Investigating lack of accretion disk eccentricity growth in a global 3D MHD simulation of a superhump system

We present the results of a 3D global magnetohydrodynamic (MHD) simulation of an AM CVn system that was aimed at exploring eccentricity growth in the accretion disc self-consistently from a first principles treatment of the MHD turbulence. No significant eccentricity growth occurs in the simulation. In order to investigate the reasons why, we ran 2D alpha disc simulations with alpha values of 0.01, 0.1, and 0.2, and found that only the latter two exhibit significant eccentricity growth. We present an equation expressing global eccentricity evolution in terms of contributing forces and use it to analyse the simulations. As expected, we find that the dominant term contributing to the growth of eccentricity is the tidal gravity of the companion star. In the 2D simulations, the alpha viscosity directly contributes to eccentricity growth. In contrast, the overall magnetic forces in the 3D simulation damp eccentricity. We also analyzed the mode-coupling mechanism of Lubow, and confirmed that the spiral wave excited by the 3:1 resonance was the dominant contributor to eccentricity growth in the 2D α = 0.1 simulations, but other waves also contribute significantly. We found that the α = 0.1 and 0.2 simulations had more relative mass at larger radii compared to the α = 0.01 and 3D MHD simulation, which also had an effective α of 0.01. This suggests that in 3D MHD simulations without sufficient poloidal magnetic flux, MRI turbulence does not saturate at a high enough α to spread the disc to large enough radii to reproduce the superhumps observed in real systems.

Emission of Joule heating events in simulations of the solar corona

Context. Nanoscale events in cooperation with steady heating from a slow heating mechanism, such as slow-burning current-sheets, could be able to heat the corona; however, their observational traces are hard to detect via current instrumentation. After we locate heating events in magnetohydrodynamic (MHD) simulations and synthesise observational data, we extract observational signatures of small-scale events. Aims. Our mission is threefold. The first goal is to observe the manifestation of small-scale events via three observational tools: intensity maps of three extreme ultraviolet (EUV) filters in the Atmospheric Imaging Assembly (AIA) instrument with resolution better than that in AIA images, emission measure (EM) analysis, and time-lag maps. The second goal is to identify the reason why we cannot quantify the energy release from observed events. The third goal is to study the differences between the radiation from isolated heating events and that from the whole corona. Methods. We employed a three-dimensional magnetohydrodynamic (3D-MHD) simulation using the Bifrost code. We simulated the atmosphere of a network embedded in the quiet Sun (QS), and we identified 3D heating events in the corona in several time-steps. Then we synthesised the three observational tools for two cases. First, we considered information from the total column mass in the corona, and then we considered only regions that exhibit heating events. Results. We report on the differences between the two regions of investigation, which also consist of the evidence to justify why observers cannot identify small-scale heating events in observations. We found that the combination of multiple heating events at different cooling phases along the line of sight gives the impression of thin elongated threads of events. For this reason, the EM as a function of temperature has a multi-thermal distribution. Both the radiation and the emission measure of the isolated heating events have values at least ten times lower than the signal calculated from the total corona. We also found that heating events move together with diffuse emission from the slow heating mechanism, and for this reason we cannot differentiate between the two. In addition, we find that the frequency of heating events and their intensity affect the EM distribution as a function of temperature. We also find that the filter’s intensity, EM, and time-lag maps of heating events are different to those incorporating information from the total column mass of the corona. However, the two regions have, on average, comparable values, which are slightly smaller than the analytical cooling timescales calculated for an optically thin and radiation-dominated atmosphere.

MHD simulations of ram pressure stripping of disk galaxies

The removal of ISM of disk galaxies through ram pressure stripping (RPS) has been extensively studied in numerous simulations (see Roediger 2009 and references therein). The models show that RPS has a significant impact on galaxy evolution (truncation of the ISM will lead to a decrease in star formation and a change in galaxy color). Nevertheless, the role of magnetic fields (MFs) on the dynamics of the gas in this process has not been sufficiently studied, although the influence of the MFs on the large scale structure is well established. This motivated us to perform a 3D MHD simulation of a disk galaxy with an isothermal, non-self gravitating and magnetized gaseous disk in equilibrium with a galaxy potential (Allen & Santillán, 1991). We model RPS on the galactic disk under the wind-tunnel approximation with the use of the RAMSES code (Teyssier, 2002) in order to understand the effects of MFs in RPS.