Sarah A. Jaeggli

Localized enhancements of Fe+10 density in the corona as observed in Fe Xi 789.2 nm during the 2006 march 29 total solar eclipse

The first ever image of the full solar corona in the Fe XI 789.2 nm spectral line was acquired during the total solar eclipse of 2006 March 29. Several striking features stand out in the processed image: (1) The emission extended out to at least 3 R☉ in streamers. (2) A bubble-like structure, occupying a cone of about 45° and reaching out to 1 R☉ above the limb, was observed southward of a bright active region complex close to the limb. (3) Localized intensity enhancements were found in different parts of the corona at heights ranging from 1.2 to 1.5 R☉. (4) Striations extended out to the edge of the field of view above an almost north-south-oriented prominence. Comparison with the corresponding white-light image taken simultaneously during the eclipse showed no evidence for these localized enhancements, and the bubble-like structure and striations, while present, did not stand out in the same manner. The extent of the Fe XI emission is attributed to the dominance of radiative over collisional excitation in the formation of that spectral line. The localized intensity enhancements, observed only in Fe XI and not in white light, are a signature of localized increases in Fe+10 density relative to electron density. These are the first observations to show direct evidence of localized heavy ion density enhancements in the extended corona. They point to the importance of implementing observations of the Fe XI 789.2 nm line with existing or future coronagraphs for the exploration of the physical processes controlling the behavior of heavy ions in different source regions of the solar wind.

On Molecular Hydrogen Formation and the Magnetohydrostatic Equilibrium of Sunspots

We have investigated the problem of sunspot magnetohydrostatic equilibrium with comprehensive IR sunspot magnetic field survey observations of the highly sensitive Fe I lines at 15650 ? and nearby OH lines. We have found that some sunspots show isothermal increases in umbral magnetic field strength which cannot be explained by the simplified sunspot model with a single-component ideal gas atmosphere assumed in previous investigations. Large sunspots universally display nonlinear increases in magnetic pressure over temperature, while small sunspots and pores display linear behavior. The formation of molecules provides a mechanism for isothermal concentration of the umbral magnetic field, and we propose that this may explain the observed rapid increase in umbral magnetic field strength relative to temperature. Existing multi-component sunspot atmospheric models predict that a significant amount of molecular hydrogen (H2) exists in the sunspot umbra. The formation of H2 can significantly alter the thermodynamic properties of the sunspot atmosphere and may play a significant role in sunspot evolution. In addition to the survey observations, we have performed detailed chemical equilibrium calculations with full consideration of radiative transfer effects to establish OH as a proxy for H2, and demonstrate that a significant population of H2 exists in the coolest regions of large sunspots.

Erratum: “Localized Enhancements of Fe+10 Density in the Corona as Observed in Fe XI 789.2 nm during the 2006 March 29 Total Solar Eclipse” (ApJ, 663, 598 [2007])

The eclipse image of Figure 3 was provided to the authors by Jackob Strikis of the Elizabeth Observatory, Athens, who claimed authorship. However, shortly after publication the authors discovered that this eclipse image was in fact a preliminary version of an image belonging to Prof. Miloslav Druckmuller, taken during the 2006 total solar eclipse from Libya at 30 56.946 0 N, 24 14.3010 E, and at an altitude of 158 m. This image can be found at http://www.zam.fme.vutbr.cz /~druck/Eclipse/index.htm. We extend our gratitude to Prof. Druckmuller, from Brno University of Technology, Czech Republic, who brought this incident to our attention, and who has graciously accepted our apology for this unintentional mishap. A forthcoming article in collaboration with Prof. Druckmuller is in preparation. The Astrophysical Journal, 670:1521, 2007 December 1

Utilization of redundant polarized solar spectra to infer the polarization properties of the new generation of large aperture solar telescopes

Spectro-polarimetry plays an important role in the study of solar magnetism and strongly influences the design of the new generation of solar telescopes. Calibration of the polarization properties of the telescope is a critical requirement needed to use these observations to infer solar magnetic fields. However, the large apertures of these new telescopes make direct calibration with polarization calibration optics placed before all the telescope optical elements impractical. It is therefore desirable to be able to infer the polarization properties of the telescope optical elements utilizing solar observations themselves. Taking advantage of the fact that the un-polarized, linearly, and circularly polarized spectra originating from the Sun are uncorrelated, we have developed techniques to utilize observations of solar spectra with redundant combination of the polarization states measured at several different telescope configurations to infer the polarization properties of the telescope as a whole and of its optical elements. We show results of these techniques applied to spectro-plarimetric data obtained at the Dunn Solar Telescope.

MULTI-WAVELENGTH STUDY OF A DELTA-SPOT. I. A REGION OF VERY STRONG, HORIZONTAL MAGNETIC FIELD

Active region NOAA 11035 appeared in 2009 December, early in the new solar activity cycle. This region achieved a delta sunspot (δ spot) configuration when parasitic flux emerged near the rotationally leading magnetic polarity and traveled through the penumbra of the largest sunspot in the group. Both visible and infrared imaging spectropolarimetry of the magnetically sensitive Fe i line pairs at 6302 and 15650 A show large Zeeman splitting in the penumbra between the parasitic umbra and the main sunspot umbra. The polarized Stokes spectra in the strongest field region display anomalous profiles, and strong blueshifts are seen in an adjacent region. Analysis of the profiles is carried out using a Milne–Eddington inversion code capable of fitting either a single magnetic component with stray light or two independent magnetic components to verify the field strength. The inversion results show that the anomalous profiles cannot be produced by the combination of two profiles with moderate magnetic fields. The largest field strengths are 3500–3800 G in close proximity to blueshifts as strong as 3.8 km s−1. The strong, nearly horizontal magnetic field seen near the polarity inversion line in this region is difficult to understand in the context of a standard model of sunspot magnetohydrostatic equilibrium.

Detection of an Extended Near-Sun Neutral Helium Cloud from Ground-based Infrared Coronagraph Spectropolarimetry

Sensitive spectropolarimetric observations from the Haleakala SOLARC coronagraph and infrared imaging spectropolarimeter have detected an extended diffuse surface brightness flux at the 1083 nm wavelength of neutral helium (He I). This has the polarization signature of light scattered by an extended He I cloud in the vicinity of the Sun. The He I scattered surface brightness appears to be consistent with a previous eclipse measurement and satellite observations of the local interstellar medium (LISM) helium wind (LISW), obtained using observations of the He I UV resonance line at 58.4 nm. The sensitivity of the infrared coronagraphic method suggests that the LISW interaction with the local solar wind can have observable consequences that may yield a useful ground-based technique for studying the coronal and interplanetary plasma.

Formation of the UV Spectrum of Molecular Hydrogen in the Sun

Ultraviolet lines of molecular hydrogen have been observed in solar spectra for almost four decades, but the behavior of the molecular spectrum and its implications for solar atmospheric structure are not fully understood. Data from the HRTS instrument revealed that H2 emission forms in particular regions, selectively excited by bright UV transition region and chromospheric lines. We test the conditions under which H2 emission can originate by studying non-LTE models sampling a broad range of temperature stratifications and radiation conditions. Stratification plays the dominant role in determining the population densities of H2, which forms in greatest abundance near the continuum photosphere. However, opacity due to photoionization of silicon and other neutrals determines the depth to which UV radiation can penetrate to excite the H2. Thus the majority of H2 emission forms in a narrow region, at about 650 km in standard 1D models of the quiet-Sun, near the tau=1 opacity surface for the exciting UV radiation, generally coming from above. When irradiated from above using observed intensities of bright UV emission lines, detailed non-LTE calculations show that the spectrum of H2 seen in the quiet-Sun SUMER atlas spectrum and HRTS light bridge spectrum can be satisfactorily reproduced in 1D stratified atmospheres, without including 3D or time dependent thermal structures. A detailed comparison to observations from 1205 to 1550 Angstroms is presented, and the success of this 1D approach to modeling solar UV H2 emission is illustrated by the identification of previously unidentified lines and upper levels in HRTS spectra.