Engineering topics

Uitenbroek, Han

Publications and source records attributed to Uitenbroek, Han.

6 records

NASA NTRS2017

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

The broadening of the hydrogen lines during flares is thought to result from increased charge (electron, proton) density in the flare chromosphere. However, disagreements between theory and modeling prescriptions have precluded an accurate diagnostic of the degree of ionization and compression resulting from flare heating in the chromosphere. To resolve this issue, we have incorporated the unified theory of electric pressure broadening of the hydrogen lines into the non-LTE radiative-transfer code RH. This broadening prescription produces a much more realistic spectrum of the quiescent, A0 star Vega compared to the analytic approximations used as a damping parameter in the Voigt profiles. We test recent radiative-hydrodynamic (RHD) simulations of the atmospheric response to high nonthermal electron beam fluxes with the new broadening prescription and find that the Balmer lines are overbroadened at the densest times in the simulations. Adding many simultaneously heated and cooling model loops as a 'multithread' model improves the agreement with the observations. We revisit the three component phenomenological flare model of the YZ CMi Megaflare using recent and new RHD models. The evolution of the broadening, line flux ratios, and continuum flux ratios are well-reproduced by a multithread model with high-flux nonthermal electron beam heating, an extended decay phase model, and a 'hot spot' atmosphere heated by an ultra relativistic electron beam with reasonable filling factors: approximately 0.1%, 1%, and 0.1% of the visible stellar hemisphere, respectively. The new modeling motivates future work to understand the origin of the extended gradual phase emission.

Kowalski, Adam F.↗

NASA NTRS2000

Understanding Stellar Light Spatial Inhomogeneities and Time Variability

We would like the opportunity to thank NASA for supporting our efforts to construct tools to analyze the spectra of spatially inhomogeneous and temporally varying stellar atmospheres. This financial support has allowed us to a versatile radiative transfer code that can be used for many different applications. With this numerical code we have written a point-and-click analysis package written in IDL that can be used to look extensively at the generated output data. Below we describe the most recent results obtained with our transfer code and list papers that have appeared with these results. Although we have not been able to produce as many time-dependent calculations as we had hoped (mainly because of programmatic reasons; Sasselov took another position halfway through the grant), we believe we have

Uitenbroek, Han↗

NASA NTRS1998

Study of Magnetic Structure in the Solar Photosphere and Chromosphere

This grant funded an observational and theoretical program to study the structure and dynamics of the solar photosphere and low chromosphere, and the spectral signatures that result. The overall goal is to learn about mechanisms that cause heating of the overlying atmosphere, and produce variability of solar emission in spectral regions important for astrophysics and space physics. The program exploited two new ground-based observational capabilities: one using the Swedish Solar Telescope on La Palma for very high angular resolution observations of the photospheric intensity field (granulation) and proxies of the magnetic field (G-band images); and the other using the Near Infrared Magnetograph at the McMath-Pierce Solar Facility to map the spatial variation and dynamic behavior of the solar temperature minimum region using infrared CO lines. We have interpreted these data using a variety of theoretical and modelling approaches, some developed especially for this project. Previous annual reports cover the work done up to 31 May 1997. This final report summarizes our work for the entire period, including the period of no-cost extension from 1 June 1997 through September 30 1997. In Section 2 we discuss observations and modelling of the photospheric flowfields and their consequences for heating of the overlying atmosphere, and in Section 3 we discuss imaging spectroscopy of the CO lines at 4.67 mu.

Noyes, Robert W.↗

NASA NTRS1995

New insight in the solar T(sub MIN) region from the CO lines at 4.67 micron

We discuss recent observations of the fundamental vibration-rotation transitions of carbon monoxide (CO) in the solar infrared spectrum. Employing a new array detector at the McMath-Pierce facility on Kitt Peak we find that the CO lines sketch a rich picture of the dynamics of the solar temperature minimum region, the lower boundary of the chromosphere. In a spectra-spectroheliogram and a time-sequence of the slit-spectra obtained during exceptional seeing conditions we observe small-scale bright, ring shaped, blueshifted features. We speculate that they are the signature of granular overshoot into the convectively stable temperature minimum. The centers of the rings are among the coolest elements seen in strong CO-line heliograms on the disk, and may be instrumental to the low temperature observed in CO close to the solar limb.

Uitenbroek, Han↗

NASA NTRS1994

Imaging spectroscopy of the solar CO lines at 4.67 microns

We analyze spatially and temporally resolved spectra of the fundamental vibration-rotation transitions of carbon monoxide (CO) in the solar spectrum at 4.67 micrometers. Our observations imply that, in the quiet Sun, spatial variations in CO intensity are largely dynamical in nature, reinforcing the suggestion that dynamical effects play a key role in the formation of the dark CO cores. Time sequences of resolved spectra exhibit mainly 3 minute power in line-core intensity but mainly a 5 minute period in Doppler shift. The weak 7-6 R68 line shows normal Evershed flow in the penumbra of a sunspot; we find evidence for the onset of inverse Evershed flow in the strong 3-2 R14 line. Spectra at the limb indicate that 3-2 R14 emission extends approximately 360 km beyond the continuum limb.

Uitenbroek, Han↗

NASA NTRS1991

Ca II H(2v) and K(2v) cell grains

The bright Ca II H(2v) and K(2v) grains, which are intermittently present in the interiors of network cells in quiet-sun areas, should provide important diagnostics of the dynamical interaction between the quiet photosphere and the chromosphere above it, but their nature has so far eluded identification. The extensive observational literature on these grains and on related phenomena is here reviewed, and various contradictions are resolved. It is concluded that the grains are a hydrodynamical phenomenon in which magnetic fields do not play a major role. The grains are due to interference between a pervasive standing oscillation and an 8 Mm horizontal wavelength in the chromosphere, and the wave trains of the evanescent p-mode interference pattern in the upper photosphere.

Rutten, Robert J.↗