arXiv subjects

Cosima Breu

Publications and source records attributed to Cosima Breu.

3 recordsLinked to original sources

arXiv2026

Polarization of decayless kink oscillations in a 3D MHD coronal loop model

Decayless kink oscillations are frequently observed in solar coronal loops and are considered potential contributors to coronal heating. Despite the ubiquity of this wave phenomenon, its driving mechanism remains unclear. Studies to derive the polarization state of these oscillations, which would be a key to identifying the drivers, have been limited due to observational constraints. We analyze a 3D MHD simulation of coronal loops using the MURaM code. Synthetic extreme ultraviolet (EUV) emission maps, combined with velocity diagnostics, are used to identify and characterize transverse wave motions in the simulated loop structures. This is the first demonstration of decayless kink waves emerging self-consistently in a 3D MHD loop-in-a-box model. The simulation produces persistent, low-amplitude, decayless kink oscillations that closely match observed properties. These oscillations arise spontaneously, without any imposed periodic driver, and exhibit clear linear polarization with oscillation planes not aligned to the principal axes. The observed coherency of linear polarization with oscillation cycles favors a self-sustained or quasi-steady type wave driver over a stochastic or broadband source.

astro-ph.SR↗

arXiv2023

How numerical treatments of the transition region modify energy flux into the solar corona

The large temperature gradients in the solar transition region present a significant challenge to large scale numerical modelling of the Sun's atmosphere. In response, a variety of techniques have been developed which modify the thermodynamics of the system. This sacrifices accuracy in the transition region in favour of accurately tracking the coronal response to heating events. Invariably, the modification leads to an artificial broadening of the transition region. Meanwhile, many contemporary models of the solar atmosphere rely on tracking energy flux from the lower atmosphere, through the transition region and into the corona. In this article, we quantify how the thermodynamic modifications affect the rate of energy injection into the corona. We consider a series of one-dimensional models of atmospheric loops with different numerical resolutions and treatments of the thermodynamics. Then, using Alfv\'en waves as a proxy, we consider how energy injection rates are modified in each case. We find that the thermodynamic treatment and the numerical resolution significantly modify Alfv\'en travel times, the eigenfrequencies and eigenmodes of the system, and the rate at which energy is injected into the corona. Alarmingly, we find that the modification of the energy flux is frequency dependent, meaning that it may be difficult to compare the effects of different velocity drivers on coronal heating if they are imposed below an under-resolved transition region, even if the sophisticated thermodynamic adaptations are implemented.

astro-ph.SR↗

arXiv2016

Maximum mass, moment of inertia and compactness of relativistic stars

A number of recent works have highlighted that it is possible to express the properties of general-relativistic stellar equilibrium configurations in terms of functions that do not depend on the specific equation of state employed to describe matter at nuclear densities. These functions are normally referred to as "universal relations" and have been found to apply, within limits, both to static or stationary isolated stars, as well as to fully dynamical and merging binary systems. Further extending the idea that universal relations can be valid also away from stability, we show that a universal relation is exhibited also by equilibrium solutions that are not stable. In particular, the mass of rotating configurations on the turning-point line shows a universal behaviour when expressed in terms of the normalised Keplerian angular momentum. In turn, this allows us to compute the maximum mass allowed by uniform rotation, M_{max}, simply in terms of the maximum mass of the nonrotating configuration, M_{TOV}, finding that M_{max} ~ (1.203 +- 0.022) M_{TOV} for all the equations of state we have considered. We further show that a universal relation can be found between the dimensionless moment of inertia and the stellar compactness. Although this relation is not surprising as it involves two quantities that have been shown to exhibit universal behaviour with other stellar properties, our parameterisation represents a refinement over a similar relation by Lattimer and Schutz (2005), where a different normalisation was used, and could provide an accurate tool to constrain the equation of state of nuclear matter when measurements of the moment of inertia become available.

gr-qc↗