On the physical origin of the second solar spectrum of the Sc II line at 4247 A

The peculiar three-peak structure of the linear polarization profile shown in the second solar spectrum by the Ba II line at 4554 A has been interpreted as the result of the different contributions coming from the barium isotopes with and without hyperfine structure (HFS). In the same spectrum, a triple peak polarization signal is also observed in the Sc II line at 4247 A. Scandium has a single stable isotope (^{45}Sc), which shows HFS due to a nuclear spin I=7/2. We investigate the possibility of interpreting the linear polarization profile shown in the second solar spectrum by this Sc II line in terms of HFS. A two-level model atom with HFS is assumed. Adopting an optically thin slab model, the role of atomic polarization and of HFS is investigated, avoiding the complications caused by radiative transfer effects. The slab is assumed to be illuminated from below by the photospheric continuum, and the polarization of the radiation scattered at 90 degrees is investigated. The three-peak structure of the scattering polarization profile observed in this Sc II line cannot be fully explained in terms of HFS. Given the similarities between the Sc II line at 4247 A and the Ba II line at 4554 A, it is not clear why, within the same modeling assumptions, only the three-peak Q/I profile of the barium line can be fully interpreted in terms of HFS. The failure to interpret this Sc II polarization signal raises important questions, whose resolution might lead to significant improvements in our understanding of the second solar spectrum. In particular, if the three-peak structure of the Sc II signal is actually produced by a physical mechanism neglected within the approach considered here, it will be extremely interesting not only to identify this mechanism, but also to understand why it seems to be less important in the case of the barium line.Comment: 8 pages, 8 figures, and 1 table. Accepted for publication in Astronomy and Astrophysic

Validity of the zero-thermodynamic law in off-equilibrium coupled harmonic oscillators

In order to describe the thermodynamics of the glassy systems it has been recently introduced an extra parameter also called effective temperature which generalizes the fluctuation-dissipation theorem (FDT) to systems off-equilibrium and supposedly describes thermal fluctuations around the aging state. Here we investigate the applicability of a zero-th law for non-equilibrium glassy systems based on these effective temperatures by studying two coupled subsystems of harmonic oscillators with Monte Carlo dynamics. We analyze in detail two types of dynamics: 1) sequential dynamics where the coupling between the subsystems comes only from the Hamiltonian and 2) parallel dynamics where there is a further coupling between the subsystems arising from the dynamics. We show that the coupling described in the first case is not enough to make asymptotically the effective temperatures of two interacting subsystems coincide, the reason being the too small thermal conductivity between them in the aging state. This explains why different interacting degrees of freedom in structural glasses may stay at different effective temperatures without never mutually thermalizing.Comment: 23 pages, REVTeX, 4 eps figure

Simulation study of the filamentation of counter-streaming beams of the electrons and positrons in plasmas

The filamentation instability driven by two spatially uniform and counter-streaming beams of charged particles in plasmas is modelled by a particle-in-cell (PIC) simulation. Each beam consists of the electrons and positrons. The four species are equally dense and they have the same temperature. The one-dimensional simulation direction is orthogonal to the beam velocity vector. The magnetic field grows spontaneously and rearranges the particles in space, such that the distributions of the electrons of one beam and the positrons of the second beam match. The simulation demonstrates that as a result no electrostatic field is generated by the magnetic field through its magnetic pressure gradient prior to its saturation. This electrostatic field would be repulsive at the centres of the filaments and limit the maximum charge and current density. The filaments of electrons and positrons in this simulation reach higher charge and current densities than in one with no positrons. The oscillations of the magnetic field strength induced by the magnetically trapped particles result in an oscillatory magnetic pressure gradient force. The latter interplays with the statistical fluctuations in the particle density and it probably enforces a charge separation, by which electrostatic waves grow after the filamentation instability has saturated.Comment: 13 pages, 8 figure

Calibration of the 6302/6301 Stokes V line ratio in terms of the 5250/5247 ratio

Four decades ago the Stokes V line ratio in the Fe I 5247.06 and 5250.22 {\AA} lines was introduced as a powerful means of exploring the intrinsic field strengths at sub-pixel scales, which led to the discovery that most of the photospheric flux is in intermittent kG form. The "green" 5247-5250 line pair is unique because it allows the magnetic-field effects to be isolated from the thermodynamic effects. No other line pair with this property has since been identified. In recent years much of the magnetic-field diagnostics has been based on the "red" Fe I 6301.5 and 6302.5 {\AA} line pair, since it was chosen in the design of the Hinode space observatory. Although thermodynamic effects severely contaminate the magnetic-field signatures for this line ratio, it is still possible to use it to extract information on intrinsic magnetic fields, but only after it has been "renormalized", since otherwise it produces fictitious, superstrong fields everywhere. In the present work we explore the joint behavior of these two line ratios to determine how the "contaminated" red line ratio can be translated into the corresponding green line ratio, which then allows for a direct interpretation in terms of intrinsic magnetic fields. Our observations are mainly based on recordings with the ZIMPOL-3 spectro-polarimeter at IRSOL in Locarno, Switzerland, complemented by data from the STOP telescope at the Sayan solar observatory (Irkutsk, Russia). The IRSOL observations are unique by allowing both the green and red line pairs to be recorded simultaneously on the same CCD sensor. We show how the line ratios depend on both the measured flux densities and on the heliocentric distance (the \mu\ value on the solar disk), and finally derive the calibration function that enables the red line ratio to be translated to the green ratio for each \mu\ value

Circularly polarized waves in a plasma with vacuum polarization effects

The theory for large amplitude circularly polarized waves propagating along an external magnetic field is extended in order to include also vacuum polarization effects. A general dispersion relation, which unites previous results, is derived.Comment: 5 pages (To appear in Physics of Plasmas