HAO 2012 Profiles In Science: Dr. Sarah Gibson
Contact:
303-497-1587
sgibson@ucar.edu
Area of expertise: Sun and Heliosphere
Specialties: mass ejections and their precursors; Solar cycle studies from Sun to Earth.
Dr. Sarah Gibson is a Senior Scientist 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, she returned to Boulder and HAO as a Scientific Visitor. Dr. Gibson's research centers on 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-to-Earth system at solar minimum. She leads and coordinates international working groups on both of these subjects, and develops and maintains community analysis tools for comparing models to data. 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
Sun-to-Earth connections are least complicated at times of low magnetic activity, when the solar atmosphere and heliosphere are structurally simple. Therefore studying solar minima has a special place in the interdisciplinary investigation of the origin of solar variability and its consequent impacts at the Earth. As chair of the International Astronomical Union (IAU) Working Group on Comparative Solar Minima, (http://ihy2007.org/IAUWG/WEBPAGES/IAUWG.shtml), Dr. Gibson organized an IAU symposium on Comparative Magnetic Minima in Mendoza, Argentina in October, 2011. Scientists studying the Sun, stars, planets, solar wind, and the Earth's upper atmosphere, climate, and space environment joined together in a truly interdisciplinary symposium, the results of which appeared in an IAU Proceedings publication (e.g., Paper 1, below). A Topical Issue of the journal Solar Physics appeared in press in December, 2011, providing a comprehensive, Sun-to-Earth observational description of the recent epochal solar minimum. Dr. Gibson was an author on four papers in this Topical Issue including two Invited Reviews (e.g., Paper 2, below). Finally, during the past year Dr. Gibson collaborated with U.S. Geological Survey scientist Dr. Jeffrey Love in a study that showed that strong periodicities during the recent solar minimum were unusual on timescales of the geomagnetic record (~150 years), due to a combination of low activity and warped heliospheric magnetic structure, a longitudinal asymmetry possibly rooted in the solar interior (Paper 3, below).
Coronal mass ejections (CMEs) and associated prominence eruptions are spectacular solar phenomena that drive potentially adverse space weather at the Earth. A particular focus of Dr. Gibson's analysis of CMEs has been observations and models of coronal prominence cavities, which are long-lived coronal structures that store the magnetic energy liberated in CMEs. This year, two papers (Publications 4 & 5) were published by members of the International Space Science Institute (ISSI) International Team on Prominence Cavities led by Gibson. These papers, along with several others published by the team in the past few years, made use of the set of forward-modeling computational tools developed by the ISSI team and maintained by Gibson as a resource for the solar community (http://people.hao.ucar.edu/sgibson/FORWARD). These tools enable a side-by-side comparison of models and multi-wavelength observations. Their application in this year's publications by the ISSI team resulted in the first quantitative analysis of temperatures within coronal cavities.
Publications
(1) Gibson, S. E. and Zhao, L., 2012: A porcupine sun? Implications for the solar wind and Earth, Proceedings of IAU S286. 10.1017/S1743921312004851
Abstract: The recent minimum was unusually long, and it was not just the case of the ``usual story'' slowed down. The coronal magnetic field never became completely dipolar as in recent Space Age minima, but rather gradually evolved into an (essentially axisymmetric) global configuration possessing mixed open and closed magnetic structures at many latitudes. In the process, the impact of the solar wind at the Earth went from resembling that from a sequence of rotating "fire-hoses" to what might be expected from a weak, omnidirectional "lawn-sprinkler". The previous (1996) solar minimum was a more classic dipolar configuration, and was characterized by slow wind of hot origin localized to the heliospheric current sheet, and fast wind of cold origin emitted from polar holes, but filling most of the heliosphere. In contrast, the more recent minimum solar wind possessed a broad range of speeds and source temperatures (although cooler overall than the prior minimum). We discuss possible connections between these observations and the near-radial expansion and small spatial scales characteristic of the recent minimum's porcupine-like magnetic field.
Figure 1 caption: TOP: Coronal magnetic field from the Predictive Science Inc. Magnetohydrodynamics on a Sphere (MAS) model for (left to right) Whole Sun Month Carrington Rotation (CR) 1913 in August-September 1996, August 1, 2008 eclipse prediction (using CR2071 and CR2072 data), and July 22, 2009 eclipse (using CR2084 and CR2085). BOTTOM: artist's conception for similar times of solar wind morphology (faster wind streams indicated in yellow emanating from coronal holes) and impact for the Earth's radiation belt (large relativistic electron population indicated by red) and cosmic rays (high levels indicated by number of squiggly red arrows).
(2) Thompson, B. J., Gibson, S. E., Schroeder, P. C., Webb, D. F., Arge, C. N. and 22 co-authors, Bisi. 2011: A snapshot of the sun near solar minimum: the Whole Heliosphere Interval, Invited Review, Solar Physics, 274, 10.1007/s11207-011-9891-6.
Abstract: We present an overview of the data and models collected for the Whole Heliosphere Interval, an international campaign to study the three-dimensional solar-heliospheric-planetary connected system near solar minimum. The data and models correspond to solar Carrington Rotation 2068 (20 March - 16 April 2008) extending from below the solar photosphere, through interplanetary space, and down to Earth’s mesosphere. Nearly 200 people participated in aspects of WHI studies, analyzing and interpreting data from nearly 100 instruments and models in order to elucidate the physics of fundamental heliophysical processes. The solar and inner heliospheric data showed structure consistent with the declining phase of the solar cycle. A closely spaced cluster of low-latitude active regions was responsible for an increased level of magnetic activity, while a highly warped current sheet dominated heliospheric structure. The geospace data revealed an unusually high level of activity, driven primarily by the periodic impingement of high-speed streams. The WHI studies traced the solar activity and structure into the heliosphere and geospace, and provided new insight into the nature of the interconnected heliophysical system near solar minimum.
Figure 2 caption:Whole Heliosphere Interval latitude-longitude (Carrington) maps for a-b) subsurface flows determined from helioseismic inversions of NSO/GONG solar surface velocity data, c) solar surface magnetic fields (GONG), coronal hole boundaries determined from d) Helium 10830 data and g) EUV and magnetic observations, f) magnetic range of influence determined from SOHO/MDI data, e, h-i) solar-wind model (WSA, MAS, GONG) footpoint data, and j) Nobeyama 17-GHz radio emission.
(3) Love, J. L., Rigler, J. E., and Gibson, S. E., 2012: Geomagnetic detection of the nonaxisymmetric solar dynamo and the historical peculiarity of minimum 23–24, GRL, 39, L04102. 10.1029/2011GL050702
Abstract: Analysis is made of the geomagnetic-activity aa index covering solar cycle 11 to the beginning of 24, 1868–2011. Autocorrelation shows 27.0-d recurrent geomagnetic activity that is well-known to be prominent during solar-cycle minima; some minima also exhibit a smaller amount of 13.5-d recurrence. Previous work has shown that the recent solar minimum 23–24 exhibited 9.0 and 6.7-d recurrence in geomagnetic and heliospheric data, but those recurrence intervals were not prominently present during the preceding minima 21–22 and 22–23. Using annual-averages and solar-cycle averages of autocorrelations of the historical aa data, we put these observations into a long-term perspective: none of the 12 minima preceding 23–24 exhibited prominent 9.0 and 6.7-d geomagnetic activity recurrence. We show that the detection of these recurrence intervals can be traced to an unusual combination of sectorial spherical-harmonic structure in the solar magnetic field and anomalously low sunspot number. We speculate that 9.0 and 6.7-d recurrence is related to transient large-scale, low-latitude organization of the solar dynamo, such as seen in some numerical simulations.
Figure 3 caption: Solar cycle averages of r(λ), cycles 11–24, 1868–2011. Also shown is the long-term average for all cycles (orange) and the anomalous autocorrelation (red) defined as the difference between each individual average and the long-term average. The amplitude scale is given in the upper right-hand corner, and the horizontal gray line for each autocorrelation shows its zero-level baseline.
(4) Reeves, K. K., Gibson, S. E., Kucera, T. A., Hudson, H. S., 2012: Thermal properties of coronal cavities observed with the X-ray telescope on Hinode, ApJ, 746, 146. 10.1088/0004-637X/746/2/146
Abstract: Coronal cavities are voids in coronal emission often observed above high latitude filament channels. Sometimes, these cavities have areas of bright X-ray emission in their centers. In this study, we use data from the X-ray Telescope (XRT) on the Hinode satellite to examine the thermal emission properties of a cavity observed during 2008 July that contains bright X-ray emission in its center. Using ratios of XRT filters, we find evidence for elevated temperatures in the cavity center. The area of elevated temperature evolves from a ring-shaped structure at the beginning of the observation, to an elongated structure two days later, finally appearing as a compact round source four days after the initial observation. We use a morphological model to fit the cavity emission, and find that a uniform structure running through the cavity does not fit the observations well. Instead, the observations are reproduced by modeling several short cylindrical cavity "cores" with different parameters on different days. These changing core parameters may be due to some observed activity heating different parts of the cavity core at different times. We find that core temperatures of 1.75 MK, 1.7 MK, and 2.0 MK (for July 19, July 21, and July 23, respectively) in the model lead to structures that are consistent with the data, and that line-of-sight effects serve to lower the effective temperature derived from the filter ratio.
Figure 4 caption: Left image shows a synthetic map of emission measure using a morphological model with an Earth-centered viewpoint for the cavity observed on 2008 July 23, derived using simulated XRT Thin-Be and Ti-poly filter intensities. The center image shows a map of the synthetic temperature calculated for the same model and filter pair. The right image shows the emission measure (gray) and temperature (black) along the arc plotted in the images, as well as the data for temperature (red) and emission measure (orange) from the Thin-Be/Ti-poly filter pair.
(5) Kucera, T. A., Gibson, S. E., Schmit, D. J., Landi, E., and Tripathi, D., 2012: Temperature and EUV intensity in a coronal prominence cavity, ApJ, 757, 73.10.1088/0004-637X/757/1/73
Abstract: We analyze the temperature and EUV line emission of a coronal cavity and surrounding streamer in terms of a morphological forward model. We use a series of iron line ratios observed with the Hinode Extreme-ultraviolet Imaging Spectrograph (EIS) on 2007 August 9 to constrain temperature as a function of altitude in a morphological forward model of the streamer and cavity. We also compare model predictions to the EIS EUV line intensities and polarized brightness (pB) data from the Mauna Loa Solar Observatory (MLSO) Mark 4 K-coronameter. This work builds on earlier analysis using the same model to determine geometry of and density in the same cavity and streamer. The fit to the data with altitude-dependent temperature profiles indicates that both the streamer and cavity have temperatures in the range 1.4-1.7 MK. However, the cavity exhibits substantial substructure such that the altitude-dependent temperature profile is not sufficient to completely model conditions in the cavity. Coronal prominence cavities are structured by magnetism so clues to this structure are to be found in their plasma properties. These temperature substructures are likely related to structures in the cavity magnetic field.
Figure 5 caption: Line ratio (temperature diagnostic) vs. plane-of-sky altitude for data used for fit and model. The points in black are data, and the blue diamonds are the intensity values of the corresponding pixel as calculated with the model. The error bars on the data points represent Poisson and dark current uncertainties. The orange and pink points represent the model if both the cavity and streamer temperature profiles are increased (orange) or decreased (pink) by 5%.