The Solar Ultraviolet Spectrum Estimated Using the Mg <Emphasis Type="SmallCaps">ii</Emphasis> Index and Ca <Emphasis Type="SmallCaps">ii</Emphasis> K Disk Activity

As part of a program to estimate the solar spectrum back to the early twentieth century, we have generated fits to UV spectral irradiance measurements from 1 – 410 nm. The longer wavelength spectra (150 – 410 nm) were fit as a function of two solar activity proxies, the Mg ii core-to-wing ratio, or Mg ii index, and the total Ca ii K disk activity derived from ground based observations. Irradiance spectra at shorter wavelengths (1 – 150 nm) where used to generate fits to the Mg ii core-to-wing ratio alone. Two sets of spectra were used in these fitting procedures. The fits at longer wavelengths (150 to 410 nm) were derived from the high-resolution spectra taken by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on the Upper Atmospheric Research Satellite (UARS). Spectra measured by the Solar EUV Experiment (SEE) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite were used for the fits at wavelengths from 1 to 150 nm. To generate fits between solar irradiance and solar proxies, this study uses the above irradiance data, the NOAA composite Mg ii index, and daily Ca ii K disk activity determined from images measured by Big Bear Solar Observatory (BBSO). In addition to the fitting coefficients between irradiance and solar proxies, other results from this study include an estimated relationship between the fraction of the disk with enhanced Ca ii K activity and the Mg ii index, an upper bound of the average solar UV spectral irradiance during periods where the solar disk contains only regions of the quiet Sun, as was believed to be present during the Maunder Minimum, as well as results indicating that slightly more than 60% of the total solar irradiance (TSI) variability occurs between 150 and 400 nm.     

Weak Emission Lines in the Wings of Solar H and K

The rare-earth ions cerium ii, lanthanum ii, dysprosium ii, and additionally zirconium ii and iron ii, are seen as weak emission features in the wings of the solar Ca ii H and K lines. The strength of these emission lines increases on the disk toward the limb. We provide recent high-resolution observations at disk center and at the limb. The identity of the weak lines is re-worked. We point out the unique role of eclipse spectra in distinguishing between the photospheric and chromospheric origins of emission lines. It is then demonstrated from our full disk (Sun-as-a-Star) and center disk archives, 1974 – 2010, that no activity cycle related signal is evident (save for the H and K lines themselves).     

The He <Emphasis Type="SmallCaps">ii</Emphasis> Lines in the Lyman Series Profiles of Solar Prominences

The hydrogen and helium lines are the most prominent lines in the solar prominences spectra. Observations with the SUMER spectrometer onboard SOHO showed that there are weak lines in the blue wings of the Lyman series which affect their profiles. They were all identified as He ii lines in the Lyman series wings, except for the Lα line whose profile was affected by the use of an attenuator. The He ii lines are the even Balmer lines of the He ii system, a set of lines that we complete with the odd ones. We characterize them by comparison with the blue wings of the Lyman series in order to improve the H Lyman series observations and modeling, on one hand and to provide He ii lines observations for further combined H – He i – He ii modeling, on the other hand.     

Do polar faculae on the sun predict a sunspot cycle?

The paper reports the results of the analysis of the data on polar faculae for three solar cycles (1960–1986) at the Kislovodsk Station of the Pulkovo Observatory and on polar bright points in Ca ii K line for two solar cycles (1940–1957) at the Kodaikanal Station of the Indian Institute of Astrophysics. We have noticed that the monthly numbers of polar faculae and polar bright points in Ca ii K line and monthly sunspot areas in each hemisphere of the following solar cycle have a correlation with each other. A new cycle of polar faculae and polar bright points in the Ca ii K line begins after the polar magnetic field reversal. We find that the smaller the period between the ending of the polar field reversal and the beginning of a new sunspot cycle is, the more intense is the cycle itself. The intensity of the forthcoming solar cycle (cycle 22) and the periods of strong fluctuations in activity expected in this cycle are also discussed.     

The Mount Wilson Ca <Emphasis Type="SmallCaps">ii</Emphasis> K Plage Index Time Series

It is well established that both total and spectral solar irradiance are modulated by variable magnetic activity on the solar surface. However, there is still disagreement about the contribution of individual solar features for changes in the solar output, in particular over decadal time scales. Ionized Ca ii K line spectroheliograms are one of the major resources for these long-term trend studies, mainly because such measurements have been available now for more than 100 years. In this paper we introduce a new Ca ii K plage and active network index time series derived from the digitization of almost 40 000 photographic solar images that were obtained at the 60-foot solar tower, between 1915 and 1985, as a part of the monitoring program of the Mount Wilson Observatory. We describe here the procedure we applied to calibrate the images and the properties of our new defined index, which is strongly correlated to the average fractional area of the visible solar disk occupied by plages and active network. We show that the long-term variation of this index is in an excellent agreement with the 11-year solar-cycle trend determined from the annual international sunspot numbers series. Our time series agrees also very well with similar indicators derived from a different reduction of the same data base and other Ca ii K spectroheliograms long-term synoptic programs, such as those at Kodaikanal Observatory (India), and at the National Solar Observatory at Sacramento Peak (USA). Finally, we show that using appropriate proxies it is possible to extend this time series up to date, making this data set one of the longest Ca ii K index series currently available.     

A Century of Solar Ca <Emphasis Type="SmallCaps">ii</Emphasis> Measurements and Their Implication for Solar UV Driving of Climate

Spectroheliograms and disk-integrated flux monitoring in the strong resonance line of Ca ii (K line) provide the longest record of chromospheric magnetic plages. We compare recent reductions of the Ca ii K spectroheliograms obtained since 1907 at the Kodaikanal, Mt. Wilson, and US National Solar Observatories. Certain differences between the individual plage indices appear to be caused mainly by differences in the spectral passbands used. Our main finding is that the indices show remarkably consistent behavior on the multidecadal time scales of greatest interest to global warming studies. The reconstruction of solar ultraviolet flux variation from these indices differs significantly from the 20th-century global temperature record. This difference is consistent with other findings that, although solar UV irradiance variation may affect climate through influence on precipitation and storm tracks, its significance in global temperature remains elusive.     

The Solar Spectral Irradiance as a Function of the Mg <Emphasis Type="SmallCaps">ii</Emphasis> Index for Atmosphere and Climate Modelling

We present a new method to reconstruct the solar spectrum irradiance in the Ly α – 400 nm region, and its variability, based on the Mg ii index and neutron-monitor measurements. Measurements of the solar spectral irradiance available in the literature have been made with different instruments at different times and different spectral ranges. However, climate studies require harmonised data sets. This new approach has the advantage of being independent of the absolute calibration and aging of the instruments. First, the Mg ii index is derived using solar spectra from Ly α (121 nm) to 410 nm measured from 1978 to 2010 by several space missions. The variability of the spectra with respect to a chosen reference spectrum as a function of time and wavelength is scaled to the derived Mg ii index. The set of coefficients expressing the spectral variability can be applied to the chosen reference spectrum to reconstruct the solar spectra within a given time frame or Mg ii index values. The accuracy of this method is estimated using two approaches: direct comparison with particular cases where solar spectra are available from independent measurements, and calculating the standard deviation between the measured spectra and their reconstruction. From direct comparisons with measurements we obtain an accuracy of about 1 to 2%, which degrades towards Ly α. In a further step, we extend our solar spectral-irradiance reconstruction back to the Maunder Minimum introducing the relationship between the Mg ii index and the neutron-monitor data. Consistent measurements of the Mg ii index are not available prior to 1978. However, we remark that over the last three solar cycles, the Mg ii index shows strong correlation with the modulation potential determined from the neutron-monitor data. Assuming that this correlation can be applied to the past, we reconstruct the Mg ii index from the modulation potential back to the Maunder Minimum, and obtain the corresponding solar spectral-irradiance reconstruction back to that period. As there is no direct measurement of the spectral irradiance for this period we discuss this methodology in light of the other proposed approaches available in the literature. The use of the cosmogenic-isotope data provides a major advantage: it provides information about solar activity over several thousands years. Using technology of today, we can calibrate the solar irradiance against activity and thus reconstruct it for the times when cosmogenic-isotope data are available. This calibration can be re-assessed at any time, if necessary.     

Fine structure in Ca ii on the solar disc

High-dispersion spectra of the core of the K line of Ca ii as seen at the center of the solar disc have been reduced. Resolution on the spectra approach 1 arc sec. Line profiles of individual elements are very asymmetric and often are peaked on only one side of the line center. Variations of the line profiles and the emission peaks are discussed. The doubly reversed mean profile of the K line is explained as a spatial average of individual profiles, and it is suggested that single peaks may be caused by Doppler-shifted discrete elements in the chromosphere.Donald H. Menzel Research Fellow in Astronomy.     

Wave reflections in the solar atmosphere

The small phase-lag between velocities observed at different chromospheric levels is interpreted as being due to acoustic waves reflected by the very hot atmospheric layers of the chromosphere-corona transition zone. We consider first an isothermal slab, then a realistic solar atmospheric model and calculate weighting functions for velocities in Ca ii lines. It is shown that taking into account these functions and integrating over horizontal wave numbers leads to a good agreement with previous observations (Mein, 1977) in the case of 8498 and 8542 Ca ii lines. For the K line, the less good agreement shows that magnetoacoustic waves become important in the upper chromospheric layers.     

Contribution of chromospheric features to UV irradiance variability from spatially-resolved Ca II K spectroheliograms

We have digitized the Ca ii K spectroheliograms, observed at the National Solar Observatory at Sacramento Peak, for the period 1980 (maximum of solar cycle 21), 1985 (minimum of solar cycle 21), 1987 (beginning of the ascending phase of solar cycle 22), 1988 and 1989 (ascending phase and maximum of solar cycle 22), and 1992 (declining phase of solar cycle 22). A new method for analyzing the K spectroheliograms has been developed and applied to the K images for the time interval of 1992. Using histograms of intensity, we have segregated and measured the cumulative intensity and area of various chromospheric features like the plages, magnetic network and intranetwork elements. Also, the full width at half maximum (FWHM) derived from the histograms has been introduced as a new index for describing the chromospheric activity in the K-line. The full-disk intensity (spatial K index) has been derived from spatially-resolved K images and compared to the spectral K index derived from the line profiles for the full disk. Both the spatial K index and FWHM have been compared to the UV irradiance measured in the Mg ii h and k lines by the NOAA9 satellite and found that they are highly correlated with the Mg ii h and k c/w ratio.NRC Resident Research Associate, on leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

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.     

Periodic oscillations found in the velocity field of a quiescent prominence

With the purpose of detecting periodic oscillations or waves in a quiescent prominence, temporal variations of a Ca ii K line profile have been studied. The most conspicuous phenomenon found here is the fact that the edge of the prominence showed, over some 20000 km along the spectrograph-slit, periodic velocity fluctuations of nearly the same phase with periods of 210–240 s and with an amplitude of up to ± 2 kms ^–1. At other portions, several different periods of peaks (160–400 s) can also be seen in the power spectra, but less distinctly. As to the intensity and the line width, however, no periodic variations have been detected.     

The Balmer 9 and Balmer 11 lines of He ii in the Sun

We identify the Balmer 9 and 11 lines of He ii at 959 Å and 942 Å in solar spectra. These lines are produced mainly by recombination following photoionization of He ii by coronal XUV radiation. From analysis of the line intensities, we confirm the theoretical model of Avrett et al. (1976), who found that an appreciable amount of He⁺⁺ is present at temperatures of 1–2 × 10⁴ K and that the anomalously strong He ii 304 line is produced primarily by collisional excitation. We also confirm the suggestion of Kohl (1977) that the photoionization-recombination process is more important in active regions than in the quiet Sun, and we find that the 304 line is produced largely by recombination in solar flares.     

Solar Heii(304 Å) and Si xi(303 Å) line profiles

Additional observations of He ii (304 Å) and Si xi (303 Å) were obtained from a high resolution rocket spectrograph flown on 30 August, 1973 and 20 January, 1975. The profile of the He ii (304 Å) line is everywhere clearly non-gaussian across the solar disk, except in bright active areas. Near the limb, the profile is distinctly reversed. The profile of the Si xi (303 Å) line is essentially gaussian for all regions across the solar disk. Measurements of the He ii/Si xi intensity ratio indicate that the average value of this ratio across the disk depends markedly on solar activity, being about 101 for a moderate level of activity and 301 for a quiet Sun.     

Comparison of UV spectra from solar-type stars

We compare high-resolution spectra from the Sun and the four solar-type stars 16 Cyg A, 16 Cyg B, HD 32008, HD 34411 obtained with IUE in the wavelength range 2650–2930 Å. The comparison is made for peak intensities between absorption lines. At the level of accuracy of the IUE observations, the stars 16 Cyg A, 16 Cyg B, HD 34411 are indistinguishable from the Sun, in particular the Mgii resonance line profiles are identical. HD 32008 is not a solar analog but is evidently of late G to early K spectral type.Based on observations with the International Ultraviolet Explorer collected at the Villafranca Satellite Tracking Station of the European Space Agency.     

On a possible explanation of chromospheric line asymmetries of solar flares

We discuss the relationship between the chromospheric downward motions and the line asymmetries in solar flares by using a simple model. It is found that both the blue asymmetry and red asymmetry of the H line can be caused by downward motions, as long as the moving material is confined to different heights in the chromosphere. The Ca ii K line, however, mainly shows a red asymmetry. The results can qualitatively explain some observations.     

The sun‐as‐a‐star solar spectrum

The Sun is the only star for which individual surface features can be observed directly. For other stars, the properties of starspots, stellar rotation, stellar flares, etc, are derived indirectly via variation of star‐integrated spectral line profiles or their luminosity measurements. Solar disk‐integrated and disk‐resolved observations allow for investigations of the contribution of individual solar disk features to sun‐as‐a‐star spectra. Here, we provide a brief overview of three sun‐as‐a‐star programs, currently in operation, and describe recent improvements in observations and data reduction for the Integrated Sunlight Spectrometer (ISS), one of three instruments comprising the Synoptic Optical Long‐term Investigations of the Sun (SOLIS) system. Next, we discuss studies employing sun‐as‐a‐star observations (including Ca II K line as proxy for total unsigned magnetic flux and 2800 MHz radio flux) as well as the effects of flares on solar disk‐integrated spectra. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The EVE Doppler Sensitivity and Flare Observations

The Extreme-ultraviolet Variability Experiment (EVE; see Woods et al., 2009) obtains continuous EUV spectra of the Sun viewed as a star. Its primary objective is the characterization of solar spectral irradiance, but its sensitivity and stability make it extremely interesting for observations of variability on time scales down to the limit imposed by its basic 10 s sample interval. In this paper we characterize the Doppler sensitivity of the EVE data. We find that the 30.4 nm line of He ii has a random Doppler error below 0.001 nm (1 pm, better than 10 km s⁻¹ as a redshift), with ample stability to detect the orbital motion of its satellite, the Solar Dynamics Observatory (SDO). Solar flares also displace the spectrum, both because of Doppler shifts and because of EVE’s optical layout, which (as with a slitless spectrograph) confuses position and wavelength. As a flare develops, the centroid of the line displays variations that reflect Doppler shifts and therefore flare dynamics. For the impulsive phase of the flare SOL2010-06-12, we find the line centroid to have a redshift of 16.8 ± 5.9 km s⁻¹ relative to that of the flare gradual phase (statistical errors only). We find also that high-temperature lines, such as Fe xxiv 19.2 nm, have well-determined Doppler components for major flares, with decreasing apparent blueshifts as expected from chromospheric evaporation flows.     

A comparison between Mgii and Caii spectroheliograms

A detailed photometric comparison between a Mgii K filterheliogram and a nearly simultaneous Caii K spectroheliogram is presented. The comparison shows a close correspondence in both location and intensity of the bright features on the Sun with a correlation coefficient of 0.92 ± 0.02 between the Mgii and the Caii intensities in active regions.A small flare is most likely observed in the Mgii heliogram giving a substantial contribution to the recorded intensity.We also estimate theoretically the heights in the solar atmosphere at which the Mgii K and Caii K lines are formed. Taking into account the general shape of the line profiles and the different passbands used in the recordings we arrive at an average height of formation of 1700–1900 km above the photosphere for these particular heliograms.     

On peculiarities of the H and K Ca ii lines of quiescent prominences

A study of the metallic lines in bright quiescent prominences indicates that the optical thickness in the K line of Ca ii may reach values as high as 10³. This is about 10 times larger than the optical thickness in the H line and may explain some peculiarities of the H and K lines in solar prominences.