Juan M. Fontenla

Formation of the Solar 10830 Å Line

One-dimensional hydrostatic-equilibrium models are shown here for faint, average, and bright components of the quiet Sun, and for a plage region, describing in each case how the atmosphere is stratified through the photosphere, chromosphere, and transition region up to a temperature of 105 K. The observed coronal line radiation is assumed to be the inward incident radiation at the 105 K boundary. This coronal radiation penetrates into the upper chromosphere causing sufficient helium ionization to populate the lower level of the He I 10830 A line, producing optically-thin absorption of the photospheric continuum at 10830A. The amount of absorption, which is proportional to the optical thickness of the upper chromosphere in the 10830 line, depends on 1) the strength of the coronal lines at wavelengths in the He I 504 A ionizing continuum, and 2) the density and geometrical thickness of the upper chromosphere. The computed 10830 A line is shown for the four atmospheric models and for three values of the coronal illumination. The calculated off-limb 10830 intensity distribution shows a minimum in the low chromosphere and a maximum at roughly 2000 km above the photosphere, in general agreement with observations, indicating that this is the predominant height of the transition region over most of the solar surface.

Imaging polarimeters for solar extreme ultraviolet astronomy

In this paper, the authors describe new EUV/FUV (100 {angstrom} {le} {lambda} {le} 1500 {Angstrom} polarimeter instrument concepts for solar research. These instruments are designed to observe linear polarization in EUV/FUV spectral lines originating in the outer solar atmosphere, specifically: a new coronagraph/polarimeter operating at 1215.7 {Angstrom} (neutral hydrogen Lyman {alpha}), which could observe this line in the near solar corona and lead to the first direct measurements of both strength and direction of coronal magnetic fields and a new multilayer EUV imaging polarimeter, operating at wavelengths of strong helium emission lines (e.g., 304 {Angstrom}, 584 {Angstrom}), which could observe impact polarization phenomena and provide information concerning the relative importance of thermal and nonthermal processes in solar flares. The emission mechanisms we will address with these instruments include resonance scattering and impact polarization. The authors discuss several instrument configurations and provide theoretical calculations and performance predictions for coronagraph/polarimeter instruments utilizing an optical design similar to the Ritchey-Chretien EUV/FUV telescopes developed for flight on the Stanford/MSFC/LLNL MultiSpectral Solar Telescope Array (MSSTA).

Solar EUV/FUV line polarimetry: observational parameters and theoretical considerations

Several mechanisms can induce a detectable amount of linear polarization (□ 1%) in spectral lines emitted by the outer solar atmosphere at EUV/FUV wavelengths (100 A< A < 1500 A): (1) Polarization in FUV lines (up to 20%) can be originated by resonance scattering of radiation anisotropically illuminating the emitting atoms. Modifications of this polarization can then result from the presence of a magnetic field (Hanle effect); (2) Impact line polarization can arise from anisotropic collisional excitation of the EUV emitting atoms by particles (electrons, protons) with non-Maxwellian velocity distributions. We suggest how new technological developments associated with the production of ultra-smooth, low scatter flow-polished mirror substrates and high quality multilayer and interference film coatings can make possible some exciting new optical instruments which should permit observations of these polarization effects. Polarization measurements have not previously been obtained in these wavelength regimes. We give the observational parameters for the development of all refledive FUV coronagraph/polarimeter and EUV imaging polarimeter instruments. A coronagraph/polarimeter, operating at hydrogen Lyman a, could provide - via the Hanle effect - the first direct measurements of coronal vector magnetic fields, of relevance to numerous fundamental questions in modern solar physics. A multilayer EUV imaging polarimeter, operating at EUV helium lines (e.g., 304 A, 584 A), could provide information on impact polarization phenomena, relevant to questions concerning the relative importance of thermal and non-thermal processes in solar flares. The reflecting polarization analyzers for these instruments will operate at the Brewster angle and will be coated with advanced thin film interference coatings or EUV multilayer coatings. We describe some new types of EUV/FUV polarimeters based on these polarization optics.

The role of particle diffusion in the lower transition region: Revised interpretation of emission measures

We have calculated energy-balance models of the solar chromosphere-corona transition region for T 25,000 K and decreases n H1 (while decreasing n p ) between 8000 and 18,000 K. Diffusion substantially increases n HeI for T > 35,000 K and decreases n HeI (while increasing n HeII) between 9000 and 25,000 K, and increases n HeIII for T < 60,000 K. These results suggest that the use of emission measure techniques that ignore the effects of diffusion may be unreliable.