More Like this

1. Assessing the Sources of the O ⁺ in the Plasma Sheet

   https://doi.org/10.1029/2024JA032635  

   Liao, J.; Kistler, L_M; Mouikis, C_G; Fuselier, S_A; Hedlund, M. (August 2024, Journal of Geophysical Research: Space Physics)

   Abstract To study the average contributions of the cusp outflow through the lobes and of the nightside auroral outflow to the O⁺in the plasma sheet (PS), we performed a statistical study of tailward streaming O⁺in the lobes, plasma sheet boundary layer|the plasma sheet boundary layer (PSBL) and the PS, using MMS/Hot Plasma Composition Analyzer (HPCA) data from 2017 to 2020. Similar spatial patterns illustrate the entry of cusp‐origin O⁺from the lobes to the PS through the PSBL. There is an Y_GSM‐dependent energy pattern for the lobe O⁺, with low‐energy O⁺streaming closer to the tail center and high energy (1–3 keV) O⁺streaming near the flanks. Low energy (1–100 eV) O⁺from the nightside auroral oval is identified in the near‐Earth PSBL/PS with high‐density (>0.02 cm⁻³), and energetic (>3 keV) streaming O⁺with similar density (∼0.013 cm⁻³) is observed further out on the duskside of the PSBL/PS. The rest of the nightside auroral O⁺in the PSBL is mixed with O⁺coming in from the lobe, making it difficult to distinguish the source. We estimated the contributions of the different sources of H⁺and O⁺ions through the PS between 7 and 17 R_E, using estimates from this work and data extracted from previous studies. We conclude that, during quiet times, the majority of the near‐Earth PS H⁺are from the cusps, the polar wind and Earthward convection from the distant tail. Similarly, while the O⁺in the same region has a mixed source, cusp origin outflow provides the highest contribution. 

   more » « less
2. Estimates of the Isotope Effect for Nitrate Assimilation in the Indian Sector of the Southern Ocean

   https://doi.org/10.1029/2023JC020830  

   Thomas, R_K; Fawcett, S_E; Forrer, H_J; Robinson, C_M; Knapp, A_N (July 2024, Journal of Geophysical Research: Oceans)

   Abstract The Southern Ocean is a high‐nutrient, low‐chlorophyll (HNLC) region characterized by incomplete nitrate (NO₃⁻) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the¹⁴N‐ versus the¹⁵N‐bearing form of NO₃⁻, quantified as the NO₃⁻assimilation isotope effect (¹⁵ε). Previous summertime estimates of the¹⁵ε from HNLC regions range from 4 to 11‰. While culture work has shown that the¹⁵ε varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the¹⁵ε varies in the field. Here we estimate the¹⁵ε from water‐column profile and surface‐water samples collected in the Indian sector of the Southern Ocean—the first leg of the Antarctic Circumnavigation Expedition (December 2016–January 2017) and the Crossroads transect (April 2016). Consistent with prior work in the mid‐to‐late summer Southern Ocean, we estimate a higher¹⁵ε (8.9 ± 0.6‰) for the northern Subantarctic Zone and a lower¹⁵ε (5.4 ± 0.9‰) at and south of the Subantarctic Front. We interpret our data in the context of coincident measurements of phytoplankton community composition and estimates of iron and light stress. Similar to prior work, we find a significant, negative relationship between the¹⁵ε and the average mixed‐layer photosynthetically active radiation flux of 30–100 μmol m⁻² s⁻¹, while above 100 μmol m⁻² s⁻¹,¹⁵ε increases again. In addition, while we observe no robust relationship of the¹⁵ε to iron availability or phytoplankton community, mixed‐layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude. 

   more » « less
3. The Enge Split-Pole Spectrograph at the University of Notre Dame

   https://doi.org/10.1051/epjconf/202430402002  

   Carmichael, Scott; O’Malley, Patrick; Bardayan, Daniel; Bailey, Thomas; Boomershine, Chevelle; Brodeur, Maxime; Coil, Sydney; Dembski, Cade; Gore, Tom; Jones, Chloe; et al (January 2024, EPJ Web of Conferences)

   Pakou, A; Souliotis, G; Moustakidis, C (Ed.)

   Nuclear reactions play a crucial role in determining the nucleosynthesis that occurs in astrophysical events. The rates of many reactions that significantly impact certain nucleosynthesis processes can not be currently measured via direct means. These reactions must be constrained in another manner, such as determining the level energies and other structure properties of the compound nuclei. In order to measure level energies of nuclei relevant to nuclear astrophysics, the Enge split-pole spectrograph has been installed and commissioned at the University of Notre Dame’s Nuclear Science Laboratory. The first scientific measurement has also been performed. Structure properties of⁵⁸Cu were measured via the reaction⁵⁸Ni(³He,t)⁵⁸Cu to provide the first experimental constraint of the⁵⁷Ni(p,γ)⁵⁸Cu reaction rate, which impacts the production of of⁴⁴Ti,⁵⁷Fe, and⁵⁹Ni in core-collapse supernovae. Preliminary analysis of this measurement confirms the level energies of states in⁵⁸Cu that could lead to significant resonances in the⁵⁷Ni(p,γ)⁵⁸Cu reaction rate, while suggesting the presence of additional states that have not been previously observed but could also lead to significant resonances. 

   more » « less
4. Phosphorus-bearing molecules PO and PN at the edge of the Galaxy

   https://doi.org/10.1038/s41586-023-06616-1  

   Koelemay, L. A.; Gold, K. R.; Ziurys, L. M. (November 2023, Nature)

   Abstract Despite its importance in planet formation and biology¹, phosphorus has been identified only in the inner 12 kpc of the Galaxy^2–19. The study of this element has been hindered in part by unfavourable atomic transitions^2,4,20. Phosphorus is thought to be created by neutron capture on²⁹Si and³⁰Si in massive stars^20,21, and released into the interstellar medium by Type II supernova explosions^2,22. However, models of galactic chemical evolution must arbitrarily increase the supernovae production²³to match observed abundances. Here we present the detection of gas-phase phosphorus in the Outer Galaxy through millimetre spectra of PO and PN. Rotational lines of these molecules were observed in the dense cloud WB89-621, located 22.6 kpc from the Galactic Centre²⁴. The abundances of PO and PN in WB89-621 are comparable to values near the Solar System²⁵. Supernovae are not present in the Outer Galaxy²⁶, suggesting another source of phosphorus, such as ‘Galactic Fountains’, where supernova material is redistributed through the halo and circumgalactic medium²⁷. However, fountain-enriched clouds are not found at such large distances. Any extragalactic source, such as the Magellanic Clouds, is unlikely to be metal rich²⁸. Phosphorus instead may be produced by neutron-capture processes in lower mass asymptotic giant branch stars²⁹which are present in the Outer Galaxy. Asymptotic giant branch stars also produce carbon²¹, flattening the extrapolated metallicity gradient and accounting for the high abundances of C-containing molecules in WB89-621. 

   more » « less
5. On the Replicability of the Thermodynamic Modeling of Spectroscopic Titration Data in the Nickel(II) En System

   https://doi.org/10.1002/cem.3619  

   Lawler, Fenton C.; Storteboom, Ryan S.; Rosales‐Lopez, Plinio D.; Hoogstra, Madison N.; Selvaggio, Katherine J.; Chen, Trevina; Zogg, Krista A.; Heule, Dafna L.; Pehrson, Noah J.; Baker, Aerin E.; et al (October 2024, Journal of Chemometrics)

   ABSTRACT Characterizing complicated solution phase systems in situ requires advanced modeling techniques to capture the intricate balances between the many chemical species. Due to the error inherent in any scientific measurement, a spectrophotometric titration experiment with nickel(II) and ethylenediamine (en) was repeated six times using an autotitrator to test the replicability of the data and the consistency of the resulting thermodynamic model. All six datasets could be modeled very tightly (R² > 99.9999%) with the following eight complexes: [Ni]²⁺, [Ni₂en]⁴⁺, [Nien]²⁺, [Ni₂en₃]⁴⁺, [Nien₂]²⁺, [Ni₂en₅]⁴⁺, [Nien₃]²⁺, and [Nien₆]²⁺. The logK values for the stepwise associative reactions agree with existing literature values for the majority species ([Nien_{n = 1–3}]²⁺) and matched expectations for the minority species; 95% confidence intervals for each logK value were determined via bootstrapping, which quantifies the variability in the binding constant value that is supported by a given dataset. The repeated experiments, which could not be successfully concatenated together, demonstrate that replication is crucial to capturing all the variability in the logK values. Conversely, bootstrapped confidence intervals across multiple experiments can be readily combined to generate an appropriate range for an experimentally determined binding constant. 

   more » « less