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).

This article discusses an unpublished paradigm by Athay that relates the general properties of the solar photosphere, chromosphere, and corona to the stream of photons, kinetic energy, and magnetic fields flowing from the solar interior. Using the Athay paradigm, we discuss the physics of the solar atmosphere and its coupling to the solar dynamo to clarify the connection of observed structures and variations in the three layers to their hydromagnetic interpretation. The details of the eleven-year cycles of solar activity are quite different, but each cycle exhibits two invariant features. First, the chromosphere and corona are always present above the photosphere in its turbulent state maintained by the radiative flux escaping at the surface as the solar luminosity. Second, the solar magnetic field is globally reversed early in each cycle, accompanied by systematic drifts in magnetic activity shown in the sunspot butterfly diagram of each cycle. We describe a scenario for the corresponding systematic changes in the upper solar atmosphere that recover the minimum-activity corona from one cycle to the next. We discuss in some detail the mechanisms that heat the atmosphere and process the magnetic flux continually emerging from the interior, providing a unified view of the interior-atmospheric system.

We have observed selected Fraunhofer lines, both integrated over the full disk and for a small circular region near the center of the solar disk, on 1215 days over the past 30 years. Our full disk results for the chromosphere show that Ca II K 3933 Å nicely tracks the 11 yr magnetic cycle based on sunspot number, with a peak amplitude in central intensity of ~37%. The wavelength of the midline core absorption feature, called K3, referenced to nearby photospheric Fe, displays an activity cycle variation with an amplitude of 3 mÅ (6 mÅ center disk). Other chromospheric lines, such as He I 10830 Å, Ca II 8542 Å, Hα, and the CN 3883 Å bandhead, track Ca II K intensity with lower relative amplitudes. In the low photosphere, temperature-sensitive C I 5380 Å appears constant in intensity to 0.2%. In the high photosphere, the cores of strong Fe I lines, Na D1 and D2, and the Mg I b lines, present a puzzling signal, perhaps indicating a role for the 22 yr Hale cycle. Solar minimum around 1985 was clearly seen, but the following minimum in 1996 was missing. Our center disk results show that both Ca II K and C I 5380 Å intensities are constant, indicating that the basal quiet atmosphere is unaffected by cycle magnetism within our observational error. A lower limit to the Ca II K central intensity atmosphere is 0.040. This possibly represents conditions as they were during the Maunder minimum. Converted to the Mount Wilson S-index (H+K index), the Sun center disk is at the lower activity limit for solar-type stars. An appendix provides instructions for URL access to both the raw and reduced data.

This paper presents and interprets observations obtained by the Spectral Irradiance Monitor (SIM) on the Solar Radiation and Climate Experiment (SORCE) over a time period of several solar rotations during the declining phase of solar cycle 23. The time series of visible and infrared (IR) bands clearly show significant wavelength dependence of these variations. At some wavelengths the SIM measurements are qualitatively similar to the Mg II core-to-wing ratio, but in the visible and IR they show character similar to the Total Solar Irradiance (TSI) variations. Despite this overall similarity, different amplitudes, phases, and temporal features are observed at various wavelengths. The TSI can be explained as a complex sum of the various wavelength components. The SIM observations are interpreted with the aid of solar images that exhibit a mixture of solar activity features. Qualitative analysis shows how the sunspots, faculae, plage, and active network provide distinct contributions to the spectral irradiance at different wavelengths, and ultimately, how these features combine to produce the observed TSI variations. Most of the observed variability appears to be qualitatively explained by solar surface features related directly to the magnetic activity.

The SORCE (Solar Radiation and Climate Experiment) SIM (Spectral Irradiance Monitor) instrument is a satellite-borne prism spectrometer that measures the solar spectrum from 200-2700 nm with a cadence of at least 2 spectra per day with a resolution of 1-33 nm. The nearly 800-day long data set provides the temporal evolution of solar irradiance throughout ultraviolet, visible and infrared spectral regions. At some wavelengths in the ultraviolet, the SIM measurements exhibit variations similar to Mg II index, and in the visible and IR they show similarities to Total Solar Irradiance (TSI) record, but with differing amplitudes, phases, and shapes relative to the monitors. The TSI can be explained a's a complex mix of the various wavelength components observed by SIM. Further insight into the solar variability observed by SIM can be gained from comparing an analysis of the distribution of solar features as measured by PSPT (Precision Solar Photometric Telescope) in conjunction with spectral synthesis. Most of the observed behavior appears to be qualitatively explained by the observed solar surface features that directly relate to the magnetic activity but some IR variations are not readily explained as was noted by Fontenla et al. (2004).

Solar empirical models based on regression of two variability indices for radiation from the photosphere and chromosphere fit total solar irradiance (TSI) observations with accuracy comparable to the precision reported for the observations themselves. However, the physical meaning of the fitting coefficients and their stability during different phases of the solar cycle has not been examined in detail. We test the stability of the coefficients in regression models of the VIRGO TSI observations over the nine years from the minimum of Cycle 23 in 1996 through the maximum to 2005. We also show how the coefficients converge to the "best fit'' using a search in the coefficient space. Analysis of TSI variability in different phases of this cycle shows little change in regression models as long as the time periods used in the regression are long enough to show the slow solar cycle variation in TSI. We extend our analysis to TSI observations from ERB, ACRIM2, ACRIM3, DIARAD, and TIM. The regression models from these time series show large systematic differences in fitting coefficients for the plage and sunspot indices that we used. These differences are significantly larger than the estimated uncertainties in the coefficients and point to the difficulty of combining observations from different instruments to create an accurate composite TSI record over several solar cycles. Our results clearly demonstrate the improvement in precision of TSI measurements from the Nimbus 7 ERB in Cycle 22 to the latest SORCE TIM data in Cycle 23.

The Solar–Stellar Irradiance Comparison Experiment (SOLSTICE) and the Spectral Irradiance Monitor (SIM) on the Solar Radiation and Climate Experiment (SORCE) both measure the solar ultraviolet irradiance surrounding the Mg II doublet at 280 nm on a daily basis. The SIM instrument's resolution (1.1 nm) is similar to the Solar Backscatter Ultraviolet instruments used to compute the standard NOAA Mg II index, while SOLSTICE's resolution is an order of magnitude higher (0.1 nm). This paper describes the technique used to calculate the index for both instruments and compares the resulting time series for the first 18 months of the SORCE mission. The spectral resolution and low noise of the SOLSTICE spectrum produces a Mg II index with a precision of 0.6%, roughly a factor of 2 better than the low-resolution index measurement. The full-resolution SOLSTICE index is able to measure short-timescale changes in the solar radiative output that are lost in the noise of the low-resolution index.

Here we report the progress in both measurements and analysis of total solar irradiance (TSI) during the last 24 years. Recent TSI measurements made by ACRIM III and VIRGO in the last two years agree to within 0.5 Wm−2 and show the same pattern of short-term variability. A 24-year composite record of TSI measurements gives estimates of its variation for two solar cycles. Such composites give the first estimates of secular variation of the solar output. Our analysis of TSI data from solar minimum to maximum for cycles 22 and 23 gives nearly identical regression equations because of improvement in VIRGO degradation corrections, thus, resolving the empirical issue raised by de Toma et al. [Astrophys. J. Lett. 549 (2001) L131]. This agreement occurs despite a decrease in cycle 23 of sunspot number by ≈33% below solar maximum values for cycles 21 and 22.

Over the past decade, regular measurement programs for parts of the solar spectrum have been established. In recent years substantial progress has also been made on the physical understanding of these measurements. To refine our understanding and to make quantitative estimates of this variability requires a study of the entire solar spectrum. Our approach to this requirement is to combine empirical image analysis with the theory for emission, absorption, and transfer of radiation in the solar atmosphere. The goal is the successful combination of observed solar images with semi-empirical models and theory for calculation of a mixed line+continuum spectrum emitted from realistic representations of the observed solar disk. We present the latest results from the SunRISE spectral synthesis model in specific spectral bands in the UV, visible, and near-IR, and compare them to related observations.

The latest SOHO VIRGO total solar irradiance (TSI) time series is analyzed using new solar variability measures obtained from full-disk solar images made at the San Fernando Observatory and the Mg II 280 nm index. We discuss the importance of solar cycle 23 as a magnetically simpler cycle and a variant from recent cycles. Our results show the continuing improvement in TSI measurements and surrogates containing information necessary to account for irradiance variability. Use of the best surrogate for irradiance variability due to photospheric features (sunspots and faculae) and chromospheric features (plages and bright network) allows fitting the TSI record to within an rms difference of 130 ppm for the period 1986 to the present. Observations show that the strength of the TSI cycle did not change significantly despite the decrease in sunspot activity in cycle 23 relative to cycle 22. This points to the difficulty of modeling TSI back to times when only sunspot observations were available.

Motivated by observed anomalous features in cycle 23, as inferred from records of photospheric magnetic flux, we develop a flux transport dynamo-based scheme in order to investigate the physical cause of such anomalies. In this first study we focus on understanding anomalies occurring in the polar field evolutionary pattern in cycle 23, namely, why the polar reversal in cycle 23 was slow, why after reversal the buildup of the polar field was slow, and why the south pole reversed approximately a year after the north pole did. We construct a calibrated flux transport dynamo model that operates with dynamo ingredients such as differential rotation, meridional circulation, and large-scale poloidal field source derived from observations. A few other dynamo ingredients, such as diffusivity and quenching pattern, for which direct observations are not possible, are fixed by using theoretical guidance. By showing that this calibrated model can reproduce major longitude-averaged solar cycle features, we initialize the model at the beginning of cycle 22 and operate by incorporating the observed variations in meridional circulation and large-scale surface magnetic field sources to simulate the polar field evolution in cycle 23. We show that a 10%-20% weakening in photospheric magnetic flux in cycle 23 with respect to that in cycle 22 is the primary reason for a ~1 yr slowdown in polar reversal in cycle 23. Weakening in this flux is also the reason for slow buildup of polar field after reversal, whereas the observed north-south asymmetry in meridional circulation in the form of a larger decrease in flow speed in the northern hemisphere than that in the southern hemisphere during 1996-2002 and the appearance of a reverse, high-latitude flow cell in the northern hemisphere during 1998-2001 caused the north polar field to reverse before the south polar field.