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ASR 2004/HAO/HAO Achievements/SAH

The Magnetic Sun and Variability -
Solar Atmosphere and Heliosphere (SAH)

Magnetic Field Observations, Interpretation and Theory
Solar Atmospheric Dynamic and Radiative Processes
Coronal Structure and Dynamics
Physics of the Solar Wind

Other HAO Sections SIV | TISO | Facilities

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.

Coronal Structure and Dynamics

Coronal Mass Ejection (CME) Studies

-Relative Magnetic Helicity in Equilibrium Magnetic Fields- Mei Zhang (NCAR Afficilate Scientist, National Astronomical Observatory, China) and Boon Chye Low (HAO) investigated the nature of relative magnetic helicity in equilibrium magnetic fields in unbounded domains. As is well known, the total magnetic energy of a force-free magnetic field in an unbounded space anchored to an inner boundary is bounded above by a value defined entirely by the boundary flux distribution. Zhang and Low are using a simple formula (discovered by Zhang) for the relative total helicity of an external, axisymmetric, force-free field anchored to a unit sphere, to theoretically determine similar upper bounds for the total relative helicity of force-free magnetic fields.

-Numerical Investigations of 2D Magnetic Equilibria- Gordon Petrie (HAO/SCD Visiting Scientist, University of Athens, Greece) is developing a finite-element code to treat two-dimensional, equilibrium magnetic fields, in order to conduct studies of such equilibria in parameter regimes not previously investigated. The theory of Low and Zhang (2002) describing the two kinds of CMEs and their possible relationship to inverse and normal prominences motivates this study. The prominence observations of Lin, Penn and Kuhn (1998) showing a reversal of sign of the axial magnetic field of the prominence suggests that the magnetic configurations in prominences may be richer than previously thought. A two-dimensional hydromagnetic study is a first step toward exploring new possibilities for prominence magnetic structures.

-Collaborative Studies of 2D and 3D Magnetic Equilibria- The collaboration between HAO (Low, Zhang), SCD (Natasha Flyer, Steve Thomas), and the University of Colorado (Bengt Fornberg) continues to study axisymmetric equilibrium fields external to a unit sphere. In a previous publication (Flyer et al, 2004), it was shown that an equatorial magnetic flux rope in a force-free magnetic field may store energy above the Aly open field limit. But the excess energy above this limit is small, of the order of less than 8 percent, confirming other similar results. A systematic parameter search of the solutions by Zhang and Flyer shows that the 8 percent limit is common, and may be related to a bound on the total relative helicity of the force-free fields in unbounded domain. A new study introducing plasma pressure and gravitational stratification of a static atmosphere in equilibrium with the field shows that flux ropes embedded in the atmosphere may break the 8 percent limit on the energy in excess of the Aly limit, provided mass is loaded in strategic parts of the magnetic structure. A high-precision numerical solver, capable of treating the governing, nonlinear, elliptic partial differential equation, was developed for this project.

This collaboration has expanded to treat force-free magnetic fields and fields in static equilibrium in the presence of pressure and gravitational forces in realistic three-dimensional geometry. A complete generalization is not possible because three-dimensional magnetostatic problems are intractable in general. Low reduced the governing equations to a more tractable form describing a large sub-set of the general solutions which retains three-dimensionality and holds the potential of modeling the rich field topologies of observed coronal structures (Low, 1991). Included in the sub-set are the force-free magnetic fields. The reduced governing equations require the proper formulation of boundary conditions which hitherto have been elusive and poorly understood. This obstacle has been removed, and the door is open for generating fully three-dimensional equilibrium magnetic field for modeling solar coronal magnetic structures (Low 2005, in prepartation). The powerful numerical solver developed by Flyer and Fornberg for treating axisymmetric force-free magnetic fields can be extended to treat this class of three-dimensional problems. Ellen Zweibel (University of Wisconsin) has joined the HAO/SCD/Colorado collaboration.

-Flux Emergence and the Evolution of the Coronal Magnetic Field- Yuhong Fan and Sarah Gibson (both of HAO) have extended the numerical simulations of coronal flux ropes to 3D spherical geometry, so as to address the question of magnetic energy storage and release in coronal mass ejections. In an earlier study, Sturrock et al. (2001) have considered a 3D coronal magnetic field that consists of a twisted magnetic flux rope anchored at both ends, confined within an external potential arcade, as a possible configuration for CME precursors. Through an order of magnitude estimate, they found that with a moderate amount of total twist (less than 2 full field line winds), the free magnetic energy associated with the twisted magnetic flux rope is sufficient for the flux rope to rupture through the arcade and extend to infinity as a CME, without opening up all the arcade field. Fan modeled this dynamic scenario by performing isothermal, low-beta MHD simulations of the evolution of the coronal magnetic field as a twisted, line-tied magnetic flux rope is transported slowly into the corona previously occupied by a potential arcade field. The simulations show two distinct phases for the evolution of the coronal magnetic field (see the accompnaying Figure and Movie). In the earlier phase, the coronal flux rope evolves quasi-statically with both the magnetic energy and twist being built up as a result of flux emergence. In this stage the rise velocity is nearly constant, being about 0.01 times the characteristic Alfven speed (top panel of the Figure), and the magnetic energy evolution accurately tracks the energy build-up due to the Poynting flux through the lower boundary (bottom panel of the Figure). When sufficient twist and magnetic energy are transported into the corona, the flux rope undergoes a significant acceleration (top panel of the Figure) and is able to rupture through the overlying arcade, producing a CME-like eruption (see the Movie). The flux rope continues to accelerate after the flux emergence is stopped at t=97 (as marked by the vertical dotted lines in the Figure), and the rise velocity reaches about 550 km/sec when the flux rope begins to exit the outer boundary of the domain at r = 6 R⊙. In this latter dynamic phase of the evolution, the flux rope becomes kinked, which facilitates the loss of confinement of the flux rope by changing its orientation at the apex such that it becomes easier for the flux rope to part and erupt through the arcade field. A sigmoid shaped current layer develops as a result of the onset of the kink instability (Fan and Gibson, 2004), and there is significant magnetic energy release in the eruption phase as can be seen in the bottom panel of the Figure.

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CME-related solar atmospheric phenomena

-CME-related Coronal and Chromospheric Waves- Although Moreton waves have historically been observed in Hα data, more recently waves have also been observed in chromospheric He I (λ=1083nm) images obtained at the Mauna Loa Solar Observatory. In an effort to better understand the nature of chromospheric waves and their relationship to coronal waves observed in EIT data, Holly Gilbert (HAO), Barbara Thompson (Goddard Space Flight Center), Tom Holzer (HAO), and Joan Burkepile (HAO) focused on two events in which waves are observed simultaneously in He I and Fe XII (λ=195Å), lines formed in the chromosphere and corona, respectively (Gilbert et al, 2004). Comparing the waves observed in these two lines allows the determination of the spatial relationship between coronal and chromospheric waves, and thus aids in the understanding of the underlying physics of waves in the solar atmosphere. The main goal of this work was to begin an investigation into whether coronal and chromospheric waves are both mechanical (e.g., MHD waves) by looking at their spatial relationship. Gilbert and colaborators found the chromospheric waves in these two events to be roughly co-spatial with their coronal counterparts, indicating they are not mechanical in nature but are chromospheric imprints of mechanical waves propagating through the corona. This conclusion is based upon the nature of the formation of the He I absorption line.

Another study involving chromospheric waves was completed by Gilbert and Holzer (2004) where the focus is on the same two wave events observed in He I from their previous work. Two interesting phenomena occur in these wave events: the waves are visible in the He I velocity data and multiple waves are observed for each event. The velocity signal provides evidence for the downward propagation from the corona to the chromosphere of a slow-mode hydromagnetic wave generated at the front of the fast coronal hydromagnetic wave. In this work, it is also suggested that the observed multiple waves indicate more than one driving mechanism may be involved.

-A New Technique for Estimating Prominence Masses- Prominences are cool condensations of partially ionized plasma in the million-degree corona. They often appear in absorption when the sun is viewed in EUV emission lines formed at coronal temperatures. The coronal EUV radiation at wavelengths less than 504Å undergoes Lyman continuum absorption by both hydrogen and helium (i.e., the coronal radiation ionizes hydrogen and helium atoms from their ground states). It is presently unclear what role prominences play in the initiation and dynamics of CMEs, although erupting prominences are strongly correlated with CMEs. The masses of prominences involved in CMEs are not generally measured, but the accurate determination of such masses may help in assessing the dynamical importance of prominences in CME events. Gilbert, Holzer, and Robert MacQueen (HAO) have developed a new technique for deriving prominence mass, one that uses observations of coronal radiation in the Fe XII (19.5 nm) spectral line, which is absorbed by prominence material. This new method makes it possible to consider the effects of both foreground and background radiation in the analysis, and it can be applied to both quiescent and erupting prominences by using two versions of the method, labelled as the "spatial-interpolative" version and the "temporal-interpolative" version. Gilbert, Holzer and MacQueen find that when both versions can be applied to the same event, the temporal-interpolative approach yields the more accurate results. They have applied both versions to an erupting prominence observed on 1999 July 12 (this prominence had an associated CME), and find that the two approaches result in similar mass determinations:(6.0 ± 2.5) X ¹⁴g for the temporal-interpolative approach and (7.4 ± 4.6) X ¹⁴g for the spatial-interpolative approach (Gilbert, Holzer and MacQueen, 2004).

-Observational Studies of Prominence Acceleration- Giuliana de Toma (HAO), Holzer, Burkepile, and Gilbert are carrying on an observational study of prominence acceleration, aimed at investigating the relationship between the acceleration of eruptive prominences and the speed of their associated CMEs, in order to better understand the acceleration phase of prominences in the low corona. This study considers events near or at the solar limb when eruptive prominences observed in Mauna Loa Solar Observatory (MLSO) ground observations show a clear association with CMEs. To determine the prominence acceleration, Hα observations from the MLSO/Polarimeter for Inner Coronal Studies (PICS) instrument are used. This instrument obtains observations off limb with a broadband filter of about 1.0 nm, and on disk with a narrower filter of about 0.05 nm. The broad filter of the MLSO/PICS instrument used with the occulting disk avoids a common problem with narrow band filters where the prominence material can disappear because it is Doppler-shifted out of the instrument band pass. The MLSO/PICS instrument also has the advantage of a wide field-of-view, extending from 1.01 solar radii to ~2.3 solar radii in the east and west directions, and a high temporal cadence of 3-minute which is crucial for measuring trajectories accurately. Previous coronagraph observations have shown that the prominence material of a CME moves slower than the leading front, but that the two appear to travel outward as a unit. It is thus expected that the prominence acceleration in the low corona will be related to the acceleration of the CME itself and can be a good indicator of the CME final velocity. To test this hypothesis, the plane-of-sky projection of the maximum radial accelerations of the eruptive prominences are compared with the projected asymptotic velocities of the corresponding CMEs. Preliminary results from this study show that, while there is a broad range of observed accelerations for prominences, there appears to be a correlation between the prominence maximum acceleration and the CME asymptotic velocity. In particular, all CMEs with v > 600 km/s have accelerations of the order of 20 m/s² or larger and all fast CMEs with v > 900 km/s have accelerations greater than 100 m/s².

-Composite Kippenhan-Schluter Prominence Models- Low and Petrie have generalized the classical Kippenhahn-Schluter prominence-sheet model. They showed that one-dimensional Kippenhahn-Schluter solutions, each representing a thin vertical layer of magnetized plasma, can be stacked against one another in a collection of such layers to form a prominence rich in small-scale structure, without losing the equilibrium among the Lorentz, plasma and gravitational forces. Moreover, for each such composite solution, each layer may be given a uniform motion of an arbitrary magnitude in its plane, in which case, this hydromagnetic state is described by the steady state equations. These solutions present an opportunity to perform forward modeling of the polarimetric signals one might expect to encounter in the observation of prominence magnetic fields. A collaboration has been set up with Roberto Casini (HAO) and Haosheng Lin (NCAR Affiliate Scientist, University of Hawaii) to exploit this opportunity in support of observational efforts at HAO and Hawaii to observe prominence magnetic fields with new techniques and instrumentation. Low and Petrie are also investigating the theoretical freedom to prescribe arbitrary sliding motions between the vertical prominence plasma sheets, both as a basic hydromagnetic problem and as representations of the motions observed in real prominences.

-Formation and Dissipation of Magnetic Tangential Discontinutities- Low, Petrie and Zhang investigated the formation of magnetic tangential discontinuities between vertical magnetic flux surfaces in a stratified atmosphere. This is generally a three-dimensional effect with variations in the two dimensions of each flux surface and variations in the third direction perpendicular to the surfaces. They have found analytical solutions describing such discontinuities in force equilibrium. These explicit solutions provide an opportunity to analyze the dynamical consequences of magnetic reconnection dissipating the tangential discontinuity. Spontaneous magnetic dissipation of tangential discontinuities within prominences is being investigated as a mechanism whereby fully ionized plasma can drain from prominences and magnetic flux escape into the atmosphere.

-Transient Coronal Holes Observed in the He I 1083 nm Line- Observations from Yohkoh/SXT and SOHO/EIT have shown that dimming regions often appear on the solar disk near the location of a CME. Brightenings in He I 1083 nm are also visible in observations made at MLSO that form at the same time and are co-spatial with the EUV intensity dimmings observed from space. The He I 1083 nm brightenings are induced by a decrease of the overlying coronal radiation. The EUV and X-ray dimmings and He I 1083 nm brightenings can thus be interpreted as different manifestations of the decreased coronal density caused by the ejection of coronal material, that is, as transient coronal holes. de Toma, Holzer, Burkepile and Gilbert have studied six cases when transient coronal holes developed in association with a CME onset and were seen in He I 1083 nm and in the EIT Fe XII 19.5 nm line. They find the transient coronal holes at the EUV wavelengths observed by EIT and in the IR He I 1083 nm line form at the same time, within the 12-minute cadence of the EIT observations and there is good agreement in both shape and size of the holes at these wavelengths. The high, 3-minute temporal cadence of the He I 1083 nm observations taken at MLSO allows them to determine the appearance and evolution of transient coronal holes accurately and to study the relationship with other manifestations of activity at the CME onset. They find the transient coronal holes typically form during the impulsive phase of the flare, and follow the CME onset by several minutes to up to an hour. Transient coronal holes tend to form within regions of weak magnetic flux (5-20 Mx/cm²) and their formation time varies from between 20 minutes to about an hour.

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Coronal Structure and its Evolution

-Multi-Wavelength Determination of Coronal Holes- In collaboration with the Center for Integrated Space Weather Modeling (CISM), de Toma, Nick Arge (NOAA) and Gibson have worked on the validation of coronal models. As part of this effort, models and observations of coronal holes have been compared. Coronal holes have been historically observed as regions of reduced emission on the solar disk in X-Ray and EUV wavelengths, or as regions of reduced brightness in coronagraph images at the limb. They can also be seen as relatively bright regions in He I 1083 nm. They are usually identified with the footpoints of magnetic field lines that open into the heliosphere, as derived from potential field model extrapolations. Since the appearance of coronal holes is different at different wavelengths, and since we do not have observational data to determine which regions on the Sun have open or closed magnetic field lines, the identification of coronal holes has always been difficult. de Toma, Arge, and Gibson have used relative intensity images in EUV wavelengths from SOHO/EIT, ground-based observations in the He I 1083 nm line, Hα line, and magnetograms in an attempt to identify coronal hole regions in a more objective way. Data are assembled in the form of synoptic maps, and a set of criteria based of the intensity contrast and the magnetic property of coronal holes are used to identify them using an automatic procedure. Hα and magnetograms are used to identify filament channels and to resolve the ambiguity with coronal holes. The observed coronal holes are then compared with the modeled ones and used to evaluate the model accuracy and the model sensitivity to the initial conditions.

-Observable Properties of Coronal Magnetic Flux Ropes- Gibson, Fan, and collaborators George Fisher (University of California, Berkeley), Cristina Mandrini (Instituto de Astronomía y Física del Espacio, IAFE, Argentina), and Pascal Demoulin (University of Paris, France) (Gibson et al., 2004) have considered the observable properties of a magnetic flux rope in the corona, as modeled in Fan and Gibson (2003, 2004). In particular, the structure, evolution, and relative location and orientation of S-shaped, or sigmoid active regions and filaments were compared to topological features of the magnetic flux rope, testing the theories that: 1) X-ray sigmoids appear at the regions of the flux rope known as bald patch-associated separatrix surfaces (BPSS), where, under dynamic forcing, current sheets can form leading to reconnection and localized heating; and that 2) filaments are regions of enhanced density contained within dips in the magnetic flux rope. Analysis demonstrates that the shapes and relative orientations and locations of the BPSS and dipped field in the emerging flux rope are consistent with observations of X-ray sigmoids and their associated filaments. Moreover, current layers indeed form along the sigmoid BPSS as the flux rope is driven by the kink instability, which is evidence in support of the theory that X-ray sigmoids appear when this critical topological surface is dynamically forced.

Figure: Comparison of t=56 BPSS (red field lines) to current sheets (yellowish-green isosurfaces). (right) Same, with t=39 BPSS also shown (purple field lines). Color contours at bottom boundary represent magnitude of current at photosphere (same color scaling as coronal isosurfaces).

Figure: Two views of t=39 BPSS (purple field lines) and dipped field representing filament (brown) (note, dipped field has been filled from the dip bottom up a distance .1 L or to where the field direction becomes horizontal, whichever comes first. L is the shorter of the horizontal lengths of the simulation domain). Color contours at lower boundary represent normal component of the magnetic field at the photosphere.

-Coronal Cavities, Flux Ropes, and CMEs- Gibson, David Foster (HAO), Burkepile, and Andrew Stanger (HAO) are investigating the currently debated question of whether or not magnetic flux ropes are present in the corona prior to a CME, or whether they are formed during eruptions. This is important to establish because a flux rope precursor is a reservoir of magnetic energy, so observations that can be clearly associated with a flux rope are strong indicators for eruption. Good candidates for flux rope precursors are observed quiescent white light coronal cavities. It has long been established that CMEs often possess three parts, a front, a cavity, and a bright core. Even in cases where no core (identified with the erupting prominence) can be found, a cavity is often present. Observations of the low solar corona, such as those taken by HAO's MLSO, demonstrate that this three-part structure can be present well before the eruption, as a helmet streamer, embedded white light cavity, and prominence core. As in the case of CMEs, sometimes the prominence itself is not visible, but the helmet and cavity are. Magnetic flux rope models provide a physical explanation for the cavity, forming a region of depleted density surrounding the prominence core (Low and Hundhausen, 1995). The quiescent cavities observed in white light are thus consistent with a precursor magnetic flux rope in the corona. Gibson, Foster, Burkepile and Stanger are currently studying quiescent cavities using MLSO Mark IV coronagraph data and tabulatng their sizes, shapes, locations, orientations, intensity contrasts, and relation to CMEs. They are also conducting an analytic study of how cavity appearance changes using a modified three-dimensional density model (Gibson et al. 2003) to quantitatively model cavity density.

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Physics of the Solar Wind

Quasi-steady processes

-New Transport Equation for Fully Ionized Gases- Mari Anne Killie, Åse Marit Janse, Øystein Lie-Svendsen (all of the University of Oslo, Norway), and Egil Leer (NCAR Affiliate Scientist, University of Oslo, Norway) have worked on the development of new transport equations for fully ionized gases, which improve the description of Coulomb collisions compared to fluid transport equations that are in common use today (Killie at al. 2004). The main motivation for this work has been to improve the description of energy transport and collisional forces in the solar corona and transition region, which in turn are important for understanding, for example, the origin of the solar wind mass flux, as well as coronal abundances of helium and minor ions.

-Solar Wind Minor Ion Energy Budget- Lie-Svendsen and Ruth Esser (University of Tromsø, Norway) have also studied the energy budget of minor ions in the corona and solar wind, using a solar wind model extending from the chromosphere to 1 AU (Lie-Svendsen and Esser 2005). The model results show that coronal heavy ions become very hot, with temperatures that may easily exceed 10⁸ K. This occurs even without strong heating of the ions, because the minor ion energy loss is small unless they become hot. However, without preferential heating of the minor ions (that is, a coronal heating rate per particle larger than for protons), the collisional coupling to protons causes extremely large minor ion abundances in the corona, which are ruled out by observations.

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Transient processes

-Transient Outflows from Closed Coronal Loops- The solar wind model mentioned in the preceding paragraph has been used by Eirik Endeve, Lie-Svendsen, Leer, and Viggo Hansteen (all of the University of Oslo, Norway) to study the outflow of solar wind plasma from closed coronal loops that periodically open into the solar wind. The main finding is that such loops do not seem to be a source of helium-rich material that can explain the high helium abundances that have occasionally been observed in situ in the solar wind (mainly in association with coronal mass ejections). The reason is that in closed regions, the high density leads to collisional coupling between α-particles and protons, causing a low α- particle temperature in the corona. The α-particles therefore do not have sufficient (thermal) energy to escape from the gravitational potential when the flux tube is opened. Only many hours, or even days, after opening will an enhanced α-particle flux emerge.

-MHD Simulations of the Inner Corona and Solar Wind- Endeve, Holzer, and Leer (Endeve et al. 2004) have used an MHD simulation to study heating of electrons and protons in an axially symmetric model of the solar corona, extending from the coronal base to 15 solar radii. To consider heating of electrons and protons separately, as well as the collisional coupling between the particle species, a two-fluid description of the electron- proton plasma is used. A steady coronal heat input, uniform base pressure, and dipole field boundary conditions produce a magnetic field configuration similar to that seen with white-light coronagraphs during quiet-Sun conditions: a helmet streamer is formed in the inner corona around the equator, surrounded by coronal holes at higher latitudes. The plasma inside the helmet streamer is in hydrostatic equilibrium, while in the coronal holes a transonic solar wind is accelerated along the field. The collisional coupling between electrons and protons becomes weak close to the coronal base. In the case of proton heating, the thermal structure along open and closed field lines is very different, and there is a large pressure jump across the streamer- coronal hole boundary. When the simulation is run on a long time scale, the helmet streamer becomes unstable, and massive plasmoids are periodically released into the solar wind. These plasmoids contribute significantly to the total mass and energy flux in the solar wind. The mass of the plasmoids is reduced when electrons are heated. In a model that is not axisymmetric, it is possible that this streamer instability can give rise to the slow solar wind.

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