Nonlinear Alfvén wave dynamics at a 2D magnetic null point: ponderomotive force

Context: In the linear, β = 0 MHD regime, the transient properties of magnetohydrodynamic (MHD) waves in the vicinity of 2D null points are well known. The waves are decoupled and accumulate at predictable parts of the magnetic topology: fast waves accumulate at the null point; whereas Alfvén waves cannot cross the separatricies. However, in nonlinear MHD mode conversion can occur at regions of inhomogeneous Alfvén speed, suggesting that the decoupled nature of waves may not extend to the nonlinear regime. Aims: We investigate the behaviour of low-amplitude Alfvén waves about a 2D magnetic null point in nonlinear, β = 0 MHD. Methods: We numerically simulate the introduction of low-amplitude Alfvén waves into the vicinity of a magnetic null point using the nonlinear LARE2D code. Results: Unlike in the linear regime, we find that the Alfvén wave sustains cospatial daughter disturbances, manifest in the transverse and longitudinal fluid velocity, owing to the action of nonlinear magnetic pressure gradients (viz. the ponderomotive force). These disturbances are dependent on the Alfvén wave and do not interact with the medium to excite magnetoacoustic waves, although the transverse daughter becomes focused at the null point. Additionally, an independently propagating fast magnetoacoustic wave is generated during the early stages, which transports some of the initial Alfvén wave energy towards the null point. Subsequently, despite undergoing dispersion and phase-mixing due to gradients in the Alfvén-speed profile (∇c_A ≠ 0) there is no further nonlinear generation of fast waves. Conclusions: We find that Alfvén waves at 2D cold null points behave largely as in the linear regime, however they sustain transverse and longitudinal disturbances - effects absent in the linear regime - due to nonlinear magnetic pressure gradients

Neutrinos and the synthesis of heavy elements: the role of gravity

The synthesis of heavy elements in the Universe presents several challenges. From one side the astrophysical site is still undetermined and on other hand the input from nuclear physics requires the knowledge of properties of exotic nuclei, some of them perhaps accessible in ion beam facilities. Black hole accretion disks have been proposed as possible r-process sites. Analogously to Supernovae these objects emit huge amounts of neutrinos. We discuss the neutrino emission from black hole accretion disks. In particular we show the influence that the black hole strong gravitational field has on changing the electron fraction relevant to the synthesis of elements.Comment: 5 pages, 5 figures, Invited talk at the 15th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics (CGS15), to appear in EPJ Web of Conference

On the periodicity of oscillatory reconnection

Context. Oscillatory reconnection is a time-dependent magnetic reconnection mechanism that naturally produces periodic outputs from aperiodic drivers. Aims. This paper aims to quantify and measure the periodic nature of oscillatory reconnection for the first time. Methods. We solve the compressible, resistive, nonlinear magnetohydrodynamics (MHD) equations using 2.5D numerical simulations. Results. We identify two distinct periodic regimes: the impulsive and stationary phases. In the impulsive phase, we find the greater the amplitude of the initial velocity driver, the longer the resultant current sheet and the earlier its formation. In the stationary phase, we find that the oscillations are exponentially decaying and for driving amplitudes 6.3−126.2 kms−1, we measure stationary-phase periods in the range 56.3−78.9 s, i.e. these are high frequency (0.01−0.02 Hz) oscillations. In both phases, we find that the greater the amplitude of the initial velocity driver, the shorter the resultant period, but note that different physical processes and periods are associated with both phases. Conclusions. We conclude that the oscillatory reconnection mechanism behaves akin to a damped harmonic oscillator

On the self-consistent physical parameters of LMC intermediate-age clusters

The LMC clusters are unique templates of simple stellar population (SSP), being crucial to calibrate models describing the integral light as well as to test the stellar evolution theory. With this in mind we analyzed HST/WFPC2 (V, B--V) colour-magnitude diagrams (CMDs) of 15 populous LMC clusters with ages between ~0.3 Gyr and ~4 Gyr using different stellar evolutionary models (Padova, PEL or Pisa, BaSTI or Teramo). Following the approach described by Kerber, Santiago & Brocato (2007), we determined accurate and self-consistent physical parameters (age, metallicity, distance modulus and reddening) for each cluster by comparing the observed CMDs with synthetic ones. We found significant trends in the physical parameters due to the choice of stellar evolutionary model and treatment of convective core overshooting. In general, models that incorporate overshooting presented more reliable results than those that do not. Comparisons with the results found in the literature demonstrated that our derived metallicities are in good agreement with the ones from the spectroscopy of red giants. We also confirmed that, independent of the adopted stellar evolutionary library, the recovered 3D distribution for these clusters is consistent with a thick disk roughly aligned with the LMC disk as defined by field stars. Finally, we also provide new estimates of distance modulus to the LMC center, that are marginally consistent with the canonical value of 18.50.Comment: 6 pages, 4 figures, conference contribution to IAU Symposium 256, van Loon J.T. & Oliviera J.M., ed