Plasma sheet electron precipitation into the diffuse aurora is critical for magnetosphere‐ionosphere coupling. Recent studies have shown that electron phase space holes can pitch‐angle scatter electrons and may produce plasma sheet electron precipitation. These studies have assumed identical electron hole parameters to estimate electron scattering rates (Vasko et al., 2018,
The mineral apatite, Ca5(PO4)3(F,Cl,OH), is a ubiquitous accessory mineral, with its volatile content and isotopic compositions used to interpret the evolution of H2O on planetary bodies. During hypervelocity impact, extreme pressures shock target rocks resulting in deformation of minerals; however, relatively few microstructural studies of apatite have been undertaken. Given its widespread distribution in the solar system, it is important to understand how apatite responds to progressive shock metamorphism. Here, we present detailed microstructural analyses of shock deformation in ~560 apatite grains throughout ~550 m of shocked granitoid rock from the peak ring of the Chicxulub impact structure, Mexico. A combination of high‐resolution backscattered electron (BSE) imaging, electron backscatter diffraction mapping, transmission Kikuchi diffraction mapping, and transmission electron microscopy is used to characterize deformation within apatite grains. Systematic, crystallographically controlled deformation bands are present within apatite, consistent with tilt boundaries that contain the <
- NSF-PAR ID:
- 10456817
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Meteoritics & Planetary Science
- Volume:
- 55
- Issue:
- 8
- ISSN:
- 1086-9379
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract https://doi.org/10.1063/1.5039687 ). In this study, we have re‐evaluated the efficiency of this scattering by incorporating realistic electron hole properties from direct spacecraft observations into computing electron diffusion rates and lifetimes. The most important electron hole properties in this evaluation are their distributions in velocity and spatial scale and electric field root‐mean‐square intensity (). Using direct measurements of electron holes during a plasma injection event observed by the Van Allen Probe at , we find that when 4 mV/m electron lifetimes can drop below 1 h and are mostly within strong diffusion limits at energies below 10 keV. During an injection observed by the THEMIS spacecraft at , electron holes with even typical intensities ( 1 mV/m) can deplete low‐energy ( a few keV) plasma sheet electrons within tens of minutes following injections and convection from the tail. Our results confirm that electron holes are a significant contributor to plasma sheet electron precipitation during injections. -
Abstract The air‐sea exchange of oxygen (O2) is driven by changes in solubility, biological activity, and circulation. The total air‐sea exchange of O2has been shown to be closely related to the air‐sea exchange of heat on seasonal timescales, with the ratio of the seasonal flux of O2to heat varying with latitude, being higher in the extratropics and lower in the subtropics. This O2/heat ratio is both a fundamental biogeochemical property of air‐sea exchange and a convenient metric for testing earth system models. Current estimates of the O2/heat flux ratio rely on sparse observations of dissolved O2, leaving it fairly unconstrained. From a model ensemble we show that the ratio of the seasonal amplitude of two atmospheric tracers, atmospheric potential oxygen (APO) and the argon‐to‐nitrogen ratio (Ar/O2), exhibits a close relationship to the O2/heat ratio of the extratropics (40–
). The amplitude ratio, / , is relatively constant within the extratropics of each hemisphere due to the zonal mixing of the atmosphere. / is not sensitive to atmospheric transport, as most of the observed spatial variability in the seasonal amplitude of APO is compensated by similar variations in (Ar/ ). From the relationship between /heat and / in the model ensemble, we determine that the atmospheric observations suggest hemispherically distinct /heat flux ratios of 3.3 0.3 and 4.7 0.8 nmol between 40 and in the Northern and Southern Hemispheres respectively, providing a useful constraint for and heat air‐sea fluxes in earth system models and observation‐based data products. -
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Key points Extreme aviation is accompanied by ever‐present risks of hypobaric hypoxia and decompression sickness. Neuroprotection against those hazards is conferred through fractional inspired oxygen (
) concentrations of 60–100% (hyperoxia). Hyperoxia reduces global cerebral perfusion (gCBF), increases reactive oxygen species within the brain and leads to cell death within the hippocampus. However, an understanding of hyperoxia's effect on cortical activity and concomitant levels of cognitive performance is lacking. This limits our understanding of whether hyperoxia could lower the brain's threshold of tolerance to physiological stressors inherent to extreme aviation, such as high gravitational forces.
This study aimed to quantify the impact of hyperoxia upon global cerebral perfusion (gCBF), cognitive performance and cortical electroencephalography (EEG).
Hyperoxia evoked a rapid reduction in gCBF, yet cognitive performance and vigilance were enhanced. EEG measurements revealed enhanced alpha power, suggesting less desynchrony, within the cortical temporal regions.
Collectively, this work suggests hyperoxia‐induced brain hypoperfusion is accompanied by enhanced cognitive processing and cortical arousal.
Abstract Extreme aviators continually inspire hyperoxic gas to mitigate risk of hypoxia and decompression injury. This neuroprotection carries a physiological cost: reduced cerebral perfusion (CBF). As reduced CBF may increase vulnerability to ever‐present physiological challenges during extreme aviation, we defined the magnitude and duration of hyperoxia‐induced changes in CBF, cortical electrical activity and cognition in 30 healthy males and females. Magnetic resonance imaging with pulsed arterial spin labelling provided serial measurements of global CBF (gCBF), first during exposure to 21% inspired oxygen (
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Abstract Mineral inclusions are ubiquitous in metamorphic rocks and elastic models for host‐inclusion pairs have become frequently used tools for investigating pressure–temperature (
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