HAO 2010 PROFILES IN SCIENCE: Dr. Bruce Lites

Contact

303-494-1517
lites@ucar.edu

Dr. Bruce Lites is a Senior Scientist in the High Altitude Observatory of the National Center for Atmospheric Research. He received is PhD in Astrophysics in 1972 from the University of Colorado, Boulder. His first position at HAO began in September, 1968 as a Graduate Research Assistant working with Dr. Grant Athay. His main research interest is the interpretation of measurements of the solar magnetic field in terms of structure, dynamics, and energetics of the solar atmosphere. Other research interests include applications of radiative transfer to solar problems and instrumentation for solar observations. He is a primary investigator on the HINODE mission.

Publications:

(1) The Multi-Component Photospheric Magnetic Field
B. W. Lites, R. Casini, R. Manso Sainz, J. Jurcák, K. Ichimoto, R. Ishikawa, T. J. Okamoto, S. Tsuneta, and L. Bellot Rubio (2010): Scattering Polarization in the Fe I 630 nm Emission Lines at the Extreme Limb of the Sun, ApJ 713 450, doi:10.1088/0004-637X/713/1/450.
(http://adsabs.harvard.edu/abs/2010ApJ...713..450L.)

Abstract:

The Spectro-Polarimeter (SP) aboard the Hinode spacecraft was developed jointly by HAO and the Lockheed Martin Solar and Astrophysics Laboratory. This instrument provides continuous, high resolution, high precision measurements of polarization of the Sun. Shown in the figure below is an example of polarization spectra (Stokes parameters I, Q, U, V) taken at the extreme edge of the Sun. It reveals for the first time a very thin layer of emission in the neutral iron spectrum lines that are normally in absorption on the solar disk. Image at left of Figure 1 is the spectral continuum image from the SP map, showing the slit position as a vertica dashed line. The Stokes I, Q, U, V spectra are shown at right. The Stokes I spectrum appears in absorption on the disk, giving way to emission just above the limb. Doppler shifts of the emission profile are clearly visible along the slit. The Stokes Q, U spectral images have been binned 2 × 2 to aid visualization above the noise level of the weak linear polarization in the spectral lines. On the disk, the unbinned Stokes V spectral image shows weak, antisymmetric profiles characteristic of the Zeeman effect.

Observed over several days, the Doppler shifts of the emission line profiles are clearly visible along the slit. With the exception of very few points, the observed Stokes Q, U spectral images revealed that linear polarization signal was predominantly radially oriented, that is, perpendicular to the natural direction of polarization. This is expected for scattering around a spherical source, and is nearly opposite to that expected from classical scattering polarization. The authors were able to reproduce the anomalous polarization orientation arises from the complex atomic structure of the neutral iron atoms giving rise to these spectrum lines. The degree of polarization observed in this emission indicates the presence of a weak magnetic field in the upper solar atmosphere with a strength of approximately 2 Gauss. This groundbreaking work demonstrates the need to implement complex multi-level atom models in order to provide a correct interpretation of the subtle polarization signatures observed in the radiation-dominated chromospheric layers, the next frontier of solar physics.

Hinode Stokes spectra observation
Figure 1 (Actual size): Hinode Stokes spectra observation at the west solar limb taken at 20:34:47 on 2007 September 26.
Modeling of the radial scattering polarization observed by Hinode SOT/SP on Sept. 9, 2007 near the solar limb at a height of 0.3
Figure 2 (Actual size): Modeling of the radial scattering polarization observed by Hinode SOT/SP on Sept. 9, 2007 near the solar limb at a height of 0.3". The atomic model adopted for these calculations included 80 levels, for a total of 532 radiative transitions between 200nm and 26,000nm. The top row of the figure shows the normalized intensity (left) and Stokes Q/Stokes I ratio (right) assuming an atmosphere free from magnetic field. Note that the maximum amplitude of the scattering polarization is about 3 times larger than the observed value. The center row shows the same quantities as the top row, but assuming that the terms a5P, a5D, and a5F in the model atom are completely depolarized by collisions. We note how the ratio between the scattering polarizations in the two lines is strongly dependent on the presence of these depolarizing collisions. The bottom row again shows the same quantities, but for a horizontal, random-azimuth (i.e., "canopy-like") magnetic field of 2 Gauss, and no depolarizing collisions. In this case, we obtain scattering polarization amplitudes that are in agreement with the observed values.