parts of these CBPs at TR temperature. A correlation between the various parameters of Mg ii resonance lines (e.g. intensity, Doppler velocity, velocity gradient) is present in the region of magnetic loops or loop segments. However, a quiet-Sun (QS) region does not show any correlation. Doppler velocities as well as the full width at half maximum (FWHM) of these lines are very prominent in the magnetic loops and loop segments compared to the Doppler velocities and FWHM in the QS region. Higher red-shifts and FWHM at TR temperatures are directly related to the dominance of the energy release process in these regions in the framework of the nanoflare model. A magnetogram from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) reveals the existence of two opposite magnetic polarities in the extent of both CBPs, which is a very well established result. We find that one CBP is formed by the convergence of two opposite magnetic polarities, while the other is triggered by the emergence of a new magnetic field prior to the onset of this CBP.     

Planning and Scheduling of Payloads of AstroSat During Initial and Normal Phase Observations

On 28th September 2015, India launched its first astronomical space observatory AstroSat, successfully. AstroSat carried five astronomy payloads, namely, (i) Cadmium Zinc Telluride Imager (CZTI), (ii) Large Area X-ray Proportional Counter (LAXPC), (iii) Soft X-ray Telescope (SXT), (iv) Ultra Violet Imaging Telescope (UVIT) and (v) Scanning Sky Monitor (SSM) and therefore, has the capability to observe celestial objects in multi-wavelength. Four of the payloads are co-aligned along the positive roll axis of the spacecraft and the remaining one is placed along the positive yaw axis direction. All the payloads are sensitive to bright objects and specifically, require avoiding bright Sun within a safe zone of their bore axes in orbit. Further, there are other operational constraints both from spacecraft side and payloads side which are to be strictly enforced during operations. Even on-orbit spacecraft manoeuvres are constrained to about two of the axes in order to avoid bright Sun within this safe zone and a special constrained manoeuvre is exercised during manoeuvres. The planning and scheduling of the payloads during the Performance Verification (PV) phase was carried out in semi-autonomous/manual mode and a complete automation is exercised for normal phase/Guaranteed Time Observation (GuTO) operations. The process is found to be labour intensive and several operational software tools, encompassing spacecraft sub-systems, on-orbit, domain and environmental constraints, were built-in and interacted with the scheduling tool for appropriate decision-making and science scheduling. The procedural details of the complex scheduling of a multi-wavelength astronomy space observatory and their working in PV phase and in normal/GuTO phases are presented in this paper.     

Thermophysical properties of Almahata Sitta meteorites (asteroid 2008 TC3) for high‐fidelity entry modeling

Asteroid 2008 TC₃ was characterized in a unique manner prior to impacting Earth's atmosphere, making its October 7, 2008, impact a suitable field test for or validating the application of high‐fidelity re‐entry modeling to asteroid entry. The accurate modeling of the behavior of 2008 TC₃ during its entry in Earth's atmosphere requires detailed information about the thermophysical properties of the asteroid's meteoritic materials at temperatures ranging from room temperature up to the point of ablation (T ~ 1400 K). Here, we present measurements of the thermophysical properties up to these temperatures (in a 1 atm. pressure of argon) for two samples of the Almahata Sitta meteorites from asteroid 2008 TC₃: a thick flat‐faced ureilite suitably shaped for emissivity measurements and a thin flat‐faced EL6 enstatite chondrite suitable for diffusivity measurements. Heat capacity was determined from the elemental composition and density from a 3‐D laser scan of the sample. We find that the thermal conductivity of the enstatite chondrite material decreases more gradually as a function of temperature than expected, while the emissivity of the ureilitic material decreases at a rate of 9.5 × 10^?5 K^?1 above 770 K. The entry scenario is the result of the actual flight path being the boundary to the load the meteorite will be affected with when entering. An accurate heat load prediction depends on the thermophysical properties. Finally, based on these data, the breakup can be calculated accurately leading to a risk assessment for ground damage.     

Constraints on the176Lu cosmochronometer

In an endeavour to resolve reported discrepancies in the value of the branching ratio of¹⁷⁶Lu at astrophysical energies, a new determineation of the¹⁷⁵Lu (nγ)^176mLu capture cross section has been measured as 958 ± 58 mb. This gives a value of the branching ratio of 0.21 ±0.05. This result indicates that some reequilibration of the ground and isomeric states of¹⁷⁶Lu occurs in stellar environments undergoing s-process nucleosynthesis, and confirms that¹⁷⁶Lu is not a reliable cosmochronometer. However the very existence of¹⁷⁶Lu in the solar system implies that the ground state of¹⁷⁶Lu was not completely depopulated, and provides the possibility of using this nuclide as a sensitive thermometer for stellar processes.     

Study of the Wolf-Rayet members of the cluster NGC 6231

Two Wolf-Rayet members of the cluster NGC 6231 are studied spectrophotometrically. HD 151932, a suspected variable, shows variations in the emission line flux as well as continuum magnitude measurements. An attempt is made to understand this variation as due to the asymmetric atmospheric structure. The other, HD 152270, a (WC7 + O) binary, shows variation of emission line flux for C III and C IV lines only. This variation is studied as a possible phenomenon of atmospheric eclipses.     

Scanner observations of P/Halley (1982i)

We present an outline to calculate the principal and indirect portions of the planetary disturbing function, based on the method of W. M. Smart. We truncate the Taylor expansion at the third power of eccentricity-inclination. The outline of a new method to obtain the negative powers of the mutual distance between two planets is also given, neglecting powers higher than the fourth in the eccentricity-inclination.