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1. Frequency dependence of Rayleigh wave amplification by variation in Earth structure investigated using the constant energy flux approximation

   https://doi.org/10.1093/gji/ggaf056  

   Menke, William; Dalton, Colleen A; Lloyd, Andrew; Lopes da Silva, Danielle; Levin, Vadim (February 2025, Geophysical Journal International)

   SUMMARY The sensitivity of Rayleigh wave amplitude to Earth structure has applications to seismic tomography, both in cases where amplitude information is used to supplement phase velocity data to improve images of elastic parameters, and to correct amplitudes for local Earth structure in attenuation tomography. We review the theoretical basis of the ray theoretical approximation, in which the wave amplitudes are controlled by a combination of geometrical spreading and local changes in energy density due to Earth structure. We focus mainly on the latter effect, which we term the constant energy flux approximation. We investigate the ray theoretical basis for this approximation, test it against a full waveform simulation that verifies its accuracy and show how it can be used to compute the sensitivity of amplitude to elastic moduli and density. We investigate how perturbing these parameters in a set of simple Earth models affects Rayleigh wave amplitudes, and demonstrate that a slow velocity heterogeneity can cause either increased or reduced amplitudes, depending upon the depth of the heterogeneity and the observation frequency. Consequently, amplitude sensitivity can be either positive or negative, and its magnitude can vary significantly with frequency. Although an added complication, the very different behaviour of phase velocity and amplitudes to changes in Earth structure implies that the two types of data are complementary and suggest the effectiveness of using both in Rayleigh wave tomography. 

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2. Crowdsourced Doppler measurements of time standard stations demonstrating ionospheric variability

   https://doi.org/10.5194/essd-15-1403-2023  

   Collins, Kristina; Gibbons, John; Frissell, Nathaniel; Montare, Aidan; Kazdan, David; Kalmbach, Darren; Swartz, David; Benedict, Robert; Romanek, Veronica; Boedicker, Rachel; et al (January 2023, Earth System Science Data)

   David Carlson (Ed.)

   Abstract. Ionospheric variability produces measurable effects in Doppler shift of HF (high-frequency, 3–30 MHz) skywave signals. These effects are straightforward to measure with low-cost equipment and are conducive to citizen science campaigns. The low-cost Personal Space Weather Station (PSWS) network is a modular network of community-maintained, open-source receivers, which measure Doppler shift in the precise carrier signals of time standard stations. The primary goal of this paper is to explain the types of measurements this instrument can make and some of its use cases, demonstrating its role as the building block for a large-scale ionospheric and HF propagation measurement network which complements existing professional networks. Here, data from the PSWS network are presented for a period of time spanning late 2019 to early 2022. Software tools for the visualization and analysis of this living dataset are also discussed and provided. These tools are robust to data interruptions and to the addition, removal or modification of stations, allowing both short- and long-term visualization at higher density and faster cadence than other methods. These data may be used to supplement observations made with other geospace instruments in event-based analyses, e.g., traveling ionospheric disturbances and solar flares, and to assess the accuracy of the bottomside estimates of ionospheric models by comparing the oblique paths obtained by ionospheric ray tracers with those obtained by these receivers. The data are archived at https://doi.org/10.5281/zenodo.6622111 (Collins, 2022). 

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3. Charm and bottom hadrons in hot hadronic matter

   https://doi.org/10.1016/j.physrep.2025.05.002  

   Das, Santosh K; Torres-Rincon, J; Rapp, R (May 2025, Physics reports)

   Heavy quarks, and the hadrons containing them, are excellent probes of the QCD medium formed in high-energy heavy-ion collisions, as they provide essential information on the transport properties of the medium and how quarks color-neutralize into hadrons. Large theoretical and phenomenological efforts have been dedicated thus far to assess the diffusion of charm and bottom quarks in the quark-gluon plasma and their subsequent hadronization into heavy-flavor (HF) hadrons. However, the fireball formed in heavy-ion collisions also features an extended hadronic phase, and therefore any quantitative analysis of experimental observables needs to account for the rescattering of charm and bottom hadrons. This is further reinforced by the presence of a QCD cross-over transition and the notion that the interaction strength is maximal in the vicinity of the pseudo-critical temperature. We review existing approaches for evaluating the interactions of open HF hadrons in a hadronic heat bath and the pertinent results for scattering amplitudes, spectral functions and transport coefficients. While most of the work to date has focused on 𝐷-mesons, we also discuss excited states as well as HF baryons and the bottom sector. Both the HF hadro-chemistry and bottom observables will play a key role in future experimental measurements. We also conduct a survey of transport calculations in heavy-ion collisions that have included effects of hadronic HF diffusion and assess its impact on various observables. 

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4. Properties of high entropy borides synthesized via microwave-induced plasma

   https://doi.org/10.1063/5.0098276  

   Storr, Bria; Moore, Luke; Chakrabarty, Kallol; Mohammed, Zaheeruddin; Rangari, Vijaya; Chen, Cheng-Chien; Catledge, Shane A. (June 2022, APL Materials)

   Microwave-induced plasma was used to anneal precursor powders containing five metal oxides with carbon and boron carbide as reducing agents, resulting in high entropy boride ceramics. Measurements of hardness, phase structure, and oxidation resistance were investigated. Plasma annealing for 45 min in the range of 1500–2000 °C led to the formation of predominantly single-phase (Hf, Zr, Ti, Ta, Mo)B2 or (Hf, Zr, Nb, Ta, Mo)B2 hexagonal structures characteristic of high entropy borides. Oxidation resistance for these borides was improved by as much as a factor of ten when compared to conventional commercial diborides. Vickers and nanoindentation hardness measurements show the indentation size effect and were found to be as much as 50% higher than that reported for the same high entropy boride configuration made by other methods, with average values reaching up to 38 GPa (for the highest Vickers load of 200 gf). Density functional theory calculations with a partial occupation method showed that (Hf, Zr, Ti, Ta, Mo)B2 has a higher hardness but a lower entropy forming ability compared to (Hf, Zr, Nb, Ta, Mo)B2, which agrees with the experiments. Overall, these results indicate the strong potential of using microwave-induced plasma as a novel approach for synthesizing high entropy borides. 

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5. Gradient drift instability and decameter ionospheric irregularities at the edge of polar holes

   Thaller, S; Hughes, J; Crowley, G; Noto, J; Blay, R; Wilson, J (May 2023, Ionospheric Effects Symposium)

   The polar and high latitude regions of the ionosphere are host to complex plasma processes involving Magnetosphere-Ionosphere (MI) coupling, plasma convection, and auroral dynamics. The magnetic field lines from the polar cusp down through the auroral region map out to the magnetosphere and project the footprint of the large-scale convective processes driven by the solar wind onto the ionosphere. This region is also a unique environment where the magnetic field is oriented nearly vertical, resulting in horizontal drifts along closed, localized, convection patterns, and where prolonged periods of darkness during the winter result in the absence of significant photoionization. This set of conditions results in unique ionospheric structures which can set the stage for the generation of the gradient drift instability (GDI). The GDI occurs when the density gradient and ExB plasma drift are in the same direction. The GDI is a source of structuring at density gradients and may give rise to ionospheric irregularities that impact over-the-horizon radars and GPS signals. While the plasma ExB drifts are supplied by magnetospheric convection and MI coupling, sharp density gradients in the polar regions will be present at polar holes. Since the GDI occurs where the density gradient and plasma drift are parallel, the ionospheric irregularities caused by the GDI should occur at the leading edge of the polar hole. If so, the resulting production of small-scale density irregularities may, if the density is high enough, give rise to scintillation of GNSS signals and backscatter on HF radars. In this study, we investigate whether these irregularities can occur at the edges of polar holes as detected by the HF radar scatter. We use the Ionospheric Data Assimilation 4-Dimentional (IDA4D) and Assimilative Mapping of Ionospheric Electrodynamics (AMIE) models to characterize the high latitude ionospheric density and ExB drift convective structures, respectively, for one of nine polar hole events identified using RISR-N incoherent scatter radar in Forsythe et al [2021]. The combined IDA4D and AMIE assimilative outputs indicate where the GDI could be triggered, e.g., locations where the density gradient and ExB drift velocity have parallel components and the growth rate is smaller than the characteristic time over which the convective pattern changes, in this case, ~1/15 min. The presence of decameter ionospheric plasma irregularities is detected using the Super Dual Auroral Radar Network (SuperDARN). SuperDARN radars are HF coherent scatter radars. The presence of ionospheric radar returns in regions unstable to GDI grown strongly suggest the GDI is producing decameter scale plasma irregularities. The statistical analyses conducted in the above investigation do not show a clear pattern of enhanced scatter with larger computed GDI growth rates. Further investigation must be conducted before concluding that the GDI does not cause irregularities detectable with HF radar at polar holes. 

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