RESEARCH OF THE PARAMETRIC SPACE OF THE THREE-MODE αω-DYNAMO MODEL

В рамках настоящей работы изучаются простая феноменологическая модель aw-динамо, с динамическим подавлением α-генератора квадратичной формой от компонент магнитного поля. В частности, рассматривается подавление спиральностью и энергией. Определяется форма ядра функционала подавления при которой интегральный член можно устранить и свести модель к системе дифференциальной лоренцевского типа. В общем случае в модели есть возможность широкого варьирования ядра функционала и вида квадратичной формы. В рассматриваемой модели потенциально могут реализовываться значительно более богатый набор динамических режимов, по сравнению с классическимслучаем лоренцевской системы In the framework of this work, we study a simple phenomenological model of a αω — dynamo, with dynamic suppression of a α-generator by a quadratic form from a magnetic field component. In particular, suppression of spiralness and energy is available. The shape of the nucleus is determined, and the system of differentiation of the Lorentz type. In general, the models have the possibility of a wide range of functional and quadratic forms. Compared with the classical case of the Lorentz system.

Magnetic properties of a long-lived sunspot

Context. In a recent statistical study of sunspots in 79 active regions, the vertical magnetic field component Bver averaged along the umbral boundary is found to be independent of sunspot size. The authors of that study conclude that the absolute value of Bver at the umbral boundary is the same for all spots. Aims. We investigate the temporal evolution of Bver averaged along the umbral boundary of one long-lived sunspot during its stable phase. Methods. We analysed data from the HMI instrument on-board SDO. Contours of continuum intensity at Ic = 0.5Iqs, whereby Iqs refers to the average over the quiet sun areas, are used to extract the magnetic field along the umbral boundary. Projection effects due to different formation heights of the Fe I 617.3 nm line and continuum are taken into account. To avoid limb artefacts, the spot is only analysed for heliocentric angles smaller than 60°. Results. During the first disc passage, NOAA AR 11591, Bver remains constant at 1693 G with a root-mean-square deviation of 15 G, whereas the magnetic field strength varies substantially (mean 2171 G, rms of 48 G) and shows a long term variation. Compensating for formation height has little influence on the mean value along each contour, but reduces the variations along the contour when away from disc centre, yielding a better match between the contours of Bver = 1693 G and Ic = 0.5Iqs. Conclusions. During the disc passage of a stable sunspot, its umbral boundary can equivalently be defined by using the continuum intensity Ic or the vertical magnetic field component Bver. Contours of fixed magnetic field strength fail to outline the umbral boundary.

Magnetic fields in massive spirals: The role of feedback and initial conditions

Context. Magnetic fields play a very important role in the evolution of galaxies through their direct impact on star formation and stellar feedback-induced turbulence. However, their co-evolution with these processes has still not been thoroughly investigated, and the possible effect of the initial conditions is largely unknown. Aims. This Letter presents the first results from a series of high-resolution numerical models, aimed at deciphering the effect of the initial conditions and of stellar feedback on the evolution of the galactic magnetic field in isolated Milky Way-like galaxies. Methods. The models start with an ordered magnetic field of varying strength, either poloidal or toroidal, and are evolved with and without supernova feedback. They include a dark matter halo, a stellar and a gaseous disk, as well as the appropriate cooling and heating processes for the interstellar medium. Results. Independently of the initial conditions, the galaxies develop a turbulent velocity field and a random magnetic field component in under 15 Myr. Supernova feedback is extremely efficient in building a random magnetic field component up to large galactic heights. However, a random magnetic field emerges even in runs without feedback, which points to an inherent instability of the ordered component. Conclusions. Supernova feedback greatly affects the velocity field of the galaxy up to large galactic heights, and helps restructure the magnetic field up to 10 kpc above the disk, independently of the initial magnetic field morphology. On the other hand, the initial morphology of the magnetic field can accelerate the development of a random component at large heights. These effects have important implications for the study of the magnetic field evolution in galaxy simulations.