A comparison of He ii 304 Å and He i 10 830 Å spectroheliograms

Spectroheliograms were obtained simultaneously in the He ii 304 Å emission line and the He i 10 830 Å absorption line with an angular resolution of approximately 5″. A negative print of the 304 Å image is matched with a positive print of the 10 830 Å image so that corresponding features of the chromospheric network (including active regions) appear identical in the two images. Differences between these images include the facts that:

1. Disk filaments and limb darkening are strongly visible in the 10 830 Å positive image, but they are weakly visible (as lightenings) in the 304 Å negative image.
2. The contrast between the chromospheric network and the network cell centers is much greater in the 10 830 Å image than in the 304 Å negative image.

These results provide constraints on models of helium line formation in various types of solar features.     

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 solar radiation between 3300 and 12500 Å

Results are presented, which follow from the merging of: (a) our previously published absolute integrals of the disk-center intensity for 20 Å wide spectral bands; (b) the ratios of mean to central intensity derived from recent observations of the center-to-limb variation of those bands ( <6600 Å); (c) the ratios of mean to central intensity derived from the observations of the center-to-limb variation at continuum-wavelengths according to Pierce and Slaughter ( >6600 Å); (d) the high resolution Fourier transform spectra obtained by J. Brault at Kitt Peak for the disk-center and the irradiance; (e) some further auxiliary data, which served mainly to eliminate the local perturbations caused by lines of telluric molecular bands.The main result is the presentation of high precision radiation data for both the integrated disk and the disk-center, concerning the line-averaged radiation and the continuum (in UV: highest window-intensities) as well.The internal accuracy (the scatter) should not be worse than that of the FTS spectra, which is less than 0.2% (mean error); local systematic deviations exceeding 0.5% are not to be expected. The absence of a significant systematic error - neutral or wavelength-dependent - has been proven already elsewhere.     

The 2200 Å extinction hump and porous graphite grains

We discuss in this paper the possibility of interpreting the 2200 Å band occurring in the interstellar extinction curves as being attributed to porous graphite. The results show that grains with radii smaller than 0.015 m and a porosity degree within the values 0.02f0.25 are able to fit satisfactorily the peak at 4.6 m^–1 and the band shape between 4 and 5.2 m^–1. Consideration of the expected density for such particles seems to confirm that interstellar grains may be porous but, at the same time, suggests that care must be taken when data concerning dust in the solar system are extrapolated to the interstellar space.     

The geometric albedos of Uranus and Neptune between 2100 and 3350 Å

Absolutely calibrated spectra of Uranus, Neptune, and the solar analog stars 16 Cyg A and B between 2100 and 3350 Å are reported. The geometric albedos of both planets are close to the curve expected for a semi-infinite Rayleigh-Raman scattering atmosphere between 2100 and 2800 Å. Longward of 2800 Å the albedos fall below the Rayleigh-Raman values and connect smoothly to the ground-based photometry of J.S. Neff, D.C. Humm, J.T. Bergstralh, A.L. Cochran, W.D. Cochran, E.S. Barker, and R.G. Tull (1984, Icarus60, 221–235).Neptune is about 5.5% brighter than Uranus and shows slightly stronger Raman scattering signatures in the MgII lines at 2800 Å in accordance with the results of Neff et al. for the CaII H and K lines. This means that the stratospheric haze on Neptune is thinner than on Uranus. The fact that the Neptunian geometric albedo between 2100 and 2800 Å is so close to the ideal semi-infinite Rayleigh-Raman scattering atmosphere could be exploited for future absolute calibrations of other Solar System objects in this wavelength region.     

Periodic variations of the Ba II 4554 Å and Ca II 8542 Å line profiles in coronal holes

The oscillations of the half-width of the Ba II 4554 ? and Ca II 8542 ? spectral lines have been analyzed using observations at the base of solar coronal holes (CHs). The observed variations (~50 m ? for Ca II and ~4 m ? for Ba II) exceed considerably the thermal broadenings of these lines calculated from the measured intensity oscillations, suggesting their nonthermal nature. We point out a number of observational facts that hamper an unambiguous interpretation of the periodic Ba II and Ca II profile variations solely by the manifestation of torsional Alfve´ n waves in the lower solar atmosphere.     

Graphite grains,carbon depletion and the 2200 Å feature

In a recent paper Millar has shown that if one assumes that the carbon depleted from the gas phase is all tied up into small graphite grains the observational data raise serious doubts against the hypothesis that these particles are responsible for the 2200 Å extinction hump. In the present paper it is shown that this problem may be overcome if the presence in the interstellar space of graphite grains with sizes greater than 0.02 is taken into account. The derived ratios between the masses of large grains, which do not contribute to the ultraviolet extinction hump, and those of the small ones varies from region to region of the sky and are consistent with those evaluated in the circumstellar shells of carbon stars. Moreover, the largest sizes of the graphite particles we find are in agreement with those needed to fit the interstellar extinction curve over the wavelength range 0.11–<1 .     

Spectral Line Selection for HMI: A Comparison of Fe I 6173 Å and Ni I 6768 Å

We present a study of two spectral lines, Fe I 6173 Å and Ni I 6768 Å, that were candidates to be used in the Helioseismic and Magnetic Imager (HMI) for observing Doppler velocity and the vector magnetic field. The line profiles were studied using the Mt. Wilson Observatory, the Advanced Stokes Polarimeter and the Kitt Peak-McMath Pierce telescope and one-meter Fourier transform spectrometer atlas. Both Fe I and Ni I profiles have clean continua and no blends that threaten instrument performance. The Fe I line is 2% deeper, 15% narrower, and has a 6% smaller equivalent width than the Ni I line. The potential of each spectral line to recover pre-assigned solar conditions is tested using a least-squares minimization technique to fit Milne-Eddington models to tens of thousands of line profiles that have been sampled at five spectral positions across the line. Overall, the Fe I line has a better performance than the Ni I line for vector-magnetic-field retrieval. Specifically, the Fe I line is able to determine field strength, longitudinal and transverse flux four times more accurately than the Ni I line in active regions. Inclination and azimuthal angles can be recovered to ≈2^° above 600 Mx cm^?2 for Fe I and above 1000 Mx cm^?2 for Ni I. Therefore, the Fe I line better determines the magnetic-field orientation in plage, whereas both lines provide good orientation determination in penumbrae and umbrae. We selected the Fe I spectral line for use in HMI due to its better performance for magnetic diagnostics while not sacrificing velocity information. The one exception to the better performance of the Fe I line arises when high field strengths combine with high velocities to move the spectral line beyond the effective sampling range. The higher g _eff of Fe I means that its useful range of velocity values in regions of strong magnetic field is smaller than Ni I.     

Spectropolarimetry of the methane and ammonia bands of Jupiter between 6800 and 8200 Å

Spectropolarimetry of Jupiter at resolutions between 22 and 35 Å reveals a strong increase of linear polarization in the 7250-$\text{A}\text{?}$ CH₄ band. This is very probably due to the decreasing contribution toward the band center of the higher orders of scattering, which have a smaller net polarization than the first few orders. The linear polarization is also enhanced in the band at 7900 $\text{A}\text{?}$ comprising the 7920-$\text{A}\text{?}$ NH₃ and 7600- to 8200-$\text{A}\text{?}$ CH₄ bands. The normalized circular polarization shows a feature at 7250 $\text{A}\text{?}$ with a dispersion shape. This is most probably produced in a double-scattering process involving either a solid or liquid aerosol with an absorption at 7250 $\text{A}\text{?}$. Methane aerosols, the obvious candidates from a spectroscopic point of view, are, however, forbidden if current estimates of the Jovian atmospheric temperature are correct.     

Comparison of Prominences in Hα and He II 304 Å

In this letter, we bring attention to prominences which show different morphology in H and Heii 304 Å, as observed simultaneously by BBSO and EIT on board SOHO. Those two lines have been thought to represent similar chromospheric structures although they are formed at significantly different temperatures. We give two examples representing two kinds of anomaly: (1) prominences showing strong H emissions in the lower part and strong Heii emissions in the upper part, and (2) erupting prominences showing extensive Heii emission, but nothing in H. Our results indicate that a part or the whole of a prominence may be too hot to emit H radiation, possibly due to heating or thermal instability. Please note that these are not just two isolated cases, many other prominences show the similar differences in H and Heii 304 Å.     

Chromospheric Sunspot Oscillations in Hα and Ca ii 8542 Å

We study chromospheric oscillations including umbral flashes and running penumbral waves in a sunspot of active region NOAA 11242 using scanning spectroscopy in Hα and Ca?ii 8542 Å with the Fast Imaging Solar Spectrograph (FISS) at the 1.6 meter New Solar Telescope at the Big Bear Solar Observatory. A bisector method is applied to spectral observations to construct chromospheric Doppler-velocity maps. Temporal-sequence analysis of these shows enhanced high-frequency oscillations inside the sunspot umbra in both lines. Their peak frequency gradually decreases outward from the umbra. The oscillation power is found to be associated with magnetic-field strength and inclination, with different relationships in different frequency bands.     

The He i 10 830 Å chromosphere and filament associated structures

Full disk, He I 10 830 Å solar spectroheliograms have been generated using the Haleakala Stokes polarimeter-spectrometer. The spectroheliograms, with spatial resolution of 10 × 16 arc sec and wavelength bandpass of 0.53 Å, were developed for the detection of coronal holes, and have been compared with nearly simultaneous H and Ca K flltergrams.Areas of reduced helium absorption have been noted in the neighborhood of filaments and neutral zones in the longitudinal solar magnetic field. The existence of these helium lanes is discussed in terms of their relationship to H filament channels or to the coronal cavities which surround prominences.     

Image tube spectra of pluto and triton from 6800 to 9000 Å

To search for a possible atmosphere on Pluto and Triton, spectra of these objects as well as comparison stars were obtained with a three-stage Varo image tube for the spectral region from 6800 to 9000 Å. Ratio spectra indicate an absorption feature near 8900 Å, although the steeply diminishing response of the image tube at that wavelength casts some doubt on the reality of this feature. The feature appears more definitive in the spectrum of Pluto and less certain in the spectrum of Triton. The absorption was analyzed using our recently determined band-model parameters for methane. Under the assumption of a pressure higher than 0.01 atm an abundance of 3 m-amagat was determined. For pressures limited by the methane abundance itself, an abundance of 50 m-amagat and a pressure of 10^?3 atm was derived (using g = 0.20 g⊕ for both Pluto and Triton). This pressure is close to the pressure that can be expected from the equilibrium vapor pressure of a methane frost. If the absorption at 8900 Å is spurious, our analysis will be applicable as an upper limit for the presence of methane gas on Pluto or Triton.     

Solar line blocking for disk-center and disk-averaged radiation from 3300 to 6860 Å

The line blocking is tabulated for 10 Å ( < 6300 Å) or 20 Å ( > 6300 Å) wide intervals. It follows from the spectral averages and the local continuum derived by Neckel and Labs from high-resolution Fourier transform spectra, which had been obtained by J. Brault at Kitt Peak. The internal accuracy (the scatter) is in the order of 0.1%. Significant systematic errors arising from local distortions of the adopted continuum level can be excluded. Larger errors are to be expected only near the Balmer limit, where the localization of the continuum is very ambiguous.     

Correlations between OH,NaD and OI 5577 Å emissions in the airglow

Simultaneous measurements of the night airglow OI 5577 Å and OH (8,3) band have been carried out at Cachoeira Paulista (23°S, 45°W), Brazil since 1976. Cross correlation analyses between the nocturnal variations of these emissions and also with the OH rotational temperature (ROT) for various time shifts are presented. It is found that OH (8,3) is well correlated with ROT but with a time lag of about 1 h. The variations of OI 5577 Å lead OH (8,3) by about 2–3 h and ROT covaries with 5577 Å with a time lag less than 1 h. For the sake of comparison, OI 5577 Å, OH and NaD data from a number of IQSY stations have been analysed. It is noted that (1) OI 5577 Å leads OH by about 2 h at mid-latitude stations and (2) OH is well correlated with Na 5893 Å with a time lag of less than 1 h. The presence of the phase lagged correlation patterns between OH/5577 Å, OH/ROT and OH/NaD indicates vertical propagation of a wavelike perturbation of the upper atmosphere.     

Transition Probabilities for the 1815 and 3344 Å Forbidden Lines of Ne iii

We have measured the radiative lifetime of the 2s22p4 1S0 metastable level of Ne2+ (Ne iii) to be 223+/-11 ms at the 90% confidence level by observing the photons emitted at 1815 ? by a decaying population of 1S0 Ne2+ ions produced and stored in a radio-frequency ion trap. This is the first lifetime measurement for an excited term of a ground configuration ion in the second row of the periodic table. The transition probabilities (A-values) for the forbidden transitions in the ground configurations of these ions are required for astrophysical line-ratio diagnostics. Using calculated branching ratios, we estimate that A&parl0;lambda1815&parr0;=1.94+/-0.17 and A&parl0;lambda3344&parr0;=2.55+/-0.19 s-1. Because these numbers have a sum with an experimentally determined uncertainty of 5%, they will provide more accurate results than the calculated A-values for determining electron temperature and density from astrophysical Ne iii line ratios.     

Interrelation between monthly mean values ofionospheric and solar parameters and their effect on 5577 Å night airglow emission

The paper presents the relations between different solar and ionospheric parameters. Variation of 5577 line intensity with the variation of solar and ionospheric parametersis also discussed. A study have been made and following important results are obtained:(i) Virtual height ofF layer is decreased exponentially with the increase of solar flare numbers, sunspot number and 10.7 cm solar flux.(ii) Linear relations between critical frequency ofF layer and different solar parameters are obtained.(iii) Empirical relations between ionospheric and solar parameters are established.(iv) It is concluded that airglow intensity will also be affected with the variation of different solar and ionospheric parameters.(v) It is concluded that airglow intensity is mainly affected by 10.7 cm solar flux among different solar parameters and virtual height plays important role than critical frequency ofF layer.     

Prominence Cavity Regions Observed Using SWAP 174 Å Filtergrams and Simultaneous Eclipse Flash Spectra

SWAP images from PROBA2 taken at 174 Å in the Fe ix/x lines are compared with simultaneous slitless flash spectra obtained during the solar total eclipse of 11 July 2010. Myriad faint low-excitation emission lines together with the He i and He ii Paschen α chromospheric lines are recorded on eclipse spectra where regions of limb prominences are obtained with space-borne imagers. We analyzed a deep flash spectrum obtained by summing 80 individual spectra to evaluate the intensity modulations of the continuum. Intensity deficits are observed and measured at the prominences boundaries in both eclipse and SWAP images. The prominence cavities interpreted as a relative depression of plasma density, produced inside the corona surrounding the prominences, and some intense heating occurring in these regions, are discussed. Photometric measurements are shown at different scales and different, spectrally narrow, intervals for both the prominences and the coronal background.     

Magnetic Activity and the Enhanced 5303 Å Emission in Solar Active Regions

Analysis of the intensity maps of Fexiv emission at =5303 Å obtained from the observations of ground-based coronagraphs during the sunspot minimum period 1985 and 1986 shows the persistent presence of localized strong emissions. Typical emission intensities associated with the active regions were found to be about 5 to 20 times stronger than the unperturbed corona. Using Stanford magnetograms we identified strong magnetic field gradients in those active regions, with a minimum threshold gradient of 3 × 10^–5 G km^–1, associated with such strong emissions. From our sample we infer that the flux emergence within or in the vicinity of a persistent active region seems to play an important role in the activity associated with the enhancement in 5303 Å emission.     

Evening and morning twilight enhancement of λ5577 Å atomic oxygen line at Allahabad

A photometer for the observation of twilight airglow emissions has been fabricated. Intensity variation in morning and evening twilight of 5577 Å line of atomic oxygen has been recorded at Allahabad for one year duration from 23rd March, 1987 to 22nd March, 1988. The enhancement in the intensity of the line is found to be irregular. Out of 40 clear evenings and 25 clear mornings from 23rd March, 1987 to 15th June, 1987 enhancement is observed in 24 evenings and 15 mornings. However, no enhancement is observed during the period from 15th September, 1987 to 15th October, 1987. It is found that rise and fall of intensity is exponential during twilight period. Considering different mechanism for the excitation of atomic oxygen green line, it is concluded that none of them are able to explain enhancement. More work is needed to be carried out for the better understanding of the problem.