Search arXiv⌕ Search
Home/Archive/arXiv

arXiv · 2608.00343

The generation of spiral density waves by MRI in accretion discs

G. Mamatsashvili·D. Kobaidze·W. Mouhali·T. Lehner·L. Burdiladze
View original source ↗Read source document ↗Export metadata ↓

Abstract

We investigate the linear dynamics of non-axisymmetric perturbations in Keplerian discs subject to a weak uniform vertical magnetic field in the shearing box approximation. Perturbations are decomposed into shearing waves and evolved by numerically integrating the linearized ideal MHD equations. The disc flow supports three basic perturbation modes: two incompressible modes - magnetic mode that undergoes magnetorotational instability (MRI) and inertia-magnetic waves - and compressible spiral density waves. The magnetic mode and inertia-magnetic waves have a low frequency of the order of Alfvén and orbital frequencies, respectively, while density waves have high-frequency. We introduce mode eigenfunctions and governing modal equations to analyze the dynamics of individual modes. For non-axisymmetric modes, the modal equations are coupled due to the shear of the Keplerian rotation of the disc, giving rise to a new shear-induced linear mode coupling process, which is rooted in the non-self-adjoint nature of shear flows. We focus on the generation of density waves by the dominant MRI-unstable magnetic mode. We show that initially imposed magnetic mode undergoes MRI growth and abruptly excites density waves when its radial wavenumber crosses zero. The density wave-MRI coupling is most efficient when the azimuthal and vertical wavelengths of perturbations are comparable to the disc scale height. Since density waves are compressible, whereas MRI is incompressible, this wave excitation process can also be regarded as a linear mechanism generating compressible motions via MRI-driven incompressible ones. Its implications for compressible nonzero net vertical field MRI-turbulence are also discussed.

Explore related subjects

astro-ph.EPastro-ph.HEastro-ph.SR

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

G. Mamatsashvili, D. Kobaidze, W. Mouhali, T. Lehner, L. Burdiladze. 2026-07-31. The generation of spiral density waves by MRI in accretion discs. https://arxiv.org/abs/2608.00343

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Bridging magnetothermal winds and photoevaporation to model discs dispersal

Protoplanetary disc dispersal is driven by two processes usually modelled separately: photoevaporative and magnetohydrodynamic (MHD) disc winds. Global simulations indicate that in the inner disc these are not distinct outflows but a single magnetothermal wind. We assemble a closed-form, two-phase model that respects it. A single-field-line wind, whose base is fixed by the irradiated temperature and penetration column, supplies the launch and feeds a secular evolution, with photoevaporation convolved on as a sink. The flux closure $B_z\proptoΣ^q$ is self-limiting: for $q\le1/2$ depletion alone cannot demagnetise the disc, so dispersal requires independent flux loss, parameterised by the magnetic Reynolds number $\mathcal{R}_m$. Integrating the coupled system yields two regimes. Efficient flux loss ($\mathcal{R}_m\lesssim1$) lets the magnetisation front recede by over an order of magnitude and opens a photoevaporative gap. Flux retention ($\mathcal{R}_m\gg1$) drives the front outward, sustains accretion, and defers dispersal by $\approx2.7$~Myr. Deriving the base from stellar irradiation instead of prescribing it, we find that the cold-launch approximation is valid during the early stages of disc evolution: anchoring the base at plasma equipartition ($β_{\rm base} \sim 1$) confines irradiation's influence on the magnetic lever arm to the magnetothermal annulus, decoupling the peak accretion rate from the incident flux. Both regimes clear the disc inside-out, through either a photoevaporatively amplified cavity wall or an expanding magnetothermal front.

astro-ph.EP↗
arXiv2026

Faithful Neural Embeddings for 3D Exoplanet Climate Modeling

With the rapid advancement of telescopes like JWST and Ariel, there is an urgent need for efficient 3D climate models to interpret observations of exoplanet atmospheres. Traditional 3D general circulation models (GCMs) are computationally intensive, prompting the development of machine learning (ML) emulators to accelerate simulations. Recent work, such as that by Plaschzug et al. 2026 \cite{plaschzug2026accelerating}, uses a dense neural network (DNN) to predict local gas temperatures and winds from input parameters, including local gas pressure, spatial coordinates (longitude and latitude), and global temperature. However, this model relies on predicting individual temperature values (points) at specific grid points, which can be limited by the resolution and constraints of the training grid. In this work, we investigate a couple of alternative frameworks based on latent-space representations of local gas temperature ($\text{T}_{\text{gas}}$) to obtain a faithful, low-dimensional representation of these profiles. This represents the first step toward developing a latent space regression model, offering a structurally cohesive alternative to the existing point-wise prediction method \cite{plaschzug2026accelerating}. By capturing the optimal embedding space of atmospheric data, our proposed framework can produce simulated profiles while maintaining computational efficiency, making it suitable for large-scale exoplanet ensemble studies.

astro-ph.EP↗
arXiv2026

Some challenges for the long-term survival of Naiad, Neptune's innermost moon

The Naiad-Thalassa 73:69 mean-motion resonance implies these moons have co-existed for $\gtrsim$1 Gyr, raising the question of how they survived to the present day. We examine three challenges to Naiad's long-term survival: tidal disruption, heliocentric bombardment, and runaway collisional erosion by planetocentric debris. We constrain Naiad's internal strength requirements from its nominal density and shape, compute present-day impact rates on Neptune's inner moons from heliocentric bombardment, and use $N$-body simulations to model the fate of ejecta produced by non-disruptive impacts. First, Naiad's nominal density of ${\sim}0.8\text{ g cm}^{-3}$ and elongated shape suggest it cannot be held together by self-gravity alone, implying a cohesive strength of $\gtrsim10$ kPa to avoid tidal disruption, although this constraint is relaxed if Naiad has a higher density of ${\gtrsim}1.3\text{ g cm}^{-3}$. Second, we show that Naiad may have been disrupted in the last 1 Gyr by heliocentric bombardment, although this depends sensitively on the size-frequency distribution of Kuiper Belt objects at small sizes and on Naiad's catastrophic disruption threshold, both of which are poorly constrained. Third, and most notably, we find that even small, non-disruptive impacts can trigger runaway collisional erosion by planetocentric debris on extremely short timescales. Avoiding this ``sesquinary catastrophe'' requires that Naiad has a collisional strength of at least several MPa, which is difficult to reconcile with being a reaccumulated ``rubble pile'', leftover from the capture of Triton and the subsequent cataclysm of Neptune's primordial satellite system. Together, these results suggest that Naiad may be a physically unusual object among small ring-moons -- possibly a largely coherent, monolithic fragment -- and that our understanding of Neptune's inner satellite system leaves much to be explained.

astro-ph.EP↗