Magnetic fields in quiescent prominences

We discuss the longitudinal component of the magnetic field, B , based on data from about 135 quiescent prominences observed at Climax during the period 1968–1969. The measurements are obtained with the magnetograph which records the Zeeman effect on hydrogen, helium and metal lines. Use of the following lines, H; Hei, D₃, Hei, 4471 Å; Nai, Di and D₂, leads to the same value for the observed magnetic field component in these prominences. For more than half of the prominences their mean field, B , satisfy the inequalities 3 G B 8 G, and the overall mean value for all the prominences is 7.3 G. As a rule, the magnetic field enters the prominence on one side and exits on the other, but in traversing the prominence material, the field tends to run along the long axis of the prominence.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Spectral Characteristics of the He <Emphasis Type="SmallCaps">i</Emphasis> D<Subscript>3</Subscript> Line in a Quiescent Prominence Observed by THEMIS

We analyze the observations of a quiescent prominence acquired by the Téléscope Heliographique pour l’Étude du Magnetisme et des Instabilités Solaires (THEMIS) in the He?i 5876 Å (He?i D₃) multiplet aiming to measure the spectral characteristics of the He?i D₃ profiles and to find for them an adequate fitting model. The component characteristics of the He?i D₃ Stokes I profiles are measured by the fitting system by approximating them with a double Gaussian. This model yields an He?i D₃ component peak intensity ratio of \(5.5\pm0.4\), which differs from the value of 8 expected in the optically thin limit. Most of the measured Doppler velocities lie in the interval ±?5 km?s^?1, with a standard deviation of ±?1.7 km?s^?1 around the peak value of 0.4 km?s^?1. The wide distribution of the full-width at half maximum has two maxima at 0.25 Å and 0.30 Å for the He?i D₃ blue component and two maxima at 0.22 Å and 0.31 Å for the red component. The width ratio of the components is \(1.04\pm0.18\). We show that the double-Gaussian model systematically underestimates the blue wing intensities. To solve this problem, we invoke a two-temperature multi-Gaussian model, consisting of two double-Gaussians, which provides a better representation of He?i D₃ that is free of the wing intensity deficit. This model suggests temperatures of 11.5 kK and 91 kK, respectively, for the cool and the hot component of the target prominence. The cool and hot components of a typical He?i D₃ profile have component peak intensity ratios of 6.6 and 8, implying a prominence geometrical width of 17 Mm and an optical thickness of 0.3 for the cool component, while the optical thickness of the hot component is negligible. These prominence parameters seem to be realistic, suggesting the physical adequacy of the multi-Gaussian model with important implications for interpreting He?i D₃ spectropolarimetry by current inversion codes.     

Brightness fluctuations in the K-line wings

A power-spectrum and cross-spectrum analysis has been made of measurements of temporal fluctuations of intensity observed in the K-line wing (2.07 Å from line center) and of simultaneous measurements of temporal fluctuations of Doppler displacement of the cores of 3931.122 Fe i and 3933 Ca ii (K₃). The measurements were made in a quiet region near the center of the Sun's disk. We find that the average power spectra of the intensity fluctuations have two significant peaks of about equal strength: one at 0.0033 Hz (300-s period); and one at about 0.001 Hz (1000-s period). The average rms value of these intensity fluctuations is 0.0435±0.0082. Maximum brightness comes before maximum violet displacement of the Fe i line. The mean of the best determined phases is 137° and of all the data 108°. At those places on the Sun where the 300-s oscillations can be identified in the k₃ core, the Doppler displacement of the Fe i line leads that of the K₃ core by a mean phase angle of 27°.     

Helium 10830 Å measurements of the Sun

Measurements of the Sun in the near-infrared He i 10830 Å absorption line were performed using the echelle spectrograph with a dispersion of 6.71 mÅ per pixel at the Vacuum Tower Telescope (German Solar Telescopes, Teide Observatory, Izaña, Tenerife, Spain) on May 26, 1993. These measurements were compared with full-disc soft X-ray images of the Sun (Japanese solar satellite Yohkoh), full-disc solar images in H (Big Bear Solar Observatory), full-disc solar images in the He i 10830 Å line (National Solar Observatory, Kitt Peak) and with full-disc microwave solar maps at 37 GHz (Metsähovi Radio Research Station). In the He 10830 Å line the Sun displays a limb darkening similar to that in the visible part of the spectrum. Active regions and H filaments show a strong absorption in the He 10830 Å line, whereas the absorption is weak in coronal holes.     

A peculiar sodium ‘flare’ on the sun

A peculiar brightening was observed near the centre of the D₂ line of Nai on July 28, 1966, in a position where no activity could be seen in the H line but where a new important active region formed during the next rotation of the sun. The brightening, first observed at 06:30 UT, progressively faded and disappeared before 07:10 UT.     

Visibility of Prominences Using the He <Emphasis Type="SmallCaps">i</Emphasis> D<Subscript>3</Subscript> Line Filter on the PROBA-3/ASPIICS Coronagraph

We determine the optimal width and shape of the narrow-band filter centered on the He?i D₃ line for prominence and coronal mass ejection (CME) observations with the ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) coronagraph onboard the PROBA-3 (Project for On-board Autonomy) satellite, to be launched in 2020. We analyze He?i D₃ line intensities for three representative non-local thermal equilibrium prominence models at temperatures 8, 30, and 100 kK computed with a radiative transfer code and the prominence visible-light (VL) emission due to Thomson scattering on the prominence electrons. We compute various useful relations at prominence line-of-sight velocities of 0, 100, and 300 km?s^?1 for 20 Å wide flat filter and three Gaussian filters with a full-width at half-maximum (FWHM) equal to 5, 10, and 20 Å to show the relative brightness contribution of the He?i D₃ line and the prominence VL to the visibility in a given narrow-band filter. We also discuss possible signal contamination by Na?i D₁ and D₂ lines, which otherwise may be useful to detect comets. Our results mainly show that i) an optimal narrow-band filter should be flat or somewhere between flat and Gaussian with an FWHM of 20 Å in order to detect fast-moving prominence structures, ii) the maximum emission in the He?i D₃ line is at 30 kK and the minimal at 100 kK, and iii) the ratio of emission in the He?i D₃ line to the VL emission can provide a useful diagnostic for the temperature of prominence structures. This ratio is up to 10 for hot prominence structures, up to 100 for cool structures, and up to 1000 for warm structures.     

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.     

Forbidden sulphur I lines in the solar spectrum

The [SI] lines are due to transitions within the 3s²3p⁴ ground configuration of neutral sulphur. The results are presented from a search for the [Si] lines in the Fraunhofer spectrum. Two identifications are proposed with faint features in the Fraunhofer spectrum: ¹D₂-¹S₀ at = 7725.02 Å and ³P₂-¹D₂ at = 10821.23 Å. Their measured equivalent widths are shown to confirm the value for the solar abundance of sulphur, logN _S = 7.21 (in the scale logN _H = 12.00), which is derived from the permitted high-excitation lines. These lines give the first convincing identification of [Si] lines in an astrophysical source.     

The Hei D 3 5876 Å line as an activity and structural indicator in F-type Hyades stars

We give the first results of a study in F-type MS stars using the Hei D ₃ absorption feature at 5876 Å as an indicator of stellar activity and structure. The observations were obtained with the QUB echelle spectrograph at the Cassegrain focus of the 4.2 m William Herschel telescope, and have spectral resolution /6×10⁴, and signal-to-noise ratios 200. We chose as a first sample 17 F-stars of the Hyades Main Sequence. It was found that Hei D ₃ and X-ray emission appear aroundB-V 0.3, indicating the onset of sub-surface convection zones. Below this value, in stars withB-V0.42–0.44 the behaviour of the Hei D ₃ equivalent width against Rossby number is different from that observed in later type stars, indicating that the presence of nonradiative mechanisms other than dynamo must be important in heating the chromospheres of these hotter stars.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.Based on observations made with the William Herschel telescope, operated on the island of La Palma by the Royal Greenwich Observatory, in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofisica de Canarias.     

Comment on the X-ray event of July 7, 1966

According to Snijders (1968) the decay profile of an X-ray burst determines the effective temperature describing the distribution of fast electrons in the emitting source. In this paper it is concluded that the observations of the hard X-ray burst of 7 July, 1966; 0038 UT are not in disagreement with the concept of thermal bremsstrahlung from electrons with a Maxwellian distribution of about 10⁸ K. Some physical parameters of the source are determined. The magnetic field strength is found to be about 1200 gauss. The initial temperature kT ₀ is approximately 40 keV.     

A proposed correction to the solar abundances of carbon and oxygen utilizing new and accurate theoretical forbidden transition probabilities

Previously published solar abundances of oxygen and carbon can be corrected to be logN(O) = 8.93 and logN(C) = 8.60 on the hydrogen log-scale when new accurate forbidden electric quadrupole transition probabilities A _Q(s^–1) are used. Such A _Q's, based on the new atomic structure and electron correlation theory, developed recently by Sinanolu and coworkers, are reported for the (¹ S ₀-¹ D ₂) lines of [C i], [N ii, [O i] and [O iii] and the (² P-² D) lines of [N i] and [O ii]. The available experimental values are also given for comparison.Work supported by Grant No. GP-29471 from the U.S. National Science Foundation.     

Height of helium emission in the chromosphere

Filtergrams of the limb show the He i D₃ chromosphere as a shell which is separated from the limb by a gap. The height of maximum D₃ contribution occurs at about 1350 km above the limb and is independent of the intensity of the D₃ emission. We interpret this effect as the height to which coronal EUV radiation is capable of penetrating the atmosphere.     

Activity in the quiet Sun

Macrospicules have been observed in H and He i D₃, on the disk and above the limb. In 1975, a rate of 1400 (A day)^–1 is inferred, and the ratio of equatorial to polar rates 2. D₃ intensities are a few × 10^–3 of the disk center, and do not decrease in coronal holes. The ratio of H to D₃ intensities is 10. The integral number of macrospicules with D₃ intensity I ₀ is proportional to I ₀ ^–1.     

LU Vel (GJ 375): A M3.5Ve Flare and Double-Lined Spectroscopic Binary

High resolution echelle spectroscopic observations taken with the FEROS spectrograph at the 2.2 m telescope ESO confirm the binary nature of the flare M3.5V star LU Vel (GJ 375, RE J0958-462) previously reported by Christian and Mathioudakis (2002). Emission of similar intensity from both components is detected in the Balmer, Na i D₁&D₂, He i D₃, Ca ii H&K, and Ca ii IRT lines. We have determined precise radial velocities by cross correlation with radial velocity standard stars, which have allowed us to obtain for the first time the orbital solution of the system. The binary consists of two near-equal M3.5V components with an orbital period shorter than 2 days. We have analyzed the behaviour of the chromospheric activity indicators (variability and possible flares). In addition, we have determined its rotational velocity and kinematics.     

Estimates of the physical parameters of supernova remnants (SNR)

The various physical parameters of a SNR can easily be worked out from graphs established on the basis of the recalibrated relation between the radio surface brightness , and the linear diameterD of a SNR (Ilovaisky and Lequeux, 1972). These graphs lead to the estimation of the distancer (kpc), linear diameterD (pc), monochromatic power at 1 GHz,P ₁ GHz (W Hz^–1); and total powerP _tot (a) (erg. s^–1) of a SNR, given its mean angular diameter <> (arc min), flux density at 1 GHz, S₁ GHz (f.u.) and spectral indexa. Three SNR (W28A₁, Monoceros SNR, W49B) are used to illustrate the case. The radio spectrum of one of these (W49B), curved at low frequencies, is explained in terms of absorption by the diffuse interstellar medium. Various cases are discussed and some physical parameters of the absorbing matter are established.     

Late-time emission of the type II Supernovae powered by the radioactive decay56Ni−56Co−56Fe

A one-zone model for the late time SN II energized by the radioactive decay⁵⁶Ni–⁵⁶Co–⁵⁶Fe is presented. The model succeeds in reproducing for the late time evolution of H and [Oi] 6300 emission lines in SN1970g for the reasonable set of parameters: mass of ejecta 4M , boundary velocityv ₀=4000 km s^–1 and amount of⁵⁶NiM _Ni=0.02M . However, a one-zone model does not account for the late time continuum. In the case of SN1980k the radioactive model fits H and [Oi] 6300 emissions att250 day satisfactory but fails at very late time, e.g.,t=670 day when the predicted value of the ratioL(H)/L(6300) is two orders of magnitude smaller than the observed one. We suggest that the strong H emission in SN1980k on the 670th day is due to the interaction of the supernova envelope with the pre-SN wind. The radioactive model for the late time SN II predicts strong Mgii 2800 line and detectable Hei 10830 line in emission and absorption.     

High resolution spectroscopy of solar activity

We describe an observing program designed to obtain high spatial resolution photographic spectra of solar active region phenomena, with time resolution as short as 6 s. The Vacuum Tower Telescope and Echelle Spectrograph at Sacramento Peak Observatory are used to make observations simultaneously in H, He D₃, Ca ii K, Mg b₁, the CN bandhead at 3883, and the magnetically-sensitive line Fe i 6302. Images reflected from the slit jaw are exposed simultaneously in white-light and H. Observations of chromospheric heating, following a high-velocity infall along an H superpenumbral filament, are presented to illustrate the capabilities of the program.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the National Science Foundation.     

Line width observation of Heii 4686 Å and Hei 4713 Å in the chromosphere

Line profiles of He ii 4686 Å and He i 4713 Å from active regions in the chromosphere were observed during the total solar eclipse of February 16, 1980, with a grazing incidence objective grating spectrograph. The Doppler width of the He i triplet line of 4713 Å increases with height and the average width is compatible with width of metallic and hydrogen lines, suggesting that the kinetic temperature of He i triplet emitting region is T 8000 K. This can only be explained by recombination after photo-ionization due to coronal UV radiation. The Doppler width of the Paschen line of He ii 4686 is, without any correction for the separation of subcomponents of the line nor non-thermal velocity, 18.4 km s^-1. This line width also shows a tendency to increase with height. After comparison with Doppler widths of He i 4713 and the EUV lines, and a necessary subtraction of non-thermal velocity, it is shown that this line is emitted in a 2 × 10⁴ K temperature region, which again supports the view that this line is emitted through the recombination process after photoionization due to coronal XUV radiation below 228 Å.     

The supergranule velocity field

A study of supergranule motions confirms horizontal velocities with peak values of typically 0.36 km s^–1 as observed in Fe i 8688 Å. These show no significant variation with height over the range of formation of C i 9111, Fe i 8688, and Mg i 8806, but there is a substantial reduction to about one-half of this at the level of Ca ii 8542.Near disk center, supergranule vertical velocities in Fe i 8688 have rms values ±0.01 km ^–1, after allowance for the residual effects of the line-of-sight component of the horizontal supergranule motions, the five-minute oscillations, granule motions, and detector drift. There is a marginally-significant association of magnetic elements, and hence of cell boundaries, with downward motions; but this requires further testing.Measurements of downward velocities 0.1 km^–1 in regions of strong magnetic field when using unpolarized light are attributed to the much higher downflow inside the elements themselves and have nothing to do with supergranule motions.Visiting Astronomer, Kitt Peak National Observatory.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

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.