The contrast of magnetic elements in synthetic CH- and CN-band images of solar magnetoconvection

We present a comparative study of the intensity contrast in synthetic CH-band and violet CN-band filtergrams computed from a high-resolution simulation of solar magnetoconvection. The underlying simulation has an average vertical magnetic field of 250 G with kG fields concentrated in its intergranular lanes, and is representative of a plage region. To simulate filtergrams typically obtained in CH- and CN-band observations we computed spatially resolved spectra in both bands and integrated these spectra over 1 nm FWHM filter functions centred at 430.5 nm and 388.3 nm, respectively. We find that the average contrast of magnetic bright points in the simulated filtergrams is lower in the CN-band by a factor of 0.96. This result strongly contradicts earlier semi-empirical modeling and recent observations, which both etimated that the bright-point contrast in the CN-band is \emph{higher} by a factor of 1.4. We argue that the near equality of the bright-point contrast in the two bands in the present simulation is a natural consequence of the mechanism that causes magnetic flux elements to be particularly bright in the CN and CH filtergrams, namely the partial evacuation of these elements and the concomitant weakening of molecular spectral lines in the filter passbands. We find that the RMS intensity contrast in the whole field-of-view of the filtergrams is 20.5% in the G band and 22.0% in the CN band and conclude that this slight difference in contrast is caused by the shorter wavelength of the latter. Both the bright-point and RMS intensity contrast in the CN band are sensitive to the precise choice of the central wavelength of the filter.Comment: 24 pages, 9 figures, submitted to Ap

Dichroic Masers due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization

The theory of the generation and transfer of polarized radiation, mainly developed for interpreting solar spectropolarimetric observations, allows to reconsider, in a more rigorous and elegant way, a physical mechanism that has been suggested some years ago to interpret the high degree of polarization often observed in astronomical masers. This mechanism, for which the name of 'dichroic maser' is proposed, can operate when a low density molecular cloud is illuminated by an anisotropic source of radiation (like for instance a nearby star). Here we investigate completely unsaturated masers and show that selective stimulated emission processes are capable of producing highly polarized maser radiation in a non-magnetic environment. The polarization of the maser radiation is linear and is directed tangentially to a ring equidistant to the central star. We show that the Hanle effect due to the presence of a magnetic field can produce a rotation (from the tangential direction) of the polarization by more that 45 degrees for some selected combinations of the strength, inclination and azimuth of the magnetic field vector. However, these very same conditions produce a drastic inhibition of the maser effect. The rotations of about 90 degrees observed in SiO masers in the evolved stars TX Cam by Kemball & Diamond (1997) and IRC+10011 by Desmurs et al (2000) may then be explainedby a local modification of the anisotropy of the radiation field, being transformed from mainly radial to mainly tangential.Comment: Accepted for publication on Ap

The Magnetic Sensitivity of the Ba II D1 and D2 Lines of the Fraunhofer Spectrum

The physical interpretation of the spectral line polarization produced by the joint action of the Hanle and Zeeman effects offers a unique opportunity to obtain empirical information about hidden aspects of solar and stellar magnetism. To this end, it is important to achieve a complete understanding of the sensitivity of the emergent spectral line polarization to the presence of a magnetic field. Here we present a detailed theoretical investigation on the role of resonance scattering and magnetic fields on the polarization signals of the Ba II D1 and D2 lines of the Fraunhofer spectrum, respectively at 4934 \AA\ and 4554 \AA. We adopt a three-level model of Ba II, and we take into account the hyperfine structure that is shown by the $^{135}$Ba and $^{137}$Ba isotopes. Despite of their relatively small abundance (18%), the contribution coming from these two isotopes is indeed fundamental for the interpretation of the polarization signals observed in these lines. We consider an optically thin slab model, through which we can investigate in a rigorous way the essential physical mechanisms involved (resonance polarization, Zeeman, Paschen-Back and Hanle effects), avoiding complications due to radiative transfer effects. We assume the slab to be illuminated from below by the photospheric solar continuum radiation field, and we investigate the radiation scattered at 90 degrees, both in the absence and in the presence of magnetic fields, deterministic and microturbulent. We show in particular the existence of a differential magnetic sensitivity of the three-peak Q/I profile that is observed in the D2 line in quiet regions close to the solar limb, which is of great interest for magnetic field diagnostics.Comment: 40 pages, 1 table and 19 figures. Accepted for publication in The Astrophysical Journal (ApJ

Recent Advances in Chromospheric and Coronal Polarization Diagnostics

I review some recent advances in methods to diagnose polarized radiation with which we may hope to explore the magnetism of the solar chromosphere and corona. These methods are based on the remarkable signatures that the radiatively induced quantum coherences produce in the emergent spectral line polarization and on the joint action of the Hanle and Zeeman effects. Some applications to spicules, prominences, active region filaments, emerging flux regions and the quiet chromosphere are discussed.Comment: Review paper to appear in "Magnetic Coupling between the Interior and the Atmosphere of the Sun", eds. S. S. Hasan and R. J. Rutten, Astrophysics and Space Science Proceedings, Springer-Verlag, 200

Recent progress in terahertz metamaterial modulators

The terahertz (0.1–10 THz) range represents a fast-evolving research and industrial field. The great interest for this portion of the electromagnetic spectrum, which lies between the photonics and the electronics ranges, stems from the unique and disruptive sectors where this radiation finds applications in, such as spectroscopy, quantum electronics, sensing and wireless communications beyond 5G. Engineering the propagation of terahertz light has always proved to be an intrinsically difficult task and for a long time it has been the bottleneck hindering the full exploitation of the terahertz spectrum. Amongst the different approaches that have been proposed so far for terahertz signal manipulation, the implementation of metamaterials has proved to be the most successful one, owing to the relative ease of realisation, high efficiency and spectral versatility. In this review, we present the latest developments in terahertz modulators based on metamaterials, while highlighting a few selected key applications in sensing, wireless communications and quantum electronics, which have particularly benefitted from these developments

Downflows in sunspot umbral dots

We study the velocity field of umbral dots at a resolution of 0.14". Our analysis is based on full Stokes spectropolarimetric measurements of a pore taken with the CRISP instrument at the Swedish 1-m Solar Telescope. We determine the flow velocity at different heights in the photosphere from a bisector analysis of the Fe I 630 nm lines. In addtion, we use the observed Stokes Q, U, and V profiles to characterize the magnetic properties of these structures. We find that most umbral dots are associated with strong upflows in deep photospheric layers. Some of them also show concentrated patches of downflows at their edges, with sizes of about 0.25", velocities of up to 1000 m/s, and enhanced net circular polarization signals. The downflows evolve rapidly and have lifetimes of only a few minutes. These results appear to validate numerical models of magnetoconvection in the presence of strong magnetic fields.Comment: Final published version. For best quality figures, please download the PS versio

Analytical maximum likelihood estimation of stellar magnetic fields

The polarised spectrum of stellar radiation encodes valuable information on the conditions of stellar atmospheres and the magnetic fields that permeate them. In this paper, we give explicit expressions to estimate the magnetic field vector and its associated error from the observed Stokes parameters. We study the solar case where specific intensities are observed and then the stellar case, where we receive the polarised flux. In this second case, we concentrate on the explicit expression for the case of a slow rotator with a dipolar magnetic field geometry. Moreover, we also give explicit formulae to retrieve the magnetic field vector from the LSD profiles without assuming mean values for the LSD artificial spectral line. The formulae have been obtained assuming that the spectral lines can be described in the weak field regime and using a maximum likelihood approach. The errors are recovered by means of the hermitian matrix. The bias of the estimators are analysed in depth.Comment: accepted for publication in MNRA

Fusion rate enhancement due to energy spread of colliding nuclei

Experimental results for sub-barrier nuclear fusion reactions show cross section enhancements with respect to bare nuclei which are generally larger than those expected according to electron screening calculations. We point out that energy spread of target or projectile nuclei is a mechanism which generally provides fusion enhancement. We present a general formula for calculating the enhancement factor and we provide quantitative estimate for effects due to thermal motion, vibrations inside atomic, molecular or crystal system, and due to finite beam energy width. All these effects are marginal at the energies which are presently measurable, however they have to be considered in future experiments at still lower energies. This study allows to exclude several effects as possible explanation of the observed anomalous fusion enhancements, which remain a mistery.Comment: 17 pages with 3 ps figure included. Revtex styl