HAO 2012 Profiles In Science: Dr. Roberto Casini

Contact:

303-497-1508
casini@ucar.edu

Dr. Roberto Casini is a Scientist III at the High Altitude Observatory of the National Center for Atmospheric Research. He holds a M.Sc. in Physics and a Ph.D. in Astronomy from the University of Florence, Italy. He specializes in spectro-polarimetric diagnostics of the solar atmosphere. He is involved with the development and maintenance of numerical codes for the forward modeling and inversion of scattering polarization in spectral lines, in the presence of electric and magnetic fields. He works on fundamental problems of polarized radiative transfer, such as the development of new diagnostics for magnetized plasmas, and a self-consistent treatment of partial redistribution for scattering polarization in complex atoms. He is also involved in the development of instrumentation and data analysis for polarimetry. He is the PI of the Visible Spectro-Polarimeter for the Advanced Technology Solar Telescope, and of the HAO Prominence Magnetometer.

Profile

Roberto Casini's main interests are in the fundamental theory underlying the formation of polarized radiation emanating from the solar atmosphere, but he also extends his efforts in this field to encompass data analysis and interpretation, as well as instrumentation development. The ultimate objective of his work is to develop and improve diagnostic methods for the interpretation of solar polarization observations, and the development of the instrumentation needed to advance our understanding of the magnetism of the solar atmosphere. Roberto's research focus responds to the growing need for diagnostics of solar magnetic fields higher in the solar atmosphere, such as in the chromosphere and in the corona. He is the Principal Investigator (PI) for the Visible Spectro-Polarimeter (ViSP) being developed for the Advanced Technology Solar Telescope (ATST). He is also the PI and Instrument Scientist of the HAO Prominence Magnetometer (ProMag) instrument. In addition, Roberto sits on the Science Working Groups for both ATST and a possible future multinational space mission, Roberto maintains a suite of computer codes for thecalculation of the scattering polarization from complexatoms in magnetized plasmas. These codes are starting to beimplemented as part of the HAO Community Spectro-polarimetric Analysis Center (CSAC).

Roberto and colleagues at HAO have recently investigated the effects of atmospheric seeing on polarization signal cross-talk (Casini, De Wijn, & Judge 2012). The results of this work allow to determine the minimum camera frame-rate and polarization modulation frequency needed to meet the desired goal for polarimetric accuracy, and they were used in the design of the modulation scheme of the HAO Chromosphere Magnetometer.

Roberto also continues development of computer tools for the design of broadband, optimally efficient, polarization modulators (Tomczyk, Casini, De Wijn, & Nelson 2010) and of super-achromatic wave-plates for polarization calibration purposes. By adding capabilities to create multi-crystalline compounds the routines improve the efficiency response across the spectral range. With these tools we have created a set of possible designs for the polarization optics that will be implemented at the ATST. These solutions are currently undergoing feasibility checks with potential vendors. The figure below shows the efficiency plots for Stokes I, Q, U, and V, for a four-element quartz-sapphire modulator that was optimized between 380 and 2500 nm. The thick curve shows the efficiency plot for the ideal solution, while the medium- and light-weight curves gives, respectively, the 1-s and 3-s results of a tolerance analysis of the solution assuming maximum errors of 1/100th waves in the elements' retardance, and 0.3 deg in the elements' relative positions. The horizontal dashed lines show the maximum optimal efficiencies attainable with a full-Stokes modulator. The solution shown is for a continuously rotating modulator, with 10 camera exposures per modulation period.

Roberto has also coded a vector efficiency calculator based on the Chandezon method, in order to study the polarization properties of diffraction gratings for solar spectro-polarimetric applications. The figure below shows the modeled efficiency curves for parallel (TE) and perpendicular (TM) polarizations to the grooves, for a stock grating with a groove density of 316 l/mm and a blaze angle of 63 deg, illuminated at the Littrow condition. The figure shows the performance of this grating over the spectral range of operation of the ATST/ViSP.

Highlights

Filtering of polarization fringes using pattern recognition. We have developed a pattern-recognition based approach to the problem of removal of polarized fringes from spectro-polarimetric data. We demonstrated that 2D Principal Component Analysis can be trained on a given spectro-polarimetric map in order to identify and isolate fringe structures from the spectra. This allows us in principle to reconstruct the data without the fringe component, providing an effective and clean solution to the problem (Casini, Judge, & Schad 2012).

The figure below shows spectro-polarimetric scans of an active region near the solar limb in the He I chromospheric lines at 1083 nm. These data were collected with the NSO/FIRS instrument in Sacramento Peak (Sunspot, NM). After data reduction and calibration, Stokes Q, U, and V still show a significant pollution of the spectro-polarimetric signal from instrumental effects, in the form of polarization fringes (first row). These instrumental artifacts can be sampled by performing a principal component analysis (PCA) of the data. This process allows to separate the spectral signal (second row) and the polarization fringes (third row) into nearly orthogonal PCA subspaces, thus enabling the filtering out of the instrumental artifacts. The bottom row shows a spatial slice (Y=70) of the three previous rows: the original data (stars), the polarization fringes (dotted line), and the filtered signal (continuous line). The filtered data was reconstructed by retaining only the first 7 eigenfeatures of the PCA decomposition of the original data.

Publications

(1) R. Casini, A.G. de Wijn, & P.G. Judge, 2012, Analysis of Seeing-Induced Polarization Cross-Talk and Modulation Scheme Performance, The Astrophysical Journal, 757, 45.

(2) B.C. Low, W. Liu, T. Berger, & R. Casini, 2012, The Hydromagnetic Interior of a Solar Quiescent Prominence. II. Magnetic Discontinuities and Cross-field Mass Transport, The Astrophysical Journal, 757, 21.

(3) R. Casini, P.G. Judge, & T. Schad, 2012, Removal of Spectro-Polarimetric Fringes by Two-Dimensional Pattern Recognition, The Astrophysical Journal, 756, 194.

(4) A. López Ariste, F. Leblanc, R. Casini, R. Manso Sainz, B. Gelly, & C. Le Men, 2012, Resonance scattering polarization in the magnetosphere of Mercury, Icarus, 220, 1104.

(5) B.C. Low, T. Berger, R. Casini, & W. Liu, 2012, The Hydromagnetic Interior of a Solar Quiescent Prominence. I. Coupling between Force Balance and Steady Energy Transport, The Astrophysical Journal, 755, 34.