arXiv subjects

Hans Goedbloed

Publications and source records attributed to Hans Goedbloed.

5 recordsLinked to original sources

arXiv2026

Once more: Leaky MHD waves in coronal magnetic flux tubes

By a detailed comparison of leaky magnetohydrodynamic waves in coronal magnetic flux tubes with leaky electromagnetic waves in dielectric media it is shown that the latter kind may be called quasi-normal modes, since they can be regularised by a normalisation which systematically cuts off the contribution of the external homogeneous region, whereas such a possibility is forbidden for the former kind by the conservation of magnetic flux. Consequently, leaky magnetohydrodynamic waves cannot be systematically applied to coronal seismology, i.e. to the inverse spectral problem of determining the different equilibrium distributions of the fields by comparing the spectra they produce with the observed ones.

astro-ph.SR↗

arXiv2025

The hydrodynamic thermal continuum, with applications to stratified atmospheres and 1D coronal loop models

Using both analytical and numerical means, we demonstrate that linear stability analysis of a hydrodynamic stratified atmosphere or a 1D coronal loop model in non-adiabatic settings features a thermal continuum corresponding to highly localized eigenfunctions. This thermal continuum can be precomputed, involving the net heat-loss function and its partial derivatives, and is the generalization of the thermal instability introduced by~\citet{Parker1953}. We account for a thermal imbalance, directly affecting thermal instability growthrates. We present completely general equations that govern all eigenmodes, including non-adiabatically affected p- and g-modes of the stratified settings. We intend to clarify how linear thermal instability is relevant for solar loops that show spontaneous in-situ condensations, and eliminate recent confusion on specific isochoric routes to linear instability alongside other thermal instability channels. The thermal continuum, previously identified as a crucial ingredient in magnetohydrodynamic eigenmode spectra for coronal loops and atmospheres, drives multithermal aspects across our universe, such as forming solar coronal rain and prominences, or cold cloud creation in intracluster to interstellar medium environments.

astro-ph.SR↗

arXiv2024

Parametric Survey of Nonaxisymmetric Accretion Disk Instabilities: Magnetorotational Instability to Super-Alfv\'enic Rotational Instability

Accretion disks are highly unstable to magnetic instabilities driven by shear flow, where classically, the axisymmetric, weak-field Magneto-Rotational Instability (MRI) has received much attention through local WKB approximations. In contrast, discrete non-axisymmetric counterparts require a more involved analysis through a full global approach to deal with the influence of the nearby magnetohydrodynamic (MHD) continua. Recently, rigorous MHD spectroscopy identified a new type of an ultra-localised, non-axisymmetric instability in global disks with super-Alfv\'enic flow. These Super-Alfv\'enic Rotational Instabilities (SARIs) fill vast unstable regions in the complex eigenfrequency plane with (near-eigen)modes that corotate at the local Doppler velocity and are radially localised between Alfv\'enic resonances. Unlike discrete modes, they are utterly insensitive to the radial disk boundaries. In this work, we independently confirm the existence of these unprecedented modes using our novel spectral MHD code Legolas reproducing and extending our earlier study with detailed eigenspectra and eigenfunctions. We calculate growth rates of SARIs and MRI in a variety of disk equilibria, highlighting the impact of field strength and orientation, and find correspondence with analytical predictions for thin, weakly magnetised disks. We show that non-axisymmetric modes can significantly extend instability regimes at high mode numbers, with maximal growth rates comparable to the MRI. Furthermore, we explicitly show a region filled with quasi-modes whose eigenfunctions are extremely localised in all directions. These modes must be ubiquitous in accretion disks, and play a role in local shearing box simulations. Finally, we revisit recent dispersion relations in the Appendix, highlighting their relation to our global framework.

astro-ph.HE↗

arXiv2022

The Super-Alfvénic Rotational Instability in accretion disks about black holes

The theory of instability of accretion disks about black holes, neutron stars or proto-planets, is revisited by means of the recent method of the Spectral Web. The cylindrical accretion disk differential equation is shown to be governed by the forward and backward Doppler-shifted continuous Alfvén spectra $Ω_{\rm A}^\pm \equiv m Ω\pm ω_{\rm A}$, where $ω_{\rm A}$ is the static Alfvén frequency. It is crucial to take non-axisymmetry ($m \ne 0$) and super-Alfvénic rotation of the Doppler frames ($|mΩ| \gg |ω_{\rm A}|$) into account. The continua $Ω_{\rm A}^+$ and $Ω_{\rm A}^-$ then overlap, ejecting a plethora of Super-Alfvénic Rotational Instabilities (SARIs). In-depth analysis for small inhomogeneity shows that the two Alfvén singularities reduce the extent of the modes to sizes much smaller than the width of the accretion disk. Generalization for large inhomogeneity leads to the completely unprecedented result that, for mode numbers $|k| \gg |m|$, any complex $ω$ in a wide neighborhood of the real axis is an approximate `eigenvalue'. The difference with genuine eigenmodes is that the amount of complementary energy to excite the modes is tiny, $|W_{\rm com}| \le c$, with $c$ the machine accuracy of the computation. This yields a multitude of two-dimensional continua of quasi-discrete modes: quasi-continuum SARIs. We conjecture that the onset of 3D turbulence in magnetized accretion disks is governed, not by the excitation of discrete axisymmetric Magneto-Rotational Instabilities, but by the excitation of modes from these two-dimensional continua of quasi-discrete non-axisymmetric Super-Alfvénic Rotational Instabilities.

astro-ph.HE↗

arXiv2004

Transonic instabilities in accretion disks

In two previous publications$^{1,2}$, we have demonstrated that stationary rotation of magnetized plasma about a compact central object permits an enormous number of different MHD instabilities, with the well-known magneto-rotational instability as just one of them. We here concentrate on the new instabilities found that are driven by transonic transitions of the poloidal flow. A particularly promising class of instabilities, from the point of view of MHD turbulence in accretion disks, is the class of {\em trans-slow Alfven continuum modes}, that occur when the poloidal flow exceeds a critical value of the slow magnetosonic speed. When this happens, virtually every magnetic/flow surface of the disk becomes unstable with respect to highly localized modes of the continuous spectrum. The mode structures rotate, in turn, about the rotating disk. These structure lock and become explosively unstable when the mass of the central object is increased beyond a certain critical value. Their growth rates then become huge, of the order of the Alfven transit time. These instabilities appear to have all requisite properties to facilitate accretion flows across magnetic surfaces and jet formation.[1] R. Keppens, F. Casse, J.P. Goedbloed, "Waves and instabilities in accretion disks: Magnetohydrodynamic spectroscopic analysis", Astrophys. J. {\bf 569}, L121--L126 (2002).[2] J.P. Goedbloed, A.J.C. Belien, B. van der Holst, R. Keppens, "Unstable continuous spectra of transonic axisymmetric plasmas", Phys. Plasmas {\bf 11}, 28--54 (2004).

physics.plasm-ph↗