HAO 2010 PROFILES IN SCIENCE: Dr. Sarah Gibson
Contact
303-497-1587
sgibson@ucar.edu
Area of expertise: Sun and Upper Atmosphere
Specialties: coronal mass ejections and their precursors; high-speed streams of solar wind that can affect Earth; the overall structure of the solar corona
Dr. Sarah Gibson is a Scientist III in the High Altitude Observatory of the National Center for Atmospheric Research. She received her PhD in Astrophysics from the University of Colorado at Boulder in 1995. Following a one year visit to Cambridge University in England as a NATO/NSF post-doctoral fellow, and nearly four years at NASA Goddard Space Flight Center, her first job at HAO was as a Scientific Visitor. Gibson uses computer models, along with ground- and space-based observations, to study the structure and evolution of coronal mass ejections. She is particularly interested in understanding twisted magnetic structures located in the Sun's corona that may be linked to mass ejections. She also studies high-speed streams of solar wind that can affect Earth even when the Sun is near a minimum stage in the solar cycle. Gibson's methods include advanced data analysis techniques that incorporate a range of solar observations to aid in forming a comprehensive, three-dimensional picture of the corona. Her research may eventually lead to better predictions of coronal mass ejections and other solar events that can buffet Earth's atmosphere and affect sensitive communications and navigational systems.
Professional Website(s): Sarah Gibson
Summary of Achievements:
Sarah Gibson’s research examines solar drivers of the terrestrial environment, from short-term space weather drivers such as coronal mass ejections (CMEs), to long-term solar cycle variation, with emphasis on the Sun-Earth system at solar minimum. She coordinates international working groups on these subjects, and develops community analysis tools and web resources.
Coronal prominence cavities. Coronal prominences and their cavities are of interest because they provide clues to magnetohydrodynamic (MHD) equilibrium states of the solar corona, and to the processes that may destabilize these equilibria and drive CMEs. We are at a critical point for understanding cavities. In particular, novel views of cavities from HAO's CoMP and NASA missions such as STEREO and SDO, along with new modeling tools (FORWARD codes described below) now allow us to choose between models of CME precursors and to understand how magnetic energy and helicity accumulate prior to eruption. Gibson was the leader of an International Space Science Institute (ISSI) International Team on Prominence Cavities. With HAO sabbatical visitor James Dove, postdoctoral associate Laurel Rachmeler, graduate student Donald Schmit, and other members of the ISSI group, she has undertaken a broad observational analysis of prominence cavities in multiple wavelengths (from radio to soft Xray), and compared these to MHD models.
Forward modeling. Solar coronal observables depend upon density, temperature, velocity, and magnetic field, all of which are important constraints on theoretical models. Comparing models to data is not always straightforward, however, since each observable depends on coronal plasma properties in different ways. Forward-modeling is the process of taking a model-defined three-dimensional distribution of coronal plasma properties, and performing line-of-sight integrals specific to a given coronal observable's dependence on these properties. This past year has seen a concerted effort led by Gibson to construct modular forward-model codes in IDL Solarsoft, to enable the community in side-by-side comparisons of model predictions and data. A suite of forward modeling software is now being tested by HAO and other community users. This “FORWARD tree” includes a set of magnetohydrodynamic models and is capable of reproducing a broad range of observables, from white light to EUV to CoMP Stokes parameters.
Comparative solar minima. Solar minima represent times of low magnetic activity and a simple heliosphere, and are thus excellent targets for interdisciplinary, system-wide studies of the origins of solar variability and consequent impacts at the Earth. The recent solar minimum extended longer and was "quieter" than any we have observed in the Space Age, inspiring both scientific and public interest. A rich variety of satellite and ground-based observations, in conjunction with theoretical and numerical modeling advances, have allowed us to probe the peculiarities of this minimum as never before. Comparisons between well-studied periods like the Whole Sun Month (WSM) and Whole Heliosphere Interval (WHI) illustrated how different minima can be. Gibson was a leader of both WSM and WHI, and has now formed an International Astronomical Union (IAU) working group on Comparative Solar Minima. The scientific goal is to characterize the coupled Sun-Earth system at its most basic, "ground state", and to understand the degree and nature of variations within and between solar minima. Under the auspices of the IAU working group, Gibson has successfully proposed an IAU symposium on "Comparative Magnetic Minima" to be held in Mendoza, Argentina, in 2011.
Publication:
Abstract:
We present a three-dimensional density model of coronal prominence cavities, and a morphological fit that has been tightly constrained by a uniquely well-observed cavity. Observations were obtained as part of an International Heliophysical Year campaign by instruments from a variety of space- and ground-based observatories, spanning wavelengths from radio to soft-X-ray to integrated white light. From these data it is clear that the prominence cavity is the limb manifestation of a longitudinally-extended polar-crown filament channel, and that the cavity is a region of low density relative to the surrounding corona. As a first step towards quantifying density and temperature from campaign spectroscopic data, we establish the three-dimensional morphology of the cavity. This is critical for taking line-of-sight projection effects into account, since cavities are not localized in the plane of the sky and the corona is optically thin. We have augmented a global coronal streamer model to include a tunnel-like cavity with elliptical cross-section and a Gaussian variation of height along the tunnel length. We have developed a semi-automated routine that fits ellipses to cross-sections of the cavity as it rotates past the solar limb, and have applied it to Extreme Ultraviolet Imager (EUVI) observations from the two Solar Terrestrial Relations Observatory (STEREO) spacecraft. This defines the morphological parameters of our model, from which we reproduce forward-modeled cavity observables. We find that cavity morphology and orientation, in combination with the viewpoints of the observing spacecraft, explains the observed variation in cavity visibility for the east vs. west limbs.
Publication:
Abstract:
The current solar minimum may not be “peculiar” when considered on scales of a century or more. However, the opportunity for discovery yielded by its extended nature in combination with the abundance of modern observations cannot be overstated. In this paper, we describe the Whole Heliosphere Interval (WHI), an in-depth study of the Sun-Earth system for a solar rotation in March/April 2008. We discuss how WHI fits within the broader context of the current deep, long, and complex solar minimum.
Publication:
Abstract:
The Whole Heliosphere Interval is an international coordinated observing and modeling effort to characterize the three-dimensional interconnected solar-heliospheric-planetary system, i.e., the “heliophysical” system. WHI was part of the International Heliophysical Year (IHY), on the 50th anniversary of the International Geophysical Year (IGY), and benefited from hundreds of observatories and instruments participating in IHY activities. WHI describes the 3-D heliosphere originating from solar Carrington Rotation 2068, March 20–April 16, 2008. The focus of IAU JD16 was on analyses of observations obtained during WHI, and simulations and modeling involving those data and that period. Consideration of the WHI interval in the context of surrounding solar rotations and/or in comparison to last solar minimum was also encouraged. Our goal was to identify connections and commonalities between the various regions of the Heliosphere.
