A document by Adrien J Mourey. Click on the document to view its contents.

We present a statistical study of large magnetic field vector residuals between Swarm observations and the 13th generation International Geomagnetic Reference Field (IGRF-13) model under quiet to weakly disturbed geomagnetic conditions. Since the vast majority of data are taken during relatively quiet geomagnetic conditions, statistics of these large residuals are important for estimating potential errors for satellite operation when using IGRF-13 as reference, as well as for magnetosphere-ionosphere-thermosphere (MIT) studies. Residuals with magnitude of vector differences between Swarm observations and IGRF-13 estimations larger than 300 nT under Dst >-50 nT are studied from 2014 to 2020. All large residuals appear in the high-latitude auroral zone region peaking around 70° (-70°) magnetic latitude (MLAT) in the northern hemisphere (southern hemisphere) but there is also a secondary occurrence peak just below 80° (-80°) MLAT. However, the two hemispheres show clear asymmetries in the magnetic longitude (MLON) distribution where both hemispheres show high concentration of large residuals around the geographic poles. Since the two geographic poles are located at different geomagnetic locations and polar satellite’s orbits give rise to highly biased number of observations around the geographic poles, it results in high occurrence of large residuals around the geographic poles but a decrease in occurrence rate with respect to the total number of measurements. Identifying the interhemispheric asymmetries resulting from the asymmetry of the geographic poles with respect to the magnetic poles under relatively quiet geomagnetic conditions is helpful for a better understanding of interhemispheric asymmetry for the MIT system.

The volume-integrated pH of seawater can be determined from the frequency and depth dependence of wind-generated ambient noise in the ocean. Over the 1 − 10 kHz frequency band, three main processes contribute to the acoustic attenuation in seawater: the chemical relaxation of boric acid and magnesium carbonate (< 3 kHz, related to pH), and magnesium sulfate (> 3 kHz, unrelated to pH). When local winds are strong (> 10 m/s), the ambient noise is dominated by locally generated surface noise, which exhibits a depth-independent directionality, and weak frequency and depth-dependent intensity. By measuring the depth-dependence of the spectral slope, the pH may be estimated from a comparison of the experimental data with an analytical model of ambient noise. Measurements of the depth-dependent ambient noise field were carried out in the Philippine Sea, Mariana Trench, and Tonga Trench from 2009 to 2021. The wideband (5 Hz - 30 kHz) acoustic data were recorded with untethered, free-falling, autonomous instrument platforms known as Deep Sound, equipped with two or four hydrophones. In all the data collected, the power spectral slopes became steeper with depth due to the stronger attenuation of high frequencies compared to low frequencies. Depth-averaged pH values, ranging from 7.68 to 8.35, were obtained from eight instrument drops. The noise spectral method, which has the potential for determining the depth-averaged value of pH, with the averaging depth being adjustable, could be suitable for the long-term passive acoustic monitoring of ocean acidity.

Seismic ambient noise has revolutionized regional crust and upper mantle imaging for broadband ocean-bottom seismometer (BBOBS) studies. However, the ocean poses unique challenges including tilt and compliance noise and presence of interface waves with considerable water-column sensitivity. Transfer function (TF) noise removal techniques enable separation of fundamental Scholte waves from overtone Rayleigh waves. However, such noise removal strategies do not always provide the expected signal improvement. Here, we leverage data from recent U.S.-led BBOBS deployments to evaluate variability in ambient-noise surface-wave quality and efficacy of TF corrections across different oceanic environments and instrumentation. We examine the 15-30 s and 3-10 s period bands to represent typical ranges for lithospheric imaging. We find that water depth, instrument design, sediment thickness, and ocean basin are the most important factors controlling ambient-noise data quality and effectiveness of TF corrections. At 3-10 s, enhancement of higher-mode Rayleigh waves is most successful in deep water with thin sediments. At 15-30 s, signal is improved both in shallow water due to infragravity wave removal and in instances of high tilt noise. Our results provide new insights into the environmental conditions and instrument designs that shape ambient noise tomography performance, offering strategies for optimizing future BBOBS studies as well as novel seafloor-sensing technologies.

Using three-dimensional MHD simulations, we explore the stability of magnetotail configurations that include a local enhancement of $B_z$ (a “$B_z$ hump”). We focus on configurations that were previously found to be unstable in 2-D, neglecting cross-tail ($y$) variations as well as a cross-tail magnetic field component $B_y$.\, but approached final 2-D stable states. Not surprisingly, the 2-D unstable configurations were also found unstable in 3-D, developing 3-D structure after an initial rise as in 2-D. This is consistent with the fact that the selected $B_z$ hump configurations are characterized also by a local tailward decrease of field line entropy, which has been found to govern ballooning/interchange (B/I) instability \cite{schi04}. The evolution of the maximum electric field of the unstable 3-D modes showed an early exponential growth, which was only modestly faster than the 2-D mode. This might suggest that the unstable ballooning regime extends from $k_y \to \infty$ (proper ballooning modes) over finite $k_y$ even to $k_y \to 0$, at least for some equilibria. When the 3-D modes had evolved they grew significantly faster than the 2-D modes. This was associated with an evolution into several narrow beams. The 3-D modes did not approach stable final configurations. The final 2-D stable configurations approached from the unstable ones were found to be unstable in 3-D. These findings are again consistent with the fact that the 2-D evolution, governed by ideal MHD, did not alter the characteristics of the entropy function.

Borneo is a key piece of the regional tectonic puzzle of southeast Asia and a unique location to study subduction termination, particularly in the north. The tectonic history of Sabah in northern Borneo includes elements of five different subduction events: 1) westward Triassic Palaeo-Pacific (~240-200 Ma); 2) southward Cretaceous Proto-South China Sea (92-86 Ma); 3) southward Late Eocene to Early Miocene (~40-20 Ma) Proto-South China Sea; 4) northwestward Celebes Sea (~17-9 Ma); and 5) southeastward Middle Miocene Sulu Sea (~15-10 Ma) subduction. Anti-clockwise (~45°) rotation of Borneo and Neogene (~23-5 Ma) uplift and subsidence events preceded Late Miocene-Early Pliocene exhumation and emplacement (7.8-7.2 Ma) of the 4095 m high Mt. Kinabalu granite pluton. We utilize the northern Borneo Orogeny Seismic Survey (nBOSS) seismological dataset to calculate teleseismic receiver functions and constrain crust and upper mantle discontinuities beneath Sabah using common conversion point migration imaging. Mid-crustal and Moho discontinuities occur beneath most of Sabah at 22-25 and 30-40 km depth, respectively, but major lateral variations delineate lithospheric boundaries that appear associated with surface ophiolites, Mt. Kinabalu granite, and sedimentary depocenters. We show evidence for an uppermost mantle slab remnant beneath Sabah in the form of a gently westward-dipping, 75-100 km wide, discontinuity at 45-55 km depth, likely composed of oceanic crust, its lithosphere detached, and associated with Celebes Sea subduction. Sabah geology, tectonics, uplift and subsidence are reexamined in light of the new lithosphere images, revealing Sabah and post-subduction tectonics in unprecedented detail.

Active eddies in the eastern Weddell Gyre (WG) facilitate water mass exchange between the WG and warmer Antarctic Circumpolar Current, driving warm inflow into the WG. Using observations and data-assimilating model output, the impact of seasonal extension of the eastern WG on the local eddy kinetic energy (EKE) field was investigated. By altering the ambient density field, the seasonal extension modulates both EKE production via baroclinic instability (BCC) and EKE redistribution through the pressure work (PW). When WG contracts in spring-summer, the EKE produced by BCC tends to accumulate at eastern WG due to the weak outward redistribution by PW. Whereas, during autumn-winter when WG extends, enhanced PW causes a greater EKE outflow than local production. Understanding these dynamics is essential for predicting future eddy-induced warm inflow into the WG and subsequent bottom water formation, especially under the scenario of a potentially extending WG due to climate change.

This study uses the Versatile Electron Radiation Belt (VERB) code to model Saturn’s radiation belt environment, investigating electron acceleration, loss, and transport mechanisms. The simulations consider various physical processes, including convection particularly also driven by a Volland-Stern (VS) electric field, radial diffusion, collisional energy loss, and local wave-particle diffusion driven by chorus and hiss waves. Starting from initial conditions derived from Cassini observations, our simulations successfully reproduce the characteristic ”zebra stripes” pattern in spectrograms of Saturn’s radiation belts. Our results suggest that radial diffusion and neutral-particle interactions have negligible effects on zebra stripe formation. However, the presence of a persistent VS electric field results in significant electron acceleration above the Corotation Drift Resonance (CDR) energies in the MeV energy range, producing flux levels exceeding typical observations. Additional local wave-particle diffusion further enhances electron acceleration in this energy range. When the VS electric field pulse is treated as transient instead of constant to imitate dynamic conditions, the overestimation of MeV electron flux is reduced but remains elevated. Despite these differences, the simulations underscore the critical role of the VS electric field and local diffusion in controlling electron acceleration and transport. Our results highlight the necessity of understanding the interplay between global electric fields and localized diffusion processes in shaping electron dynamics within Saturn’s radiation belts.

Syn- to late-orogenic extension caused by gravitational collapse is a process that has been documented in most orogens. In this paper, we investigate the contemporaneous stress field in Taiwan to examine syn-orogenic extensional faulting and the possible gravitational collapse modes by which it is taking place. We use a large, well-constrained, clustered earthquake focal mechanism data set from which we calculate the contemporaneous stress field (σ1, σ2, σ3), the direction of the maximum compressive horizontal stress (SH), and determine the most likely fault plane orientations and kinematics. Our results show that, in the upper 21 km of the crust, shearing, with lesser compression are the dominant stress regimes in the Western Foothills and Hsuehshan Range, resulting in a complex interaction of strike-slip (with transpression and transtension) and thrust faulting. Extension is predominately localised in the upper 14 km of the crust in the Central Range, where it is roughly orogen-parallel. SH and the best-fit fault planes in the Central Range strike roughly NW-SE, at a high angle to the structural and topographic grain. At depths greater than 21 km, compression and shear (with transpression and minor extension) are the dominant stress regimes throughout Taiwan. Along the eastern flank of the mountain belt, mixed compression and shear stress regimes at depths greater than 50 km are related to the Manila and Ryukyu subduction zones. Our results suggest that Taiwan is undergoing orogen-parallel extension as a result of lateral extrusion with free-boundary gravitational collapse.

Europa’s leading hemisphere is composed of water-ice and hydrated salts, indicated by near-IR water-related spectral features. NaCl also exists on Europa’s surface [1], which has significant implications for the habitability of the subsurface ocean. Hydrohalite (NaCl•2H2O) is expected to be stable, but has not been detected; yet the process of desiccating to NaCl is not understood. We explore this process through laboratory experiments involving near-IR spectra of hydrated salts under simulated Europa surface desiccation processes, specifically micrometeoroid bombardment and thermal and vacuum processing. Epsomite (MgSO4•7H2O), hydrated NaCl (containing adsorbed molecular water), and hydrohalite (NaCl•2H2O) were analyzed. Samples were prepared by grinding, wet sieving under liquid nitrogen, and pressing samples into pellets for analysis in an ultra-high vacuum chamber. The pellets were cooled to ~<140K and lased with 355 nm, 200 mJ, 5.25 ns pulse, 500 μm beam diameter laser to replicate micrometeoroid bombardment [2]. The samples were then thermally desiccated at room temperature under vacuum overnight. The initial laser bombardment disrupted the surface of the epsomite pellet, resulting in surface whose spectra are consistent with finer-grained material, evidenced by a larger volume-scattering reflectance peak near 4 and 5-μm. Warming over 4 hrs to 224K partially desiccated the epsomite, as evidenced by shallowing of the 1.5 and 2-μm bands and the narrowing of the 2-μm band. Leaving it in vacuum overnight at room temperature led to further desiccation, with both bands shallowing and the 1.5-μm narrowing. For the hydrated NaCl, a color center formed at 460 nm, likely the result of the UV illumination, disappearing as the temperature increased to 207K. Although the surface was highly disrupted, there was little evidence for desiccation in the NIR spectra due either to lasing or exposure to vacuum at >200K (it is expected that room temperature will result in desiccation). These results indicate that micrometeoroid bombardment is primarily a mechanical weathering process. The NaCl on Europa’s surface may contain adsorbed H2O robust against desiccation by micrometeoroid bombardment and thermal processing.[1] Trumbo, S.K., et al., 2022, PSJ, 3:27.[2] Gillis-Davis, J., 2022, EPSC, 16, 1193.

A document by Savannah Lynn. Click on the document to view its contents.

Vertical mode and rotary-with-depth statistics are compiled from a global dataset of full-depth lowered ADCP baroclinic velocity profiles. Mode-1 standing waves make up half (~0.5) profiles at the equator, increasing to ~0.9 at 60-70ºS and in boundary currents and the Antarctic Circumpolar Current. However, this implies that higher modes dominate in almost half of the profiles. Rotary clockwise-and counterclockwise-with-depth profiles each make up ~0.2 of the profiles at mid and low latitudes, mostly in modes 2 and higher. The mode-1 zero-crossing or node is often offset relative to WKB-stretched mid-depth. Surface and bottom anomalies are present in ~0.9 of the profiles, particularly those with higher energy, with bottom anomalies slightly more common over rougher topography. Low modes are responsible for redistributng and homogenizing the internal-wave field from localized sources but the practice of fitting coarsely-resolved profile data to mode-1 standing-wave structure is not valid everywhere, particularly near the equator.

Magnetic reconnection in Earth’s magnetotail is thought to create bursty bulk flows (BBFs), short-lived plasma bulk velocity enhancements in the magnetotail’s central plasma sheet (CPS) region. Closely related to BBFs are dipolarization fronts (DFs), sudden increases in Bz, the magnetic field component aligned with Earth’s magnetic dipole axis. Both phenomena affect energy distribution and flux transport in the magnetotail.   We demonstrate novel methods of identifying BBFs and DFs in 3D global magnetospheric simulations and present results for multiple case studies. We search for BBFs and DFs in a simulation conducted using the 3D global magnetospheric hybrid-Vlasov code Vlasiator. DFs are identified using a magnetic field time derivative threshold, whereas BBF are defined based on a velocity threshold. Tailward DFs (anti-dipolarization fronts) are found at magnetic islands, while earthward DFs are mostly seen in finger-like structures of high earthward bulk velocity alongside BBFs. We show that detections of fast flows meeting the BBF criteria in virtual spacecraft time series also originate due to moving reconnection locations and movement of the current sheet within the CPS region, while the reconnection outflow stays roughly constant. The results show that rapid Bz variations in the simulation have multiple sources, and similar satellite measurements of BBFs can arise from different physical phenomena. Our findings may help with interpreting satellite observations in the magnetotail.

Mixed-phase clouds are important for simulating precipitation formation and cloud radiative effects in Numerical Weather Prediction (NWP) and climate models. One challenge to reduce model uncertainties is how to best represent the sub-grid inhomogeneity of liquid and ice within a model grid-box. This is poorly constrained by observations, yet is key for representing microphysical process rates that grow ice crystals at the expense of liquid droplets. This study uses in-situ airborne observations from stratiform, shallow cumulus, deep convective and frontal clouds to investigate the spatial overlap of liquid and ice phases on length scales ~1 km, which are appropriate for current regional NWP models. We place observational constraints on a simple parametrisation that describs the mixed-phase fraction as a function of sub-grid liquid and ice cloud fractions and demonstrate that most of the observations show that when ice and liquid are present they are close to fully overlapped.

• A parametric study of electrostatic analyzers based on the donut topology with a 0-20keV energy range was conducted • A set of 3 construction parameters can be used to carefully tune the geometric factor and energy resolution at a specific angular resolution • It suggests to be a versatile concept for a variety of applications from space weather to performance intensive science missions.

Climate change disproportionately impacts vulnerable communities around the globe. Previous work has shown that more socially vulnerable communities at the city and regional level within the contiguous United States (CONUS) are exposed to higher temperatures, but the impact of future warming on this unequal heat exposure remains unknown. We assess past and future average daily maximum temperature exposure across the CONUS in high- and low-vulnerability locations using a social vulnerability index (SVI). We find that the disparity in temperature exposure for high-SVI areas increased from ~1.3°C (1951-1980) to ~1.6°C (1994-2023). Using an ensemble of downscaled climate models, we assess future impacts at global warming levels of 1.5°C, 2°C, and 3°C. Specifically, we compare the number of days/year that exceed air temperature thresholds recommended for safe fan use (35°C, 39°C) in high-SVI and low-SVI areas. We find that for exposure to a 35°C air temperature threshold, at 1.5°C global warming there is an average annual exposure difference of ~16 days, at 2°C ~18 days, and at 3°C ~22 days. For exposure to a 39°C threshold, at 1.5°C global warming there is an exposure difference of ~5 days, at 2°C ~6 days, and at 3°C ~9 days. These results highlight that in many low-elevation, high-SVI areas future warming will disproportionately increase the number of days exceeding cooling and adaptation-relevant temperature thresholds. These findings further support the need to slow global warming to lessen adverse impacts for all communities, and they emphasize the need to direct adaptation efforts towards more vulnerable communities.

In this study, we investigate the effective resolution of deterministic AI weather prediction models. We find that an ideal, perfectly trained AI model follows the mean of the predictive distribution for the lead time interval which is used in its loss function during training. We demonstrate the consequences and limitations of this result with forecast data from various AI models, including Aurora, Pangu, GraphCast and GenCast and we compare them to ensemble and deterministic forecasts from the European Centre for Medium Range Weather Forecasting. We further demonstrate the impact of the resolution on mean-square error scores and suggest a method for a fairer comparison of two models with different effective resolution.

▪ Moss et al. 2019. An investigation into different correlation methods between NMR T2 distributions and primary drainage capillary pressure curves using an extensive sandstone database. E3S Web of Conferences. 89(2019), pp. 02003.