Global distribution of filaments during solar cycle No. 20

By tracing the positions of filaments on the solar disk for a series of consecutive Carrington rotations, one can make a compact representation of the changes in general topology of photospheric magnetic fields during the course of a solar cycle. Examples are shown for the time period 1964–1974, which may provide some insight into the long-term relationship of the mid-latitude diagonal filaments and the high latitude polar crown.     

Variations of the radiation signatures of noise storm-emitting sunspot groups during a solar cycle

The flux density of a noise storm continuum is known to depend on importance parameters of the associated sunspot group, e.g., its total area A. A study of the continua at 287, 234, 113, and 64 MHz, however, reveals in case of the two cycles Nos. 20 and 21 that the radiation signatures of sunspot groups, with a value of A kept fixed, vary systematically with time indicating regular changes of relevant parameters of the overlying loop systems with the phase of the solar cycle. A trend of intense continua at high frequencies (for definition, cf. Figures 1, 2(b)) to occur preferably during the first activity maximum of a solar cycle has been obtained in either case suggesting a decrease of the emissivity of sunspot groups with time. Vice versa, intense continua at lower frequencies (for definition, cf. Figures 1, 2(b)) were mainly observed during the later phase of both cycles. The latter effect is shown to be attributed rather to a long-term variation of the spectral characteristics of the type-I continua than to an enhanced number of intense type-III continua. From the result obtained it follows that non-potential loops extending to great heights into the corona or developing at least conditions favourable for the generation of an intense type-I continuum even at the frequencies < 100 MHz tend to occur more frequently above sunspot groups during the later phase of a solar cycle than above the comparable groups of its first activity maximum.Furthermore, characteristic periods have been found for both cycles during which the emissivity, especially of the very large sunspot groups, was significantly diminished with reference to the comparable groups of the adjacent time intervals.     

Short-term periodicities in sunspot areas during solar cycle 22

We have analyzed the daily record of sunspot areas during the current cycle 22 looking for the short-term periodicity of around 155 days which was present during some previous solar cycles. Two different methods have been used to compute the power spectra and the results indicate that such periodicity has been absent during the current solar cycle, which confirms the results obtained by other authors who used flares or flare-related data.However, we have found that, during some intervals of time, a periodicity close to 86 days is statistically significant. A similar periodicity was found by Landscheit (1986) in energetic X-ray flares, between 1970 and 1982 (second and first half of solar cycles 20 and 21, respectively), and by Bai (1992b) for important solar flares during solar cycle 20.     

Simulations of the gross solar magnetic field during sunspot cycle 21

Regarding new bipolar magnetic regions as sources of flux, we have simulated the evolution of the radial component of the solar photospheric magnetic field during 1976–1984 with a spatial resolution of about 34 000 km, and have derived the corresponding evolution of its absolute value averaged over the visible disk. For nominal values of the transport parameters, this simulated gross field is in close, though imperfect, agreement with the observed gross field and its associated indices of solar activity. By analyzing the response of the simulated gross field to variations in the transport parameters and the source properties, we find that the simulated field originates in newly erupted bipolar regions. The lifetimes of these regions are almost always less than 3 mo. Consequently, the strength of the simulated gross field is a measure of the current level of solar activity, and any recurrent patterns with lifetimes in excess of 6 mo must reflect the continuing eruption of new flux at active longitudes rather than the persistence of old flux in long-lived magnetic structures.E. O. Hulburt Center for Space Research.Laboratory for Computational Physics.Berkeley Research Associates, Springfield, VA.     

Simulations of the mean solar magnetic field during sunspot cycle 21

Regarding new bipolar magnetic regions as sources of flux, we have computed the evolution of the photospheric magnetic field during 1976–1984 and derived the corresponding evolution of the mean line-of-sight field as seen from Earth. We obtained a good, but imperfect, agreement between the observed mean field and the field computed for a nominal choice of flux transport parameters. Also, we determined the response of the computed mean field to variations in the transport parameters and the source properties. The results lead us to regard the mean-field evolution as a random-walk process with dissipation. New eruptions of flux produce the random walk, and together differential rotation, meridional flow (if present), and diffusion provide the dissipation. The net effect of each new source depends on its strength and orientation (relative to the strength and orientation of the mean field) and on the time elapsed before the next eruption (relative to the decay time of the field). Thus the mean field evolves principally due to the contributions of the larger sources, which produce a strong, gradually evolving field near sunspot maximum but a weak, sporadically evolving field near sunspot minimum.E. O. Hulburt Center for Space Research.Laboratory for Computational Physics.     

The north-south asymmetry of sunspot areas during solar cycle 22

We have analyzed the asymmetry of sunspot areas during the current solar cycle 22, finding that it has been statistically significant and that the shape of the underlying trend within the full asymmetry time series (1874–1993) indicates that the dominance of solar activity has started to shift, during the current cycle, from the northern hemisphere to the southern one.     

The solar wind velocity in the eleven-year cycle no. 20 and the solar radar cross-section

The annual average values of the solar wind velocity over the period 1962–1972 were investigated on the basis of data obtained from different space probes. The comparison of the pattern of the annual average solar wind velocities observed by the Vela and Pioneer 6 satellites indicates that the pattern presented by Gosling et al. (1971) is realistic. The long-range trend in the solar wind velocity during the 11-year cycle is governed by the number and intensity of irregularities occurring in the corona. These irregularities may represent motions of mass or some types of MHD shock waves and they are responsible for the increased heating of the corona which then in turn causes an increase in the values of the solar radar cross-section and of the solar wind velocity. A close relation is demonstrated between the monthly and annual average values of the solar wind velocity and of the cross-section.     

Coronal rotation during solar cycle 20

Using K-coronameter observations made by the High Altitude Observatory at Haleakala and Mauna Loa, Hawaii during 1964–1976, we determine the apparent recurrence period of white-light solar coronal features as a function of latitude, height, and time. A technique based on maximum entropy spectral analysis is used to produce rotational period estimates from daily K-coronal brightness observations at 1.125R _S and 1.5R _S from disk center and at angular intervals of 5° around the Sun's limb. Our analysis reaffirms the existence of differential rotation in the corona and describes both its average behavior and its large year-to-year variations. On the average, there is less differential rotation at the greater height. After 1966–1967 we observe a general increase in coronal rotation rate which may relate to similar behavior reported for the equatorial photospheric Doppler rate. However, the coronal rate increase is significantly greater than the photospheric. If K-coronal features reflect the rotation at depth in the Sun, the long-term rate increase and the variable differential rotation may be evidence for dynamically important exchanges of energy and momentum in the upper convection zone.     

Positions of sunspot groups and solar rotation

The question is studied whether the one-year solar oscillation found by V. F. Chistyakov for the years 1965–1973 can be traced in the observations of sunspots of 1874–1971 published by Greenwich Observatory. The result is negative. But the study leads to the following two conclusions: (1) The average observable centres of gravity of spot groups are variably displaced towards the central meridian or towards the limb, the time scale of this variability being of the order of 70 years. Thus the angular velocity should be determined from recurrent groups in transit of the central meridian only. (2) The angular velocity will be smaller when determined from older spots.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

Statistical behavior of sunspot groups on the solar disk

In the present study we have produced a diagram of the latitude distribution of sunspot groups from the year 1874 through 1999 and examined statistical characteristics of the mean latitude of sunspot groups. The reliability of the observed data set prior to solar cycle 19 is found quite low as compared with that of the data set observed after cycle 19. A correlation is found between maximum latitude at which first sunspot groups of a new cycle appear and the maximum solar activity of the cycle. It is inferred that solar magnetic activity during the early part of an extended solar cycle may contain some information about the strength of forthcoming solar cycle. A formula is given to describe latitude change of sunspot groups with time during an extended solar cycle. The latitude-migration velocity is found to be largest at the beginning of solar cycle and decreases with time as the cycle progresses with a mean migration velocity of about 1.61° per year.     

A survey of coronal holes and their solar wind associations throughout sunspot cycle 20

To gain insight into the relationships between solar activity, the occurrence and variability of coronal holes, and the association of such holes with solar wind features such as high-velocity streams, a study of the period 1963–1974 was made. This period corresponds approximately with sunspot cycle 20. The primary data used for this work consisted of X-ray and XUV solar images obtained from rockets. The investigation revealed that:

1. The polar coronal holes prominent at solar minimum, decreased in area as solar activity increased and were small or absent at maximum phase. This evolution exhibited the same phase difference between the two hemispheres that was observed in other indicators of activity.
2. During maximum, coronal holes occurred poleward of the sunspot belts and in the equatorial region between them. The observed equatorial holes were small and persisted for one or two solar rotations only; some high latitude holes had lifetimes exceeding two solar rotations.
3. During 1963–74 whenever XUV or X-ray images were available, nearly all recurrent solar wind streams of speed ?500 km s^?1 were found associated with coronal holes at less than 40° latitude; however some coronal holes appeared to have no associated wind streams at the Earth.

     

Limb-darkening and solar cycle variation of sunspot intensities

New observations of the umbral limb-darkening are presented. We find a real and significant decrease in the umbra/photosphere intensity ratio towards the limb. This result contrasts the findings of previous authors and we believe this to be the first time such a decrease is reported. Our conclusion is based on broad band pinhole photometer intensity observations of 22 large sunspots covering the spectral region 0.387–2.35 m. The data are selected from measurements on approximately 600 days during the last 15.5 yr. The application of the limb-darkening data to the study of the temperature stratification in the umbra is briefly discussed. The observations confirm the suggestion that the umbra/photosphere intensity ratio seems to be a linear function of the phase in the solar cycle.     

The solar wind cycle,the sunspot cycle,and the corona

Recent theories of the solar cycle and of coronal heating strongly suggest that solar cycle variations of different quantities (i.e. sunspots, coronal green line, etc.) ought not to be expected to be in phase with one another. In agreement with this notion we note that the shape of the corona typical of a maximum eclipse occurs 1.5yr before sunspot maximum, compared with 2 yr as might be expected from Leighton's standard model. Further, we argue that the phase of the solar wind cycle can be determined from geomagnetic observations. Using this phase, a solar cycle variation of 100 km s^–1 in the solar wind velocity and 1 in the magnetic field intensity becomes apparent. In general, the solar wind cycle lags the coronal-eclipse-form cycle by 3 yr, compared with the 2 yr that might be expected from model calculations.     

On the latitude drift of sunspot groups and solar rotation

The N-S drift of sunspot groups has been studied in a different way than previously, using positions of recurrent groups of the years 1874–1976. The existence of the meridional motions, the general shape of the drift curves, and the dissimilarity between these curves around sunspot maxima and minima, are all confirmed. In addition, also for the angular velocity of the Sun the same material gives differences around the times of sunspot maxima and minima.     

Prediction of maximum sunspot number in solar cycle 23

Instantaneous predictions of the maximum monthly smoothed sunspot number in solar cycle 23 have been made with a linear regressive model, which gives the predicted maximum value as a function of the smoothed sunspot numbers corresponding to a given month from the minimum in all preceding cycles. These predictions indicate that the intensity of solar activity in the current cycle will be at an average level.     

The 44–60 Å flux during the ascending period of the solar cycle no. 20 (1964–67)

An attempt is made to separate the true 44–60 Å flux from the nominal one measured by the Solrad 8 satellite during 1966.The 44–60 Å flux recorded at Arcetri through the telemetry of the Solrad 6, 7 and 8 satellites is also examined in order to study its time variations in comparison with other indices of solar activity.     

North-south distribution of solar flares during cycle 23

In this paper, we investigate the spatial distribution of solar flares in the northern and southern hemispheres of the Sun that occurred during the period 1996 to 2003. This period of investigation includes the ascending phase, the maximum and part of the descending phase of solar cycle 23. It is revealed that the flare activity during this cycle is low compared to the previous solar cycle, indicating the violation of Gnevyshev-Ohl rule. The distribution of flares with respect to heliographic latitudes shows a significant asymmetry between northern and southern hemisphere which is maximum during the minimum phase of the solar cycle. The present study indicates that the activity dominates the northern hemisphere in general during the rising phase of the cycle (1997–2000). The dominance of northern hemisphere shifted towards the southern hemisphere after the solar maximum in 2000 and remained there in the successive years. Although the annual variations in the asymmetry time series during cycle 23 are quite different from cycle 22, they are comparable to cycle 21.     

Statistics of the largest sunspot and facular areas per solar cycle

The statistics of extreme values is used to investigate the statistical properties of the largest areas of sunspots and photospheric faculae per solar cycle. The largest values of the synodic-solar-rotation mean areas of umbrae, whole spots and faculae, which have been recorded for nine solar cycles, are each shown to comply with the general form of the extreme value probability function. Empirical expressions are derived for the three extreme value populations from which the characteristic statistical parameters, namely the mode, median, mean and standard deviation, can be calculated for each population. These three extreme value populations are also used to find the expected ranges of the extreme areas in a group of solar cycles as a function of the number of cycles in the group. The extreme areas of umbrae and whole spots have a dispersion comparable to that found by Siscoe for the extreme values of sunspot number, whereas the extreme areas of faculae have a smaller dispersion which is comparable to that found by Siscoe for the largest geomagnetic storm per solar cycle. The expected range of the largest sunspot area per solar cycle for a group of one hundred cycles appears to be inconsistent with the existence of the prolonged periods of sunspot minima that have been inferred from the historical information on solar variability. This inconsistency supports the contention that there are temporal changes of solar-cycle statistics during protracted periods of sunspot minima (or maxima). Indeed, without such temporal changes, photospheric faculae should have been continually observable throughout the lifetime of the Sun.