Predoctoral Projects, 2015

Project Title: Calculation of new models of the solar atmosphere and spectrum based on spacecraft observations in the extreme ultraviolet, and improved determination of solar elemental abundances

Project Advisor: Dr. Eugene H. Avrett

Background: A recently published atlas of the solar extreme-ultraviolet spectrum obtained by SOHO, a NASA-ESA spacecraft, provides extremely valuable observational data that can be used to determine the structure of the chromosphere, transition region, and corona of the solar atmosphere. A state-of-the art atmospheric modeling program is now being used to determine atmospheric models and calculated spectra consistent with these observations, providing new models of the solar atmosphere in much better agreement with observations than before and allowing new interpretations of physical processes in the solar atmosphere.

Scientific Questions: Our proximity to the Sun gives us the opportunity to observe the entire electromagnetic spectrum of a star with far greater spectral, spatial, and temporal resolution than is possible with any other star or astronomical source, and to understand the physical processes that are responsible for the emitted radiation. The general processes are reasonably well understood, but there have been only primitive attempts to understand the spectrum in detail. The observational data and computational tools are at hand to determine 1) the detailed structure of the solar atmosphere, 2) the non-radiative heating that causes the outward increase in temperature, and 3) an improved set of solar elemental abundances.

Scientific Methodology: The solar extreme-ultraviolet spectrum consists of emission continua and many thousands of emission lines from neutral to highly ionized atoms. The existing atmospheric modeling program, called Pandora, can be described as follows: Given a set of atmospheric parameters, the program calculates the line and continuous spectrum at all wavelengths from the radio/infrared range to the EUV/X-ray region. Comparisons between calculated and observed spectra are used to infer, by trial and error, the characteristics of the emitting region. The program takes into account, for a time-independent, 1-D planar or spherical medium with a given internal velocity and mass-flow distribution, the optically thick non-LTE transfer of line and continuum radiation for multilevel atoms and multiple stages of ionization, with line blending and interlocking, partial frequency redistribution, particle diffusion, incident illumination, radiative energy balance or non-radiative heating, and hydrostatic or pressure equilibuium or other hydrodynamic constranints. The program also makes use of the extensive atomic and molecular opacity data compiled and calculated at SAO by Robert Kurucz. In addition to accounting for radiative and collisional ionization, recombination, excitation, and de-excitation, the program includes autoionization, dielectronic recombination, charge transfer, and the excitation and ionization due to collisions with neutral hydrogen as well as with electrons. No other program has all these capabilities. Nevertheless, there are many uncertainties in the rates and cross sections and atomic model parameters that require careful study in each application. The program also can be applied to the interpretation of spectra from stars other than the Sun and from other astronomical sources.

Getting Started: If you are interested, stop by my office, P-340, for some recent research papers to read, and, if you like, a set of notes (287 pp) from the last time (1984) the Department offered the graduate course on Solar and Stellar Atmospheres. There are some immediately publishable research opportunities based on the available observations and the calculations that can be carried out with the Pandora program. A good background in atomic physics and an interest in the analysis of astronomical spectra would be needed. A realistic project would be to resolve a recent controversy over the solar abundance of oxygen, neon, and other elements.


Clay Fellow Warren Brown