Kasiviswanathan Sankarasubramanian

The Diffraction Limited Spectro-Polarimeter: a new instrument for high-resolution solar polarimetry

The National Solar Observatory in collaboration with the High-Altitude Observatory is developing a new solar polarimeter, the Diffraction Limited Spectro-Polarimeter. In conjunction with a new high-order adaptive optics system at the NSO Dunn Solar Telescope, the DLSP design facilitates very high angular resolution observations of solar vector magnetic fields. This project is being carried out in two phases. As a follow-on to the successful completion of the first phase, the ongoing DLSP Phase II implements a high QE CCD camera system, a ferro-electric liquid crystal modulator, and a new opto-mechanical system for polarization calibration. This paper documents in detail the development of the modulator system and its performance, and presents preliminary results from an engineering run carried out in combination with the new NSO high-order AO system.

Diffraction-limited spectropolarimeter: Phase I

A diffraction limited spectro-polarimeter is under construction at the National Solar Observatory in collaboration with the High Altitude Observatory. The scientific objective of the project is to measure the magnetic fields on the Sun up to the diffraction limit of the Dunn Solar Telescope. The same instrument would also measure the magnetic field of large sunspots or sunspot groups with reasonable spatial resolution. This requires a flexible image scale which cannot be obtained with the current Advanced Stokes Polarimeter (ASP) without loosing 50% of the light. The new spectro-polarimeter is designed in such a way that the image scale can be changed without loosing much light. It can work either in high-spatial resolution mode (0.09 arcsec per pixel) with a small field of view (FOV: 65 arcsec) or in large FOV mode (163 arcsec) with low-spatial resolution (0.25 arcsec per pixel). The phase-I of this project is to design and build the spectrograph with flexible image scale. Using the existing modulation, calibration optics of the ASP and the ASP control and data acquisition system with ASP-CHILL camera, the spectrograph was tested for its performance. This paper will concentrate on the performance of the spectrograph and will discuss some preliminary results obtained with the test runs.

Bisector Analysis of Stokes Profiles: Effects Due to Gradients in the Physical Parameters

The asymmetry of Stokes V profiles of the spectral lines λ6301.5 and λ6302.5 was utilized to systematically study a sunspot observed close to the disk center. The Stokes spectra were taken with the National Solar Observatory (NSO)/High Altitude Observatory Advanced Stokes Polarimeter (ASP). The NSO low-order adaptive optics system was used to record a data set of consistently high resolution. We find the following results from this analysis: (1) a strong correlation between the center-of-gravity (COG) velocity derived from the intensity profiles with the V-profile asymmetry; (2) the amplitude asymmetry is much more sensitive to changes in the COG velocity than the area asymmetry; and (3) plotting area versus amplitude asymmetry for the entire active region results in a bimodal distribution. Different areas within the active region, such as penumbra, umbra, light bridge, and small-scale fields outside the sunspot, are clearly separated in this plot. The light bridges and the small-scale magnetic fields surrounding the observed sunspot show larger amplitude asymmetry compared to the area asymmetry, whereas the penumbra shows larger area asymmetry. In comparison, the Stokes V spectra measured in the umbra show little area and amplitude asymmetry. In this paper, we use bisector analysis of the V intensity profile as a new tool to determine the gradients in the physical parameters in a more direct way. The gradients derived from the bisector method provide further and more direct evidence for the physical picture derived from the study of the asymmetries. For light bridges we find that the data is consistent with a picture of small convective cells confined to lower layers of the atmosphere. Asymmetries in the penumbra are caused by steep vertical gradients in the Evershed velocities in combination with the gradient in the line-of-sight inclination angle, confirming the earlier observations. For small-scale fields, the picture is consistent with the canopy effect. We also compare the average velocity and the magnetic field strength derived from this bisector analysis with the velocity and magnetic field strength derived from the ASP inversion and find excellent agreement between these independent methods. Apart from these asymmetries, we also observe extremely asymmetric V profiles, such as one-lobed profiles and multiple reversals, mostly at the edges of the limb-side penumbra. In these regions, we also find differences in the V profiles of λ6301.5 and λ6302.5 that suggest steep gradients in the physical parameters. The asymmetries derived from a single scan match well with the ones derived from the time-averaged properties obtained by averaging 14 such scans separated by 7.5 minutes. This suggests that, in a statistical sense, the Stokes V asymmetries do not vary with time and describe a global/general property of magnetic features found in regions such as light bridge, umbra, penumbra, and small-scale fields.