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Results for “NONEQUILIBRIUM IONIZATION”

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Observational tests for nonequilibrium ionization in the solar corona

Nonequilibrium ionization may be produced by a variety of processes in the solar corona, for example, by mass flows through the large temperature gradients of the transition region or by impulsive heating and cooling. Any deviation from equilibrium ionization would have a strong effect on the radiation from the corona and on the interpretation of solar observations; hence, it is important to determine observational signatures of nonequilibrium. The temperature-sensitive line ratios can be used as such signatures. We examine the line ratios: C IV I(1548.2 A)/I(312.4 A), O IV I(789.4 A)/I(554.4 A), O V I(629.7 A)/I(172.2 A), O VI I(1031.9 A)/I(173.0 A) and O VI I(1031.9 A)/I(150.1 A). These line ratios are calculated for four coronal loop models that have a steady flow and that are known to have significant departures from equilibrium ionization. Our results indicate that, in general, nonequilibrium causes a considerable reduction in the line ratios, more than an order of magnitude in the downflowing leg of the loop model with the largest mass flows. We find that the C IV line ratio is the most sensitive to nonequilibrium. We discuss the implications of our results for observations, specifically, the observations expected from the upcoming SOHO mission.

Spadaro, D.↗
NASA NTRS2005

Calculation of H2-He Flow with Nonequilibrium Ionization and Radiation: an Interim Report

The nonequilibrium ionization process in hydrogen-helium mixture behind a strong shock wave is studied numerically using the detailed ionization rate model developed recently by Park which accounts for emission and absorption of Lyman lines. The study finds that, once the avalanche ionization is started, the Lyman line is self-absorbed. The intensity variation of the radiation at 5145 Angstroms found by Leibowitz in a shock tube experiment can be numerically reproduced by assuming that ionization behind the shock wave prior to the onset of avalanche ionization is 1.3%. Because 1.3% initial ionization is highly unlikely, Leibowitz s experimental data is deemed questionable. By varying the initial electron density value in the calculation, the calculated ionization equilibration time is shown to increase approximately as inverse square-root of the initial electron density value. The true ionization equilibration time is most likely much longer than the value found by Leibowitz.

Furudate, Michiko↗
NASA NTRS1991

Nonequilibrium ionization effects in asymmetrically heated loops

The effects of nonequilibrium ionization on magnetic loop models with a steady siphon flow that is driven by a nonuniform heating rate are investigated. The model developed by Mariska (1988) to explain the observed redshifts of transition region emission lines is examined, and the number densities of the ions of carbon and oxygen along the loop are computed, with and without the approximation of ionization equilibrium. Considerable deviations from equilibrium were found. In order to determine the consequences of these nonequilibrium effects on the characteristics of the EUV emission from the loop plasma, the profiles and wavelength positions of all the important emission lines due to carbon and oxygen were calculated. The calculations are in broad agreement with Mariska's conclusions, although they show a significant diminution of the Doppler shifts, as well as modifications to the line widths. It is concluded that the inclusion of nonequilibrium effects make it more difficult to reproduce the observed characteristics of the solar transition region by means of the asymmetric-heating models.

Spadaro, D.↗
NASA NTRS1972

Nonequilibrium ionization measurements in hydrogen-helium mixtures

Time-resolved emission measurements of several atomic line and continuum radiation channels were made behind the incident shock wave of an electric arc shock tube. Test times and nonequilibrium ionization times were obtained for shock velocities up to 2.5 X 10,000 m/sec in a 0.2 H2 - 0.8 He gas mixture. The shock-heated test gas was found to be free from driver gas contamination, and the test times were adequate to achieve steady state conditions. An activation energy of 4 eV was obtained from the nonequilibrium ionization time measurements. Modifications of experimental technique to determine the effect of test gas impurity level on ionization time measurements are discussed.

Leibowitz, L. P.↗
NASA NTRS1977

Design of closed-cycle MHD generator with nonequilibrium ionization and system

A method is developed to include the nonequilibrium ionization process in the MHD generator duct design equations, and these equations are coupled to the thermodynamic conditions of the closed cycle system. This is used to relate MHD generator size, configuration and gas conditions to the overall thermodynamic efficiency of the system. The system studied consists of an MHD loop (Ar + Cs or He + Cs) topping a steam bottoming plant.

Voshall, R. E.↗
NASA NTRS1995

High-resolution measurements of the K-alpha spectra of low-ionizationm species of iron: A new spectral signature of nonequilibrium ionization conditions in young supernova remnants

We present the first systematic laboratory measurements of high-resolution K-alpha spectra of intermediate ions of iron, Fe X-XVII. These lines are not produced in collisional equilibrium plasmas because of the relevant charge states cannot exist at the high electron temperatures required for appreciable excitation of the K-alpha transitions. However, they can provide excellent spectral diagnostics for nonequilibrium ionization conditions, such the ionizing plasmas of young supernova remnants. To facilitate the line identifications, we compare our spectra with theoretical atomic calculations performed using multiconfiguration parametric potential and Dirac-Fock atomic codes. Our measurements also allow direct comparison with time-dependent ionization balance calculations for ionizing plasmas, and good agreement is found.

Decaux, V.↗
NASA NTRS2011

The Impact of Nonequilibrium Ionization on SDO/AIA and Hinode/EIS Observations of Impulsively Heated Plasmas

Most plasma diagnostics assume the emitting material is in a state of ionization equilibrium. For example, the AIA temperature response functions have been derived on this basis. The assumption is reasonable whenever the plasma is evolving slowly or is very dense, but these are not the conditions that apply during impulsive heating events. It is now widely believed that many coronal loops are bundles of unresolved strands that are heated quasi-randomly by nanoflares. Full blown flares are thought to have similar sub-structure. We have studied the importance of nonequilibrium effects in these circumstances by modeling nanoflare-heated loops and simulating their observation by AIA and the EIS spectrometer on Hinode. We find that the intensities of hot emission lines can be highly suppressed and that the net emission from the loop tends to be dominated by strands that have entered a slow cooling phase, well after the impulsive energy release has ended. The hottest strands are relatively invisible, both because they are tenuous and because they cool rapidly by thermal conduction. Thus, AIA channels that are normally thought of as being sensitive to hot plasma, such 131 and 94, are in fact frequently not able to detect the hot plasma that is present. The magnitude of the effect is case dependent. Great care must be exercised when using the standard temperature response functions in situations where nonequilibrium ionization is likely to be important.

Klimchuk, James A.↗
NASA NTRS1979

Nonequilibrium ionization in solar and stellar winds

Substantial and systematic departures from ionization equilibrium can occur in the solar transition region and corona when mass outflows are present. Modeling calculations illustrate the general characteristics of the ionization balance in such regions. The presence of nonequilibrium conditions suggests a natural explanation for the extended region of EUV line emission that is observed above the solar limb. Comparison with observations of a coronal hole on the disk indicates that outflow may not start until temperatures of about 250,000 K are reached. Additional consequences include a diminution of the density discrepancy between ultraviolet and radio observations of coronal holes, and potential effects on the energy balance in solar and stellar atmospheres undergoing mass loss.

Dupree, A. K.↗
NASA NTRS1990

The effects of nonequilibrium ionization on the radiative losses of the solar corona

The emissivity of the ions of carbon and oxygen has been recalculated for a set of solar coronal loop models with a steady state siphon flow. The ion densities were calculated from the plasma velocities, temperatures, and densities of the models, and large departures from equilibrium were found. For purposes of comparison, the emissivity was calculated with and without the approximation of ionization equilibrium. Considerable differences in the radiative loss function Lambda(T) curve between equilibrium and nonequilibrium conditions were found. The nonequilibrium Lambda(T) function was then used to solve again the steady state flow equations of the loop models. The differences in the structure of these models with respect to the models calculated adopting the Lambda(T) curve in equilibrium are discussed.

Spadaro, D.↗
NASA NTRS1979

X-ray evidence for electron-ion equilibrium and ionization nonequilibrium in young supernova remnants

The A-2 spectroscopy experiment on HEAO 1 detected X-ray emission up to 25 keV from the supernova remnants Cas A and Tycho. The spectra must include continuum components with effective temperature equivalent or 10 to the 8th power K which could arise from optically thin plasmas in the collisionless shock fronts. This is the first indication of electron-ion temperature equilibrium in the expanding shell of young remnants. Measurements of the equivalent widths of the K alpha and K beta iron line blends in Cas A, show that their ratio is not compatible with the measured X-ray temperature in the collisional ionization equilibrium model. The search for hard X-ray pulsars in both remnants was unsuccessful.

Pravdo, S. H.↗
NASA NTRS1979

X-ray evidence for electron-ion equilibrium and ionization nonequilibrium in Young supernova remnants

X-ray emission up to 25 keV from the supernova remnants Cas A and Tycho has been detected with the A-2 spectroscopy experiment on HEAO 1. The spectra must include continuum components with effective temperature of about 10 to the 8th K which could arise from optically thin plasmas in the collisionless shock fronts. This is the first indication of electron-ion temperature equilibrium in the expanding shell of young remnants. The equivalent widths of the K-alpha and K-beta iron line blends in Cas A have also been measured and it is found that their ratio is not compatible with the measured X-ray temperature in the collisional ionization equilibrium model. Finally, an unsuccessful search has been conducted for hard X-ray pulsars in both remnants.

Pravdo, S. H.↗