Large scale coherent magnetohydrodynamic oscillations in a sunspot.

Although theoretically predicted, the simultaneous excitation of several resonant modes in sunspots has not been observed. Like any harmonic oscillator, a solar magnetic flux tube can support a variety of resonances, which constitute the natural response of the system to external forcing. Apart from a few single low order eigenmodes in small scale magnetic structures, several simultaneous resonant modes were not found in extremely large sunspots. Here we report the detection of the largest-scale coherent oscillations observed in a sunspot, with a spectrum significantly different from the Sun's global acoustic oscillations, incorporating a superposition of many resonant wave modes. Magnetohydrodynamic numerical modeling agrees with the observations. Our findings not only demonstrate the possible excitation of coherent oscillations over spatial scales as large as 30-40 Mm in extreme magnetic flux regions in the solar atmosphere, but also paves the way for their diagnostic applications in other astrophysical contexts.

Spectropolarimetric fluctuations in a sunspot chromosphere.

The instrumental advances made in this new era of 4 m class solar telescopes with unmatched spectropolarimetric accuracy and sensitivity will enable the study of chromospheric magnetic fields and their dynamics with unprecedented detail. In this regard, spectropolarimetric diagnostics can provide invaluable insight into magneto-hydrodynamic (MHD) wave processes. MHD waves and, in particular, Alfvénic fluctuations associated with particular wave modes were recently recognized as important mechanisms not only for the heating of the outer layers of the Sun's atmosphere and the acceleration of the solar wind, but also for the elemental abundance anomaly observed in the corona of the Sun and other Sun-like stars (also known as first ionization potential) effect. Here, we take advantage of state-of-the-art and unique spectropolarimetric Interferometric BIdimensional Spectrometer observations to investigate the relation between intensity and circular polarization (CP) fluctuations in a sunspot chromosphere. Our results show a clear link between the intensity and CP fluctuations in a patch which corresponds to a narrow range of magnetic field inclinations. This suggests the presence of Alfvénic perturbations in the sunspot. This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

A semi-empirical approach to the evaluation of the relative biological effectiveness of therapeutic proton beams: the methodological framework.

We address the problem of the evaluation of the relative biological effectiveness (RBE) of therapeutic proton beams for cell inactivation. We consider a general approach to the evaluation of the lethal effect of protons which is applicable to different situations, including those in which an extended Bragg peak is obtained via a modulated beam. Our approach combines two kinds of information: (1) the experimental results available in the literature for the response of Chinese hamster V79 cells to monoenergetic beams and (2) the energy spectrum of the beam in the target volume, computed through a Monte Carlo algorithm. We have applied this method to a simple Bragg peak produced by a broad-field proton beam of about 70 MeV initial energy, such as those that, after attenuation, are typically used for treatment of ocular tumors. We have found that the RBE increases with depth, even beyond the Bragg peak, up to a value close to 2, when evaluated at the same surviving fraction as that resulting after exposure to 2 Gy of X rays.