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SPECTROPOLARIMETRIC OBSERVATIONS OF THE HELIUM 10830 A˚ LINE: A SEARCH FOR THE SIGNATURE OF OPTICAL PUMPING Nathan Goldbaum University of Colorado at Boulder Advisor: Jeff Kuhn University of Hawaii, Institute for Astronomy ABSTRACT In order to characterize the center-to-limb and latitudinal variation of the polarization of the He I 10830 A˚ line we observed the limb of the sun at several position angles using the Scatter-Free Observatory for Limb Active Regions and Coronae (SOLARC), an imaging spectropolarimeter located on the summit of Haleakala, Maui. The data were reduced and analyzed to produce profiles of the magnitude of the Stokes Q/I and U/I signals as a function of latitude and solar radius. Modeling of the observed profiles should allow us to detect whether the observed linear polarization signal is due entirely to scattering polarization or possibly includes absorptively polarized light transmitted through an optically pumped gas. Subject headings: Sun, Optical Pumping, He I 10830 A,˚ Spectropolarimetry 1. Introduction Spectropolarimetric observations offer a unique means to measure many properties of the solar atmosphere. Observing the second solar spectrum (Stenflo & Keller 1998) | the term used to describe the sun's polarized spectrum | makes possible direct measurements of many important aspects of the sun's atmosphere, particularly with respect to the vector magnetic field. Stokes I, Q, and U spectra can be used to map the photospheric magnetic field through the transverse Zeeman effect (Mickey et al. 1996). These maps are commonly used as the boundary conditions for force-free extrapolation of the chromospheric and coronal magnetic field (Guo et al. 2008). Spectropolarimetric data also enables direct observation of the coronal magnetic field by using both the Hanl´eand Zeeman effects (Lin et al. 2000, 2004), a necessary ingredient in fully understanding the nature of flares, the coronal heating problem, and many other open issues in solar physics (Judge 2003). The Helium I 10830 A˚ line is particularly attractive for studying structures in the chromosphere and corona due to its sensitivity to the Hanl´eand Zeeman effects (Centeno et al. 2008). { 2 { In this work, we examine the linear polarization of the He I 10830 A˚ line using data obtained with SOLARC (Scatter-Free Observatory for Limb Active Regions and Coronae), a half-meter off- axis imaging spectropolarimeter designed for coronal observations and located on the summit of Haleakala, Maui. First, we discuss the physics of the transition and the mechanisms that could produce the observed linear polarization signal. Next, we describe the instrument and some of its unique properties. This is followed by a discussion of the observations and data reduction. Finally, we discuss the results of our observations and the key features which must be reproduced in any models which attempt to simulate this spectral line. 2. Optical Pumping While observing the Hα line of a sample of Herbig Ae/Be stars, Harrington & Kuhn (2007) found a small change in the linear polarization signal in the absorptive component of the P-Cygni Hα profiles of several of the observed stars. Previous models, based on scattering polarization in the accretion disks surrounding these young and massive stars, predicted a linear polarization signal in the emissive component due to disk scattering but could not account for observed linear polarization observed in the absorptive component. Kuhn et al. (2007) proposed a model that employed a resonant absorption mechanism to explain the observed Stokes profiles. In this model, an anisotropic radiation field induces population imbalances in the degenerate sublevels of the lower level of an atomic transition. A gas prepared in this way is said to be pumped. If a second beam of light is transmitted through a pumped gas at an angle to the pumping radiation, the emergent light will be linearly polarized. Kuhn et al. (2007) modeled the atomic physics of hydrogen and the performed the radiative transfer calculations necessary to predict the Stokes Q and U Hα profiles. The predictions included a significant polarization signal in the absorptive component of the Hα profiles. If optical pumping is the explanation for the observed line profiles in the Herbig Ae/Be stars, we should be able to see the signature of optical pumping in solar transitions when looking at the limb of the sun where there is a similar geometry. The He I 10830 A˚ line has been well studied in the context of solar spectropolarimetric obser- vations and atmospheric diagnostics (Trujillo Bueno et al. 2002; Trujillo Bueno & Asensio Ramos 3 2007; Centeno et al. 2008). This spectral line arises as a transition between the 2 P0;1;2 (Ju = 0; 1; 2) 3 3 state and the 2 S1 (Jl = 1) state of the neutral helium atom. The 2 S1 state is the metastable ground level of orthohelium and can only be populated through collisional excitation from the 1 ground level 1 S1 parahelium state. The 10830 A˚ line is a triplet, but it is observed as a doublet in the solar spectrum since the transitions with Ju=2,1 appear thermally blended at plasma tem- peratures (Trujillo Bueno et al. 2002). The observations presented here do not resolve the `blue' component, a common occurrence in observations of the quiet sun He I 10830 A˚ line (Sanz-Forcada & Dupree 2008). This transition is susceptible to optical pumping since the lower level has Jl = 1, which means there are three degenerate substates corresponding to mJ = ±1,0. If we consider the { 3 { transition with Ju=0 and Jl = 1, as in Figure 1, the upper level will have only one substate and the lower level will have three substates. Resonant polarized absorption in this particular quantum system can be explained using an interesting semiclassical model. Consider a pumping beam traveling in the z direction, incident on an isotropic gas that is susceptible to a transition with Ju=0 and Jl = 1. We assume the pumping beam has been tuned to excite this transition and that all of the degenerate lower level sublevels are equally populated. Since the pumping beam cannot excite oscillations along the beam's direction of propagation, those transitions that correspond to oscillations in z, with ∆mJ =0, are not allowed. Transitions with ∆mJ = ± 1 may proceed, however. The pumping beam will induce any electrons in the levels with jmJ j = 1 | corresponding to induced oscillations in the x and y directions | to quickly jump to the upper level. After a short time in the upper level, the electrons undergo a spontaneous de-excitation with an equal probability for the electron to end up in any of the degenerate sublevels (mj=0 or ±1). Electrons in the state with mJ = 0 will only transition to a higher level due to collisions or through some other `forbidden' mechanism. Over time the system will reach an equilibrium in which most of the electrons are in the state with mJ = 0 (Kuhn et al. 2007). The optical pumping mechanism can also operate on the states in which Ju = 2,1 (Trujillo Bueno 2002). If another unpolarized beam is incident on the gas in the y direction, the pumped gas absorbs and scatters the component of the unpolarized beam that is polarized along the z direction. The emergent beam will be polarized in the x direction due to resonant polarized absorption in an optically pumped gas. The observations considered here were of regions at the limb of the sun. In this geometry the pumping field originates at the photosphere directly below the atmospheric helium. The transmitted absorptively polarized light, which we observe as linearly polarized light aligned with the limb, originates on the disk at the extreme limb. The radiation field is anisotropic due both to the geometry and the limb darkening. It also bears mentioning that we will be observing polarized photons that originate in classical scattering events as well as ones which were polarized by passing through a pumped gas. Only modeling of the relevant atomic physics and radiative transfer will determine whether an optical pumping mechanism is in operation in our data. 3. SOLARC SOLARC (Kuhn et al. 2003) is a 45 cm off-axis Gregorian telescope (Figure 3). The 11◦ off-axis angle places the primary focus out of the telescope's line of sight, removing the secondary mirror and support structures from the field of view. This minimizes scattered light, allowing longer integration times that are critical for photon-limited observations. The telescope is fitted with a liquid-crystal-based fixed retarder system (Figure 4) that modulates the polarization state of the incoming light so that one Stokes parameter is sure to be transmitted through a linearly polarized analyzer and the orthogonal (negative) Stokes parameter is sure to be blocked. This allows a fixed linearly polarized analyzer to block, say, +Q polarized light but allow -Q, along with the unpolarized, U, and V polarized components of the beam. The modulated beam is focused { 4 { onto a fiber bundle array. The fiber array is made up of 128 fibers arranged in a 16×8 rectangular pattern. A lenslet on each fiber focuses light into the fiber core, minimizing lost photons due to thick fiber claddings. Each fiber images a 22-arcsecond field of view in the current configuration. The fibers pipe the light to an optical bench where the OFIS spectrograph (Figure 5) is located. At the fiber bundle exit the fibers have been rearranged into a 2×64 array, using the mapping scheme illustrated in Figure 2. The light exits the fiber bundle, passes through a collimating lens, and then is dispersed by an echelle grating ruled to 79 lines/mm with a blaze angle of 63.5◦.

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