HAO 2011 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
Sarah Gibson is a Scientist III Section Head 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
Dr. Sarah Gibson, over the past year, has worked to bring to fruition several projects embarked upon by the International Space Science Institute (ISSI) international working group on Coronal Prominence Cavities (http://www.issibern.ch/teams/coronalprom/index.html) that she led. Prominence 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 coronal mass ejections (CMEs). With members the ISSI group, and also HAO sabbatical visitor James Dove and postdoctoral associate Laurel Rachmeler, she has performed observational analyses of prominence cavities in multiple wavelengths (from radio to soft Xray), and compared them to magnetohydrodynamic model predictions. Of particular note is the first direct observational evidence for a magnetic flux rope in the corona as observed by HAO's Coronal Multichannel Polarimeter (CoMP) (see Publication 1).
Gibson has also continued her work on comparative solar minima over the past year. 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. Gibson led the Whole Heliosphere Interval (WHI) (http://ihy2007.org/WHI/WHI.shtml) campaign in 2008, and the International Astronomical Union (IAU) Working Group on Comparative Solar Minima (http://ihy2007.org/IAUWG/WEBPAGES/IAUWG.shtml) beginning in 2009. This year the WHI and IAU WG efforts culminated in a WHI Topical Issue of 28 papers in the journal of Solar Physics (see overview paper, Publication 2), and an IAU symposium on Comparative Magnetic Minima in Mendoza, Argentina that brought together scientists studying the Sun, stars, planets, solar wind, and the Earth's upper atmosphere, climate, and space environment.
Publications
Figure 1:
(1) Dove, J. B., Gibson, S. E., Rachmeler, L. A., Tomczyk, S., and Judge, P. 2011: Coronal magnometry: Observational signatures of magnetic flux ropes, ApJ Letters, 731, L1, 2011
Abstract: Coronal prominence cavities may be manifestations of twisted or sheared magnetic fields capable of storing the energy required to drive solar eruptions. The Coronal Multi-Channel Polarimeter (CoMP), recently installed at Mauna Loa Solar Observatory, can measure polarimetric signatures of current-carrying magnetohydrodynamic (MHD) systems. For the first time, this instrument offers the capability of daily full-Sun observations of the forbidden lines of Fe XIII with high enough spatial resolution and throughput to measure polarimetric signatures of current-carrying MHD systems. By forward-calculating CoMP observables from analytic MHD models of spheromak-type magnetic flux ropes, we show that a predicted observable for such flux ropes oriented along the line of sight is a bright ring of linear polarization surrounding a region where the linear polarization strength is relatively depleted. We present CoMP observations of a coronal cavity possessing such a polarization ring.
Figure 1 caption: Line-of-sight-integrated Stokes linear polarization P/I for (a) forward-calculated spheromak configuration and (b) CoMP observations of April 21, 2005 southwest cavity. The range indicated by the color bar for (a) corresponds to 5–28% linear polarization, and for (b) corresponds to 1–11% linear polarization. Red lines indicate direction of P/I vectors.
(2) Gibson, S. E., de Toma, G., Emery, B., Riley, P., Zhao, L., Elsworth, Y., Leamon, R. J., Lei, J., McIntosh, S., Mewaldt, R. A., Thompson, B. J., and Webb, D. F. 2011: WHI in the context of current solar minimum, Solar Physics, in press.
Abstract: Throughout months of extremely low solar activity during the recent extended solar-cycle minimum, structural evolution continued to be observed from the Sun through the solar wind and to the Earth. In 2008, the presence of long-lived and large low-latitude coronal holes meant that geospace was periodically impacted by high-speed streams, even though solar irradiance, activity, and interplanetary magnetic fields had reached levels as low or lower than observed in past minima. This time period, which includes the first Whole Heliosphere Interval (WHI~1: Carrington Rotation (CR) 2068), illustrates the effects of fast solar-wind streams on the Earth in an otherwise quiet heliosphere. By the end of 2008, sunspots and solar irradiance had reached their lowest levels for this minima (e.g., WHI~2: CR 2078), and continued solar magnetic-flux evolution had led to a flattening of the heliospheric current sheet and the decay of the low-latitude coronal holes and associated Earth-intersecting high-speed solar-wind streams. As the new solar cycle slowly began, solar-wind and geospace observables stayed low or continued to decline, reaching very low levels by June-July 2009. At this point (e.g., WHI~3: CR 2085) the Sun--Earth system, taken as a whole, was at its quietest. In this article we present an overview of observations that span the period 2008-2009, with highlighted discussion of CRs 2068, 2078, and 2085. We show side-by-side observables from the Sun's interior through its surface and atmosphere, through the solar wind and heliosphere and to the Earth's space environment and upper atmosphere, and reference detailed studies of these various regimes within this topical issue and elsewhere.
Figure 2 caption: WHI 1 (CR 2068: 20 March–16 April 2008) rotation. SOHO/Extreme Ultraviolet Imaging Telescope (EIT) Carrington map, with overlaid sub-Earth field lines. Fieldlines are calculated by ballistically mapping back along a radial trajectory from the ACE spacecraft at 1 AU to the solar source surface (r = 2.5R) using wind velocities measured at ACE, and then following the field line to the solar surface using a potential-field-source-surface (PFSS) extrapolation. Fieldlines are represented as a straight line between the coordinates (latitude, longitude) of their footpoints on the solar disk (small asterisks) and the coordinates (latitude, longitude) of the other end of the field line at the source surface (black "+" symbols). Blue dots represent neutral line (HCS) at the source surface. Color-coding represents solar-wind velocity.