Showing papers by "Bruce W. Lites published in 2006"

An Overview of Existing Algorithms for Resolving the 180° Ambiguity in Vector Magnetic Fields: Quantitative Tests with Synthetic Data

Abstract: We report here on the present state-of-the-art in algorithms used for resolving the 180° ambiguity in solar vector magnetic field measurements. With present observations and techniques, some assumption must be made about the solar magnetic field in order to resolve this ambiguity. Our focus is the application of numerous existing algorithms to test data for which the correct answer is known. In this context, we compare the algorithms quantitatively and seek to understand where each succeeds, where it fails, and why. We have considered five basic approaches: comparing the observed field to a reference field or direction, minimizing the vertical gradient of the magnetic pressure, minimizing the vertical current density, minimizing some approximation to the total current density, and minimizing some approximation to the field's divergence. Of the automated methods requiring no human intervention, those which minimize the square of the vertical current density in conjunction with an approximation for the vanishing divergence of the magnetic field show the most promise.

Spectral Line Selection for HMI: A Comparison of Fe I 6173 Å and Ni I 6768 Å

Abstract: We present a study of two spectral lines, Fe I 6173 A and Ni I 6768 A, that were candidates to be used in the Helioseismic and Magnetic Imager (HMI) for observing Doppler velocity and the vector magnetic field. The line profiles were studied using the Mt. Wilson Observatory, the Advanced Stokes Polarimeter and the Kitt Peak-McMath Pierce telescope and one-meter Fourier transform spectrometer atlas. Both Fe I and Ni I profiles have clean continua and no blends that threaten instrument performance. The Fe I line is 2% deeper, 15% narrower, and has a 6% smaller equivalent width than the Ni I line. The potential of each spectral line to recover pre-assigned solar conditions is tested using a least-squares minimization technique to fit Milne-Eddington models to tens of thousands of line profiles that have been sampled at five spectral positions across the line. Overall, the Fe I line has a better performance than the Ni I line for vector-magnetic-field retrieval. Specifically, the Fe I line is able to determine field strength, longitudinal and transverse flux four times more accurately than the Ni I line in active regions. Inclination and azimuthal angles can be recovered to ≈2° above 600 Mx cm−2 for Fe I and above 1000 Mx cm−2 for Ni I. Therefore, the Fe I line better determines the magnetic-field orientation in plage, whereas both lines provide good orientation determination in penumbrae and umbrae. We selected the Fe I spectral line for use in HMI due to its better performance for magnetic diagnostics while not sacrificing velocity information. The one exception to the better performance of the Fe I line arises when high field strengths combine with high velocities to move the spectral line beyond the effective sampling range. The higher g eff of Fe I means that its useful range of velocity values in regions of strong magnetic field is smaller than Ni I.

Spectral Line Selection for HMI: A Comparison of Fe I 6173 and Ni I 6768

Abstract: We present a study of two spectral lines, Fe I 6173 Angstroms and Ni I 6768 Angstroms, that were candidates to be used in the Helioseismic and Magnetic Imager (HMI) for observing Doppler velocity and the vector magnetic field. The line profiles were studied using the Mt. Wilson Observatory, the Advanced Stokes Polarimeter and the Kitt Peak McMath telescope and one meter Fourier transform spectrometer atlas. Both Fe I and Ni I profiles have clean continua and no blends that threaten instrument performance. The Fe I line is 2% deeper, 15% narrower and has a 6% smaller equivalent width than the Ni I line. The potential of each spectral line to recover pre-assigned solar conditions is tested using a least-squares minimization technique to fit Milne-Eddington models to tens of thousands of line profiles that have been sampled at five spectral positions across the line. Overall, the Fe I line has a better performance than the Ni I line for vector magnetic field retrieval. We selected the Fe I spectral line for use in HMI due to its better performance for magnetic diagnostics while not sacrificing velocity information.

Spinor: Visible and Infrared Spectro-Polarimetry at the National Solar Observatory

Abstract: The Spectro-Polarimeter for Infrared and Optical Regions (SPINOR) is a new spectro-polarimeter that will serve as a facility instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic polarimetry over a very broad range of wavelengths, from 430 to 1600 nm, allowing for the simultaneous observation of several visible and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc.).

Spectro-polarimetric observations and non-LTE Modeling of Ellerman bombs

Abstract: Ellerman bombs are bright emission features observed in the wings of Hα, usually in the vicinity of magnetic concentrations. Here we show that they can also be detected in the Ca II infrared triplet lines, which are easier to interpret and therefore allow for more detailed diagnostics. We present full Stokes observations of the 849.8 and 854.2 nm lines acquired with the new spectro-polarimeter SPINOR. The data show no significant linear polarization at the level of 3 × 10−4. The circular polarization profiles exhibit measureable signals with a very intricate pattern of peaks. A non-LTE analysis of the spectral profiles emerging from these features reveals the presence of strong downflows (∼10 {km s−1}) in a hot layer between the upper photosphere and the lower chromosphere.

On the fine structure of sunspot penumbrae - III. The vertical extension of penumbral filaments

Abstract: In this paper we study the fine structure of the penumbra as inferred from the uncombed model (flux tube embedded in a magnetic surrounding) when applied to penumbral spectropolarimetric data from the neutral iron lines at 6300 A. The inversion infers very similar radial dependences in the physical quantities (LOS velocity, magnetic field strength etc.) as those obtained from the inversion of the Fe I 1.56 μ m lines. In addition, the large Stokes V area asymmetry exhibited by the visible lines helps to constrain the size of the penumbral flux tubes. As we demonstrate here, the uncombed model is able to reproduce the area asymmetry with striking accuracy, returning flux tubes as thick as 100-300 kilometers in the vertical direction, in good agreement with previous investigations.

A Detailed Analysis of an Ephemeral Region .

Abstract: In order to improve the understanding of the process of emergence of magnetic flux on the solar surface, we studied the temporal evolution of an ephemeral region using Advanced Stokes Polarimeter data We adopted two different approaches: first, we used a Milne-Eddington inversion to obtain mean parameters of the emerging bipole magnetic configuration Then, we considered the full radiative transfer equation, and we studied the trend of all the previous parameters as a function of the optical depth τ We pointed out peculiar flows, such as an initial upflow of 15 km s−1 where the zenith angle is essentially horizontal, and downflows decreasing in time in footpoints, characterized by a vertical field These results seem to confirm the emerging bipole topology, due to magnetic flux tube emergence The results obtained with this inversion confirm the structure found with MilneEddington code However we found regions in which the presence of two distinct magnetic components is highly significant It also seems very interesting the trend of the temperature with optical depth: the plasma temperature appears to grow up in the high photosphere above the emerging bipole

The Diffraction Limited Spectro-Polarimeter

Abstract: The Diffraction Limited Spectro-Polarimeter (DLSP) is a collaboration between the National Solar Observatory (NSO) and the High Altitude Observatory (HAO) to provide a stable instrument for precision measurements of solar vector magnetic fields at high angular resolution. The DLSP is integrated with the new high-order Adaptive Optics (HOAO) system at the Dunn Solar Telescope (DST) and provides Stokes spectra of the Fe i 630 nm lines approaching the 0. 2 diffraction limit of the DST. It is configured as a fixed, well-calibrated instrument that may be used simultaneously with G-band (1 nm bandpass) and a Ca K imagers (0.1 nm bandpass). The 2K×2K G-band imager allows fast frame selection and includes a burst mode for speckle imaging. The setup of DLSP and its imagers require only about 10min of preparation before start of observations. This fixed setup facilitates standardized data reduction. The DLSP permits observations with 0. 09 sampling in high resolution mode. In wide-field mode, the 0. 27 sampling allows one to map regions about 3 on a side. The achieved continuum S/N is 500 (1500) in high resolution (wide-field) mode for a 4 s integration. It is possible to achieve higher S/N by integrating longer. Data reduction routines are now available in IDL for post-observation processing, and parallel analysis routines in FORTRAN77 are being developed to allow “on-the-fly” data reduction and inversion.

Spectral Line Selection for HMI

Abstract: We present information on two spectral lines, Fe I 6173 A and Ni I 6768 A, that were candidates for use in the Helioseismic and Magnetic Imager (HMI) instrument. Both Fe I and Ni I profiles have clean continuum and no blends that threaten performance. The higher Lande factor of Fe I means its operational velocity range in regions of strong magnetic field is smaller than for Ni I. Fe I performs better than Ni I for vector magnetic field retrieval. Inversion results show that Fe I consistently determines field strength and flux more accurately than the Ni I line. Inversions show inclination and azimuthal errors are recovered to ≈ 2° above 600 Mx/cm2 for Fe I and above 1000 Mx/cm2 for Ni I. The Fe I line was recommended, and ultimately chosen, for use in HMI.