The abundance of helium in prominences and in the chromosphere

The abundance of helium relative to hydrogen is spectroscopically determined in prominences and in the chromosphere by using 1952, 1958, 1962 and 1966 eclipse data. Care is taken in the intensity calibration of emission lines, the self-absorption, and the departure from local thermodynamic equilibrium. We find from the line profiles and intensities of prominences and the chromosphere that the neutral helium lines are emitted in the metal-hydrogen emitting region where the kinetic temperature is low enough, 6000 8000 K, so that only the ionization due to UV radiation from the corona can explain the intensity of neutral helium emission. Also we find that the intensity ratio of Hei 3888.65 to H₈ 3889.05 increases towards the upper boundaries of prominences and of the chromosphere and that it approaches to a universal limiting value, both in various prominences or in the chromosphere, where it is considered that the ionization of neutral helium and hydrogen is nearly complete. From these facts the helium to hydrogen number ratio is found to be 6.5 ± 1.5%.A new schematic model of the chromosphere is presented where spicules have no hot region of emitting neutral helium lines. Here it is suggested that the kinetic temperature of spicules, 6000 8000 K, would be primarily determined by the radiation temperature of the corona and the transition region beyond the Lyman continuum of hydrogen which happens to be around those temperatures.     

Interpretation of XUV spectroheliograms

Some parameters of chromospheric structure are drawn from recently published XUV spectroheliograms. The HeII emission above the limb arises from the small amount of He⁺ still existing at 10⁶°. The larger amounts of He⁺ in the cooler corona at the poles explain the polar cap absorption in 304. The flat distribution of emission in Oiv and Ov, with a sharp spike at the limb, is caused by the rough structure of the chromosphere and the variable excitation in the emitting spicules. The intensity of the Nevii lines shows that the transition zone between chromosphere and corona is very sharp.This research was supported by the National Aeronautics and Space Administration under Grant NASA NGR 05 002 034.     

Ionized helium in prominences and in the chromosphere

Quiescent prominences It is found that Heii 4686 is emitted in the same cold region of 10000 K as hydrogen, metal and neutral helium emission lines. This conclusion is based on the finding that the observed width of 4686 is the same as the calculated width of 4686. The calculated width is derived from the observed widths of hydrogen and metallic lines. The large intensity of Heii 4686 in 10000 K can be explained by the ionization of Heii due to the UV radiation below 228 Å that comes from the corona and the transition region.Loop prominences The very broad width (30 to 50 km s^–1) of 4686 for two post-flare loop prominences shows that the Heii line is emitted in hot regions different from regions of hydrogen and metal emission. From the widths of the Balmer lines and many metallic lines the kinetic temperature for one loop is found to be 16000 K in one part and 7600 K in another part. The electron densities are 10^12.0 cm^–3 and less than 10^11.0 cm^–3 respectively.Chromosphere The intensity of 4686 in the chromosphere can be interpreted in terms of a temperature of 10000 K with the ionization due to UV radiation. But, since observations of the width of 4686 are not available, a definitive conclusion for the chromosphere cannot be reached.     

The fine structure of the solar atmosphere in the far ultraviolet

High resolution spectroheliograms in the ultraviolet emission lines He i, He ii, O iv, O v, and Ne vii have been photographed during a sounding rocket flight. Simultaneously, broad band filtergrams of the far ultraviolet solar corona were obtained from the same flight. This paper describes qualitatively the spatial distribution of the UV emission. A comparison with an H filtergram is made. The most significant results can be summarized as follows: We find most of the ultraviolet emission concentrated around spicules, with different degree of concentration, decreasing with higher temperatures. 4 different areas of ultraviolet emission can be distinguished. (1) The normal network, bright in all UV emission lines from the chromosphere into the corona. (2) The coronal holes, bright in all UV emission lines up to 600 000 K but depressed in coronal lines from 1 million degrees upward. (3) The coronal depressions near active centers, absence of all ultraviolet emissions and (4) Active regions, where ultraviolet emission comes from plages, sunspots and coronal loops. High non-thermal Doppler velocities can be found in certain plage kernels around 10⁵ to 2 × 10⁵ K. Sunspots are bright in the ultraviolet, but do not exhibit He i or He ii emission. The corona above sunspots is weak. Sunspots do not show high non-thermal Doppler velocities. The He i and He ii emission does not follow either chromospheric, transition zone or coronal pattern; one can recognize some typical behavior of each.     

The He+ λ 4686 line in the low chromosphere

We report an unsuccessful search for the He⁺ 4686 line in the low chromosphere. However, at the location of this line we detect a number of other chromospheric emission lines. This leads us to the conclusion that the He⁺ 4686 identification made in the past, as well as other identifications, are probably in error. Additionally the region of the neutral helium 4713 line is also studied.On leave from Tokyo Astronomical Observatory, Mitaka, Japan.     

Wavelength-dependence of EUV continuum absorption

The weakening of EUV line emission due to continuum absorptions of neutral hydrogen and neutral helium is investigated to examine its wavelength-dependence. After convolving the intensities predicted from multilevel calculations over an instrumental profile, we found a systematic, linear weakening for lines shortward of 912 Å, which can be attributed to Lyman continuum absorption in the cool chromospheric cloud. The degree of the weakening at the quiet Sun seems to be constant in the temperature range of 4.3 < log T < 5.4. We also find that the lines shortward of 504 Å are somewhat weakened by He i continuum absorption. From the comparison of both weakenings the temperature of the absorber is estimated to be rather low (T _e 7 × 10³ K).     

Chromospheres of flare stars

The present paper contains an attempt to formulate a theory, based on fast electrons hypothesis, of the chromospheres of flare stars. At the same time we shall undertake a survey of observations on the emission lines of flare stars and comparisons with the theory. The general discussion in the introduction concerning the civilian right of fast electron hypothesis is followed by sections in which the following problems are tackled: the anatomy of the light curve of the flare in UV Cet-type stars or its division into two components; the sources of radiation, ionizing hydrogen and other elements, and the estimation of their power in cold dwarf stars with emission lines; conditions for the excitation of emission lines in the chromospheres of those stars; the problem of duration of luminescence of flare stars in the emission lines (observations and theory); the electron temperature and electron concentration in the chromospheres of flare stars; the problem of the luminescence of emission lines in the quiescent star; the degree of ionization and the role of inelastic collisions of the electrons in the chromosphere of flare stars; the profiles of emission lines, their broadening and intensification during the flare; the dependence of the infensity of emission lines on the flare amplitude; the nature and peculiarities of two types of the Haro flares; the impact of radiation dilution and the spectral class of the star on the equivalent width of the emission lines; the possibility of exciting the forbidden lines; the problem of generation of the emission lines of neutral and ionized helium; the possibility of the Lyman-alpha emission in flare stars, the expected parameters of such emission — the radiation power, the equivalent width and profile of the Lyman-alpha line; the possibility of the presence of the strongest (after the -line) emission line — of doublet 2800 Mgii in the spectra of flare stars, the expected value of the intensity and equivalent width of that line.     

Some newly discovered coronal emission lines from high altitude infrared observations of the 7 March, 1970, solar eclipse

A survey of the infrared coronal spectrum between 1 and 3 was made from a high altitude aircraft during the 7 March, 1970, solar eclipse. The observations were made with a Fourier transform spectrometer and were confined to the outer chromosphere and inner corona. In addition to well known chromospheric lines of Hi and Hei, nine additional lines were seen. Evidence is presented for the tentative assignment of these lines to forbidden transitions in highly ionized atoms of magnesium, aluminum, silicon, sulphur, and chromium.     

A phenomenological model for disparitions brusques followed by flarelike chromospheric brightenings

Here I present a simple gravitational model for the flarelike brightenings of the chromosphere that follow most disparitions brusques (disappearing filaments). I assume the ascending prominence material is lifted out of the initially stable magnetic dips that characterize quiescent prominences and falls along the arched field lines into the chromosphere where the kinetic energy of fall is dissipated in the bright areas. The examination of prominence and chromospheric characteristics leads naturally to many predictions and relations during and after prominence eruptions. In general the predictions are specific, but the observations of necessary detail and quality are nonexistent; however, the predictions appear to agree with the data that are available. The model appears to explain all non-active-region brightenings of the chromosphere that follow disparitions brusques and an unknown fraction of active-region flares. The conclusion is that two-ribbon flares are due to the disparitions brusques chromospheric flarelike brightening mechanism. In this paper it will become clear that many specific observations in and out of active regions will be necessary to test the predictions of the model given here.     

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.     

Physical properties of solar chromospheric plages

We propose chromospheric models of plages to explain profiles of the Ca ii H, K, λl8498, λ8542, and λ8662 lines described in Paper I. These models are consistent with boundary conditions imposed by the photosphere and the Lyman continuum. We find that increasing emission in these lines is consistent with a picture of increasing temperature gradient in the low chromosphere and the resulting increase in pressure and electron density at similar line optical depths. With this picture we suggest how to empirically determine the distribution of chromospheric parameters across the solar disk directly from Ca ii filtergrams. We also propose that the high density aspects of solar activity are produced by steep temperature gradients in the low chromosphere and thus by the enhanced heating mechanisms that steepen these gradients.     

Solar oscillations in strong and weak fraunhofer lines over a quiet region

We have analysed a 35-min-long time sequence of spectra in the Caii H line, Nai D1 and D2 lines, and in a large number of strong and weak Fei lines taken over a quiet region at the center of the solar disk. The time series of these spectra have been observed simultaneously in these lines under high spatial, spectral, and temporal resolution at the Vacuum Tower Telescope (VTT) of the Sacramento Peak Observatory. We have derived the line profiles and their central intensity values at the sites of the chromospheric bright points, which are visible in the H line for easy identification. We have done a power spectrum analysis for all the lines, using their central intensity values to determine the period of oscillations. It is shown that the 3 Fei lines, present 23 Å away from the core of the H line representing the pure photospheric lines, Nai D1 and D2 lines, 6 Fei lines at the wings of H line, and Can H line exhibit 5-min, 4.05-min, 3.96-min, and 3.2-min periodicity in their intensity oscillations, respectively. Since all these lines form at different heights in the solar atmosphere from low photosphere to middle chromosphere and show different periodicities in their intensity oscillations, these studies may give an idea about the spatial and temporal relation between the photospheric and chromospheric intensities. Therefore these studies will help to better understand the physical mechanisms of solar oscillations. It is clearly seen that the period of intensity oscillations decreases outward from the low photosphere to the middle chromosphere. Since we have studied a single feature at a time on the Sun (i.e., bright points seen in the H line) in all these spectral lines simultaneously, this may explain about the footpoints of the bright points, the origin of 3-min oscillations, and the relation to other oscillations pertaining to these locations on the Sun. We have concluded that 80% of the bright points are associated with dark elements in the true continuum, and they may seem to have a relationship with the dark intergranular lanes of the photosphere, after carefully examining the brightness (bright threads) extending from the core to the far wings of the H line at the locations of a large number of bright points, using their time sequence of spectra.NRC Resident Research Associate, on leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Emission curve of growth for Feii in chromospheric limb spectra

The 270 chromospheric emission lines of Feii ranging between 2000 and 3200 Å observed by Skylab at a height of 4 (2900 km) above the limb of the quiet Sun are analyzed by the emission curve of growth method, using newly calculated gf-values. It is derived that the excitation temperature is 7.2 × 10³ K and that the turbulent velocity is consistent with the previous results that the microturbulent velocity is lower than 10 km s^–1 in the cool (<10⁴ K) region of the chromosphere.Contributions from the Kwasan and Hida Observatories, University of Kyoto, No. 270.     

A phenomenological interpretation of stellar chromospheres

An attempt is made to develop a phenomenological interpretation of stellar chromospheres. The following problems are examined: observed emission powers of magnesium chromospheres on stars based on the ultraviolet doublet, 2800 Mgii, observations; dependence of chromosphere emission on spectral and luminosity classes; stellar chromospheres as an accidental event; chromospheres of stars-components of binary systems; stars with the chromospheres of solar type (S) and nonsolar (NS) type; distribution of stars by means of the type of their chromosphere on luminosity class; stars with superpower magnesium emission; emission measures for both the magnesium and calcium chromospheres; interrelation between chromosphere, transition zone and corona; chromospheric activity and rotation of stars; possibility of the existence of chromospheres on hot stars; phenomenological picture of stellar chromospheres; stars without the line 2800 Mgii, in emission or in absorption; syndrome of red giant HD 4174. At the end, the problem of heating of stellar chromospheres is discussed.     

Solar chromosphere in the D<Subscript>3</Subscript> helium line from spectra of the eclipse of March 29, 2006

We present the processing results of slitless chromospheric spectra in a D₃ helium line obtained during the solar eclipse of March 29, 2006. A “Cascade-Photometrics 650” CCD matrix photodetector with a rate of 16 frames/s was used for the observations. Altitude distribution of absolute brightness of the helium chromosphere has been mapped in a latitude interval 30 N-10 S on the western limb. A correlation between absolute brightness of the helium chromosphere and a solar cycle phase has been confirmed. It has been revealed that the altitude distribution of helium emission has one maximum at an altitude of approximately 2000 km. In the lower chromosphere, helium emission is undetectable. This confirms our assumption about absence of an additional maximum at 300 km in the D₃ line brightness distribution in the chromosphere during sunspot maximum periods. The results also prove that X-ray coronal radiation with λ < 6 nm is the source of helium excitation in the lower chromosphere.     

Configuration of the chromospheric magnetic field in a unipolar sunspot region

A set of H chromospheric magnetograms at various wavelengths near the line center, chromospheric Dopplergrams, and photospheric vector magnetograms of a unipolar sunspot region near the solar limb were obtained with the vector video magnetograph at the Huairou Solar Observing Station. The superpenumbral chromospheric magnetic field is almost parallel to the surface at the outside of the sunspot penumbra, where the magnetic lines of force are mainly concentrated in the superpenumbral filaments. In the gaps between the filaments the chromospheric horizontal field is weak.     

On physical properties of solar spicules

New observations of solar spicules at Pulkovo, made simultaneously in 4 spectral regions, are described. The profiles of the H, D3, H, H and K lines were derived for 23 spicules.The spicules occur usually in narrow bundles and can be resolved into separate objects only due to a dispersion of radial velocities inside the bundle. The spicules in D3 are diffuse with faint interspicular emission.The emission of hydrogen, helium and Caii originate in different parts of spicules with different radial and turbulent velocities. The core of a spicule with Caii emission is a narrow rope several tens of km in diameter for the normal section. The optical thickness of a spicule in H is approximately 1.0 and the atomic density varies from 10¹¹ to 10¹² cm^–3.The helium emission in spicules seems to be produced by different mechanisms, one of them being the external radiation of corona and subcoronal matter.The model of a solar spicule is discussed.     

The spectra of near-vertical structures on the solar disk

Bright emission arches in the spectra of H and the Ca II (H and K lines) are identified as the spectroscopic picture of the chromospheric network as it appears near the solar limb. Analysis of the geometrical properties of these spectroscopic arches indicates that the average network is a diverging sheet with a divergence angle of 50°. This sheet extends to 2600 km and 2000 km as an opaque emission feature in H and the Ca II (H and K) lines, respectively.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Spectral investigation of the chromosphere

Highly resolved H spectra and filtergrams obtained at the Fraunhofer Observatory on Capri were analysed by a method whose principles have been described before. As a result the tentative conclusion of our previous work has been confirmed: The mottles of the chromospheric fine structure are clouds superimposed on the low chromosphere. Furthermore, it is proposed that the latter is identical with the interior of the supergranular cells whose spatial averages lend themselves to an interpretation in terms of a spherically symmetric model. On the other hand, we present evidence that the boundary regions which are formed by the mottles of the fine structure pattern and which constitute the upper chromosphere are not adequately described by spatial averages. Instead, the properties of the individual structural elements should be taken into account by an appropriate theory, if only in a statistical fashion. Some of these properties, viz. source function, optical thickness, Doppler width and velocity, were measured for a large number of elements. On the basis of indirect evidence we suggest that the spicules seen on limb photographs of chromospheric lines are identical with the elements of the supergranule boundary structure.Mitteilung aus dem Fraunhofer Institut Nr. 116.     

Physical properties of solar chromospheric plages

We compute a new grid of plage models to determine the difference in temperature versus mass column density structure T(m) between plage regions and the quiet solar chromosphere, and to test whether the solar chromosphere is geometrically thinner in plages. We compare partial redistribution calculations of Mg ii h and k and Ca ii K to NRL Skylab observations of Mg ii h and k in six active regions and Ca ii K intensities obtained from spectroheliograms taken at approximately the same time as the Mg ii observations. We find that the plage observations are better matched by models with linear (in log m) temperature distributions and larger values of m ₀ (the mass column density at the 8000 K layer in the chromosphere), than by models with larger low chromosphere temperature gradients but values of m ₀ similar to the quiet Sun. Our derived temperature structures are in agreement with the grid originally proposed by Shine and Linsky, but our analysis is in contrast to the study by Kelch which implies that stellar chromospheric geometrical thickness is not affected by chromospheric activity. We conclude that either the stellar Mg ii observations upon which the Kelch study was based are of poorer quality than had been assumed, or that the spatial averaging of inhomogeneous structures, which is inherent in the stellar data, does not lead to a best fit one-component model similar in detail to that of a stellar or a solar plage.Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Staff member, Quantum Physics Division, National Bureau of Standards.