Spectral investigation of chromospheric fine structure

H spectra and effectively simultaneous filtergrams were taken at the Fraunhofer Observatory on Capri with the 35 cm domeless Coudé. The spatial resolution of the 19 best spectra selected for analysis was estimated to be 1–2 arc sec. The comparison of several hundred H line profiles emitted by typical chromospheric structure elements with theoretical prediction yielded strong evidence to suggest that the chromosphere consists of two parts: A lower, rather uniform layer at rest superposed by clouds (condensations of great spatial variability) which constitute the well-known structure pattern of H filtergrams. For most image points the line-of-sight velocity, optical thickness, source function and Doppler broadening of these clouds could be determined. While the values of the latter three quantities were found to be similar to what Beckers (1968) has found for limb spicules the velocity of the bright and of the dark mottles is considerably smaller than to be expected if these features were the spicules as seen on the disk. However, our results do not rule out the possibility that the spicules rise at the centers of rosettes where they are difficult to detect.Mitteilung aus dem Fraunhofer Institut No. 105.     

Hα fine structure and the chromospheric field

The physical characteristics of the H structures previously defined as fibrils and threads are studied. 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 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.     

Morphological relationships in the chromospheric Hα 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.     

A note on chromospheric fine structure at active region polarity boundaries

High resolution H filtergrams from Big Bear Solar Observatory reveal that some filamentary features in active regions have fine structure and hence magnetic field transverse to the gross structure and the zero longitudinal field line. These features are distinct from the usual active region filament, in which fine structure, magnetic field and filament are all parallel to the zero longitudinal field line. The latter occur on boundaries between regions of weaker fields while the former occur at boundaries between regions of stronger field.     

On the relation between chromospheric and photospheric fine structure in an active region

A comparative study was done on the bright fine structure in the upper photosphere and in the lower chromosphere of an active region. The results are shown in the following: (a) The bright points in the H wing are cospatial to the facular points, which confirms the result of Wilson (1981). (b) Some points bright in the H wing are associated with the facular granules which have larger sizes than the facular points, (c) The brightness enhancement in the Ha wing is positively correlated to the enhancement in the blue continuum. However, the correlation is not so strong. (d) The moustache points are also cospatial to the facular features. (e) The geometrical shape of a moustache point is like a funnel and diverging upward in the upper photospheric and the lower chromospheric levels.Some discussions are given on the nature of the heating mechanism of the fine structure based upon these results.On leave from Kyoto University and Hida Observatory, Japan.     

The temperature structure of chromospheric flares heated by non-thermal electrons

Heating of the deep chromosphere by a vertically descending beam of non-thermal electrons with power-law energy spectrum, in flares, is analysed. In lower regions of the flare, radiative losses can balance the energy input and the flare structure is described in terms of instantaneous quasi-steady temperature/depth profiles. Motion of the optical flare material is at constant pressure and is constrained to be purely vertical by a vertical magnetic field. The ionisation of hydrogen is determined by the same non-LTE processes as in the quiet chromosphere. Temperature profiles are obtained for a wide range of electron beam intensities and spectral indices and are discussed in terms of optical flare observations. Due to the steepness of the electron spectra, typical densities in the optical flare vary only over a narrow range, despite the diversity of beam intensities, in agreement with observation.Above a certain region, the flare material cannot attain a radiatively steady state against the electron input but evaluation of the level at which this occurs leads to an estimate of the mass of material involved in the high temperature flare plasma in this model. Results, which are again insensitive to the electron beam parameters, are found to be in satisfactory agreement with observations of the mass of flare ejecta and of soft X-ray flare emission measures.     

Intensity measurements of chromospheric fine structures in Lyman-α

The intensity of the sun was measured in the Lyman- emission line with 2.5 arc-seconds of resolution. The experiment was flown in an Aerobee-150 rocket on April 28, 1966. It contained a Cassegrain telescope with a pinhole aperture placed at the focus followed by a gas-gain ionization chamber whose spectral response was 1050 Å to 1350 Å.An isophote map 1.5 by 3 arc-minutes in size made from a composite of 90 linear scans shows an enhanced region and adjacent to it a prominent dark lane 20 arc-seconds wide. The measured intensity ratio of these two regions is nine. Bright features between 6 and 20 arc-seconds in size showed typical peak intensities of 20% greater than the surrounding chromosphere. The smallest features observed were 2.5 arc-seconds in size. A direct measurement of the absolute intensity at 1216 Å gave a value of 5.9 × 10⁴ erg cm^–2 s^–1 sterad^–1 in the quiet chromosphere.Based on observations made by the author at the E. O. Hulburt Center for Space Research (supported jointly by the Office of Naval Research and the National Science Foundation) at the Naval Research Laboratory.     

Observations of solar chromospheric fine structures in the light of Lyman-α

A Cassegrain telescope with a resolution of 2 sec of arc was successfully flown in an Aerobee-150 rocket from White Sands Missile Range on October 20, 1965. A pinhole, 33 in diameter, was placed at the focus of the telescope, followed by a photo-ionization detector with a lithium-fluoride window. The instrument was kept pointed at the sun by a biaxial solar pointing control.Results indicate that in Lyman- the solar surface shows structures whose characteristic dimensions can be as small as 2 sec of arc, which corresponds to the limit of instrumental resolution. Larger structures with very sharp gradients have also been found. Intensity ratios between bright and dark areas are typically a factor of 1.7. Isophote maps of two small selected areas are discussed in this paper. The results were obtained in an undisturbed (free of plages) portion of the solar disk.Jointly sponsored by the Office of Naval Research and the National Science Foundation.     

Interpretation of Hα contrast profiles of chromospheric fine structures

Recent observations of the Hα contrast profiles of identifiable chromospheric fine structures are interpreted in terms of an empirical model. It is shown that the parameters inferred from an application of Beckers' ‘cloud’ model are unreliable, and the problem of line asymmetries is re-examined. Quantitative models for the Hα chromosphere near the limb suggest that the ‘dark band’ phenomenon is due to low opacity in the neighbourhood of the temperature minimum, while the peculiar appearance of mottle contrast profiles near the limb is explained in terms of foreground absorption.     

Combined study between the chromospheric flare models and hard X-ray observation

By comparison between SMM HXRBS observation and ground observation of H and Caii K lines for the 2B flare on February 3, 1983, we found that there was a temporal correlation between H intensity and hard X-ray flux at the early stage of the impulsive phase while different peaks in the hard X-ray flux curve represented bursts at different locations. When we combined SMM HXRBS observation with chromospheric flare models, we further found that the temporal coincidence between H intensity and hard X-ray flux could be explained quantitatively by the fact that the H flare was indeed due to the heating by non-thermal electron beams responsible for the emission of hard X-rays. Together with the discussion on coronal density based on chromospheric flare models, it was also shown that the source of electrons seemed to be situated around the top of the flare loop and the column density at the top of the chromosphere in semi-empirical flare models could not be taken as the total material above the top of the chromosphere.     

The fine structure of prominences

Faint, Doppler shifted, emission features are detected in high resolution spectra of limb prominences. Their average line-of-sight velocity is about 3 × 10⁶ cm s^-1, their average life time is 300 s, and their angular sizes are 10⁸ cm in our spectrograms. The emission line width of the spectral features increases with increasing line shift.Some implications on the stability of prominences by these structures are discussed briefly.Visiting Astronomer, on leave from the Department of Astro-Geophysics, University of Colorado, Boulder, Colorado.     

The fine structure of prominences

Ions falling in vertically aligned magnetic structures of quiescent prominences may experience a vertical Lorentz force as flux ropes are distorted from the force-free condition. The terminal velocity of such ions may be sub-Alfvénic and may correspond to the 5–15 km s^–1 velocity of down falling material observed in many quiescent prominences. The higher velocities of down falling material found in active prominences and coronal rain may occur because of higher terminal velocities occurring in stronger magnetic fields.Visiting Astronomer, on leave from the Department of Astro-Geophysics, University of Colorado, Boulder, Colorado 80309.     

The fine structure of prominences

The fine structure of nonspot prominences are studied from H filtergrams. The size of the smallest prominence structures increases with height above the chromosphere. Some prominences contain structures close to 1/2 arc second, which is the spatial resolution in the present data. The effective thickness of many nonspot prominences ranges between 4 × 10⁷ cm and 1.5 × 10⁸ cm. An apparent downward directed motion is observed in the majority of the prominences. No preferred direction of the motion is seen in regions composed of comparatively large diffuse structures. Some bright threads are visible for 1 hr and longer. Bright knots have an average observed lifetime of about 8 min. The process of condensation and subsequent destruction of prominence fine structure appears to take place on a very short time scale compared to the life time of the regions where prominences may exist. The observed H brightness of the prominences in the present data may be accounted for as scattered chromospheric radiation.     

The fine structure of prominences

High spatial resolution spectral observations of five hedgerow prominences were made in H, He i D₃ and Ca ii H and K.The observed relations between the lines were not the same in all prominences. The Ca ii H and K lines were 2–4 times brighter relative to H and D₃ than predicted theoretically. The optical thickness of H was less than for the H and K lines, the H was optically thin in medium faint prominence structures. Faint structures appeared slightly hotter than bright structures.On leave from Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, Oslo 3, Norway.     

The fine structure of the quiet Sun

The observed properties of the small-scale features visible in the quiet photosphere — the granulation, of convective origin, and the network bright points, associated with kG magnetic fields — are described. The known properties of the magnetic flux tubes associated with network bright points are also presented. Empirical models derived from the observations are discussed, as well as a few theoretical models of particular importance for the understanding of the origin of the small-scale features of the quiet photosphere. Finally, the observational evidences showing that the structure of the granulation and of the photospheric network are varying over the solar cycle are reported.     

The fine structure of the Saturnian ring system

A dust disc within a planetary magnetosphere constitutes a novel type of dust-ring current. Such an azimuthal current carrying dust disc is subject to the dusty plasma analog of the well known finite-resistivity ‘tearing’ mode instability in regular plasma current sheets, at long wavelengths. It is proposed that the presently observed fine ringlet structure of the Saturnian ring system is a relic of this process operating at cosmogonic times and breaking up the initial proto-ring (which may be regarded as an admixture of fine dust and plasma) into an ensemble of thin ringlets. It is shown that this instability developes at a rate that is many orders of magnitude faster than any other known instability, when the disc thickness reaches a value that is comparable to its present observed value.     

The EUV chromospheric network in the quiet Sun

Investigations on the structure and intensity of the chromospheric network from quiet solar regions have been carried out with EUV data obtained from the Harvard spectroheliometer on the Apollo Telescope Mount of Skylab. The distribution of intensities within supergranulation cell interiors follows a near normal function, where the standard deviation exceeds the value expected from the counting rate, which indicates fine-scale structure below the 5 arc sec resolution of the data. The intensities from the centers of supergranulation cells appear to be the same in both quiet regions and coronal holes, although the network is significantly different in the two types of regions. The average halfwidth of the network elements was measured as 10 arc sec, and was independent of the temperature of formation of the observing line for 3.8 < log T _e < 5.8. The contrast between the network and the centers of cells is greatest for lines with log T _e 5.2, where the network contributes approximately 75% of the intensity of quiet solar regions. The contrast and fractional intensity contributions decrease to higher and lower temperatures characteristic of the corona and chromosphere.     

The fine structure of a nanoflare-heated corona

Implications of the conjecture that the solar corona is heated by small-scale events (nanoflares) are examined. It is shown that even if these nanoflares are small-scale (heating areas much smaller than 1 arc sec²) and impulsive, the corona may still be rather homogeneous. In particular, the filling factor (defined as the ratio of the coronal volume radiating in X-rays and EUV to the total volume) can easily be of order unity. Future experimental determination of the filling factor could prove useful in estimating the volume of coronal material that is heated during nanoflares.Work performed at Beam Physics Branch, Plasma Physics Division, Naval Research Laboratory, Washington, D.C., U.S.A.Mailing Address: Code 6790, NRL, Washington, D.C. 20375, U.S.A.     

The fine structure of light-bridges in sunspots

High resolution photographs obtained at the Pic du Midi Observatory show that there are three types of sunspot light bridges according to their morphological structures: the photospheric ones, the penumbral ones and the umbral ones. Consequently there are no specific structures in light bridges; it results that they should not be due to specific physical properties. Properties of the fine structure of a penumbral light bridge are described.     

The formation of flare loops by magnetic reconnection and chromospheric ablation

Slow-mode shocks produced by reconnection in the corona can provide the thermal energy necessary to sustain flare loops for many hours. These slow shocks have a complex structure because strong thermal conduction along field lines dissociates the shocks into conduction fronts and isothermal subshocks. Heat conducted along field lines mapping from the subshocks to the chromosphere ablates chromospheric plasma and thereby creates the hot flare loops and associated flare ribbons. Here we combine a non-coplanar compressible reconnection theory with simple scaling arguments for ablation and radiative cooling, and predict average properties of hot and cool flare loops as a function of the coronal vector magnetic field. For a coronal field strength of 100 G the temperature of the hot flare loops decreases from 1.2 × 10⁷ K to 4.0 × 10⁶ K as the component of the coronal magnetic field perpendicular to the plane of the loops increases from 0% to 86% of the total field. When the perpendicular component exceeds 86% of the total field or when the altitude of the reconnection site exceeds 10⁶km, flare loops no longer occur. Shock enhanced radiative cooling triggers the formation of cool H flare loops with predicted densities of 10¹³ cm^–3, and a small gap of 10³ km is predicted to exist between the footpoints of the cool flare loops and the inner edges of the flare ribbons.