HAO 2011 Profiles In Science: Dr. Alfred de Wijn

Contact:

Dr. Alfred de Wijn is a Project Scientist I at the High Altitude Observatory of the National Center for Atmospheric Research. He received his PhD in Astrophysics in 2006 from Utrecht University. He began working at HAO in December 2006. His research has always been focussed on observational studies of magnetism and dynamics in the photosphere, chromosphere, and transition region. More recently, he has become heavily involved in several major instrument projects at HAO.

Summary of Achievements

Project: COSMO ChroMag (Role: Instrument Scientist)

The COSMO ChroMag instrument is a filter polarimeter designed to measure chromosphere and prominence magnetic fields. Synoptic measurements of the magnetic field through the chromosphere and at the base of the corona are key to advancing our understanding of a multitude of solar phenomena, such as the role of the chromosphere in the mass and energy balance of the corona, the origin and acceleration of the solar wind, and the onset and evolution of CMEs.

Project: COSMO K-Coronagraph (Role: Instrument Co-Scientist)

The white-light K-coronagraph is an instrument to measure white light coronal polarization brightness due to electron scattering of photospheric light by the K-corona. It will replace the aging Mauna Loa Solar Observatory Mk4 K-coronameter in early 2013. The K-Coronagraph will greatly improve upon the measurements currently being made with the Mk4 instrument. Significant improvements will be made in the signal-to-noise as well as cadence of the observations. Combined with occultation to very low height, the K-Coronagraph will provide a greatly improved handle on CME acceleration and interaction with surrounding coronal structures. The K-coronagraph passed its Preliminary Design Review in August 2011.

Publications

(1) Casini, Roberto, Alfred G. de Wijn, and Philip G. Judge. 2011: Analysis of Seeing-Induced Polarization Cross-Talk and Modulation Scheme Performance. Submitted to Applied Optics. See online article.

Abstract: Polarimetric measurements of the sun made with ground-based instruments suffer from cross-talk induced by atmospheric seeing. We analyze the generation this cross-talk, and its effects on the noise statistics of spectropolarimetric measurements for both single-beam and dual-beam instruments, from a statistical point of view. We investigate the time evolution of seeing-induced correlations between different states of one modulation cycle, and compare the response to these correlations of two popular polarization modulation schemes in a dual-beam system.

(2) Rutten, R.J., J. Leenaarts, L.H.M. Rouppe van der Voort, A.G. de Wijn, M. Carlsson, and V. Hansteen. 2011: Quiet-Sun imaging asymmetries in Na I D1 compared with other strong Fraunhofer lines. Astronomy & Astrophysics, 531. See online article.

Abstract: Imaging spectroscopy of the solar atmosphere using the Na I D1 line yields marked asymmetry between the blue and red line wings: sampling a quiet-Sun area in the blue wing displays reversed granulation, whereas sampling in the red wing displays normal granulation. The Mg I b2 line of comparable strength does not show this asymmetry, nor does the stronger Ca II 8542 Å line. In this paper, we explain it with line-formation insights from classical 1D modeling and with a 3D magnetohydrodynamical simulation combined with NLTE spectral line synthesis that permits detailed comparison with the observations. The cause of the imaging asymmetry is the combination of correlations between intensity and Dopplershift modulation in granular overshoot and the sensitivity to these of the steep profile flanks of the Na I D1 line. The Mg I b2 line has similar core formation but much wider wings due to larger opacity buildup and damping in the photosphere.

(3) Tomczyk, Steven, Roberto Casini, Alfred G. de Wijn, and Peter G. Nelson. 2010: Wavelength-diverse polarization modulators for Stokes polarimetry. Applied Optics, 49, 18, p. 3580. See online article.

Abstract: Information about the three-dimensional structure of solar magnetic fields is encoded in the polarized spectra of solar radiation by a host of physical processes. To extract this information, solar spectra must be obtained in a variety of magnetically sensitive spectral lines at high spatial, spectral, and temporal resolution with high precision. The need to observe many different spectral lines drives the development of Stokes polarimeters with a high degree of wavelength diversity. We developed a new way of designing polarization modulators that operate over a wide wavelength range with near optimal polarimetric efficiency. They are directly applicable to the next generation of multi-line Stokes polarimeters. These modulators are not achromatic in the usual sense because their polarimetric properties vary with wavelength, but they do so in an optimal way. We refer to these modulators as polychromatic. In this paper, we presented the theory behind polychromatic modulators, illustrated the concept with design examples, and presented the performance properties of a prototype polychromatic modulator.