Theoretical astrophysics and plasma physics at RPC

Abstract:

This work investigates the nonlinear transition to large heat fluxes observed in local gyrokinetic (GK) simulations of electromagnetic (EM) turbulence in STEP (Meyer 2024 Phil. Trans. R. Soc. 382 20230406). Using the stress-balance framework of (Zhang et al 2026 J. Plasma Phys.) we confirm that the onset of extreme transport correlates with a critical value of q2βe, where q is the safety factor and βe is the ratio of electron thermal pressure to magnetic pressure, and relate this to a limit on the poloidal beta βpol. Crucially, this critical value lies below any relevant linear stability limit in the (q,βe) space (e.g. the onset of ideal or kinetic ballooning modes). Using an extensive set of nonlinear GK simulations, we demonstrate that the transition to large fluxes in STEP is governed by a balance between the electrostatic and magnetic-flutter stresses. We argue, and also show numerically, that larger-major-radius tokamaks reach the EM non-zonal regime at lower βe, making this MHD-controlled saturation limit more accessible in reactor-scale devices than in small spherical tokamaks. We also demonstrate that access to a second-stable regime enables re-saturation at larger values of β′. We further show that the ideal ballooning mode (IBM) threshold serves as a useful proxy for delineating this second-stable region and also tracks the onset of large fluxes. These results provide a predictive framework for identifying no-go zone predictions from local GKs and offer new insight into the EM saturation physics relevant to STEP and other high- βe devices.

Abstract:

Abstract Mercury's magnetosphere is an extremely dynamic environment, where substorm‐like activity, including dipolarization fronts and particle acceleration, periodically disrupts the stability and radial extent of the closed field line region. The common narrowband ∼1 Hz waves at Mercury, near the local proton gyrofrequency are preferentially observed on closed field lines during quieter intervals, but their relationship to magnetospheric dynamics remains poorly understood. Here, we combine MESSENGER data sets of ∼1 Hz wave activity and dipolarization fronts to investigate whether newly closed field lines can sustain wave growth. We find that ∼1 Hz waves on the nightside are systematically absent during orbits with dipolarization intervals and typically confined within 1.3 R M , compared to greater downtail extents during quiet intervals. The results indicate that ∼1 Hz waves require long‐lived, stable closed field lines to grow; therefore, when they are observed, they can serve as diagnostics of Mercury's magnetospheric state, establishing a clear connection between field‐line history, magnetospheric dynamics, and wave occurrence in Mercury's magnetotail.

Abstract:

Abstract The origin of the complex orbital structure of young massive stars at the Galactic centre remains an open question. If these stars formed in a single episode from a gaseous accretion disc, they may initially have constituted a single, coherently rotating stellar disc. We investigate whether perturbations from an unseen intermediate-mass black hole (IMBH) could fragment and/or disrupt such a disc into the multiple orbital components observed today. First, we derive a theoretical criterion for when and where the IMBH’s torque overcomes the disc’s self-torque and tears it apart. We then test this picture with direct N-body simulations of a stellar disc interacting with an inclined IMBH around a central supermassive black hole. We find that the outcome depends strongly on the IMBH’s orbit and mass. A prograde IMBH rapidly aligns with the stellar disc, while a massive retrograde IMBH (m• ≃ 0.67 Md) anti-aligns relative to the radially overlapping stars and efficiently fragments the original disc into three components in angular-momentum space: an inner disc, a misaligned overlapping region, and an unperturbed outer disc. The IMBH also excites eccentricities in the overlapping region, driving stars away from initially circular orbits. These features emerge for an IMBH mass of 2000 ⊙ and a disc mass of 3000 ⊙ within 10–20 Myr, a timescale comparable to the age of the young Galactic centre stellar population, and provide a plausible explanation for the observed multiple orbital planes, warped geometry, and broad eccentricity distribution.

Abstract:

Abstract Multiscale temperature fluctuations are abundant in the intracluster medium (ICM) outside of galaxy cluster cores (∼100 kpc). Their origin is often attributed to turbulent stirring by subhalos or accreting baryons crossing the virial radius. However, their apparent resistance to mixing and thermal conduction in a collisional medium has not been explained. We propose a new mechanism by which steady-state temperature fluctuations can form and persist outside the cluster core. Local thermal instability, or Field instability, is used to explain filamentary condensates in cluster cores but is usually dismissed outside them because thermal conduction should suppress instability. In weakly collisional or collisionless plasmas, however, thermal conduction can be anomalously suppressed by heat-flux-driven plasma instabilities triggered in the presence of a local magnetic field, leading to two effects: (i) condensates form in a new parameter regime that overlaps with conditions outside the core, and (ii) condensates reach a steady state as in the hydrodynamic limit. This extends the regime of instability-driven fluctuations to over ≳50% (depending on hot plasma temperature) of the cluster. We use one-dimensional hydrodynamic simulations of condensates to test our analytical ideas.