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1. Scaling of Magnetic Reconnection Electron Bulk Heating in the High-Alfvén-speed and Low-β Regime of Earth’s Magnetotail

   https://doi.org/10.3847/1538-4357/acdf44  

   Øieroset, M; Phan, T D; Oka, M; Drake, J F; Fuselier, S A; Gershman, D J; Maheshwari, K; Giles, B L; Zhang, Q; Guo, F; et al (August 2023, The Astrophysical Journal)

   Abstract We have surveyed 21 reconnection exhaust events observed by Magnetospheric MultiScale in the low-plasma-βand high-Alfvén-speed regime of the Earth’s magnetotail to investigate the scaling of electron bulk heating produced by reconnection. The ranges of inflow Alfvén speed and inflow electronβ_ecovered by this study are 800–4000 km s⁻¹and 0.001–0.1, respectively, and the observed heating ranges from a few hundred electronvolts to several kiloelectronvolts. We find that the temperature change in the reconnection exhaust relative to the inflow, ΔT_e, is correlated with the inflow Alfvén speed,V_Ax,in, based on the reconnecting magnetic field and the inflow plasma density. Furthermore, ΔT_eis linearly proportional to the inflowing magnetic energy per particle, ${m_{\mathrm{i}}{V}_{\mathrm{Ax},\mathrm{in}}}^2$, and the best fit to the data produces the empirical relation ΔT_e= 0.020 ${m_{\mathrm{i}}{V}_{\mathrm{Ax},\mathrm{in}}}^2$, i.e., the electron temperature increase is on average ∼2% of the inflowing magnetic energy per particle. This magnetotail study extends a previous magnetopause reconnection study by two orders of magnitude in both magnetic energy and electronβ, to a regime that is comparable to the solar corona. The validity of the empirical relation over such a large combined magnetopause–magnetotail plasma parameter range ofV_A∼ 10–4000 km s⁻¹andβ_e∼ 0.001–10 suggests that one can predict the magnitude of the bulk electron heating by reconnection in a variety of contexts from the simple knowledge of a single parameter: the Alfvén speed of the ambient plasma. 

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2. Electron Acceleration at Quasi-parallel Nonrelativistic Shocks: A 1D Kinetic Survey

   https://doi.org/10.3847/1538-4357/ad7c4c  

   Gupta, Siddhartha; Caprioli, Damiano; Spitkovsky, Anatoly (November 2024, The Astrophysical Journal)

   Abstract We present a survey of 1D kinetic particle-in-cell simulations of quasi-parallel nonrelativistic shocks to identify the environments favorable for electron acceleration. We explore an unprecedented range of shock speedsv_sh≈ 0.067–0.267c, Alfvén Mach numbers ${\mathcal{M}}_{\mathrm{A}}=5–40$, sonic Mach numbers ${\mathcal{M}}_{\mathrm{s}}=5–160$, as well as the proton-to-electron mass ratiosm_i/m_e= 16–1836. We find that high Alfvén Mach number shocks can channel a large fraction of their kinetic energy into nonthermal particles, self-sustaining magnetic turbulence and acceleration to larger and larger energies. The fraction of injected particles is ≲0.5% for electrons and ≈1% for protons, and the corresponding energy efficiencies are ≲2% and ≈10%, respectively. The extent of the nonthermal tail is sensitive to the Alfvén Mach number; when ${\mathcal{M}}_{\mathrm{A}}\lesssim 10$, the nonthermal electron distribution exhibits minimal growth beyond the average momentum of the downstream thermal protons, independently of the proton-to-electron mass ratio. Acceleration is slow for shocks with low sonic Mach numbers, yet nonthermal electrons still achieve momenta exceeding the downstream thermal proton momentum when the shock Alfvén Mach number is large enough. We provide simulation-based parameterizations of the transition from thermal to nonthermal distribution in the downstream (found at a momentum around $p_{\mathrm{i},\mathrm{e}}/m_{\mathrm{i}}{v}_{\mathrm{sh}}\approx 3\sqrt{m_{\mathrm{i},\mathrm{e}}/m_{\mathrm{i}}}$), as well as the ratio of nonthermal electron to proton number density. The results are applicable to many different environments and are important for modeling shock-powered nonthermal radiation. 

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3. Imaging nuclear shape through anisotropic and radial flow in high-energy heavy-ion collisions

   https://doi.org/10.1088/1361-6633/ae0fc3  

   Collaboration, The STAR (October 2025, Reports on Progress in Physics)

   Abstract Most atomic nuclei exhibit ellipsoidal shapes characterized by quadrupole deformationβ₂and triaxialityγ, and sometimes even a pear-like octupole deformationβ₃. The STAR experiment introduced a new ‘imaging-by-smashing’ technique ((STAR Collaboration) 2024Nature63567; Jia 2025Rep. Prog. Phys.88092301) to image the nuclear global shape by colliding nuclei at ultra-relativistic speeds and analyzing outgoing debris. Features of nuclear shape manifest in collective observables like anisotropic flowv_nand radial flow via mean transverse momentum $\left[p_{\mathrm{T}}\right]$. We present new measurements of the variances ofv_n(n = 2, 3, and 4) and $\left[p_{\mathrm{T}}\right]$, and the covariance of $v_n^2$with $\left[p_{\mathrm{T}}\right]$, in collisions of highly deformed²³⁸U and nearly spherical¹⁹⁷Au. Ratios of these observables between the two systems effectively suppress common final-state effects, isolating the strong impact of uranium’s deformation. By comparing results with state-of-the-art hydrodynamic model calculations, we extract ${\beta }_{2\mathrm{U}}$and ${\gamma }_{\mathrm{U}}$values consistent with those deduced from low-energy nuclear structure measurements. Measurements ofv₃and its correlation with $\left[p_{\mathrm{T}}\right]$also provide the first experimental suggestion of a possible octupole deformation for²³⁸U. These findings provide significant support for using high-energy collisions to explore nuclear shapes on femtosecond timescales, with implications for both nuclear structure and quark-gluon plasma studies. 

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4. Suppression of Collisionless Magnetic Reconnection in the High Ion β, Strong Guide Field Limit

   https://doi.org/10.3847/1538-4357/ad9274  

   Giai, Carlos A; Haggerty, Colby C; Shay, Michael A; Cassak, Paul A; Davis, Zachary K (December 2024, The Astrophysical Journal)

   Abstract In magnetic reconnection, the ion bulk outflow speed and ion heating have been shown to be set by the available reconnecting magnetic energy, i.e., the energy stored in the reconnecting magnetic field (B_r). However, recent simulations, observations, and theoretical works have shown that the released magnetic energy is inhibited by upstream ion plasma betaβ_i—the relative ion thermal pressure normalized to magnetic pressure based on the reconnecting field—for antiparallel magnetic field configurations. Using kinetic theory and hybrid particle-in-cell simulations, we investigate the effects ofβ_ion guide field reconnection. While previous works have suggested that guide field reconnection is uninfluenced byβ_i, we demonstrate that the reconnection process is modified and the outflow is reduced for sufficiently large ${\beta }_i>(B_r^2+B_g^2)/B_r^2$. We develop a theoretical framework that shows that this reduction is consistent with an enhanced exhaust pressure gradient, which reduces the outflow speed as $v_{\mathrm{out}}\propto 1/\sqrt{{\beta }_i}$. These results apply to systems in which guide field reconnection is embedded in hot plasmas, such as reconnection at the boundary of eddies in fully developed turbulence like the solar wind or the magnetosheath as well as downstream of shocks such as the heliosheath or the mergers of galaxy clusters. 

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5. The Three-mm Ultimate Mopra Milky Way Survey. III. Data Release 6, An Atlas of Physical Conditions, Global Mass Conversion Laws, and 3D Physical Architecture of the Molecular Interstellar Medium in the Fourth Quadrant

   https://doi.org/10.3847/1538-4365/adf32b  

   Barnes, Peter J; Barnes, Dylan_G H; Hernández, Audra K; Lopez, Sebastian; Muller, Erik (September 2025, The Astrophysical Journal Supplement Series)

   Abstract We present Data Release 6 of ThrUMMS, consisting of complete data cubes and various moments of line emission (¹²CO,¹³CO, C¹⁸O) from molecular clouds, across 60° × 2° of the Fourth Quadrant (4Q) of the Milky Way at a resolution of 72″ in (l,b) and 0.09 km s⁻¹inV_LSR. From LTE radiative transfer analysis of the data cubes, we compute cubes and moments of the lines’ opacity, excitation temperature, and column density $N_{{}^{12}\mathrm{CO}}$. Combining $I_{{}^{12}\mathrm{CO}}$and $N_{{}^{12}\mathrm{CO}}$data, we derive a global mass conversion lawN=N₀I^p, whereN₀≈ 10¹⁸mol m⁻²andp= 2 at this resolution. We argue that the standard linearN= XI is only approximately valid:p∼ 1.5–1.0 at coarser resolutions or in atypical locations, such as Galactic center clouds. The velocity dispersion distributions are very different between $I_{{}^{12}\mathrm{CO}}$and $N_{{}^{12}\mathrm{CO}}$, the former preferentially tracing more diffuse molecular gas. We reevaluated Galactic rotation parameters for the 4Q, defining a new “BGT” model, and deprojected the (l,V) data onto (l,d) and (x,y) grids using standard kinematic procedures. To automate distance disambiguation inside the solar circle, we developed a simpleζ⁺discriminator function and applied it to our deprojections. We discovered two previously unrecognised features of the molecular cloud population: widespread ripples in the midplane of wavelength 4 kpc and amplitude 50 pc, potentially generated by the last perigalactic passage of the Sgr dwarf; and three distant, massive molecular structures, the Far Ara Clouds, two of which exhibit an exceptional velocity gradient, possibly lying near the bar’s far end or in a gas-rich dwarf galaxy ∼20–300 kpc beyond the disk. 

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