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Foukal, P.

Publications and source records attributed to Foukal, P..

30 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Magnetic loops, downflows, and convection in the solar corona

Optical and extreme-ultraviolet observations of solar loop structures show that flows of cool plasma from condensations near the loop apex are a common property of loops associated with radiations whose maximum temperature is greater than approximately 7000 K and less than approximately 3,000,000 K. It is suggested that the mass balance of these structures indicates reconnection by means of plasma motion across field lines under rather general circumstances (not only after flares). It is shown that the cool material has lower gas pressure than the surrounding coronal medium. The density structure of the bright extreme ultraviolet loops suggests that downflows of cool gas result from isobaric condensation of plasma that is either out of thermal equilibrium with the local energy deposition rate into the corona, or is thermally unstable. The evidence is thought to indicate that magnetic fields act to induce a pattern of forced convection.

Foukal, P.↗
NASA NTRS1975

The temperature structure and pressure balance of magnetic loops in active regions

EUV observations show many active region loops in lines formed at temperatures between 10,000 and 2,000,000 K. The brightest loops are associated with flux tubes leading to the umbrae of sunspots. It is shown that the high visibility of certain loops in transition region lines is due principally to a sharp radial decrease of temperature to chromospheric values toward the loop axis. The plasma density of these cool loops is not significantly greater than in the hot gas immediately surrounding it. Consequently, the internal gas pressure of the cool material is clearly lower. The hot material immediately surrounding the cool loops is generally denser than the external corona by a factor 3-4. When the active region is examined in coronal lines, this hot high pressure plasma shows up as loops that are generally parallel to the cool loops but significantly displaced laterally.

Foukal, P.↗
NASA NTRS1975

On the rotation of gas and magnetic fields at the solar photosphere

We point out that observations of a 5 percent velocity difference between photospheric gas and magnetic structures at a given latitude may simply result from angular momentum conservation by fluid elements in the convection zone. Estimates of the viscosity and magnetic drag are considered, and we conclude that they probably are not large enough to enforce strictly rigid rotation.

Foukal, P.↗
NASA NTRS1974

A three-component concept of the chromosphere and transition region

It is proposed that present observations of the chromosphere and transition region in EUV, optical and mm wavelengths are best described by a three-component concept. The three components are taken to be the interiors of supergranular cells, the hot plagettes overlying faculae, and the cooler, transient mottles which surround them in the network boundaries. The enhanced emission of the hot plagettes in transition ions is interpreted as a direct result of the increased pressure scale height over faculae relative to the cell interiors.

Foukal, P.↗
NASA NTRS1972

Magnetic coupling of the active chromosphere to the solar interior.

Evidence is summarized to show that the configuration of e lines which governs the appearance of H-alpha fine structure in active regions is set mainly by motions in the subphotosphere where these lines are anchored. It is shown that H-alpha fine structure is directly coupled to a layer probably more than 5000 km below the photosphere, and little distortion of the strong fields is expected in the ines. The shorter rotation period of active regions observed by Howard and others (compared to the photospheric gas) reted as a result ofthis direct coupling of the strong field to a more rapidly rotating solar interior. The effects of dragging such a field through a photosphere of finite resistivity are briefly considered for features of various observed cross-sections.

Foukal, P.↗
NASA NTRS1971

H alpha fine structure and the chromospheric field.

Study of the physical characteristics of the H alpha structures previously defined as fibrils and threads. The interpretation of the fibrils as ends of flux tubes is useful in tracing the behavior of the transverse field component over the solar surface. The observed properties of fibrils and threads are consistent with the hypothesis that they are produced by a shock wave mechanism similar to that advanced by Parker (1963) to explain spicules. It is suggested that the undisturbed magnetic field in the chromosphere of an active region is confined to a thin sheath, while the field of the quiet regions extends into the corona.

Foukal, P.↗
NASA NTRS1971

Morphological relationships in the chromospheric H-alpha fine structure

A continuous relationship is proposed between the basic elements of the dark fine structure of the quiet and active chromosphere. A progression from chromospheric bushes to fibrils, then to chromospheric threads and active region filaments, and finally to diffuse quiescent filaments, is described. It is shown that the horizontal component of the field on opposite sides of an active region quiescent filament can be in the same direction and closely parallel to the filament axis. Consequently, it is unnecessary to postulate twisted or otherwise complex field configurations to reconcile the support mechanism of filaments with the observed motion along their axis.

Foukal, P.↗