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1. Kinetic simulations of the filamentation instability in pair plasmas

   https://doi.org/10.1093/mnras/stad1100  

   Iwamoto, Masanori; Sobacchi, Emanuele; Sironi, Lorenzo (April 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The non-linear interaction between electromagnetic waves and plasmas attracts significant attention in astrophysics because it can affect the propagation of Fast Radio Bursts (FRBs) – luminous millisecond-duration pulses detected at radio frequency. The filamentation instability (FI) – a type of non-linear wave–plasma interaction – is considered to be dominant near FRB sources, and its non-linear development may also affect the inferred dispersion measure of FRBs. In this paper, we carry out fully kinetic particle-in-cell simulations of the FI in unmagnetized pair plasmas. Our simulations show that the FI generates transverse density filaments, and that the electromagnetic wave propagates in near vacuum between them, as in a waveguide. The density filaments keep merging until force balance between the wave ponderomotive force and the plasma pressure gradient is established. We estimate the merging time-scale and discuss the implications of filament merging for FRB observations. 

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2. A fully kinetic model for orphan gamma-ray flares in blazars

   https://doi.org/10.1093/mnras/stab562  

   Sobacchi, Emanuele; Nättilä, Joonas; Sironi, Lorenzo (March 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Blazars emit a highly variable non-thermal spectrum. It is usually assumed that the same non-thermal electrons are responsible for the IR-optical-UV emission (via synchrotron) and the gamma-ray emission (via inverse Compton). Hence, the light curves in the two bands should be correlated. Orphan gamma-ray flares (i.e. lacking a luminous low-frequency counterpart) challenge our theoretical understanding of blazars. By means of large-scale two-dimensional radiative particle-in-cell simulations, we show that orphan gamma-ray flares may be a self-consistent by-product of particle energization in turbulent magnetically dominated pair plasmas. The energized particles produce the gamma-ray flare by inverse Compton scattering an external radiation field, while the synchrotron luminosity is heavily suppressed since the particles are accelerated nearly along the direction of the local magnetic field. The ratio of inverse Compton to synchrotron luminosity is sensitive to the initial strength of turbulent fluctuations (a larger degree of turbulent fluctuations weakens the anisotropy of the energized particles, thus increasing the synchrotron luminosity). Our results show that the anisotropy of the non-thermal particle population is key to modelling the blazar emission. 

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3. Modeling the energetic tail of a dusty plasma's electron energy distribution and its effect on dust grain charge and behavior

   https://doi.org/10.1063/5.0145209  

   Nicolov, André; Bellan, Paul M (August 2023, Physics of Plasmas)

   A model for a weakly ionized dusty plasma is proposed in which UV or x-ray radiation continuously creates free electrons at high energy, which then cool through collisions with a cold neutral gas before recombining. The transition of a free electron from high energy at birth to low energy at demise implies that the electron energy distribution is not the simple Maxwellian of an isolated system in thermal equilibrium, but instead has a high-energy tail that depends on the recombination time. This tail can have a major effect on dust grain charging because the flux of tail electrons can be substantial even if the density of tail electrons is small. Detailed analytic and numerical calculations of dust grain charging show that situations exist in which a small high-energy tail dominates charge behavior. This implies that dust grain charge in terrestrial and space dusty plasmas may be significantly underestimated if a Maxwellian distribution is assumed and the non-thermal dynamics are neglected. 

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4. Self-consistent calculations of the electric charge, ion drag force, and the drift velocity of spherical grains using Langevin dynamics and comparisons against canonical experiments

   https://doi.org/10.1063/5.0164245  

   Madugula, Venkata; Suresh, Vikram; Liu, Zhibo; Ballard, Davis; Wymore, Logan; Gopalakrishnan, Ranganathan (December 2023, Physics of Plasmas)

   We present trajectory simulation-based modeling to capture the interactions between ions and charged grains in dusty or complex plasmas. Our study is motivated by the need for a self-consistent and experimentally validated approach for accurately calculating the ion drag force and grain charge that determine grain collective behavior in plasmas. We implement Langevin dynamics in a computationally efficient predictor–corrector approach to capture multiscale ion and grain dynamics. Predictions of grain velocity, grain charge, and ion drag force are compared with prior measurements to assess our approach. The comparisons reveal excellent agreement to within ±20% between predicted and measured grain velocities [Yaroshenko et al., Phys. Plasmas 12, 093503 (2005) and Khrapak et al., Europhys. Lett. 97, 35001 (2012)] for 0.64, 1.25 μm grains at ∼20−500 Pa. Comparisons with the measured grain charge [Khrapak et al., Phys. Rev. E 72, 016406 (2005)] under similar conditions reveal agreement to within ∼20% as well. Measurements of the ion drag force [Hirt et al., Phys. Plasmas 11, 5690 (2004); IEEE Trans. Plasma Sci. 32, 582 (2004)] are used to assess the viability of the presented approach to calculate the ion drag force experienced by grains exposed to ion beams of well-defined energy. Excellent agreement between calculations and measurements is obtained for beam energies >10 eV, and the overprediction below 10 eV is attributed to the neglect of charge exchange collisions in our modeling. Along with critical assessments of our approach, suggestions for future experimental design to probe charging of and momentum transfer onto grains that capture the effect of space charge concentration and external fields are outlined. 

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5. Leveraging automated mineralogy analysis to assess paleosol chemical weathering

   Weislogel, A; Pfaff, K; Knapp, J (July 2025, Goldschmidt 2025)

   Paleosol weathering commonly is characterized by an index of alteration determined from bulk rock elemental abundances. Although a variety of indices exist, they all essentially compare the proportion of immobile major elemental oxides associated with refractory minerals (mainly Al2O3 in phyllosilicate clays) to mobile major elements associated with labile minerals (i.e., CaO, Na2O). Higher proportions of immobile major elemental oxides reflects more intense chemical weathering, which is used to infer warmer, wetter climate conditions. However, bulk rock alteration indices are known to be influenced by variability in particle size, effects of quartz dilution, and authigenic mineralization, which are challenging to account for when destructive analytical approaches are used. We tested a non-destructive method of assessing chemical weathering intensity using automated mineralogy analysis, which relies on SEM BSE imaging and EDS spectrum acquisition with automated matching of whole spectra to reference spectra to generate quantitative mineralogy estimates. Our case study focused on 9 Upper Pennsylvanian Spodosol samples from the Appalachian basin, and the dominant mineralogic group identified by automated mineralogy in all samples were phyllosilicate clays (50-85%). Six samples showed >20% feldspar: however, grain shape analysis indicates often micron-scale clay-sized grains with prismatic crystal habit were assigned a potassium feldspar spectra. This is due to the automated mineralogy algorithm identifying a mixed signal from sub-micron quartz/amorphous silica and illite in the X-ray volume as a feldspar x-ray spectrum. Furthermore, the Chemical Index of Alteration calculated from bulk elemental estimates of automated mineralogy results ranged from ~60-65, and A-CN-K ternary plots indicate samples were influenced by K-metasomatism. Using an algorithm that assigns automated mineral identification on the basis of both whole-spectrum matching and particle morphology attributes (including grain shape/crystal habit and size), we can better constrain mineralogical interpretation of fine-grained sedimentary lithologies. This allows automated mineralogy analysis to be used in a new approach for assessing weathering intensity by analyzing the proportion of mobile versus immobile elements in prismatic grains versus equant grains across a distribution of grain sizes in a way that can mitigate or minimize known limitations to the accuracy of alteration indices (e.g., authigenic skins, K-metasomatism). 

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