Calcium Plage Research Articles

Wavelet Analysis of Hidden Periodicities in Some Indexes of Solar Activity

The wavelet transform is used to analyze periodicities in the Wolf numbers, calcium plage areas, coronal intensities, net solar magnetic field intensities, fluxes of solar radiation at a wavelength of 10.7 cm, and geomagnetic planetary indexes. The periodicities with scales varying between 5 and 100 years have been detected. A correlation between the time series of temperature in Tomsk and solar activity is analyzed. No correlation is found between the initial series; however, a significant correlation is established between the periodicities in the time series of temperature and individual solar activity indexes.

Kinematics and evolution of local features of the large-scale magnetic field - II. Relation to Active Regions

Relationships have been studied between the background magnetic field and the distribution of active regions over the solar surface and time. A series of magnetic-field synoptic maps covering a 20-year period has been cross-correlated with spatio-temporal distributions of three types of active formations (sunspots, calcium plages, and solar flares) used as indicators of the active regions. To make the data analysis more effective, we expanded both the magnetic-field and the active-region distributions in terms of Fourier series in longitude, and then cross-correlated the latitude-dependent Fourier harmonics. Cross-correlation functions calculated from the lower-order Fourier harmonics exhibit prolonged maxima of the amplitude. For the first-order harmonic, the maxima can be tracked throughout a long time interval of at least 13 Carrington rotations, but the time of cross-correlation decreases down to 2 rotations, as the harmonic order increases up to 8. The maxima of the cross-correlation functions indicate moreover a poleward directed drift of the magnetic features that occurred with a velocity of 10–15 m s−1. The cross-correlation functions calculated separately by using the three types of active formations as indicators of the active regions are similar to each other, although they differ in some details of minor significance. The results of the data analysis make it possible to conclude that the cross-correlation between the magnetic-field and the active-region distributions displays long-term evolution of the magnetic features emerged in the photosphere in the form of the active regions, and that the evolution occurs in accordance with Leighton's (1964) concept known at present as the flux transport model. In order to verify this conclusion, we applied the cross-correlation technique to analyze a magnetic field distribution simulated by means of the flux transport equation by using an ensemble of local-scale magnetic bipoles as a source of magnetic flux. Results of the simulated magnetic field analysis exhibit a substantial qualitative agreement with those obtained by examining the observational data.

Meter wavelength observations of the quiet sun and the slowly varying component during declining phase of the solar cycle

Abstract Two dimensional maps of the Sun at 164 MHz, obtained with the Nancay Radioheliograph from April to August 1984 are presented. The quiet sun emission and the slowly varying component far from sunspots are studied. The association of local emissions with the chromospheric calcium plages is pointed out and interpreted. The altitude of the radio sources is estimated. The comparison of the location of the sources with K-corona observations confirms previous results obtained for the minimum of the cycle. The brightness temperature of the coronal plateau, which is the radio counterpart of the coronal plasma sheet, is given.

Calcium plage intensity and solar irradiance variations

We have established a statistical relation between the facular contribution to the solar irradiance and the intensity of the associated calcium plage. For the solar irradiance data the ACRIM measurements were used. The quiet-Sun level of the irradiance was determined as a function of the time for the period studied. A sample of plages in the period of the solar activity minimum was selected, during the periods when no spots were present on the solar disc. We have expressed the dependence studied through the parameter C p in the ‘proxy’ PFI concept. The parameter C p could be related to the plage intensity (I) as C p = 0.006I + 0.003. The mean value of the parameter C p ranged between 0.015 and 0.017 depending on the choice of samples.

Ratio of calcium plage to sunspot areas of solar active regions

Published sunspot and calcium plage areas were examined for about 1,700 solar active regions (ARs) between 1971 and 1982. With these data, averaged AR properties, the possibility of energy balance between spot and facular emission over the lifetimes of activity complexes, and changes in AR properties over an 11‐year sunspot cycle were studied. Overall energy balance can neither be established nor ruled out. Apparently, real temporal changes in the nature of ARs, however, imply either that energy balance cannot hold over periods shorter than a few years or that standard models for estimating irradiance fluctuations from spot and/or plage areas must be modified.

The old calcium plages differential rotation latitudinal profile: Its gross and fine structure evolution with the solar cycle

This work is a study of the rotational properties of the solar calcium plages, during the time interval 1967–1977; only plages older than 4 days have been the object of this research. We have looked systematically for any significant change occurring during the course of the solar cycle, and any kind of ‘anomaly’ or fine structure in the differential rotation latitudinal profile. We find that such a profile undergoes a cyclic transformation, making it assume the highest steepness at the solar maximum; a sudden flattening then occurs in the first years of declining activity; the last years of the cycle, as the first years of the next one, are characterized by intermediate steepness values. Moreover, we find that, in spite of the general belief that the angular rotation rate is continuously decreasing with increasing heliographic latitude, at least two inversions do exist of such an overall tendency:

The impact of aging on the rotation rate of calcium plages in the years 1967?1977

An analysis of Ca ii spectroheliograms obtained at Catania Astrophysical Observatory throughout the years 1967–1977 has been carried out, to throw light on the complex relationship linking (Ternullo, 1986) the angular rotation rate of Ca plages with their age, as well as with solar cycle phase, and with latitude. Given the rotation law w =, a + b sin2I, solar-cycle-related oscillatory properties both of a and b coefficients are described, both for young and old Ca plages.

Sunspot cycle variations of ensemble-averaged active regions

We examine published sunspot and calcium plage areas for 1620 solar active regions between 1974 and 1985. With these data we study the properties of ensemble-averaged active regions. The average sunspot area per region, the average plage to sunspot area ratio, and the average plage intensity of regions all vary significantly with the sunspot cycle and in correlation with one another. The average plage area per region varies significantly but is uncorrelated with the sunspot cycle and with the other quantities. While the plage and sunspot areas and the plage intensities of individual active regions observed over a two-year period are strongly correlated, the relationship among these quantities appears to change over an 11-yr period. These results suggest the existence of some energetic connection between active region sunspot areas and plage intensities. Further, if energy balance between sunspot luminosity deficits and facular luminosity excesses holds, then standard models relating these quantities to sunspot and plage areas will have to be modified. Overall energy balance can neither be established nor ruled out.

Time-integrated energy budget of a solar activity complex

The 0.1–0.3% dips in solar irradiance during disk passages of large sunspot groups suggest the possibility of fluctuations in the solar luminosity. This raises the question of whether the energy not radiated by the dark sunspots is stored within the Sun for long periods of time, or is radiated by faculae during the several-month lifetime of a solar activity complex1–3. Here we examine the sunspot and facular contributions to luminosity fluctuations due to a solar activity complex over its lifetime from June to November 1982. Both direct, photometric observations of irradiance fluctuations and modelled ‘proxy’ fluctuations based on published sunspot and calcium plage areas are used. We find that the total facular energy excess is between 70 and 120% of the sunspot deficit of ∼1037 erg. Thus, at a minimum, a major portion of the missing sunspot flux is radiated by faculae, and energy balance or even an excess is possible. This work differs from earlier studies2–9 in that our data cover a longer period of time, more photometric data are included, and our analysis considers the effect of partial occultation of active regions by the solar limb.

Solar irradiance variations from photometry of active regions

The Extreme Limb Photometer (ELP) has been used to measure the irradiance fluctuation of the Sun due to selected active regions. Forty-five active regions that were completely scanned at various disk positions are included in the analysis. The contribution of these active regions to a global solar irradiance fluctuation has been correlated with photometric sunspot and facular indices (PSI and PFI) using published values of sunspot and calcium plage areas. The measured ELP fluctuations are converted to a global brightness fluctuation, ΔB/B. The sunspot component of ΔB/B correlates with PSI with r = 0.95. The facular component of ΔB/B correlates with PFI with r - 0.72. The expression for PFI is important to the question of energy balance between sunspots and faculae and the results presented here are not incompatible with energy balance between the two phenomena; that is the energy deficit of sunspots may be balanced by the energy excess of faculae.

Periodic behaviour of solar flare activity

The periodic nature of solar activity has been studied using parameters such as the sunspot Wolf numbers, calcium plage areas and flare indices. The magnitude of the solar activity based on these parameters reveals periodicities other than the most pronounced 11-yr one. Any absolute detection of periodicity in active phenomena would have fundamental significance for our understanding of solar activity. Here we investigate the temporal variation of the flare activity of the Sun using the data of 8,821 Hα flares which occurred during the period January 1965 to February 1984, and show new evidence for 155-day and 17-month periodicities of the flare activity. The 155-day periodicity is examined by taking into account the location of the flare on the Sun. It is suggested that the 155-day period may be related to the timescale for the storage and/or the escape of the magnetic field.

Variations in the solar rotation rate derived from Ca+ K plage areas

Daily calcium plage areas for the period 1951–1981 (which include the solar cycle 19 and 20) have been used to derive the rotation period of the Sun at latitude belts 10–15 ° N, 15–20 ° N, 10–15 ° S, and 15–20 ° S and also for the entire visible solar disk. The mean rotation periods derived from 10–20 ° S and N, total active area and sunspot numbers were 27.5, 27.9, and 27.8 days (synodic), respectively. A power spectral analysis of the derived rotation rate as a function of time indicates that the rotation rate in each latitude belt varies over time scales ranging from the solar activity cycle, down to about 2 years. Variations in adjacent latitude belts are in phase, whereas those in different hemispheres are not correlated. The rotation rates derived from sunspot numbers also behave similarly though the dependence over the solar cycle are not very apparent. The total plage areas, integrated over the entire visible hemisphere of the Sun shows a dominant periodicity of 7 years in rotation rate, while the other time scales are also discernible.

Correlations between thermospheric density and temperature, solar EUV flux, and 10.7‐cm flux variations

Simultaneously measured thermospheric N2 densities and solar EUV fluxes obtained by the AE‐E satellite are compared with ground‐based solar 10.7‐cm fluxes and calcium plage measurements. Short‐wavelength (coronal) EUV emissions correlate better with thermospheric density than the 10.7‐cm flux, although the reduction in density residuals is small. Correlation of density with a calcium plage index was somewhat worse than with 10.7. Although the overall correlation of 10.7‐cm flux with EUV fluxes is high, the best fit slopes are different for short‐ and long‐term variations, and there are instances where the short‐term behavior of various EUV emissions and that of the 10.7‐cm flux are distinctly different. For both density and EUV, a two‐factor formula based on daily 10.7‐cm flux and a running mean of the 10.7‐cm flux provides a better fit than using the daily 10.7‐cm flux alone.

A mechanism for solar ultraviolet flux variability

Solar UV emission observed by a filter photometer on Nimbus IV from 1969 to 1973 is examined in an attempt to understand the short term (27 day) and secular variability. Two models are discussed to explain the variations - a calcium plage model and a chromospheric network (faculae and spicule) structure model. Both relate to the remnant magnetic fields of active regions. An association between UV brightenings and the large scale magnetic field has been found consistent with the network model. An increase in UV emittance can be achieved by raising the effective chromospheric temperature closer to a photospheric level. If the Sun's luminosity is constant on these time intervals, the enhanced UV radiation could be partiallly offset by an overall decrease in photospheric temperature as measured by Livingston in visible photospheric profiles. Total solar luminosity may then show less variability, however, the UV to visible luminosity variation may have significant planetary influences. Lockwood and Thompson (1979) report a relation between solar activity and planetary albedos, and Schatten (1979) discussed a long-suspected relationship between solar activity and the Great Red Spot appearance.

“TAIYO” Solar Hydrogen Ly-α Measurement

From the currently observed data, the absolute intensity of the solar hydrogen Lyman-alpha (H Ly-α) was estimated as 3.2×1015 photons/m2 sec at 1 A. U. and constant within ±4.2% during the period from February 24 to September 30 with some lacks of observation only. No remarkable correlations with other indices such as F10.7 and Calcium plage indices were observed. The density distribution of hydrogen atoms in the altitude range from about 800km to 3, 000km was analysed. The observed absorption profile data agreed with the simulated calculation by referring to the CIRA '72 Model Atmosphere.

The relationship between solar activity and coronal hole evolution

We examine the relationship between coronal hole evolution and solar active regions during the Skylab period. We find a tendency for holes to grow or remain stable when the activity nearby, seen as calcium plages and bright regions in X-rays, is predominantly large, long-lived regions. This is consistent with results of previous studies, using somewhat different methods. We also find that there is a significantly higher number of small, short-lived active regions, as indicated by X-ray bright points, in the vicinity of decaying holes than there is near other holes. We interpret this to mean that holes disappear at least in part because they become filled with many small scale, magnetically closed, X-ray emitting features. This interpretation, together with the previously reported observation that the number of X-ray bright points was much larger near solar minimum than it was during the Skylab period, provides a possible explanation for the disappearance of the large, near-equatorial coronal holes at the time of solar minimum.

Green corona emission, calcium plage activity and solar sector magnetism

The relationship between coronal green line emission and solar sector magnetism has been studied statistically for the years 1965–1969. This period includes the rising portion and the maximum phase of solar cycle no. 20. In the years around solar maximum the results suggest the existence of longitudinal magnetic arcades at the solar sector boundaries. The arcades extend from at least 50°N to 50°S and are flanked by north-south oriented coronal holes about 90° apart. In the rising portion of the cycle the general picture consists of a high green line intensity structure to the west of the boundary and a region of low intensity several days wide to the east of it. Analyses of the calcium plage distribution in the years 1962–1969 show that, on the average, there is a tendency for the plage activity to peak near the sector boundaries. It is further concluded that the activity distribution suggested by Wilcox (1971a, b) is not typical of the behaviour of solar activity relative to the sector boundaries.

Comments on the course of solar activity during the declining phase of solar cycle 20 (1970?74)

In the declining phase of solar cycle 20 (1970–74) three pulses of activity occurred and resulted in two well defined ‘stillstands’ in the smoothed means of sunspot, 2800 MHz, and calcium plage data. Marked diminutions in spot and 2800 MHz flux took place in 1970 and 1971, respectively, and were accompanied by concomitant decreases in flare-occurrence. Studies of the latitude distribution of spots and flares show the extent of the dominance of the northern hemisphere in cycle 20 and the marked phase shift between northern and southern hemispheres. In the years studied, the longitudes of centers of activity clustered in identifiable zones or hemispheres for relatively long intervals of time. From mid-1973 to mid-1974 the Sun had a relatively inactive hemisphere centered on ∼0° longitude. The relationship of certain well defined ‘coronal holes’ to this inactive hemisphere of the chromosphere is noted. The first two spot groups of the new cycle formed in November 1974 and January 1975 in the longitude zone associated with relatively high levels of old cycle activity, a repetition of the pattern observed in 1963–64.

Solar coronal holes as sources of recurrent geomagnetic disturbances

Observations of the solar corona by Oso 7 have been used in a superposed epoch analysis to study the relationships between classes of coronal features and geomagnetic activity. Both bright coronal regions (associated with solar activity) and regions of less than average brightness (coronal holes) were investigated. It was found that for the period from January 1972 through January 1973 a significant enhancement in geomagnetic activity (Ap increase of nearly a factor of 2) occurred 2–3 days after central meridian passage of large coronal holes that extended to within 5° of the solar subearth point when they were on the meridian. Neither bright coronal features nor sunspots nor bright calcium plage regions showed such a correlation. Large coronal holes, which are frequently recurrent long-lived features, appear to satisfy the requirements for ‘M regions,’ which were hypothesized to be responsible for recurrent geomagnetic disturbances (Bartels, 1934). If solar wind high-speed streams originate preferentially in these regions, their velocity at the base of the corona will be substantially higher than that expected from an axisymmetric solar wind model.

Comparison of the correlation of incoherent scatter and ionosonde measurements of temperature with calcium plage and 2800-megahertz intensities

The variation in the diurnal thermospheric neutral temperatures has been studied. Minimum and maximum temperatures were calculated from 20 sets of diurnal incoherent scatter measurements taken at Arecibo, Puerto Rico, which corresponded to a range of 2800-MHz intensity of 120 to 167×10−22watt m−2 sec−1. Minimum temperatures were also derived from 850 reduced ionosonde records obtained from Ramey, Puerto Rico, for which the 2800-MHz intensity varied between 74 and 252×10−22 watt m−2 sec−1. Both sets of measurements were seasonally distributed. Various combinations of the monthly mean and daily values of the 2800-MHz and Ca II solar indices were fitted to the data. A formula incorporating the difference between the daily and monthly average Ca II index and the monthly average 2800-MHz index provided a better fit to all the data than did formulas incorporating only the Ca II index or the 2800-MHz index alone. The phase and amplitude of the semiannual component and the term dependenton Kp were found to remain almost the same for the maximum and minimum temperature. The term dependent on the 27-day component of the solar activity was found to be about 4 times as large for the diurnal maximum as for the minimum.