Improvements in the Determination of ISS Ca <Emphasis Type="SmallCaps">ii</Emphasis> K Parameters

Measurements of the ionized Ca ii K line are one of the major resources for long-term studies of solar and stellar activity. They also play a critical role in many studies related to solar irradiance variability, particularly as a ground-based proxy to model the solar ultraviolet flux variation that may influence the Earth’s climate. Full disk images of the Sun in Ca ii K have been available from various observatories for more than 100 years and latter synoptic Sun-as-a-star observations in Ca ii K began in the early 1970s. One of these instruments, the Integrated Sunlight Spectrometer (ISS) has been in operation at Kitt Peak (Arizona) since late 2006. The ISS takes daily observations of solar spectra in nine spectra bands, including the Ca ii K and H lines. We describe recent improvements in data reduction of Ca ii K observations, and present time variations of nine parameters derived from the profile of this spectral line.     

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.     

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.     

Deep Convection and the Solar Chromosphere

The manifestation of convection in deep layers of the Sun has been found in the dynamics of solar surface activity (Arkhypov, Antonov, and Khodachenko in Solar Phys. 270, 1, 2011). Some chromospheric phenomena could be connected with deep convection, too. We justify this hypothesis with sunspot, Ca ii, Hα, and millimeter-wave radio data. It is argued that large-scale (20 to 25 deg) bright regions in the chromosphere, surrounded by dark halos with diameters of 40° to 50°, can be manifestations of giant convection cells. The ascending and descending flows in such cells modulate the emergence of magnetic tubes generating the high-temperature regions and low-temperature halo in the chromosphere. Our estimates of the rotation rate of such features confirm their association with deep (≳ 35 Mm) layers of the solar convection zone.     

Granular-Scale Elementary Flux Emergence Episodes in a Solar Active Region

We analyse data from Hinode spacecraft taken over two 54-minute periods during the emergence of AR 11024. We focus on small-scale portions within the observed solar active region and discover the appearance of very distinctive small-scale and short-lived dark features in Ca ii H chromospheric filtergrams and Stokes I images. The features appear in regions with close-to-zero longitudinal magnetic field, and are observed to increase in length before they eventually disappear. Energy release in the low chromospheric line is detected while the dark features are fading. Three complete series of these events are detected with remarkably similar properties, i.e. lifetime of ≈ 12 min, maximum length and area of 2 – 4 Mm and 1.6 – 4 Mm², respectively, and all with associated brightenings. In time series of magnetograms a diverging bipolar configuration is observed accompanying the appearance of the dark features and the brightenings. The observed phenomena are explained as evidencing elementary flux emergence in the solar atmosphere, i.e. small-scale arch filament systems rising up from the photosphere to the lower chromosphere with a length scale of a few solar granules. Brightenings are explained as being the signatures of chromospheric heating triggered by reconnection of the rising loops (once they have reached chromospheric heights) with pre-existing magnetic fields, as well as being due to reconnection/cancellation events in U-loop segments of emerging serpentine fields. The characteristic length scale, area and lifetime of these elementary flux emergence events agree well with those of the serpentine field observed in emerging active regions. We study the temporal evolution and dynamics of the events and compare them with the emergence of magnetic loops detected in quiet Sun regions and serpentine flux emergence signatures in active regions. The physical processes of the emergence of granular-scale magnetic loops seem to be the same in the quiet Sun and active regions. The difference is the reduced chromospheric emission in the quiet Sun attributed to the fact that loops are emerging in a region of lower ambient magnetic field density, making interactions and reconnection less likely to occur. Incorporating the novel features of granular-scale flux emergence presented in this study, we advance the scenario for serpentine flux emergence.     

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).     

Multiple-Scale Pattern Recognition Applied to Faint Intergranular G-band Structures

Small-scale solar magnetic flux concentrations are studied in two-dimensional G-band images at very high spatial resolution and compared with Ca ii H enhancements. Among 970 small-sized G-band intergranular structures (IgS), 45% are co-spatial with isolated locations of Ca ii H excess and thus considered as magnetic (MIgS); they may be even twice as frequent as the known G-band bright points. The IgS are recognized in the G-band image by a new algorithm operating in four steps: (1) A set of equidistant detection levels yields a pattern of primary “cells”; (2) for each cell, the intrinsic intensity profile is normalized to its brightest pixel; (3) the cell sizes are shrunk by a unitary single-intensity clip; (4) features in contact at an appropriate reference level are merged by removal of the respective common dividing lines. Optionally, adjoining structures may be excluded from this merging process (e.g., chains of segmented IgS), referring to the parameterized number and intensity of those pixels where enveloping feature contours overlap. From the thus recognized IgS pattern, MIgS are then selected by their local Ca ii H contrast and their mean G-band-to-continuum brightness ratio.     

Solar Magnetic Activity and Total Irradiance Since the Maunder Minimum

We develop a model for estimating solar total irradiance since 1600 AD using the sunspot number record as input, since this is the only intrinsic record of solar activity extending back far enough in time. Sunspot number is strongly correlated, albeit nonlinearly with the 10.7-cm radio flux (F _10.7), which forms a continuous record back to 1947. This enables the nonlinear relationship to be estimated with usable accuracy and shows that relationship to be consistent over multiple solar activity cycles. From the sunspot number record we estimate F _10.7 values back to 1600 AD. F _10.7 is linearly correlated with the total amount of magnetic flux in active regions, and we use it as input to a simple cascade model for the other magnetic flux components. The irradiance record is estimated by using these magnetic flux components plus a very rudimentary model for the modulation of energy flow to the photosphere by the subphotospheric magnetic flux reservoir feeding the photospheric magnetic structures. Including a Monte Carlo analysis of the consequences of measurement and fitting errors, the model indicates the mean irradiance during the Maunder Minimum was about 1 ± 0.4 W m⁻² lower than the mean irradiance over the last solar activity cycle.     

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.     

Short-Term Periodicities in Solar Indices

The purpose of the present communication is to identify the short-term (few tens of months) periodicities of several solar indices (sunspot number, Caii area and K index, Lyman , 2800 MHz radio emission, coronal green-line index, solar magnetic field). The procedure used was: from the 3-month running means (3m) the 37-month running means (37m) were subtracted, and the factor (3m – 37m) was examined for several parameters. For solar indices, considerable fluctuations were seen during the ± 4 years around sunspot maxima of cycles 18–23, and virtually no fluctuations were seen in the ± 2 years around sunspot minima. The spacings between successive peaks were irregular but common for various solar indices. Assuming that there are stationary periodicities, a spectral analysis was carried out which indicated periodicities of months: 5.1–5.7, 6.2–7.0, 7.6–7.9, 8.9–9.6, 10.4–12.0, 12.8–13.4, 14.5–17.5, 22–25, 28 (QBO), 31–36 (QBO), 41–47 (QTO). The periodicities of 1.3 year (15.6 months) and 1.7 years (20.4 months) often mentioned in the literature were seen neither often nor prominently. Other periodicities occurred more often and more prominently. For the open magnetic flux estimated by Wang, Lean, and Sheeley (2000) and Wang and Sheeley (2002), it was noticed that the variations were radically different at different solar latitudes. The open flux for < 45 solar latitudes had variations very similar (parallel) to the sunspot cycle, while open flux for > 45 solar latitudes had variations anti-parallel to the sunspot cycle. The open fluxes, interplanetary magnetic field and cosmic rays, all showed periodicities similar to those of solar indices. Many peaks (but not all) matched, indicating that the open flux for < 45 solar latitudes was at least partially an adequate carrier of the solar characteristics to the interplanetary space and thence for galactic cosmic ray modulation.     

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.     

Solar Feature Catalogues In Egso

The Solar Feature Catalogues (SFCs) are created from digitized solar images using automated pattern recognition techniques developed in the European Grid of Solar Observation (EGSO) project. The techniques were applied for detection of sunspots, active regions and filaments in the automatically standardized full-disk solar images in Caii K1, Caii K3 and Hα taken at the Meudon Observatory and white-light images and magnetograms from SOHO/MDI. The results of automated recognition are verified with the manual synoptic maps and available statistical data from other observatories that revealed high detection accuracy. A structured database of the Solar Feature Catalogues is built on the MySQL server for every feature from their recognized parameters and cross-referenced to the original observations. The SFCs are published on the Bradford University web site http://www.cyber.brad.ac.uk/egso/SFC/ with the pre-designed web pages for a search by time, size and location. The SFCs with 9 year coverage (1996–2004) provide any possible information that can be extracted from full disk digital solar images. Thus information can be used for deeper investigation of the feature origin and association with other features for their automated classification and solar activity forecast.     

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.     

Comparison of computed fluxes for Fex and Fexiv lines with observed values at 1980 eclipse

Fluxes have been computed for Fex (6374 Å) and Fe xiv (5303 Å) lines as a function of solar radii and at various coronal tempratures. The electron density derived from the white light corona during the total solar eclipse of 1980 were used in the computations. Fluxes in adjacent continua have also been computed. The computed ratios of line flux to the square of continuum flux at a coronal temperature of 1.6 × 10⁶ K show a good fit with the observed values for Fex line. Further, radiative excitation seems to dominate over collisional excitation beyond 1.3 solar radius.     

The photospheric barium spectrum: Solar abundance and collision broadening of Baii lines by hydrogen

A study of the solar Ba ii spectrum leads to a solar abundance of barium of log ba = = 2.11±0.12, on the scale log h = 12. The observed asymmetry of the resonance line 4554 is consistent with an isotopic abundance ratio equal to the terrestrial one. The meteoritic Ba/Si abundance ratio found in carbonaceous chondrites appears to exceed the solar ratio by 0.1 to 0.2 dex (Section 5).The broadening by collisions with hydrogen atoms is determined from the solar spectrum (Section 4). Damping half-widths, h, of the three stronger Ba ii lines turn out to be larger by a factor of about 3.0 than predicted from pure van der Waals interaction of dipoles. Departures from LTE appear to be present in the cores of the resonance lines and of the lines arising from the metastable 5D levels (Section 6). The equivalent widths, however, remain practically unaffected.Equivalent widths of neutral barium lines are predicted and some new identifications of photospheric Ba i lines are suggested (Section 7).     

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.     

Solar EUV emission line profiles of Si ii and Si iii and their center to limb variations

Spectral line profiles of Si ii and Si iii are presented which were observed both at solar center and near the quiet solar limb with the Naval Research Laboratory EUV spectrograph of ATM/SKYLAB. Absolute intensities and line profiles are derived from the photographic data. A brief discussion is given of their center-to-limb variations and of the optical thickness of the chromosphere in these lines. Nonthermal broadening velocities are found for the optically thin lines from their full width at half maximum intensity (FWHM).Also at High Altitude Observatory for part of this work.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

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.     

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.