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1. Spatial Intermittency of Particle Distribution in Relativistic Plasma Turbulence

   https://doi.org/10.3847/1538-4357/accd73  

   Vega, Cristian ; Boldyrev, Stanislav ; Roytershteyn, Vadim ( June 2023 , The Astrophysical Journal)

   Relativistic magnetically dominated turbulence is an efficient engine for particle acceleration in a collisionless plasma. Ultrarelativistic particles accelerated by interactions with turbulent fluctuations form nonthermal power-law distribution functions in the momentum (or energy) space,f(γ)dγ∝γ^−αdγ, whereγis the Lorenz factor. We argue that in addition to exhibiting non-Gaussian distributions over energies, particles energized by relativistic turbulence also become highly intermittent in space. Based on particle-in-cell numerical simulations and phenomenological modeling, we propose that the bulk plasma density has lognormal statistics, while the density of the accelerated particles,n, has a power-law distribution function,$P(n)\mathit{dn}\propto n^{-\beta }\mathit{dn}$. We argue that the scaling exponents are related asβ≈α+ 1, which is broadly consistent with numerical simulations. Non-space-filling, intermittent distributions of plasma density and energy fluctuations may have implications for plasma heating and for radiation produced by relativistic turbulence.
2. Kinetic-scale Current Sheets in the Solar Wind at 1 au: Scale-dependent Properties and Critical Current Density

   https://doi.org/10.3847/2041-8213/ac4fc4  

   Vasko, I. Y. ; Alimov, K. ; Phan, T. ; Bale, S. D. ; Mozer, F. S. ; Artemyev, A. V. ( February 2022 , The Astrophysical Journal Letters)

   We present analysis of 17,043 proton kinetic-scale current sheets (CSs) collected over 124 days of Wind spacecraft measurements in the solar wind at 11 samples s⁻¹magnetic field resolution. The CSs have thickness,λ,from a few tens to one thousand kilometers with typical values around 100 km, or within about 0.1–10λ_pin terms of local proton inertial length,λ_p. We found that the current density is larger for smaller-scale CSs,J₀≈ 6 nAm⁻²· (λ/100 km)^−0.56, but does not statistically exceed a critical value,J_A,corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units:$J_0/J_A\approx 0.17·{(\lambda /{\lambda }_p)}^{-0.51}$. The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale:$\mathrm{\Delta }\theta \approx 19°·{(\lambda /{\lambda }_p)}^{0.5}$. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale CSs in the solar wind, while the mechanisms limiting the current density remain to be understood.
3. Microphysics of Relativistic Collisionless Electron-ion-positron Shocks

   https://doi.org/10.3847/1538-4357/ac713e  

   Grošelj, Daniel ; Sironi, Lorenzo ; Beloborodov, Andrei M. ( July 2022 , The Astrophysical Journal)

   We perform particle-in-cell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electron-positron pairs. Various external magnetizationsσ≲ 10⁻⁴and pair-loading factorsZ_±≲ 10 are studied, whereZ_±is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσexceeds a critical valueσ_Lthat decreases withZ_±. Atσ≲σ_Lthe shock is mediated by particle scattering in the self-generated microturbulent fields, the strength and scale of which decrease withZ_±, leading to lowerσ_L. (2) The energy fraction carried by the post-shock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per post-shock electron scales as${\overline{E}}_{\mathrm{e}}\propto {\left(Z_{\pm }+1\right)}^{-1}$. (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low asσ≈ 5 × 10⁻⁶. The ions then become essentially thermal with mean energy${\overline{E}}_{\mathrm{i}}$, while electrons form a nonthermal tail, extending from$E\sim {\left(Z_{\pm }+1\right)}^{-1}{\overline{E}}_{\mathrm{i}}$to${\overline{E}}_{\mathrm{i}}$. Whenσ= 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here,more »the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gamma-ray burst afterglows.

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4. Turbulence in the Sub-Alfvénic Solar Wind

   https://doi.org/10.3847/2041-8213/ac51da  

   Zank, G. P. ; Zhao, L. -L. ; Adhikari, L. ; Telloni, D. ; Kasper, J. C. ; Stevens, M. ; Rahmati, A. ; Bale, S. D. ( February 2022 , The Astrophysical Journal Letters)

   The Parker Solar Probe (PSP) entered a region of sub-Alfvénic solar wind during encounter 8, and we present the first detailed analysis of low-frequency turbulence properties in this novel region. The magnetic field and flow velocity vectors were highly aligned during this interval. By constructing spectrograms of the normalized magnetic helicity, cross-helicity, and residual energy, we find that PSP observed primarily Alfvénic fluctuations, a consequence of the highly field-aligned flow that renders quasi-2D fluctuations unobservable to PSP. We extend Taylor’s hypothesis to sub- and super-Alfvénic flows. Spectra for the fluctuating forward and backward Elsässer variables (z^±, respectively) are presented, showing thatz⁺modes dominatez⁻by an order of magnitude or more, and thez⁺spectrum is a power law in frequency (parallel wavenumber)f^−3/2($k_{\parallel }^{-3/2}$) compared to the convexz⁻spectrum withf^−3/2($k_{\parallel }^{-3/2}$) at low frequencies, flattening around a transition frequency (at which the nonlinear and Alfvén timescales are balanced) tof^−1.25at higher frequencies. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wavenumber anisotropy$k_{\perp }\sim k_{\parallel }^{3/4}$, that thez⁺fluctuations experience primarily nonlinear interactions, and that the minorityz⁻fluctuations experience both nonlinear and Alfvénic interactions withz⁺fluctuations. The density spectrum is a powermore »law that resembles neither thez^±spectra nor the compressible magnetic field spectrum, suggesting that these are advected entropic rather than magnetosonic modes and not due to the parametric decay instability. Spectra in the neighboring modestly super-Alfvénic intervals are similar.

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5. Now You See It, Now You Don’t: Star Formation Truncation Precedes the Loss of Molecular Gas by ∼100 Myr in Massive Poststarburst Galaxies at z ∼ 0.6

   https://doi.org/10.3847/1538-4357/ac3dfa  

   Bezanson, Rachel ; Spilker, Justin S. ; Suess, Katherine A. ; Setton, David J. ; Feldmann, Robert ; Greene, Jenny E. ; Kriek, Mariska ; Narayanan, Desika ; Verrico, Margaret ( February 2022 , The Astrophysical Journal)

   We use ALMA observations of CO(2–1) in 13 massive (M_*≳ 10¹¹M_⊙) poststarburst galaxies atz∼ 0.6 to constrain the molecular gas content in galaxies shortly after they quench their major star-forming episode. The poststarburst galaxies in this study are selected from the Sloan Digital Sky Survey spectroscopic samples (Data Release 14) based on their spectral shapes, as part of the Studying QUenching at Intermediate-z Galaxies: Gas, angu$$\overrightarrow{L}\mathrm{ar}$$momentum, and Evolution ($\mathit{SQuIGG}\vec{L}E$) program. Early results showed that two poststarburst galaxies host large H₂reservoirs despite their low inferred star formation rates (SFRs). Here we expand this analysis to a larger statistical sample of 13 galaxies. Six of the primary targets (45%) are detected, with$M_{{\mathrm{H}}_2}\gtrsim {10}^9$M_⊙. Given their high stellar masses, this mass limit corresponds to an average gas fraction of$〈f_{{\mathrm{H}}_2}\equiv M_{{\mathrm{H}}_2}/M_{*}〉\sim 7\%$or ∼14% using lower stellar masses estimates derived from analytic, exponentially declining star formation histories. The gas fraction correlates with theD_n4000 spectral index, suggesting that the cold gas reservoirs decrease with time since burst, as found in local K+A galaxies. Star formation histories derived from flexible stellar population synthesis modeling support thismore »empirical finding: galaxies that quenched ≲150 Myr prior to observation host detectable CO(2–1) emission, while older poststarburst galaxies are undetected. The large H₂reservoirs and low SFRs in the sample imply that the quenching of star formation precedes the disappearance of the cold gas reservoirs. However, within the following 100–200 Myr, the$\mathit{SQuIGG}\vec{L}E$galaxies require the additional and efficient heating or removal of cold gas to bring their low SFRs in line with standard H₂scaling relations.

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