An analytic mean-field α2\alpha^2-dynamo with a force-free corona

Stellar dynamos are affected by boundary conditions imposed by stellar coronae. Under some approximations it is possible to find analytical solutions. Interior dynamo models often consider a current-free coronae without taking into account the constraints imposed by the presence of currents in the corona. We aim to analytically evaluate the effect of coronal currents and of an outer boundary condition on the efficiency of an $\alpha^2$ dynamo. We intend to estimate the change in geometry and dinamo excitation numbers with respect to the current-free case. We analytically solve the turbulent dynamo induction equation for a homogeneous, non-mirror symmetric turbulence, in a spherical domain surrounded by a linear force-free corona with magnetic field B satisfying curl B = \beta B. The main result is that the dynamo number is a decreasing function of \beta. Moreover, if the current is parallel to the field (\beta > 0) the dynamo number is smaller than in the force-free case. On the contrary (\beta < 0) the dynamo number is greater than the in force-free case. We conclude that the presence of currents in the corona needs to be taken into account because it affects the condition for excitation of a dynamo.Comment: Accepted for publication in A&

Turbulent diffusion and galactic magnetism

Using the test-field method for nearly irrotational turbulence driven by spherical expansion waves it is shown that the turbulent magnetic diffusivity increases with magnetic Reynolds numbers. Its value levels off at several times the rms velocity of the turbulence multiplied by the typical radius of the expansion waves. This result is discussed in the context of the galactic mean-field dynamo.Comment: 2 pages, 1 figure, to appear in "Magnetic Fields in Diffuse Media", proceedings of Joint Discussion at the 2009 XXVII IAU General Assembly in Rio de Janeiro from 12 to 14 August, 200

Vorticity from irrotationally forced flow

In the interstellar medium the turbulence is believed to be forced mostly through supernova explosions. In a first approximation these flows can be written as a gradient of a potential being thus devoid of vorticity. There are several mechanisms that could lead to vorticity generation, like viscosity and baroclinic terms, rotation, shear and magnetic fields, but it is not clear how effective they are, neither is it clear whether the vorticity is essential in determining the turbulent diffusion acting in the ISM. Here we present a study of the role of rotation, shear and baroclinicity in the generation of vorticity in the ISM.Comment: 2 pages, 1 figure, to be published in Proceedings of IAU Symp. 271, Astrophysical Dynamics: from Stars to Galaxies, ed. N. Brummell and A.S. Brun, CU

How can vorticity be produced in irrotationally forced flows?

A spherical hydrodynamical expansion flow can be described as the gradient of a potential. In that case no vorticity should be produced, but several additional mechanisms can drive its production. Here we analyze the effects of baroclinicity, rotation and shear in the case of a viscous fluid. Those flows resemble what happens in the interstellar medium. In fact in this astrophysical environment supernovae explosion are the dominant flows and, in a first approximation, they can be seen as spherical. One of the main difference is that in our numerical study we examine only weakly supersonic flows, while supernovae explosions are strongly supersonic.Comment: 3 pages, 3 figures, to appear in Proceedings of IAU Symp. 274, Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and A. Kosoviche

Influence of Magnetic Helicity in MHD

Observations have shown that the Sun's magnetic field has helical structures. The helicity content in magnetic field configurations is a crucial constraint on the dynamical evolution of the system. Since helicity is connected with the number of links we investigate configurations with interlocked magnetic flux rings and one with unlinked rings. It turns out that it is not the linking of the tubes which affects the magnetic field decay, but the content of magnetic helicity.Comment: 2 pages, 3 figures, proceedings of IAU Symp. 271, Astrophysical Dynamics: from Stars to Galaxies, ed. N. Brummell and A.S. Brun, CU

Magnetic field decay of three interlocked flux rings with zero linking number

The resistive decay of chains of three interlocked magnetic flux rings is considered. Depending on the relative orientation of the magnetic field in the three rings, the late-time decay can be either fast or slow. Thus, the qualitative degree of tangledness is less important than the actual value of the linking number or, equivalently, the net magnetic helicity. Our results do not suggest that invariants of higher order than that of the magnetic helicity need to be considered to characterize the decay of the field.Comment: 7 pages, 10 figure

Stabilizing Effect of Magnetic Helicity on Magnetic Cavities in the Intergalactic Medium

We investigate the effect of magnetic helicity on the stability of buoyant magnetic cavities as found in the intergalactic medium. In these cavities we insert helical magnetic fields and test whether or not helicity can increase their stability to shredding through the Kelvin–Helmholtz instability and, with that, their lifetime. This is compared to the case of an external vertical magnetic field that is known to reduce the growth rate of the Kelvin–Helmholtz instability. By comparing a low-helicity configuration with a high-helicity one with the same magnetic energy, we find that an internal helical magnetic field stabilizes the cavity. This effect increases as we increase the helicity content. Stabilizing the cavity with an external magnetic field requires instead a significantly stronger field at higher magnetic energy. We conclude that the presence of helical magnetic fields is a viable mechanism to explain the stability of intergalactic cavities on timescales longer than 100 Myr

Leggi scala per lo studio di magnetosfere di pianeti extrasolari

In questa tesi viene effettuato uno studio generale della dinamica delle magnetosfere planetarie. Viene esaminata la struttura della magnetosfera terrestre, la sua dinamica, le correnti ionosferiche e i fenomeni di emissione di aurora. Verrà quindi trattato il problema dell'interazione tra la magnetosfera e un vento stellare in generale, riferendosi non solo al caso terrestre ma anche ai diversi altri pianeti del sistema solare in generale. L'interazione tra il campo magnetico planetario ed il vento stellare dà vita ad una serie di fenomeni, quali assorbimento, ricombinazione e riconnessione magnetica, che consentono di ottenere informazioni sui plasmi che circondano un pianeta e popolano la sua magnetosfera grazie allo studio degli spettri tipici di emissione. Tramite tali fenomeni si possono dedurre leggi scala sull'andamento delle peculiari emissioni magnetosferiche, in particolare grazie alle osservazioni nella zona radio dello spettro. L'analisi di queste relazioni consente la predizione di fenomeni magnetosferici su pianeti extrasolari e la predizione sulle eventuali possibili osservazioni. I pianeti extrasolari sono attualmente scoperti essenzialmente mediante il metodo della velocità radiale, che è un metodo di detezione indiretto. L'osservazione di linee spettrali tipiche delle emissioni planetarie è invece un metedo diretto di osservazione, come ad esempio il metodo dell'osservazione del transito sul disco stellare. Lo studio dell'emissione nella parte ultravioletta dello spettro dà informazioni sulla presenza e abbondanza di elementi nell'atmosfera consentendo, tra l'altro, di avanzare ipotesi sull'abitabilità di pianeti extrasolari