Energetic flare zones on the Sun

In this investigation, we have studied the latitudinal, longitudinal (northern and southern hemispheric) distributions based on 1737 major flares observed during solar cycles 19 and 20 (see subsequent paragraphs) and have arrrived at some interesting results which go to show that as far major flares are concerned latitudewise 11–20° belts, and longitudewise 5–8 places are most prolific in producing major flares in each hemisphere. During the above cycles at least 5 flare zones are present in each hemisphere. In fact these zones seem to produce more than 50% of the total number of energetic flares investigated by us and occupy only <4% area of the Sun.     

New results concerning the global solar cycle

We derive the poleward migration trajectory diagram of the filament bands for the years 1915–1982 from the H-alpha synoptic charts. We find that the global solar activity commences soon after the polar field reversal in the form of two components in each hemisphere. The first component we identify with the polar faculae that appear at latitudes 40–70° and migrate polewards. The second and the more powerful component representing the sunspots shows up at 40° latitudes 5–6 years later and drifts equatorward giving rise to the butterfly diagram. Thus the global solar activity is described by the faculae and the sunspots that occur at different latitude belts and displaced in time by 5–6 years. This gives rise to the prolonged duration for the global solar activity lasting for 16–18 years as against the 11 years which has come about based only on the spots. The two components match with the pattern of the coronal emission in 5303 Å line. Finally, we show that the two components of activity also match with the pattern of excess shear associated with the torsional oscillations on the Sun and this provides a link between the torsional oscillations and the magnetic activity.     

DEPENDENCE OF ROTATION VELOCITY OF INDIVIDUAL SUNSPOTS ON THEIR LIFETIME

By using a large number (452) of individual sunspots or individual sunspots with small spots around them, taken from the Greenwich Photoheliographic Results (GPR) for the years 1964–1976 that cover solar cycle No. 20, it is shown that the rotation velocity of the sunspots varies with their lifetimes. This investigation indicates that at the equator, the rotation rate for the last three days (of the lifetime) is about 1.3% slower than that over the whole lifetime and about 0.5% slower than during the first three days, but this is reversed at high and low latitudes, and the difference is much larger in the northern hemisphere than in the southern hemisphere. These results confirm to the fact that the rotation rate of the solar layers increases with depth.     

Distribution of LDE flares on the Sun during solar cycles 20 and 21

We have studied the latitude and longitude (northern and southern hemispheric) distributions based on 2277 LDE flares observed during the period from 1966 to 1986. We have found that there exist active zones, in which the LDE flare occurrence rate is much higher. Latitudinal belts between 11–20° and longitudinal belts around 80–100° are the most prolific places to produce LDE flares. During cycles 20 and 21 these active zones produced 36% of the total number of LDE flares by occupying only 6% area of the Sun.     

Sunspot and Auroral Activity During Maunder Minimum

The extremely low sunspot activity during the period of the Maunder minimum 1645–1715 was confirmed by group sunspot numbers, a new sunspot index constructed by Hoyt and Schatten (1998a,b). Neither sunspots nor auroral data time behavior indicate the presence of 11-year solar cycles as stated by Eddy (1976). The evidence for solar cycles was found in the butterfly diagram, constructed from observations made at Observatoire de Paris. After Clivier, Boriakoff, and Bounar (1998) the solar cycles were reflected also in geomagnetic activity. Results are supported by the variation of cosmogenic isotopes ¹⁰Be and ¹⁴C. The majority of the observed 14 naked-eye sunspots occurred on days when telescopic observations were not available. A part of them appeared in the years when no spot was allegedly observed. Two-ribbon flares appear in plages with only very small or no sunspots. Some of these flares are geoactive. Most aurorae (90%), which were observed during the Maunder minimum, appeared in years when no spot was observed. Auroral events as a consequence of proton flares indicate that regions with enhanced magnetic field can occur on the Sun when these regions do not produce any sunspots.     

Effect of Large-Scale Magnetic Fields on Total Solar Irradiance

The effect of large-scale magnetic fields on total solar irradiance (TSI) was studied both in time–frequency and in time–longitude aspects. A continuous wavelet analysis revealed that the energy of thermomagnetic disturbances due to sunspots and faculae cascades into the magnetic network and facular macrostructure. A numerical technique of time–longitude analysis was developed to study the fine structure of temporal changes in the TSI caused by longitudinal brightness inhomogeneities and rotation of the Sun. The analysis facilitates mapping large-scale thermal inhomogeneities of the Sun and reveals patterns of radiative excesses and deficits relative to the undisturbed solar photosphere. These patterns are organized into 2- and 4-sector structures that exhibit the effects of both activity complexes and magnetically active longitudes. Large-scale patterns with radiative excess display a facular macrostructure and bright patterns in the magnetic network caused by the dissipation of large-scale thermomagnetic disturbances. Similar global-scale temperature patterns were found in the upper solar atmosphere. These temperature patterns are also causally related to long-lived magnetic fields of the Sun. During activity cycles 21–23 the patterns with radiative excess tend to be concentrated around the active longitudes which are centered at about 60° and 230° in the Carrington system.     

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.     

Poleward migration of the magnetic neutral line and the reversal of the polar fields on the Sun

Poleward migration of the magnetic neutral line on the Sun has been calculated for the periods 1945–1950 and 1955–1981 using synoptic charts based on H observations. Epochs of sign reversal of the solar magnetic field at latitudes 50° to 90° have been determined for these periods. During the cycles 19 and 20 a threefold sign reversal took place in the northern hemisphere. During all the above cycles both the solar poles were of one polarity for a period ranging from 0.5 to 1 year. The poleward drift velocity of the magnetic neutral line varies from 6 to 29 m s^–1 and seems to depend on the strength of the cycle.     

On the distribution and asymmetry of solar active prominences

The paper presents the results of a study of the distribution and asymmetry of solar active prominences (SAP) for the period 1957–1998 (solar cycles 19–23). The east-west (E-W) distribution study shows that the frequency of SAP events in the 81–90° slice (in longitude) near the east and west limbs is up to 10 times greater than in the 1–10° slice near the central meridian of the Sun. The north-south (N-S) latitudinal distribution shows that the SAP events are most prolific in the 11–20° slice in the northern and southern hemispheres. Further, the E-W asymmetry of SAP events is not significant. The N-S asymmetry of SAP events is significant and it has no relation with the solar maximum year or solar minimum year during solar cycles. Further, the present study also shows that the N-S asymmetry for cycles 19–23 follows and confirms the trend of N-S asymmetry cycles as reported by Verma (1992).     

Periodic variation of the north-south asymmetry of solar activity phenomena

We report here a study of various solar activity phenomena occurring in both north and south hemispheres of the Sun during solar cycles 8&amp;#x2013;23. In the study we have used sunspot data for the period 1832&amp;#x2013;1976, flare index data for the period 1936-1993, H&amp;#x03B1; flare data 1993&amp;#x2013;1998 and solar active prominences data for the period 1957&amp;#x2013;1998. Earlier Verma reported long-term cyclic period in N-S asymmetry and also that the N-S asymmetry of solar activity phenomena during solar cycles 21, 22, 23 and 24 will be south dominated and the N-S asymmetry will shift to north hemisphere in solar cycle 25. The present study shows that the N-S asymmetry during solar cycles 22 and 23 are southern dominated as suggested by Verma.     

Solar cycle general magnetic fields of 1959–1974 and dynamical structure of the convection zone

The concept of the solar general magnetic field is extended from that of the polar fields to the concept of any axisymmetric fields of the whole Sun. The poloidal and toroidal general magnetic fields are defined and diagrams of their evolutionary patterns are drawn using the Mount Wilson magnetic synoptic chart data of Carrington rotation numbers from 1417 to 1620 covering approximately half of cycle 19 and cycle 20. After averaging over many rotations long-term regularities appear in the patterns. The diagrams of the patterns are compared with the Butterfly Diagram of sunspots of the same period. The diagram of the poloidal field shows that the Sun behaves like a magnetic quadrupole, each hemisphere having two branches of opposite polarities with mirror images on the other hemisphere. This was predicted by a solar cycle model driven by the dynamo action of the global convection by Yoshimura and could serve as a verification of the model. The diagram of the toriodal field is similar to the Butterfly Diagram of sunspots. The slight differences which do exist between the two diagrams seems to show that the fields responsible for the two may originate from different zones of the Sun. Common or different characteristics of the three diagrams are examined in terms of dynamical structure of the convection zone referring to the theoretical model of the solar cycle driven by the dynamo action of the global convection.     

The revival of solar activity after Maunder minimum in reports and observations of E. Manfredi

We present information, published here for the first time, concerning observations of sunspots made at the beginning of the XVIIIth century. The information has been taken from specola's archives in Bologna. It concerns the years just after the end of the Maunder minimum. The data confirms the presence of a double maximum in 1704–1707, and shows a high asymmetry in sunspots latitude distribution, possibly related to the abnormal Sun activity in the second half of XVIIth century.     

Global solar cycle in the distribution of the green coronal emission period: 1940–1989

We have studied the latitude-time distribution of the green (5303 Å) coronal line emission for 1940–1989 from observations by Waldmeier (1957), Kislovodsk, Lomnický tít, Norikura, and Pic-du-Midi - Q.B.S.A. (1955–1987). We have compared these data with the distributions of the weak magnetic field (Stenflo, 1988), of polar faculae and sunspots, and have given our interpretation of the results. We have found that a new cycle of coronal activity commences after the polar field reversal in the form of two components in each hemisphere. We identify the first component with the polar faculae that appear at latitude 40° and migrate polewards. The second component representing sunspots shows up at 40° latitude 5–6 years after and drifts equatorward. Thus the global coronal activity cycle has a duration of 16–17 years and is described by two components that reflect the activity of polar faculae and sunspots.     

On diurnal variations of the solar rotation rate as derived from sunspot tracings

The heliographic positions of more than 100 sunspots were accurately measured several times a day from 1974 until 1979 by means of the computer-controlled tracing method described by Schröter and Wöhl (1975). A striking degree of constancy of the solar rotation rate (about 0.15% or 3 m s^–1) is found, when east-west proper motion components of each individual stable sunspot is considered. However, large differences of the rotation rate are observed (up to 7% or 130 m s^–1) when comparing different sunspots. We found no significant correlation of these fluctuations with characteristics of the sunspots (age, evolution, etc.).Mitteilungen aus dem Kiepenheuer-Institut Nr. 191.     

N–S Asymmetry in the Solar Differential Rotation During 1957–1993

The properties of the differential rotation of the Sun are investigated by using H filaments as tracers. Annual average angular velocities of 716 quiescent filaments are determined from H photoheliograms of the Abastumani Astrophysical Observatory film collection for the years 1957–1993. The existence of north-south (N–S) asymmetry in H filaments rotation is confirmed statistically. The connection of asymmetry with the solar activity cycles is established. It is found that the northern hemisphere rotates faster during the even cycles (20 and 22) while the rotation of southern hemisphere dominates in odd ones (cycles 19 and 21). The mechanism of the solar activity should be responsible for the N–S asymmetry of the solar differential rotation. A theoretical explanation for the N–S asymmetry in the Suns rotation is offered. It is suggested that the asymmetry in the rotation of the two hemispheres of the Sun is balanced by the dynamo mechanism, which acts in parallel to the mechanism offered here. It is concluded that the N–S asymmetry of the solar rotation should cause a difference in activity level between the northern and southern hemispheres.     

Sunspot Plumes and Flow Channels

It is well known that sunspots are dark. This statement is not correct in the sunspot atmosphere between the chromosphere and the corona, where sunspots often are brighter than their surroundings. The brightest feature in the sunspot transition region is called a sunspot plume. Not all sunspots contain a plume. We find that 20 out of 21 sunspots show a plume when one magnetic polarity dominates the sunspot region out to a distance of 50 from the sunspot. Most sunspots show downflows that exceed 25 km s^–1 in the sunspot plumes at temperatures close to 250000 K. This downflow is not maintained by inflow from the corona, but by gas at transition region temperatures, streaming in flow channels from locations well outside the sunspot. We suggest that this inflow is a necessary requirement for the sunspot plume to occur and present a working hypothesis for the origin of sunspot plumes. This paper is the first thorough spectral analysis of sunspot plumes. It is based on simultaneous observations of ten or six EUV emission lines in 42 sunspot regions with the Coronal Diagnostic Spectrometer – CDS on the Solar and Heliospheric Observatory – SOHO. The line profiles are studied in detail with another SOHO instrument, the Solar Ultraviolet Measurements of Emitted Radiation – SUMER.     

Open magnetic fields and the solar cycle

Models of open magnetic structures on the Sun are presented for periods near solar minimum (CR 1626–1634) and near solar maximum (CR 1668–1678). Together with previous models of open magnetic structures during the declining phase (CR 1601–1611) these calculations provide clues to the relations between open structures, coronal holes, and active regions at different times of the solar cycle. Near solar minimum the close relation between active regions and open structures does not exist. It is suggested that near solar minimum the systematic emergence of new flux with the proper polarity imbalance to maintain open magnetic structures may occur primarily at very small spatial scales. Near solar maximum the role of active regions in maintaining open structures and coronal holes is strong, with large active regions emerging in the proper location and orientation to maintain open structures longer than typical active region lifetimes. Although the use of He I 10830 Å spectroheliograms as a coronal hole indicator is shown to be subject to significant ambiguity, the agreement between calculated open structures and coronal holes determined from He I 10830 Å spectroheliograms is very good. The rotation properties of calculated open structures near solar maximum strongly suggest two classes of features: one that rotates differentially similar to sunspots and active regions and a separate class that rotates more rigidly, as was the case for single large coronal holes during Skylab.     

Solar meridional motions derived from sunspot observations

Sunspot drawings obtained at the National Astronomical Observatory of Japan during the years 1954–1986 were used to determine meridional motions of the Sun. A meridional flow of a few ms^–1 was found, which is equatorward in the latitude range from -20° to +15° and is poleward at higher latitudes in both hemispheres. A northward flow of 0.01° day^–1 or 1.4 ms^–1 at mid-latitudes (between 10° and 20°) was also detected. From our limited data-set of three solar cycles, an indication of solar-cycle dependence of meridional motions was found.     

Rotation of Plasma in Sunspots

In this paper we present the results of a sunspot rotation study using Abastumani Astrophysical Observatory photoheliogram data for 324 sunspots. The rotation amplitudes vary in theinebreak 2–64° range (with maximum at 12–14°), and the periods around 0–20 days (with maximum atinebreak 4–6 days). It could be concluded that sunspot rotations are rather inhomogeneous and asymmetric, but several types of sunspots are distinguished by their rotational parameters.During solar activity maximum, sunspot average rotation periods and amplitudes slightly increase. This can be affected by the increase of sunspot magnetic flux tube depth. So we can suppose that sunspot formation during solar activity is connected to a rise of magnetic tubes from deeper layers of the solar photosphere, strengthening the processes within the tube and causing variations in rotation.There is a linear relation between tilt-angle oscillation periods and amplitudes, showing higher amplitudes for large periods. The variations of those periods and especially amplitudes have a periodical shape for all types of sunspots and correlate well with the solar activity maxima with a phase delay of about 1–2 years.     

Measurement of Kodaikanal White-Light Images – III. Rotation Rates and Activity Cycle Variations

The Kodaikanal sunspot data set covering the interval 1906–1987 is analyzed for differential rotation of sunspots of different sizes. As is known, smaller sunspots rotate faster than larger sunspots, and this result is verified in the analysis of this data set. These results agree well with the Mount Wilson sunspot results published earlier. The activity cycle dependence of sunspot rotation is studied. An increase in this rate at the minimum phase is seen, which has been reported earlier. It is demonstrated that this cycle variation is seen for sunspots in all size categories, which suggests that it is not a relative increase in the number of the faster-rotating small sunspots that causes the cycle dependence. These results are discussed as they may relate to subsurface dynamic properties of the Sun.