Micromonas pusilla (Butcher) Manton et Parke, a marine prasinophyte, was used to investigate how cell growth and division affect optical properties of phytoplankton over the light:dark cycle. Measurements were made of cell size and concentration, attenuation and absorption coefficients, flow cytometric forward and side light scattering and chl fluorescence, and chl and carbon content. The refractive index was derived from observations and Mie scattering theory. Diel variations occurred, with cells increasing in size, light scattering, and carbon content during daytime photosynthesis and decreasing during nighttime division. Cells averaged 1.6 μm in diameter and exhibited phased division, with 1.3 divisions per day. Scattering changes resulted primarily from changes in cell size and not refractive index; absorption changes were consistent with a negligible package effect. Measurements over the diel cycle suggest that in M. pusilla carbon‐specific attenuation varies with cell size, and this relationship appears to extend to other phytoplankton species. Because M. pusilla is one of the smallest eukaryotic phytoplankton and belongs to a common marine genus, these results will be useful for interpreting in situ light scattering variation. The relationship between forward light scattering (FLS) and volume over the diel cycle for M. pusilla was similar to that determined for a variety of phytoplankton species over a large size range. We propose a method to estimate cellular carbon content directly from FLS, which will improve our estimates of the contribution of different phytoplankton groups to productivity and total carbon content in the oceans.

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Binary-black-hole orbits precess when the black-hole spins are misaligned with the binary's orbital angular momentum. The apparently complicated dynamics can in most cases be described as simple precession of the orbital angular momentum about an approximately fixed total angular momentum. However, the imprint of the precession on the observed gravitational-wave signal is yet more complicated, with a nontrivial time-varying dependence on the black-hole dynamics, the binary's orientation and the detector polarization. As a result, it is difficult to predict under which conditions precession effects are measurable in gravitational-wave observations, and their impact on both signal detection and source characterization. We show that the observed waveform can be simplified by decomposing it as a power series in a new precession parameter b ¼ tanðβ=2Þ, where β is the opening angle between the orbital and total angular momenta. The power series is made up of five harmonics, with frequencies that differ by the binary's precession frequency, and individually do not exhibit amplitude and phase modulations. In many cases, the waveform can be well approximated by the two leading harmonics. In this approximation we are able to obtain a simple picture of precession as caused by the beating of two waveforms of similar frequency. This enables us to identify regions of the parameter space where precession is likely to have an observable effect on the waveform, and to propose a new approach to searching for signals from precessing binaries, based upon the two-harmonic approximation.

Variability in upper ocean optical properties is often driven by changes in the particle pool. We investigated the effects of such changes by characterizing individual particles. For particles in natural assemblages, we used a combination of Mie theory and flow cytometry to determine diameter (D), complex refractive index (n ϩ inЈ), and optical cross-sections at 488 nm. Particles were grouped into categories of eukaryotic pico/nanophytoplankton, Synechococcus, heterotrophic prokaryotes, detritus, and minerals to interpret variability in concurrently measured bulk inherent optical properties (IOPs) in New England continental shelf waters during two seasons. The summed contributions of individual particles to phytoplankton absorption and particle scattering were close to values for these properties measured independently using bulk methods (87% and 107%, respectively). In surface waters during both seasons, eukaryotic phytoplankton were responsible for the majority of both total particle absorption and total particle scattering. Mineral particles contributed the most to backscattering (b b ) in the spring, whereas in the summer both mineral and detrital particles were important. Synechococcus and heterotrophic prokaryotes never contributed more than 14% to IOPs. Our findings emphasize that the measurement of nonliving particles, including detritus and minerals, is necessary for understanding variability in b b in the ocean, an important quantity in the interpretation of satellite ocean color.

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We present an analysis of the chemical and ionization conditions in a sample of 100 weak Mg ii absorbers identified in the VLT/UVES archive of quasar spectra. In addition to Mg ii, we present equivalent width and column density measurements of other low ionization species such as Mg i, Fe ii, Al ii, C ii, Si ii, and also Al iii. We find that the column densities of C ii and Si ii are strongly correlated with the column density of Mg ii, with minimal scatter in the relationships. The column densities of Fe ii exhibit an appreciable scatter when compared with the column density of Mg ii, with some fraction of clouds having N (Fe ii) $ N (Mg ii), in which case the density is constrained to n H > 0:05 cm À3. Other clouds in which N (Fe ii)TN (Mg ii) have much lower densities. From ionization models, we infer that the metallicity in a significant fraction of weak Mg ii clouds is constrained to values of solar or higher, if they are sub-Lyman-limit systems. Based on the observed constraints, we hypothesize that weak Mg ii absorbers are predominantly tracing two different astrophysical processes/structures. A significant population of weak Mg ii clouds, those in which N (Fe ii)TN (Mg ii), identified at both low (z $ 1) and high (z $ 2) redshift, are likely to be tracing gas in the extended halos of galaxies, analogous to the Galactic high-velocity clouds. These absorbers might correspond to -enhanced interstellar gas expelled from star-forming galaxies, in correlated supernova events. The N(Mg ii) and N(Fe ii)/N(Mg ii) in such clouds are also closely comparable to those measured for the high-velocity components in strong Mg ii systems. An evolution is found in N(Fe ii)/N(Mg ii) from z ¼ 2:4 to z ¼ 0:4, with an absence of weak Mg ii clouds with N (Fe ii) $ N (Mg ii) at high-z. The N (Fe ii) $ N (Mg ii) clouds, which are prevalent at lower redshifts (z < 1:5), must be tracing Type Ia enriched gas in small, high-metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic structures.

After eleven gravitational-wave detections from compact-binary mergers, we are yet to observe the striking general-relativistic phenomenon of orbital precession. Measurements of precession would provide valuable insights into the distribution of black-hole spins, and therefore into astrophysical binary formation mechanisms. Using our recent two-harmonic approximation of precessing-binary signals [S. Fairhurst et al., Phys. Rev. D 102, 024055 (2020)], we introduce the "precession signal-to-noise ratio", ρ p. We demonstrate that this can be used to clearly identify whether precession was measured in an observation (by comparison with both current detections and simulated signals), and can immediately quantify the measurability of precession in a given signal, which currently requires computationally expensive parameter-estimation studies. ρ p has numerous potential applications to signal searches, source-property measurements, and population studies. We give one example: assuming one possible astrophysical spin distribution, we predict that precession has a one in ∼25 chance of being observed in any detection.

We advance a method to determine the diameter D and the complex refractive index (n + n'i) of marine particles from flow cytometric measurements of forward scattering, side scattering, and chlorophyll fluorescence combined with Mie theory. To understand better the application of Mie theory with its assumptions to flow cytometry (FCM) measurements of phytoplankton cells, we evaluate our flow cytometric-Mie (FCM-Mie) method by comparing results from a variety of phytoplankton cultures with independent estimates of cell D and with estimates of n and n' from the inversion of bulk measurements. Cell D initially estimated from the FCM-Mie method is lower than independent estimates, and n and n' are generally higher than bulk estimates. These differences reflect lower forward scattering and higher side scattering for single-cell measurements than predicted by Mie theory. The application of empirical scattering corrections improves FCM-Mie estimates of cell size, n, and n'; notably size is determined accurately for cells grown in both high- and low-light conditions, and n' is correlated with intracellular chlorophyll concentration. A comparison of results for phytoplankton and mineral particles suggests that differences in n between these particle types can be determined from FCM measurements. In application to natural mixtures of particles, eukaryotic pico/nanophytoplankton and Synechococcus have minimum mean values of n' in surface waters, and nonphytoplankton particles have higher values of n than phytoplankton at all depths.