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Coronal condensation spectra during 4 February 1962 total eclipse, determining abundances, ionization equilibria, electron densities, etc, from Fe and Ca XV lines analysis
Total intensities of continuum and of red, green and yellow coronal lines emitted by sporadic coronal condensation shown to yield lower limits to Fe and Ca abundances
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The series of self-consistent, two-dimensional, MHD calculations used to explore the reconnection dynamics implicit in the two-ribbon flare model of Kopp and Pneumann (1976) indicate that there exists, in addition to the standard, slow mode MHD shocks generated by reconnection, a standing, fast mode MHD shock. Due to thermal conduction, the slow shocks generate an evaporative upflow of chromospheric plasma into the reconnection region; an analysis of these effects of radiative and conductive cooling suggests that at least some of this evaporated plasma will undergo thermal condensation in passing through the fast shock.
A model of filament formation based on the condensation of coronal arches is described. The condensation results from initiating the radiative instability within an arch by superimposing a transient energy supply upon the steady state heating mechanism. The transient energy supply increases the density within the arch so that when it is removed the radiative losses are sufficient to lead to cooling below the minimum in the power loss curve. Times from the initial formation of the condensation to its temperature stabilization as a cool filament have been calculated for various initial conditions. They lie in the range 10,000-100,000 s with the majority of the time spent above a temperature of 1 x 10 to the 6th K. Under the assumption that the condensation of a single arch forms an element of the filament, a complete filament requires the condensation of an arcade of loops. Using experimentally derived parameters, filament densities of 10 to the 11th to 10 to the 12th per cu cm can be obtained.
The presence of condensations in active regions has the potential to be an important diagnostic of coronal heating. We present the results of models of nanoflare heated coronal loops using the 1-D hydrodynamic ARGOS code. The nanoflares are modeled by discrete pulses of energy along the loop. We explore the occurrence of cold condensations due to the effective equivalent of thermal non-equilibrium (TNE) in loops with steady heating, and examine its dependence on nanoflare timing and intensity and also nanoflares location along the loop, including of randomized distributions of nanoflares. We find that randomizing nanoflare distributions, both in time/intensity and location, tends to diminish the likelihood of condensations compared to regularly occurring nanoflares with the same average properties, but that condensations can sometimes occur in regimes where regular nanoflares would not produce TNE. Also, the condensations stay in the loop for a shorter amount of time when the nanoflares distributions are random. These properties can be used in the future to investigate diagnostics of coronal heating mechanisms.
Hudson and Ohki (1972) pointed out that the increase of the soft X-ray emission measure during flares might be accounted for in two different ways, either by 'coronal condensation', or by what they termed 'chromospheric rarefaction', now more commonly called 'chromospheric evaporation'. They ruled out coronal condensation on the basis of cornal mass content arguments. Moore et al. (1980) found it highly probable that the bulk of the mass of the soft X-ray emitting plasma is supplied during the rise phase by chromospheric evaporation from the feet of the soft X-ray loops. On the other hand, Cheng et al. (1981) argued that chromospheric evaporation is not important as a source of soft X-ray plasma. The present investigation is concerned with an event in which direct chromospheric observations contradict the conclusions reached by Cheng et al. Up to now chromospheric evaporation has always been an inference, without compelling positive evidence. In the current investigation, observations are considered which constitute such evidence.
The work completed under this project, 'Evolution and Activity in the Solar Corona: A Comparison of Coronal and Chromospheric Structures Seen in Soft X-Rays, White Light and H-Alpha Emission', includes the following presentations: (1) Analysis of H-alpha Observations of High-altitude Coronal Condensations; (2) Multi-spectral Imaging of Coronal Activity; (3) Measurement and Modeling of Soft X-ray Loop Arcades; (4) A Study of the Origin and Dynamics of CMEs; and various poster presentations and thesis dissertations.
Thermal non-equilibrium (TNE) is the likely cause of most coronal condensations, including prominences and non-flaring coronal rain. It is therefore extremely important. It is also highly intriguing, being in some respects counter intuitive. In this talk I will review the basic properties of TNE, summarize the history of TNE research, and point to some of the outstanding remaining issues.
Comparison of models of coronal condensations derived from nonoptical and optical observations. Recent empirical models of coronal active regions, based on observations outside the optical wavelength band, are reviewed. Models derived from optical observations are then considered, with special emphasis on the optical forbidden line spectrum. The formation of sporadic condensations and loop prominences following a flare is discussed.
A model is investigated for the decay of flare heated coronal loops in which rapid radiative cooling at the loop base creates strong pressure gradients which, in turn, generate large (supersonic) downward flows. The coronal material cools and 'condenses' onto the flare chromosphere. The features which distinguish this model from previous models of flare cooling are: (1) most of the thermal energy of the coronal plasma may be lost by mass motion rather than by conduction or coronal radiation; (2) flare loops are not isobaric during their decay phase, and large downward velocities are present near the footpoints; (3) the differential emission measure q has a strong temperature dependence.
Coronal condensations associated with active regions are investigated by comparing their X-ray and EUV line intensities. The EUV line measurements were made with the OSO-H satellite, and the X-ray measurements were made with an ATM support rocket. The data obtained from these observations are used to find the emission measure distribution at low temperatures.
VLA observations of a solar plage region at 6 and 20 cm wavelengths are presented. The high frequency 6 cm emission correlates well with the associated sunspots, whereas 20 cm emission shows good correlation with the H-alpha plage. Large temperature variations over a period of one day are observed in the plage-associated component without any significant changes in the sunspots. The dominant emission mechanisms at 6 and 20 cm are found to be gyroresonance radiation and bremsstrahlung, respectively. It is concluded that the coronal condensation above the chromospheric H-alpha plage has an electron density of about 5 x 10 to the 9th/cu cm and it extends to a height of 50,000 km.
A model is investigated for the decay of flare heated coronal loops in which rapid radiative cooling at the loop base creates strong pressure gradients which, in turn, generate large (supersonic) downward flows. The important features of this model which distinguish it from previous models of flare cooling are: (1) Most of the thermal energy of the coronal plasma may be lost by mass motion rather than by conduction or coronal radiation. (2) Flare loops are not isobaric during their decay phase, and large downward velocities are present near the footpoints. (3) The differential emission measure has a strong temperature dependence. These results can account for recent observations of compact flare loops that are not consistent with the previous cooling models.
The formation of the normal polarity solar prominences in a magnetic arcade by photospheric shearing motions was studied using a two-and-a-half-dimensional MHD code, which includes the gravity, radiative cooling, simplified coronal heating, and thermal conduction along the field lines. It was found that a footpoint shear induces an expansion of the magnetic arcade and cooling of the plasma in it; simultaneously, the denser material from the lower part of the arcade is pulled up by the expanding field lines. Thus, a local enhancement of radiative cooling is effected, leading to the onset of thermal instability and the condensation of coronal plasma. The condensed material grows vertically to form a sheetlike structure leading eventually to the formation of prominence.
Nonlinear treatment of prominence condensation from solar corona by thermal instability
Stability of dilute gas in mechanical and thermal equilibrium with application to nongravitational condensation phenomena in astronomy, particularly solar corona
The results of a model dynamic simulation of the formation and support of a narrow prominence at the apex of a coronal magnetic loop or arcade are described. The condensation process proceeds via an initial radiative cooling and pressure drop, and a secondary siphon flow from the dense chromospheric ends. The antibuoyancy effect as the prominence forms causes a bending of the confining magnetic field, which propagates toward the semirigid ends of the magnetic loop. Thus, a wide magnetic 'hammock' or well (of the normal-polarity Kippenhahn-Schlueter-type) is formed, which supports the prominence at or near the field apex. The simplicity of this 1.5-dimensional model, with its accompanying diagnostics, elucidates the various contributions to the nonlinear dynamics of prominence condensation and levitation.