Engineering topics

Antiochos, S. K.

Publications and source records attributed to Antiochos, S. K..

89 records · Page 5

NASA NTRS1980

The evolution of active region loop plasma

The adjustment of coronal active-region loops to changes in their heating rate is investigated numerically. The one-dimensional hydrodynamic equations are solved subject to boundary conditions in which heat flux-induced mass exchange between coronal and chromospheric components is allowed. The calculated evolution of physical parameters suggests that (1) mass supplied during chromospheric evaporation is much more effective in moderating coronal temperature excursions than when downward heat flux is dissipated by a static chromosphere, and (2) the method by which the chromosphere responds to changing coronal conditions can significantly influence coronal readjustment time scales. Observations are cited which illustrate the range of possible fluctuations in the heating rates.

Krall, K. R.↗

NASA NTRS1980

Radiative-dominated cooling of the flare corona and transition region

Recent observations of some compact flares indicate that the differential emission measure, q, of flare coronal and transition region plasma has a much steeper dependence on temperature than in nonflare regions. It is noted that this result is not compatible with models for a flare loop in which conduction to the chromosphere dominates the cooling, even in the case where the loop has a large divergence in its cross-sectional area. Only by a combination of many loops is it possible to reproduce the observations. Hence, models in which radiation dominates the evolution are investigated. It is found that the radiative models predict that q varies as T to the power (l + 1) where l measures the dependence of the radiative loss coefficient on temperature. It is concluded that the radiative models are also incapable of explaining the observations (unless, again, a combination of many loops is postulated) and it is suggested that large mass motions with velocities of the order of the sound speed may be required.

Antiochos, S. K.↗

NASA NTRS1980

The minimum flux corona; theory or concept

The reply to the criticisms of the minimum flux theory is discussed. These criticisms are correct in substance, as well as in detail. Counter arguments that the minimum flux corona theory is untenable, because of errors in its formulation, are presented.

Underwood, J. H.↗

NASA NTRS1980

A model of active prominences

A one-dimensional numerical model of active loop prominences is investigated. The model includes the effects of gravity, the geometry of the magnetic field, and conduction losses to the chromosphere. Calculations indicate that, as originally proposed by Goldsmith, the thermal instability mechanism is, by itself, sufficient to account for the appearance of bright H-alpha knots in postflare loops. Under certain conditions, initial perturbations in the loop temperature and density profiles of small, but finite, amplitude (approximately 5%) and large size scale (greater than or approximately equal to 10 to the 9th cm) can grow into condensations with temperature and density differences of over an order of magnitude and size scales of less than 10 to the 8th cm. In agreement with observations, the conditions that must be satisfied are such that loop prominence systems are likely to occur only in large flares and such that knots preferentially form at the tops of loops. The velocities, densities, and lifetimes calculated for the loop material are also in agreement with observations. It is concluded that in order for H-alpha knots to occur, heating of some form must continue into the late cooling phase of a flare loop, and that this heating is more intense near the loop base than near the apex.

Antiochos, S. K.↗

NASA NTRS1980

The thermal X-ray flare plasma

Following a review of current observational and theoretical knowledge of the approximately 10 to the 7th K plasma emitting the thermal soft X-ray bursts accompanying every H alpha solar flare, the fundamental physical problem of the plasma, namely the formation and evolution of the observed X-ray arches, is examined. Extensive Skylab observations of the thermal X-ray plasmas in two large flares, a large subflare and several compact subflares are analyzed to determine plasma physical properties, deduce the dominant physical processes governing the plasma and compare large and small flare characteristics. Results indicate the density of the thermal X-ray plasma to be higher than previously thought (from 10 to the 10th to 10 to the 12th/cu cm for large to small flares), cooling to occur radiatively as much as conductively, heating to continue into the decay phase of large flares, and the mass of the thermal X-ray plasma to be supplied primarily through chromospheric evaporation. Implications of the results for the basic flare mechanism are indicated.

Moore, R.↗

NASA NTRS1979

Numerical modeling of quasi-static coronal loops. I - Uniform energy input

A quasi-static numerical model for coronal loops is considered for the case of a uniform energy input per unit volume into the loops. A line dipole model is used to represent the loop magnetic field, and the variations in loop cross section observed in X-ray photographs are parameterized by the ratio between the cross-sectional areas at the loop apex and base. The results of numerical modeling indicate that for an area ratio greater than unity, increases in the area ratio of a loop with a given length and apex area cause a general rise in electron density and a fall in the temperature gradient, leading to large increases in the differential emission factor at high temperatures. The differential function obtained is significantly different from that predicted by analytical models; however, analytical predictions for the temperature-electron density relations are comparable to numerical results. It is also concluded that even a symmetrical loop may have a maximum temperature away from the apex.

Vesecky, J. F.↗

NASA NTRS1979

The stability of solar coronal loops

The stability of the 'quasi-static' models of coronal loops was examined. It was found that all models in which the heat flux at the base of the loop is assumed to vanish are unstable to the growth of thermal perturbations. The growth rates and the profiles of the unstable modes indicate that the instability involves primarily the low-temperature, transition-region plasma. The models can be made stable only by assuming that the heat flux at the base of the loop is large, of the order of 13% of the maximum flux in the loop. The results imply that the transition region must be intrinsically dynamic.

Antiochos, S. K.↗

NASA NTRS1979

Radiative dominated cooling of the flare corona and transition region

Models in which radiation dominates cooling flare loops are investigated. The radiative models are found to predict a differential emission measure (Q) proportional to T to the (l+1) power, where l measures the dependence of the radiative loss coefficient on temperature, lamda (T) approximately T to the (-l) power. It is concluded that the radiative models are incapable of explaining the observed temperature dependence of Q for flare coronal and transitional plasma. The models suggest that large mass motions (velocities of the order of the sound speed) may be required.

Antiochos, S. K.↗

NASA NTRS1979

The evolution of soft X-ray-emitting flare loops

We have constructed a numerical model for a cooling flare loop in which the complete set of single-fluid equations in a one-dimensional geometry (i.e., parallel to the magnetic field) is solved. Both evaporative and static boundary conditions for the chromosphere-corona interface have been developed. This model is used to investigate the effects of initial temperature and density, loop geometry, and boundary conditions on the form of the plasma evolution and the soft X-ray emission. The results are then compared with Skylab S-056 observations of the 1973 August 9 flare. For this comparison, and under the present assumptions, we conclude that even highly compact flares must have a multiloop structure similar to large flares, and that both radiative and conductive cooling are necessary to explain the observations. The data appear to be consistent with the predicted emission from a combination of evaporative cooling loops.

Antiochos, S. K.↗

NASA NTRS1978

Evolution of the coronal and transition-zone plasma in a compact flare - The event of 1973 August 9

X-ray and extreme ultraviolet observations of a compact flare were analyzed to determine the relative importance of radiation, thermal conduction, and 'evaporation' in the evolution of the temperature and density structure of the plasma. In the event studied (1973 August 9), the electron density was relatively high (5 x 10 to the eleventh to 1 x 10 to the twelfth) and radiation was evidently an important energy-loss and cooling mechanism. The light curves of ultraviolet lines formed at temperatures between 10 to the fifth to 10 to the seventh K indicate a time-varying emission measure gradient, and hence temperature gradient, during the flare. Radiative instability evidently played an important role in determining the steepness of these gradients during the rise and fall phases, and caused strong downward motions of material during the cooling phase. Toward the end of the event, the coronal electron density decreased and the temperature gradient relaxed toward that expected from a conduction-dominated plasma. For this flare, evaporative cooling did not appear to be a significant factor.

Underwood, J. H.↗

NASA NTRS1978

The analysis of high spatial resolution UV and X-ray images by computational modeling

Very high resolution stereoscopic images of high temperature loop structures observed at UV and X-ray wavelengths in the solar corona can be used to understand physical processes in the corona. An existing computational model is described and sample results are given to demonstrate that computational modeling of coronal structures can indeed take advantage of very high resolution images. The sample results include the run of temperature and number density along a typical loop and the variation of the differential emission measure with temperature. The integration of the differential emission measure with temperature along a column commensurate with an instrument's spatial resolution is the relevant parameter obtained from UV and X-ray observations. The effects of loop geometry and energy input are examined.

Vesecky, J. F.↗

NASA NTRS1978

Comments on the 'minimum flux corona' concept

Hearn's (1975) models of the energy balance and mass loss of stellar coronae, based on a 'minimum flux corona' concept, are critically examined. First, it is shown that the neglect of the relevant length scales for coronal temperature variation leads to an inconsistent computation of the total energy flux F. The stability arguments upon which the minimum flux concept is based are shown to be fallacious. Errors in the computation of the stellar wind contribution to the energy budget are identified. Finally we criticize Hearn's (1977) suggestion that the model, with a value of the thermal conductivity modified by the magnetic field, can explain the difference between solar coronal holes and quiet coronal regions.

Antiochos, S. K.↗

NASA NTRS1978

Evaporative cooling of flare plasma

We investigate a one-dimensional loop model for the evaporative cooling of the coronal flare plasma. The important assumptions are that conductive losses dominate radiative cooling and that the evaporative velocities are small compared with the sound speed. We calculate the profile and evolution of the temperature and verify the accuracy of our assumptions for plasma parameters typical of flare regions. The model is in agreement with soft X-ray observations on the evolution of flare temperatures and emission measures. The effect of evaporation is to greatly reduce the conductive heat flux into the chromosphere and to enhance the EUV emission from the coronal flare plasma.

Antiochos, S. K.↗

NASA NTRS1976

Thermal instability in post-flare plasmas

The cooling of post-flare plasmas is discussed and the formation of loop prominences is explained as due to a thermal instability. A one-dimensional model was developed for active loop prominences. Only the motion and heat fluxes parallel to the existing magnetic fields are considered. The relevant size scales and time scales are such that single-fluid MHD equations are valid. The effects of gravity, the geometry of the field and conduction losses to the chromosphere are included. A computer code was constructed to solve the model equations. Basically, the system is treated as an initial value problem (with certain boundary conditions at the chromosphere-corona transition region), and a two-step time differencing scheme is used.

Antiochos, S. K.↗

NASA NTRS1976

Evaporative cooling of flare plasma

A one-dimensional loop model for the evaporative cooling of the coronal flare plasma was investigated. Conductive losses dominated radiative cooling, and the evaporative velocities were small compared to the sound speed. The profile and evolution of the temperature were calculated. The model was in agreement with soft X-ray observations on the evolution of flare temperatures and emission measures. The effect of evaporation was to greatly reduce the conductive heat flux into the chromosphere and to enhance the EUV emission from the coronal flare plasma.

Antiochos, S. K.↗

NASA NTRS1976

Influence of magnetic field structure on the conduction cooling of flare loops

A simple model facilitates calculation of the influence of magnetic-field configuration on the conduction cooling rate of a hot post-flare coronal plasma. The magnetic field is taken to be that produced by a line dipole or point dipole at an arbitrary depth below the chromosphere. For the high temperatures (at least 10 million K) produced by flares, the plasma may remain static and isobaric. The influence of the field is such as to increase the heat flux (per unit area) into the chromosphere, but to decrease the total conduction cooling of the flare plasma. This leads to a significant enhancement of the total energy radiated by the flare plasma.

Antiochos, S. K.↗

NASA NTRS1976

Influence of magnetic field structure on the conduction cooling of flare loops

A simple model facilitates calculation of the influence of magnetic field configuration on the conduction cooling rate of a hot post-flare coronal plasma. The magnetic field is taken to be that produced by a line dipole or point dipole at an arbitrary depth below the chromosphere. For the high temperatures (T greater than or equal to 10 to the 7th power K) produced by flares, the plasma may remain static and isobaric. The influence of the field is such as to increase the heat flux (per unit area) into the chromosphere, but to decrease the total conduction cooling of the flare plasma. This leads to a significant enhancement of the total energy radiated by the flare plasma.

Antiochos, S. K.↗