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Executive Summary
The various research activities of the High Altitude Observatory (HAO) are collectively directed toward the broad scientific goal of developing a comprehensive, quantitative understanding of the coupled Sun-Earth system. HAO scientists make progress in pursuit of this objective by investigating and elucidating the basic physics underlying the production of the Sun's magnetism and activity, the structure and dynamics of the terrestrial magnetosphere, ionosphere, and upper atmosphere, and the mechanisms by means of which the Earth's atmosphere and near-space environment are influenced by the radiative, particulate, and magnetic outputs of the Sun. HAO researchers utilize a variety of approaches in studying the processes and phenomena that affect the state of the Sun-Earth system, employing instrumental, observational, interpretive, computational, and theoretical expertise to address some of the fundamental problems of solar-terrestrial physics. Reflecting its position as a part of a national center, HAO is an active participant in joint projects with other NCAR science divisions, while assiduously fulfilling its committment of service to the community by providing access to instruments and observing facilities and the data acquired thereform, maintaining a suite of models that are available to outside users, and facilitating interaction and collaboration by hosting short- and long-term scientific visitors. The nature and scope of HAO's research program affords it a unique scientific situation, enabling it to bridge the disciplinary gap between the geosciences and astrophysics, contributing to an improved understanding of both the Sun's role in weather and climate, and of the Earth and Sun within the wider astronomical context, through efforts in planetary atmospheres, the detection of extrasolar planets, and stellar physics.
The foundation upon which the overall HAO program rests comprises four cornerstone research areas:
the solar dynamo, with focus on the internal physical processes that contribute to the generation and evolution of the Sun's time-dependent magnetic field;
the transport of the generated field through the convection zone, its emergence at the solar surface, and the inference of its properties in the photosphere and overlying atmospheric layers through spectro-polarimetric techniques;
the energetics and dynamics of the solar corona and wind, including the eruptive events which are the major drivers of space weather and geomagnetic activity;
the observation and modeling of the Earth's upper atmosphere, including its extensions and couplings to both the magnetosphere and interplanetary medium, and to lower atmospheric regions.
Research in these areas of emphasis is carried out within the three scientific sections that are the basic units of HAO's organizational structure. At the present time, staff members in the Solar Interior and Variability (SIV) section study the solar dynamo, the transport of magnetic flux, and related physical processes, while those in the Solar Atmosphere and Heliosphere (SAH) section are engaged in activities pertaining to the remote sensing of the Sun's magnetic field and studies of coronal dynamics and activity and its influences on the interplanetary medium. Studies of the Earth's upper atmosphere, the ionosphere, and the magnetosphere are conducted by those in the Terrestrial Impacts of Solar Output (TISO) section. HAO researchers have made substantial progress in all of the cornerstone areas during the past year, through numerous efforts that have yielded important new results concerning the origins and properties of solar magnetism and related activity, and the dynamics and electrodynamics of the terrestrial upper atmosphere and the solar forcing thereof.
The Magnetic Sun and Variability - Solar Interior and Variability (SIV)
The complex, time-dependent magnetic field of the Sun derives from processes that operate within the convective envelope that occupies the outer 30 % of the solar interior, and in the tachocline, the thin rotational shear layer that straddles the radiative-convective interface. HAO researchers were among the first to recognize that the large-scale meridional circulation that exists in the solar convection zone could have a significant impact on the way in which the dynamo operates, with the timescale of the circulatory flow essentially establishing the period of the Sun's magnetic activity cycle. They are now exploring the predictive capabilites of such "flux transport" dynamos, using observations of the surface meridional flow and poloidal field as model inputs, and examining the consequences for the behavior of subsequent cycles. Simulations performed in this way have made it possible to understand the unusually slow rise and delayed polar field reversal of cycle 23, and to tentatively predict a later onset for the upcoming cycle 24. In addition to its role in regulating the dynamo, the origins and properties of the meridional circulation have also been the focus of active research over the last year. A mean-field model of convection zone dynamics, based on a parameterized description of turbulent energy and angular momentum transport, was used to investigate the conditions leading to solar-like meridional flow and differential rotation, and a theoretical analysis of the circulation component at the bottom of the convective envelope revealed that the extent of its penetration into underlying radiative interior was likely quite limited, contrary to the assumed flow morphology of some dynamo models. The presence of strong, concentrated toroidal fields at the radiative-convective interface was investigated as a possible disruptive influence on a flux transport dynamo, through the effect of the Lorentz force on the deep meridional flow. Results obtained using both kinematic and dynamic models indicate that although the circulation is deflected around regions of strong toroidal field, it still transports the weak poloidal field required for continued functioning of the dynamo, testimony to the robustness of the basic mechanism. Significant progress was also made toward enhancing the realism of numerical treatments of the buoyancy-driven rise of toroidal magnetic flux tubes through the solar convective envelope. The development of a new 3D anelastic MHD code made possible the first such simulations using a spherical shell as the computational domain. The results of test runs that include the effects of the Sun's rotation in modeling the ascent of axisymmetric flux rings were in good agreement with the trajectories derived from previous studies based on a simplified thin flux tube model.
HAO's efforts to detect and study planets in orbit about nearby solar-type stars also yielded noteworthy results during the last year. The Transatlantic Exoplanet Survey (TrES) network, composed of HAO's STellar Astrophysics and Research on Exoplanets (STARE) telescope (sited on the island of Tenerife, Spain) and two similar instruments (located in Flagstaff, Arizona and Mt. Palomar, California) identified a transiting planet from observations of periodic eclipses of a star in the constellation Lyra. The planet (TrES-1) has a mass and radius about 0.75 and 1.04 times the corresponding values for Jupiter, respectively, and is the fifth such object with observable transits.
Additional Accomplishments Include:
---Global MHD instabilites of the solar tachocline were treated using more realistic models that include the effects of diffusion, and the hydrodynamic stability of this layer was investigated using a newly developed nonlinear shallow-water code.
The Magnetic Sun and Variability - Solar Atmosphere and Heliosphere (SAH)
The magnetic fields that emerge from the convection zone into the photosphere in the form of concentrated flux tubes play a pre-eminent role in structuring and forcing the overlying atmosphere, contributing to the dynamics and heating of the chromosphere and high-temperature corona, and influencing the acceleration of the solar wind. Magnetic fields are also the dominant source of activity in the Sun's atmosphere, driving energetic events such as flares and coronal mass ejections (CMEs), which can significantly perturb the interplanetary medium and produce disturbed conditions in the upper atmosphere of the Earth. Throughout its history, HAO has pioneered the development and use of spectro- polarimetric instruments to investigate the emergence, evolution, and properties of solar magnetic fields. This tradition of leadership was continued during the past year with HAO involvement in a variety of collaborative efforts to design and build new ground- and space-based instrumentation for solar spectro-polarimetry. Prominent among these undertakings has been the Coronal Multi-channel Polarimeter (CoMP), a project funded as an NCAR Strategic Initiative which will enable observers to routinely measure magnetic fields in the corona, a region where direct knowledge of magnetic properties has heretofore been scant. Following its deployment at the National Solar Observatory (NSO) earlier in the year, the CoMP instrument obtained exciting observations of a prominence eruption. HAO also continued its efforts in support of the 4 meter, ground-based Advanced Technology Solar Telescope (ATST), contributing to site survey testing, the design of polarimetric calibration techniques, and the development of Visible Spectrograph (ViSP) instrument, and, in collaboration with the Lockheed Martin Solar and Astrophysics Laboratory (LMSAL), provided the Focal Plane Package for the 50 cm telescope on board the joint Japan/US/UK Solar-B mission. This instrument will be the first precision, high-resolution solar spectro-polarimeter in space. The data acquired by HAO instruments was used in a variety of interpretive and theoretical studies which have revealed more about the dynamic character of the emerging fields that have been implicated in the generation of activity and that influence the large-scale magnetic structure of the corona. A detailed observational study of evolving magnetic fields in the photosphere under active region filaments showed evidence for a field geometry that was consistent with that expected from an emerging, twisted magnetic flux rope. Three-dimensional MHD simulations were used to investigate the response of pre-existing coronal fields to the rise of such a flux concentration, providing insights into the heating, emission, and magnetic helicity injection associated with the emergence process. Twisted magnetic flux ropes confined within closed magnetic structures are also possible precursors to CMEs, allowing the magnetic energy required to drive these events to be built up and stored. HAO researchers conducted theoretical studies to determine the energy storage capacities of two- and three-dimensional magnetic equilbria with embedded flux ropes, and performed numerical simulations to examine the conditions under which an emerging, twisted flux rope can push apart the confining field lines of an overlying coronal arcade to erupt into a CME. Significant progress toward understanding CME-related processes and phenomena was also made through observational studies carried out using the instruments of the Advanced Coronal Observing System (ACOS) of HAO's Mauna Loa Solar Observatory (MLSO). These efforts included a search for for evidence of the presence of flux ropes in the corona prior to the occurrence of CMEs, an investigation of a possible correlation between the accelerations of eruptive prominences and the asymptotic speeds of associated CMEs, a study of the relation between CME-related coronal and chromospheric waves, and an examination of the properties and evolution of transient coronal holes that can develop following the onset of CMEs.
Additional Accomplishments Include:
---A theory for the shapes of spectral lines that show scattering polarization when viewed near the solar limb (the "second solar spectrum") was formulated, and a novel laboratory experiment, supported through an NCAR Opportunity Fund grant, was devised and built to test it.
---Work on the development of fast, robust techniques for inverting spectro-polarimetric observations continued, including the use of artificial neural networks, a method which holds great potential for reliable, near real-time retrieval of magnetic field strengths from data.
---Supported by an NCAR Opportunity Fund grant, an effort was undertaken to measure vector magnetic fields in prominences using filter-polarimetric techniques at the NSO Evans Coronagraphic facility. The Kippenhahn-Schluter prominence-sheet solution was extended and generalized to permit construction of models with small-scale structure that can be used to simulate polarimetric signals for comparison with observations.
The Coupled Geospace System - Terrestrial Impacts of Solar Output (TISO)
The mesosphere, thermosphere, ionosphere, and magnetosphere comprise a region wherein the dynamical, thermodynamical, and compositional conditions are subject to forcings transmitted from both above and below. The waves and tides that are prominent dynamical features of the innermost of these layers are affected by processes occurring lower in the atmosphere, while the outer layers, by virtue their ionization state, are electromagnetically coupled and respond to the prevailing conditions in the solar wind and the state of the interplanetary magnetic field. HAO supports a vigorous and broad program of research aimed at understanding the physics of the Earth's upper atmospheric regions, with particular attention paid to the influence of the Sun's variable radiative and particulate emissions. Central to these efforts are the suite of geospace models that HAO researchers have developed and maintain both as investigative tools and as a resource for the solar-terrestrial community. In the past year, upgrades were made to the Thermosphere-Ionosphere-Mesosphere Electrodynamics General Circulation Model (TIME-GCM), enhancing vertical and horizontal resolution and updating aeronomic components. Among the investigations yielding noteworthy results that were carried out using the TIME-GCM: an examination of the circumstances leading up to the occurrence of a simulated stratospheric warming, revealing the role of wave-induced changes in the mesosphere as precursors to the event; a detailed study of the factors contributing to the observed seasonal variations of the amplitude of the 6.5-day, zonal wavenumber 1 planetary wave; and, an exploration of the effects of more realistic, spatially (in longitude and latitude) and temporally variable gravity wave sources on mesospheric dynamics. The dynamics and chemistry modules from the TIME-GCM have been adapted and incorporated into the Whole Atmosphere Community Climate Model (WACCM), a collaborative undertaking involving HAO and the Climate and Global Dynamics (CGD) and Atmospheric Chemistry (ACD) Divisons of NCAR. Within the last year, WACCM was extended through the inclusion of a newly developed parameterization for the spectrum of convectively generated gravity waves, and an improved treatment of solar radiation that gives solar heating and photodissociation rates for a large number of chemical species. A WACCM simulation was performed to study the effects of sea surface temperature variations on the structure of the middle and upper atmosphere, and the model is currently being prepared to run over a complete solar cycle to examine the chemical, dynamical, and structural changes that are produced by realistic solar forcing. Significant progress was also made in HAO's efforts to understand the couplings, energetics, and dynamics of the ionosphere and magnetosphere. The feasibility of a new implementation of the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure was demonstrated through application of the method to analysis of a magnetic cloud event from January, 1997. This work represents an important step toward understanding how scale-dependent properties of electromagnetic energy and momentum transfer processes affect estimates for global thermospheric Joule heating. Global-scale, MHD simulations of the solar wind-magnetosphere interaction were performed that enabled HAO researchers to compare computational results for magnetic reconnection in the magnetopause and the field-aligned current density at low altitudes with observations, and to study and compare the properties of isolated substorms with those that occur during magnetic storms. Initial results were also obtained from the new Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model, a product of HAO's collaborative participation in the NSF-supported Center for Integrated Space Weather Modeling (CISM). The CMIT model combines HAO's Thermosphere-Ionosphere Nested Grid (TING) model with the Lyon-Fedder-Mobarry magnetospheric model; results obtained from comparative tests show a more realistic distribution of ionospheric conductances, a more clearly defined auroral oval, and Joule heating enhancements that affected the neutral atmosphere.
Additional Accomplishments Include:
---The new Fabry-Perot Interferometer, designed to measure mesospheric and lower thermospheric winds and tides and the upper thermospheric polar cap convection pattern, was deployed at Resolute, Canada, the future site of the NSF Advanced Modular Incoherent Scatter Radar (AMISR).
---A study of the thermospheric-ionospheric responses to geomagnetic storms at different solar cycle phases found that the initial thermal and compositional recoveries from storm-time perturbations occur more rapidly at solar maximum than at solar minimum.
---An examination of the relative importance of direct penetration and disturbance dynamo electric fields in the storm-time equatorial ionosphere showed that the direct penetration electric field can modify the ionospheric dynamo by changing the neutral wind and conductivity in the F-region ionosphere.