HAO 2012 Profiles In Science: Dr. Michael Thompson

Contact

303-497-1500
mjt@ucar.edu

Specialties:

Helioseismology and asteroseismology; Solar physics (external link to current solar images); Inverse problems; Stellar structure and evolution; Astrophysical fluid dynamics.

Dr. Michael Thompson is the Director of the High Altitude Observatory of the National Center for Atmospheric Research. He is responsible for the overall scientific and strategic mission, productivity, and excellence of the Observatory in the areas of Solar and Heliospheric physics, and the effects of solar variability on the Earth's magnetosphere, ionosphere, and upper atmosphere. Dr. Thompson's main scientific interests are in the structure and dynamics of the interior of the Sun and other stars, and in helio- and asteroseismology.

Summary of Achievements:

Dr. Michael Thompson's research continues in the areas of helioseismology and asteroseismology. In helioseismology, his recent focus has continued to be on inferences from the temporal variability of the global-mode frequency splittings regarding the evolving zonal flows within the solar interior. Thompson was involved during the year on several papers featuring research on solar-like stars mostly using data from the Kepler satellite, and with P.G. Judge he published a review titled "Solar and stellar activity: diagnostics and indices" for the proceedings of IAU Symposium 286.

Publications

(1) D.R. Reese, J. P. Marques, M.J. Goupil, M.J. Thompson & S. Deheuvels, 2012: Estimating stellar mean density through seismic inversions, Astronomy and Astrophysics, 539, id.A63. 18 pages. See online article.

Abstract: Context. Determining the mass of stars is crucial both for improving stellar evolution theory and for characterising exoplanetary systems. Asteroseismology offers a promising way for estimating the stellar mean density. When combined with accurate radii determinations, such as are expected from Gaia, this yields accurate stellar masses. The main difficulty is finding the best way to extract the mean density of a star from a set of observed frequencies. Aims: We seek to establish a new method for estimating the stellar mean density, which combines the simplicity of a scaling law while providing the accuracy of an inversion technique. Methods: We provide a framework in which to construct and evaluate kernel-based linear inversions that directly yield the mean density of a star. We then describe three different inversion techniques (SOLA and two scaling laws) and apply them to the Sun, several test cases and three stars, α Cen B, HD 49933 and HD 49385, two of which are observed by CoRoT. Results: The SOLA (subtractive optimally localised averages) approach and the scaling law based on the surface correcting technique described by Kjeldsen et al. (2008, ApJ, 683, L175) yield comparable results that can reach an accuracy of 0.5% and are better than scaling the large frequency separation. The reason for this is that the averaging kernels from the two first methods are comparable in quality and are better than what is obtained with the large frequency separation. It is also shown that scaling the large frequency separation is more sensitive to near-surface effects, but is much less affected by an incorrect mode identification. As a result, one can identify pulsation modes by looking for an l and n assignment which provides the best agreement between the results from the large frequency separation and those from one of the two other methods. Non-linear effects are also discussed, as is the effects of mixed modes. In particular, we show that mixed modes bring little improvement to the mean density estimates because of their poorly adapted kernels.

(2) P.G. Judge & M.J. Thompson, 2012: Solar and stellar activity: diagnostics and indices, Proceedings of the International Astronomical Union, IAU Symposium, 286, pp. 15-26. See online article.

Abstract: We summarize the fifty-year concerted effort to place the "activity" of the Sun in the context of the stars. As a working definition of solar activity in the context of stars, we adopt those globally-observable variations on time scales below thermal time scales, of ~10^5 yr for the convection zone. So defined, activity is dominated by magnetic-field evolution, including the 22-year Hale cycle, the typical time it takes for the quasi-periodic reversal in which the global magnetic-field takes place. This is accompanied by sunspot variations with 11 year periods, known since the time of Schwabe, as well as faster variations due to rotation of active regions and flaring. "Diagnostics and indices" are terms given to the indirect signatures of varying magnetic-fields, including the photometric (broad-band) variations associated with the sunspot cycle, and variations of the accompanying heated plasma in higher layers of stellar atmospheres seen at special optical wavelengths, and UV and X-ray wavelengths. Our attention is also focussed on the theme of the Symposium by examining evidence for deep and extended minima of stars, and placing the 70-year long solar Maunder Minimum into a stellar context.