Lower Solar Atmosphere

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Magnetic Field Observations, Interpretation and Theory

Diagnostic Techniques for Solar Magnetic Fields

HAO continues to explore new techniques for inferring magnetic fields in the solar atmosphere. This year progress was realized in two areas: the atomic physics of resonance scattering of polarized radiation in the presence of magnetic fields, and in the techniques used to extract vector magnetic fields from Zeeman-induced polarization.

-The Second Solar Spectrum of the Sodium D Lines-

The "second solar spectrum" (i.e., polarization of the solar spectrum, primarily due to scattering, seen prominently near the solar limb) has drawn a great deal of attention during the past decade as a promising new avenue for exploration of both solar magnetic fields and the physics of scattering of polarized radiation in an ionized plasma. Much of the detailed quantum physics of radiation scattering is, however, still not well understood, and there has been an ongoing debate on the source of the "enigmatic" observed shapes of the neutral sodium D1 and D2 polarization profiles. This year, Roberto Casini (HAO) derived a formulation of resonance scattering in a collisionless, magnetized plasma, in the limit of complete frequency redistribution, in order to investigate in more depth the formation of the second solar spectrum. In particular, he realized that the combination of quantum interferences between fine structure levels and the magnetic Hanle effect could provide an explanation of the line shape of Stokes Q/I of neutral sodium (Na) D1. An earlier attempt by Landi Degl'Innocenti (1998) to explain the second solar spectrum of the Na D1 and D2 lines relied instead on the effect of atomic polarization in the hyperfine structure levels of the ground state, leaving out the possible role of magnetic fields. That attempt was successful in reproducing the spectral line shape of D2, but required an ad-hoc assumption on the amount of ground-level atomic polarization, which could not be computed self-consistently. It also provided for an anti-symmetric shape of the Q/I profile of D1, which has been questioned by several observations. In the present formulation, Casini verified that the presence of a magnetic field can change the symmetry of the D1 profile, so it might become possible to finally explain why different observations of D1 have shown different degrees of anti-symmetry in that profile. Besides providing a possible qualitative explanation of the observations, the magnetic dependence of the symmetry of D1 is also of interest as a potential new diagnostic of weak magnetic fields in the chromosphere. Casini, in collaboration with Rafael Manso Sainz (HAO postdoc) and Javier Trujillo Bueno (IAC), is working to implement radiative transfer in the formalism, to verify if this can be sufficient to reproduce the observations for both D1 and D2.

-Laboratory Experiment For Scattering Polarization- In recent years, improvements in observational techniques for solar polarization have resulted in detailed spectral profiles of solar lines that show scattering polarization when viewed near the solar limb ( the "second solar spectrum"). The behavior of this polarization, especially in the presence of solar magnetic fields, is not well understood, as theorists have difficulty matching the features of the solar profiles with synthetic ones. With support through the NCAR Opportunity Fund, Steve Tomczyk (HAO) and Casini have assembled an optical pumping experiment to measure resonance scattering polarization in the sodium D-lines under controlled conditions of illumination and in the presence of an ambient magnetic field. The experiment consists of a laboratory sodium vapor embedded in a pair of Helmholtz coils with optics for illuminating the vapor and collecting the light scattered at 90 degrees. The polarization of the scattered light as a function of wavelength is analyzed with a Liquid Crystal polarimeter and a tunable Fabry-Perot spectrometer. The experimental setup is now complete and measurements will be made in the near future. This project also involves E. Landi Degl'Innocenti (University of Florence, Italy) and A. Lopez Ariste (CNRS, France) as scientific collaborators.

-Hydrogen Resonance Scattering with Magnetic and Electric Fields-
Casini developed a formalism to treat resonance scattering polarization in hydrogen lines when both a magnetic field and an electric field are present. This work is driven in part by new observations indicating the presence of anomalous Stokes V polarization profiles in H α. The interest for this problem comes from the significant sensitivity of hydrogen polarization to the electric Hanle effect, that is, to the modification of radiation-induced atomic polarization determined by the presence of an electric field. This was studied by Favati, Landi Degl'Innocenti, and Landolfi (1987) in the case of Lyman α, neglecting atomic polarization in the ground level. Casini generalized that problem, allowing for the possibility of an arbitrary number of hydrogen levels (only limited by computational resources), and allowing for atomic polarization of the ground level. This represents a fundamental advance in that both magnetic and electric fields are included in the formalism. From the work of Favati et al., it was known that significant levels of atomic orientation can be created for sufficiently large electric strengths (of the order of 100 V/cm). As a consequence, one should expect a significant amount of net circular polarization from resonance scattering in hydrogen lines. However, the required electric strengths to attain observable levels of atomic orientation are of little interest for solar applications, as the very low resistivity of solar plasmas is not conducive to the creation of strong electric fields. Things are different if a magnetic field is also present.In such a case, the electric field can act as a "catalyst" of the atomic orientation induced by the magnetic field itself. In particular, the rearranging of the atomic level structure induced by even a small electric field (of the order of 1 V/cm) is sufficient to enhance the effect of the alignment-to-orientation conversion mechanism induced by the magnetic field, so that observable levels of net circular polarization can be produced for field configurations that are compatible with the physics of solar plasmas.

-Anomalous H α V-profiles in Prominences- In collaboration with A. Lopez-Ariste (CNRS, France), and several others Casini investigated the possible physical mechanisms that could be responsible for the anomalous (i.e., non-Zeeman) Stokes V profiles of H α found in several spectro-polarimetric datasets of prominences, obtained with the Advanced Stokes Polarimeter (ASP) between 1996 and 2003. The occurrence of such anomalous profiles was also confirmed by recent observations with THEMIS (Telescopio Heliografico para el Estudio del Magnetismo y de Inestabilidades Solares). Casini and collaborators concluded that these anomalies in Stokes V line shapes are not likely to be due to instrumental polarization because they are not consistently present within each dataset, and also, because simultaneous observations in He D3 (whose formation in prominences is rather well understood) showed the theoretically expected polarization profiles. Computational tests on the degree of net circular polarization induced by the prominence magnetic field, via the alignment-to-orientation conversion mechanism, ruled out the possibility that this could be the physical process responsible for the observed signals. In order to achieve a sufficient amount of atomic orientation, very large magnetic strengths (of order 1000 G) must be invoked, whose occurrence in prominences is highly improbable. Furthermore, at such large field strengths the contribution to Stokes V from the Zeeman effect would completely dominate. A more plausible physical explanation could be that the H α radiation from the photosphere, incident on the prominence, is circularly polarized (e.g., because it comes from a magnetically active region), while the prominence is subject to bulk motions such that the moving atom "collects" only one state of the circular polarization of the incident radiation. This would tend to produce an anomalously high level of atomic orientation directly from irradiation, so the scattered radiation from the atom would naturally possess a large degree of net circular polarization (even in the absence of a prominence magnetic field). However, the conditions that need to be met for such a process to be efficient (a large Zeeman-effect circular polarization from the photospheric layers) probably do not occur in those quiescent prominences where the anomalous V signals have been observed. An alternative, and more intriguing, explanation could be that they reveal the "catalytic" effect of microscopic electric fields for the atomic orientation induced by the prominence magnetic field.

-The HeI 1083 nm Multiplet as a Chromospheric Magnetic Field Diagnostic- In collaboration with Trujillo-Bueno, Collados, Centeno-Elliot (all of the IAC), and Landi Degl'Innocenti (Arcetri), Hector Socas-Navarro (HAO) continued work on a theoretical study of the formation of polarization profiles in the interesting He I 1083 nm multiplet (Socas-Navarro et al. 2004b), and on the analysis of observations from the TIP infrared polarimeter in Tenerife. This work is particularly timely, given the capabilities of the new HAO/NSO visible/infrared polarimeter SPINOR to observe this multiplet in conjunction with other photospheric or chromospheric lines.

-New Spectro-Polarimetric Inversion Strategies- During the past few years, Socas-Navarro and others at HAO have been actively pursuing new fast (ideally, real-time), robust techniques for inverting spectro-polarimetric observations. The new generation of instruments currently under development will deliver data at enormous rates that cannot be processed using existing least-squares inversion procedures. Socas-Navarro has conducted research both to optimize existing techniques (Socas-Navarro 2004b), and to develop new strategies based on machine learning and pattern recognition. In particular, the use of artificial neural networks shows great promise of becoming a new standard for the analysis of spectro-polarimetric data (see the accompanying Figure).

Inversion tests with an Artificial Neural Network (ANN). An ASP map was processed using an ANN with 4 layers and 80 neurons per layer. The entire map was inverted in about 5 seconds using a Pentium 4 processor at 2.4 GHz. Upper panels: Test with synthetic data. Lower panels: Test with real observations. Left: Intrinsic magnetic strength retrieved by the ANN. Right: Median and standard deviation of the scatter plot.

-Community Spectro-Polarimetric Analysis Center (CSAC)- A new NCAR Strategic Initiative, the Community Spectro-Polarimetric Analysis Center (CSAC), was approved in FY04. Through this initiative, HAO and NCAR will continue their leadership role in the analysis of solar polarimetry for the inference of the magnetic field vector in the solar atmosphere. A host of new instruments, both space- and ground-based, will be producing large amounts of precision spectro-polarimetric data in the near future. The CSAC will standardize and optimize analysis of these data, and provide the analysis tools to the solar community. The program also plans significant involvement of the solar community. The initiative will also support continued investigation into advanced methods of analysis, including pattern recognition and artificial intelligence methods for extracting the field vector from polarimetric data. Of particular importance is the CSAC effort in support of the Solar-B mission.

Magnetic Observational Studies

-Observational Evidence for Photospheric Flux Rope Emergence- Bruce Lites (HAO) has completed a study of vector magnetic field evolution in the photosphere under active region filaments (prominences as seen against the solar disk). Examination of several years of data from the Advanced Stokes Polarimeter (ASP) has revealed the evolution of narrow, low-lying filaments as seen in H α, which occur away from sunspots in active regions. Such filaments occur commonly in active regions, and if their occurrence can be associated with the development of twist in the photospheric magnetic field, this would be strong evidence in support of the supposition that ropes of magnetic flux, born in the solar interior, rise through the solar surface into the atmosphere to inject the magnetic helicity that is now believed to influence the large-scale development of the solar corona. Two cases were examined in detail by Lites, one of which is shown in the accompanying Figure. These cases indeed show strong evidence for twisted magnetic fields in the photosphere immediately under the filaments, in that the fields have a concave-upward geometry and significant shear along the filament.

-High-Angular Resolution Studies of Quiet Sun Magnetism- Magnetic fields present in the quietest regions of the Sun, the solar "internetwork" regions, may play a crucial role in heating the upper solar atmosphere away from active regions. A recent measurement using filtergraph polarimetry and post-observation image reconstruction suggests that most intergranular lanes are occupied by kiloGauss magnetic field concentrations. The net unsigned flux of the quiet internetwork regions from that study is more than double that of prior estimates based on spectro-polarimetry from ASP, albeit at a somewhat lower angular resolution. As these filtergraph data have much lower sensitivity to polarization than the prior ASP observations, it was deemed worthwhile to repeat those ASP quiet Sun measurements using the higher angular resolution of the DLSP (Diffraction-Limited Spectro-Polarimeter). Lites and Socas-Navarro (2004) reported on such observations obtained with the DLSP and the NSO low-order adaptive optics system in September 2003. Compared to the filter-based observations these new observations are better than 10 times more sensitive to polarization, and they have similar angular resolution. The spectro-polarimetric observations of Lites and Socas-Navarro neither reveal ubiquitous occurrence of strong flux concentrations in all intergranular lanes, nor does the net unsigned flux show a large increase over the prior ASP measurements. This study then suggests that there is not a lot of hidden unsigned flux distributed over all size scales, otherwise the new observations would show a large increase in unsigned flux.

-Supersonic Upflows in the Quiet Photosphere- Socas-Navarro and Rafael Manso Sainz (HAO) have found evidence of supersonic upflows in the quiet photosphere. {{reference figure supersonic.eps here}} They speculate that these events might be the signature of exploding magnetic elements during an aborted field amplification process by convective collapse. The small-scale structure of the magnetic field in the quiet Sun is also a subject in which HAO scientists have made significant advances during the past year. (Socas-Navarro et al. 2004a; Socas-Navarro 2004a).

Stokes I (upper panels) and V (lower panels) profiles corresponding to two different locations harboring supersonic upflows in the quiet photosphere. Notice the double-peaked structure of the Stokes V blue lobe, clearly showing a highly Doppler-shifted component.

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MHD Theory and Models

-Simulations of Flux Rope Emergence into a Magnetized Atmosphere- Fan and Gibson (2003, 2004) have developed 3D MHD simulations that demonstrate the emergence of a twisted magnetic flux tube into a pre-existing coronal potential magnetic arcade. The motivation is to investigate the dynamic evolution of a coronal magnetic field in response to the emergence of significantly twisted structures, and to understand the nature of observed X-ray sigmoids. In these MHD simulations, the treatment of the energy equation and the thermodynamics of the coronal plasma are drastically simplified by using an isothermal equation of state. The focus is the evolution of the coronal magnetic field under conditions of high electric conductivity and low plasma β. The simulations show that the line-tied emerging flux tube becomes kink unstable when sufficient amount of twist is transported into the corona. For an emerging tube with a left-handed twist (which is the preferred sense of twist for active regions in the northern hemisphere), the writhing of the tube resulting from the kink instability is also left-handed, producing a forward S-shape for the tube axis as viewed from the top, opposite to the inverse S-shaped X-ray sigmoid morphology preferentially seen in the northern hemisphere. However, the writhing motion of the tube and its interaction with the ambient coronal magnetic field also drives the formation of an intense current layer, which displays an inverse S-shape, consistent with the morphology of X-ray sigmoids.

Panels [1a]-[4a] and [1b]-[4b] show the 3D evolution of the coronal magnetic field, when driven at the lower boundary by the emergence of a twisted flux rope into an overlying coronal arcade (red field lines). Field lines in the rope are color-coded based on the flux surfaces of the initial tube they belong to. When the emerged rope became substantially kinked (panels [4a] and [4b]), a curved layer of highly concentrated current formed an inverse-S shape as viewed from the top (panels [4c] and [4d]) (From Fan and Gibson 2003).

-Magnetic Free Energy in Coronal Flux Ropes- Gibson, Boon Chye Low (HAO), and collaborator Rekha Jain (University of Sheffield, UK) are working on a parameterized study of magnetic free energy, using an analytic flux rope model in a coronal atmosphere (Gibson et al. 2002). A potential field configuration is analytically determined with the same boundary condition as the non-potential model field, allowing calculation of the magnetic energy difference between the two systems. This free energy is a critical indicator of the amount of energy that can be released in a coronal mass ejection (CME). Because the model is analytic, it will be possible to do a parameter study to investigate how properties such as magnetic field strength, region size, and degree of magnetic twist combine to affect free energy.

-Testing Extrapolations of Photospheric Fields into the Corona- It is fairly standard procedure to use the photospheric magnetic field, which is well-observed, to extrapolate a coronal magnetic field. Gibson, Fan, and collaborators K. D. Leka and G. Barnes (both of CoRA), Cristina Mandrini (IAFE), Pascal Demoulin (University of Paris, France) and Tom Metcalf (LMSAL) are directly testing such methods by providing the lower boundary condition of the Fan and Gibson (2003, 2004) emerging flux rope simulation as proxy for the photospheric field, and comparing resulting predictions to properties of the, a priori known, coronal field. Preliminary results show that Fourier-transform-constant α extrapolation techniques fail to reproduce the flux rope field. In particular, the amount of twist required to reproduce the extent and height of the flux rope leads to unphysical artifacts, due to limitations of the numerical techniques.

-Simulations of Magnetic Helicity Injection into the Corona- Gibson, Fan, and collaborators George Fisher (University of California, Berkeley), Mandrini, and Demoulin (Gibson et al. 2004) have studied how apparent horizontal motions of magnetic elements at the photosphere caused by an emerging flux rope might be interpreted. In particular, they showed that local correlation tracking (LCT) analysis (Welsch et al. 2004) of a time-series of magnetograms taken from the lower boundary condition of the Fan and Gibson (2003, 2004) simulation leads to an underestimate of the amount of magnetic helicity transported into the corona by the flux rope, largely because of undetectable twisting motions along the magnetic flux surfaces which do not change the normal magnetic field. High-resolution observations of rotating sunspots may provide better information about such rotational motions, and Gibson and co-workers found that if the separated flux rope legs were taken as proxies for fully formed sunspots, the amount of rotation that would be observed before the region became kink unstable would be in the range 40-200 degrees per leg/sunspot, consistent with observations (Brown et al. 2003). However, this amount of observed rotation is again well short of the total amount of twist carried into the corona by the flux rope, and in general the study indicated that using photospheric motions to determine helicity injection is prone to significant underestimation, at least to the extent such injection arises from emerging twisted fields. This is important because estimates of helicity input to the corona have been used to quantify its energetic state, and, on longer time-frames, to estimate the source term of the "helicity budget" which traces magnetic twist from emergence all the way out to twist in magnetic clouds observed passing the Earth.

Apparent photospheric horizontal velocities, calculated explicitly for the Fan and Gibson (2003, 2004) simulation. The velocity vectors of the actual motion of photospheric foot points of magnetic field lines (bottom panels) and the velocity vectors calculated using LCT analysis of magnetogram time series (top panels) are shown with arrows. Vectors represent direction of velocity tangent to the photosphere; the length of vectors in this figure are uniform and do not represent velocity magnitude.

Total helicity injected through the photosphere as a function of time, as calculated exactly using known vertical velocity of the simulated flux rope emergence in combination with the normal magnetic field (solid line), and as calculated using Local Correlation Tracking of a time-series of the normal magnetic field to establish an apparent horizontal velocity of magnetic elements (triangles). Helicities are normalized to the total flux in the twisted flux tube.

-Modeling Coronal Flux Tube Emergence and Evolution- In July, 2004, a workshop on observational constraints for coronal magnetic flux rope modeling was held at the University of California, Berkeley Space Science Laboratories to gain input on how best to use observations for a data-constrained run of the Fan and Gibson (2003, 2004) simulation of a flux rope emerging into a coronal atmosphere. Yuhong Fan, Sarah Gibson (both HAO), Graham Barnes (Colorado Research Associates), and collaborators Janet Luhmann, Steve Ledvina, Bill Abbett, Yan Li, Loraine Lundquist, Brian Welsch (all of the University of California), Rich Nightingale (LMSAL), and David Alexander (Rice Univ.) attended. As a result of this workshop, a run of the flux rope simulation based on AR 8038, May 12, 1997 is planned. Observations of global line-of-sight magnetograms will constrain the size, location, and field strength of the emerging flux rope relative to a background dipole coronal field into which the rope emerges. Once the simulation is run, the model results will be compared a posteriori to a range of observations. The distribution and evolution of observed vector magnetic field can be compared to the evolving model field. The separatrix surfaces and current sheets formed in the simulation will be compared to X-ray sigmoids, and dipped magnetic field to the region's filament. The solar disk projection of the density-depleted flux rope cavity will be compared to dimming events seen in X-ray and EUV. Apparent photospheric magnetic field element motions will also be compared to LCT analysis of magnetograms, and sunspot rotation observed in white light to model magnetic field line foot point motion.

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Instrumentation for Inference of Solar Magnetic Fields (Ground-based Instrumentation | Space Instrumentation)

HAO continues its role as a leader in development of instrumentation for remote sensing of the magnetic field in the solar atmosphere. This tradition dates back nearly 40 years when HAO programs were initiated to measure precisely the spectral variation of the polarization of light emitted by the Sun: it is the polarization of light that carries the most prominent signature of the strength of the magnetic field as well as its orientation. The approach of precise, spectrally-resolved observations ("spectro-polarimetry") has proven to yield the most accurate measures of the field vector, along with the important accompanying information on the thermodynamic state of the solar plasma. Currently, a major fraction of new solar instrumentation programs world-wide embraces spectro-polarimetry, indicating the central role that magnetic fields demand for understanding solar variability. This trend is due in no small measure to HAO's long-term commitment to solar spectro-polarimetry.

At HAO, a number of instrumentation programs now underway focus on various aspects of observations of the solar magnetic field. Most of these efforts are collaborations with other institutions, both nationally and internationally. It is clear that the community continues to look to HAO for expertise in both the engineering and scientific aspects of solar magnetic field observation. Presented in the following is a brief description of the various HAO programs to measure fields in the lower solar atmosphere (photosphere and chromosphere), along with their current status. New instrumentation to measure fields in the corona and in solar prominences is also presented in the Coronal Structure and Dynamics section.

Ground-Based Instrumentation

-ASP: Advanced Stokes Polarimeter- The ASP is perhaps the one single instrument that spurred development of the array of new solar spectro-polarimetric instrumentation. Observations from this instrument provided the first highly quantitative measurements of the full field vector with spatial resolution adequate to isolate, if not fully resolve, some solar structures. Deployed in 1991 at the National Solar Observatory (NSO) Dunn Solar Telescope (DST), its continued usage over the past 12 years as a facility research instrument operated by NSO has proven of enormous scientific benefit. It continues to be in high demand by many observers, but now suffers from aging electronics. It will continue to be operated until the replacing instrumentation, SPINOR and DLSP described below, become fully operational. Bruce Lites is the lead scientist for ASP.

-SPINOR: Spectro-Polarimeter for Infrared and Optical Regions- The infrared spectrum offers substantial scientific advantage over the visible for studies of solar magnetism: magnetic splittings of spectrum lines increase linearly with wavelength relative to their intrinsic widths, and a few spectral features offer better sensitivity or more easily understood interpretation for chromospheric magnetic fields. There is a large potential scientific pay-off for observations combining simultaneous visible and IR spectro-polarimetry. These issues, along with increasing demand for flexible spectro-polarimetry and the need to ultimately replace ASP, have led this year to the development of the new SPINOR facility instrument. It will permit simultaneous observations of multiple lines anywhere in the wavelength range 0.4 to 1.6 microns. Furthermore, it is mated to the new NSO adaptive optics system, permitting measurements of consistently better angular resolution than ASP. The system recycles some hardware from ASP, but most of the optics and electronics of ASP will be replaced. A very successful observing run of the preliminary configuration of SPINOR was completed in June, 2004, with further modifications and improvements due in the near future. Hector Socas-Navarro is the lead scientist for SPINOR.

These initial observations of infrared Stokes spectra from SPINOR demonstrate the power of the new instrument to provide diagnostics of chromospheric vector magnetic fields through observations of the ionized calcium infrared lines and the infrared neutral helium line. Rows from top to bottom are Stokes I,Q,U,V, and columns are (left to right): Ca II 849.8 nm, Ca II 854.2 nm, and He I 1083.0 nm. Stokes Q,U panels saturate at +/- 1%, Stokes V at +/- 2%. The variation of the polarized Stokes Q,U,V profiles along the slit as viewed at high angular resolution suggest a high degree of structuring of the chromospheric vector field.

-DLSP: Diffraction-Limited Spectro-Polarimeter- As described in the 2003 Annual Report, the DLSP is intended to expand the capabilities of ASP in another direction -- toward very high angular resolution. Much of the demand for ASP observing time is for observations of photospheric vector magnetic fields. The DLSP is optimized to provide the highest angular resolution spectro-polarimetry for the photospheric Zeeman-sensitive neutral iron absorption lines at 630 nm. The DLSP is an instrument whose configuration will remain fixed and stable, and which is mated to the NSO second-generation adaptive optics system. Its fixed optical configuration will permit standard data reduction techniques to be developed, thus greatly reducing the effort needed for visiting observers to analyze their data. During FY04 the second phase of the DLSP hardware development has been implemented: new cameras and data system (the first phase version used the ASP cameras and data system), and integration into the new NSO adaptive optics system. Several observing runs concentrated on optimizing the performance of the system, and it should be ready soon for release as a facility instrument at NSO. Bruce Lites is the lead scientist from HAO, and K. Sankarasubraminan is the lead scientist at NSO.

-POLIS: POlarimetric LIttrow Spectrograph- POLIS is a ground-based prototype of the spectro-polarimeter to be flown on the Sunrise high altitude balloon platform. Stationed at Tenerife, Canary Islands, Spain, it is a collaborative program between the Kiepenheuer-Institut für Sonnenphysik (KIS), Freiburg, Germany and HAO. In late 2003, simultaneous observations were carried out of photospheric vector fields and the ultraviolet ionized calcium chromospheric lines at 397 nm. Like the instruments at the NSO DST, POLIS also is mated to an adaptive optics system. It is anticipated that soon POLIS and SPINOR or DLSP will be able to carry out sustained measurements of developing active regions over more than half of a 24-hour period, thus considerably aiding the study of the evolution of active region vector magnetic fields. Bruce Lites is the HAO lead scientist for POLIS, and in Germany, Thomas Kentischer and Wolfgang Schmidt lead the effort at the KIS.

-ATST: Advanced Technology Solar Telescope- The ATST is a project for a large-aperture solar telescope. With its 4 meter diameter primary mirror, the ATST will have a collecting area 16 times larger than the largest existing solar telescope and will be able to operate at the diffraction limit thanks to an advanced adaptive optics system. This project has been ranked by the Decadal Survey of Astronomy and Astrophysics as the most important ground-based initiative for the next decade. HAO is involved in this project at several levels. One of the most important milestones this year is the production of the final Site Survey report, due in October 2004, which will be used by the project to select on of the three remaining candidate sites (Big Bear, CA; Haleaka, HI; or La Palma, Spain). HAO has developed one of the inversion codes for the analysis of the image quality data, and has coordinated NCAR-wide efforts to validate the measurements of the site survey instrumentation (Hill et al. 2004). The NCAR Atmospheric Technology Division (ATD) has participated in the site survey testing efforts with Mike Susedik, Tom Horst, Steve Oncley, and Gordon MacLean, among others, contributing, and support has been received from NOAA through the Boulder Atmospheric Observatory. The polarimetric calibration of a 4 meter telescope is another problem that the ATST team has had to address. Kim Streander and Paul Seagraves (both HAO) presented a technique that makes use of sunspot observations to calibrate the telescope. Additionally, Socas-Navarro has developed two other techniques that are purely instrumental and do not require any assumptions about polarization of sunlight. HAO is responsible for the development of one of the most important instruments of the ATST, namely, the Visible Spectrograph (ViSP). This instrument will allow for high-precision, sensitivity and angular resolution spectro-polarimetry simultaneously in several spectral domains in the visible and near infrared up to 1.6 microns. David Elmore (HAO) and Socas-Navarro are leading the design efforts for ViSP. Some of the concepts (achromatic optics and wavelength diversity) have been proven by SPINOR up to 1 micron. Future work should confirm these concepts for the entire ViSP range.

-HAO Initiative to Measure Coronal Magnetic Fields- This is a project funded as an NCAR Strategic Initiative which aims at developing instrumentation to measure magnetic fields in the solar corona. The Coronal Multi-channel Polarimeter (CoMP) instrument consists of an electro-optically tunable birefringent filter capable of observing the coronal emission lines of Fe XIII at 1074.7 and 1079.8 nm and the prominence line of He I at 1083 nm, coupled with a polarimeter and a large format HgCdTe detector. The instrument is able to create simultaneous images of the corona in two wavelengths corresponding to the emission line and continuum. Significantly, the instrument was deployed at the 20-cm "One Shot" coronagraph at the NSO Sacramento Peak Observatory in January of 2004 with the first observations with the instrument obtained during an observing run in March, followed by additional observations in campaigns carried out in May and August. The observations in March produced exciting observations of a prominence eruption which were the subject of a press release at the June meeting of the American Astonomical Society.

-Prominence Magnetometer at the NSO Evans Coronographic Facility- As described in the HAO Annual Scientific Report for 2003, new diagnostic techniques for measuring magnetic fields in solar prominences, along with increased interest in prominence fields in association with coronal activity and CMEs, have heightened the importance of renewed efforts to measure the magnetic field vector in solar prominences. Casini (lead scientist), Elmore, Greg Card (HAO), and Tomczyk devised a blueprint for an upgrade of the Evans coronagraph to enable a program of prominence observations using filter-polarimetry in both the D3 and 1083 nm lines of neutral helium. An NCAR Opportunity Fund grant is allowing the purchase of optics and mechanical parts. This project also involves Manso-Sainz, Lopez-Ariste, M. Semel (Observatoire de Paris, France), and J. Trujillo-Bueno (IAC, Spain) as scientific collaborators.

-Polarimeter Errors at High Angular Resolution- Solar polarimeters operating at high angular resolution are affected by time-variable image motion and blurring due to seeing, if the rate of modulation of the polarization signal is less than about 400 Hz. This effect is well-known to produce "cross talk" among the Stokes polarization parameters, and some precision solar polarimeters, like ASP, are designed to minimize the seeing-induced cross talk. Now that many ground-based high-resolution solar observing facilities are being fitted with adaptive optical (AO) correction for seeing, it is important to understand how such systems might influence the polarimetric accuracy. Phil Judge (HAO), along with Elmore, Lites, Christoph Keller and Thomas Rimmele ( both of the National Solar Observatory), have carried out a study of cross talk effects to be expected from observations with AO systems (Judge et al. 2004). They find that the tip-tilt correction is essential for high resolution polarimetry.

Space Instrumentation

-Solar-B- The Solar-B space mission is currently the centerpiece of HAO's magnetic field measurement program. Solar-B is a Japanese space mission with substantial collaborations from the US and the UK. It will fly the first ever precision high-resolution solar spectro-polarimeter in space. Lockheed Martin Solar and Astrophysics Laboratory (LMSAL) and HAO are providing the Focal Plane Package (FPP) for the 50 cm Solar-B optical telescope. To be launched in the summer of 2006, this mission will provide the first continuous, seeing-free, high-resolution measurements of the solar vector magnetic field. In FY04, the instrument was mated to the telescope (see the accompanying Figure) and extensive testing and calibration have been carried out. The instrumentation and telescope appear to exceed expectations so far. Solar-B promises to be a milestone mission for solar physics. Bruce Lites is the lead scientist from HAO in the Solar-B program.

The Solar-B Optical telescope and associated instrumentation is shown illuminated by sunlight during cleanroom tests at the National Astronomical Observatory of Japan (NAOJ) in August, 2004. The 50-cm aperture telescope is the large, vertically mounted central structure. To the left, behind the stepladder, is the telescope Focal Plane Package (FPP), also mounted vertically. The red objects on the side of the FPP are thermal radiators for the electronics. Sunlight tests were carried out to verify the full system performance and to calibrate the instrument.

-Sunrise- Even the high angular resolution of Solar-B cannot access the crucial spatial scales of the solar atmosphere comparable to a photospheric scale height (about 70 km). Processes crucial to the heating of the upper solar atmosphere occur on these size scales, and must be observed in quantitative detail in order to reveal the physics of solar variability. With its 1 meter diameter aperture (twice that of Solar-B), the Sunrise telescope will achieve this resolution in the visible, and it will achieve a resolution about twice that in the ultraviolet (220 nm) accessible at balloon altitude. The Sunrise mission will fly a precision spectro-polarimeter for visible wavelengths (the descendant of POLIS), a spectrometer for the interesting chromospheric ionized Magnesium resonance lines at 280 nm, an ultraviolet imager to achieve the highest angular resolution solar observations to date, and a precision imaging vector magnetograph on a long-duration Antarctic balloon flight. All of these tools are needed to operate simultaneously to explore the physics of the interaction of fine-scale magnetic structures with the solar atmosphere. HAO and ATD are providing the balloon gondola, telescope pointing system, and the solar power system. HAO is providing electronic cameras and a data system for the spectro-polarimeter. Spain is providing the imaging magnetograph, and Germany is providing the telescope, the ultraviolet imager, the spectrograph, and the fine pointing and active telescope alignment systems. HAO and ATD are on schedule to provide a gondola and other systems for a test flight from the continental US in May, 2005. The telescope and other systems have met with some delays in Europe. It is anticipated now that a second test flight from the continental US will be needed to verify the telescope system. On this schedule, the Antarctic flight will probably occur in late 2008. Bruce Lites is the HAO lead scientist for Sunrise.

-SDO/HMI: Solar Dynamics Observatory/Helioseismic and Magnetic Imager- The HMI instrument on SDO will be the first space instrument to provide routine measurements of the full vector magnetic field over the entire solar disk, in addition to providing helioseismic measurements that continue the time series started by the Michelson Doppler Imager (MDI) on the SOHO spacecraft. Tomczyk (Lead scientist), along with Elmore, Lites, Norton, Johnathan Graham (HAO Graduate Research Assistant) and Tony Darnell (HAO) have participated in the program to develop the filter-based magnetograph based at Stanford University. HAO brings to this program its expertise in instrumentation for precision polarimetry and in analysis of polarimetric data. SDO is expected to be launched in April, 2008.

Solar Atmospheric Dynamic and Radiative Processes

Radiative Diagnostics

-Formation of the Helium Ultraviolet Resonance Lines- Neutral and ionized helium lines are important diagnostics of conditions in the upper solar chromosphere. Because of the high abundance of He and the high excitation potential of the He I and II lines, they sample regimes of temperature and density for which other atomic species in the solar atmosphere have little sensitivity. The formation of the ultraviolet lines and continua of helium is also intimately tied to the formation of helium lines in the visible and near infrared that are useful diagnostics of chromospheric magnetic fields. In two papers (Pietarila and Judge 2004, Judge and Pietarila 2004), Anna Pietarila (HAO Newkirk Graduate Assistant) and Judge have examined afresh the formation of the helium spectrum. Using a multi-faceted approach embracing radiative transfer and detailed analyses of spacecraft observations, they examined the long-standing issue of why the helium resonance lines are anomalously brighter than is predicted by various models which give good agreement for intensities of other species. They suggest that neutral helium atoms from cool regions may diffuse across magnetic field lines to hot regions where they can emit strongly, a process that does not occur for charged ions.

-Time-dependence of Atomic Level Populations in Evolving Plasmas- Phil Judge has investigated the general algebraic properties of the system of rate equations describing the radiative and collisional statistical equilibrium (SE) of an atomic system in the non-LTE regime. Using Gershgoring's theorem, he was able to identify properties of the eigenvalue problem for the SE matrix that clarify how different time-scales in the evolution of an atomic system come about, when in the presence of the simultaneous action of ambient radiation and atomic collisions in an evolving plasma. This analytic approach allows one to foresee the fundamental behavior of an atomic system, thereby making it possible to decide a priori which atomic levels are necessary to consider to properly describe the evolution of the atomic populations, given the thermodynamic properties of the plasma.

-Magnetic and Acoustic Heating in Solar and Stellar Atmospheres- Continuing an effort to understand the dynamics of the upper atmosphere of the Sun (chromosphere, transition region, low corona), Phil Judge studied UV spectra of the Sun, taken with the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument on board SOHO, and of the Sun-like star α Cen A, taken with the STIS (Space Telescope Imaging Spectrograph) instrument of the Hubble Space Telescope (HST). In collaboration with Mats Carlsson (University of Oslo, Norway) and Robert Stein (Michigan State University), he used radiation hydrodynamics simulations of the emission in the C II lines at 1335 Å, to test the hypothesis that acoustic wave dissipation could be the origin of the "basal" component of chromospheric heating. Comparison of these simulations with spectral and time-series data taken with SUMER shows that acoustic wave dissipation can only be responsible for a fraction of the observed UV intensity of the chromospheric internetwork in the C II lines, and also that the predicted oscillation frequency of the radiation output is several times larger than the observed one. Judge, Carlsson, and Stein argued that magnetic heating, rather than acoustic wave dissipation, is more likely responsible for the observed basal and internetwork emission in the C II lines. Judge, in collaboration with Steven Saar (Harvard/Smithsonian Center for Astrophysics), Carlsson, and Thomas Ayres (University of Colorado), converted SUMER intensity data to Sun-as-a-star fluxes, and compared them with STIS data for the solar-type star α Cen A and the low-activity star τ Cet. They found strong evidence of magnetic heating of the corona of τ Cet, and concluded that the heating mechanisms responsible for the chromospheres of all three stars must be similar.