Investigation of the solar differential rotation by hydrogen filaments in 1976–1986

Based on observational data on chromosphere filaments, certain characteristics of solar differential rotation during solar activity cycle No. 21 are determined at Abastumani Astrophysical Observatory.In the northern hemisphere of the Sun, propagation of a quasi-bi-annual impulse of the rotation residual from high latitudes to the equator is found in 1979–1981. It is supposed that this phenomenon might be related to the polarity reversal in the northern hemisphere of the Sun in 1981.0.     

Propagation of a quasi bi-annual impulse close to the moment of the solar magnetic field polarity changing

Some particular features of solar differential rotation have been recently revealed in Abastumani Astrophysical Observatory: in the northern hemisphere of the Sun a propagation of a quasi bi-annual impulse of the rotation residual from the high latitudes to the equator, during the time interval of 1979–1981, was statistically demonstrated. Japaridze and Gigolashvili (1992) proposed that this event might be related with the change of magnetic polarity in the northern hemisphere of the Sun in 1981.To prove this hypothesis an investigation of the MHD equations in the local system of coordinates has been carried out. A homogeneous equation with partial derivatives in the linear approximation was obtained. Its solution is presented analytically. The disturbance of velocity enhances especially at the moment of reversal of magnetic field polarity.     

Investigation of the Solar Differential Rotation of Compact Magnetic Elements for 1966 – 1986

The differential rotation of compact magnetic elements during activity cycles 20 and 21 (1966 – 1986) is studied by using solar synoptic charts. For each hemisphere the compact magnetic elements with the polarity of the circumpolar magnetic field have larger rotation rates than the elements with the opposite polarity. This difference in rotation rates is present during the whole cycle except during the polarity reversal of the circumpolar field.     

Dynamics of the circumpolar magnetic field of the Sun at a maximum of cycle 24

Data on the value and sign of the circumpolar magnetic field of the Sun at a maximum of its activity in cycle 24 have been analyzed. The data were obtained from observations at the Wilcox Solar Observatory and from synoptic maps of the magnetic field built in the SOLIS project (SOLIS stands for Synoptic Optical Long-term Investigations of the Sun) and with the Helioseismic and Magnetic Imager (HMI). We studied the dynamics of the total magnetic fields in the circumpolar latitudinal zones of different extension in the northern and southern hemispheres. The epochs of the sign reversal of the polar magnetic field were determined. It was found that, in cycle 24, the magnetic field polarity changed three times in the northern hemisphere and only once in the southern one. In the northern hemisphere, the reversal of the polar magnetic field finished approximately a year earlier than that in the southern one. The obtained results are compared to the data on the sign reversal of the polar magnetic field of the Sun reported for the previous solar cycles.     

Systematic Time Delay of Hemispheric Solar Activity

Five solar-activity indices – the monthly-mean sunspot numbers from January 1945 to March 2008, the monthly-mean sunspot areas during the period of May 1874 to March 2008, the monthly numbers of sunspot groups from May 1874 to May 2008, the monthly-mean flare indices from January 1966 to December 2006, and the numbers of solar filaments per Carrington rotation in the time interval of solar rotations 876 to 1823 – have been used to show a systematic time delay between northern and southern hemispheric solar activities in a cycle. It is found that solar activity does not occur synchronously in the northern and southern hemispheres, and there is a systematic time lag or lead (phase shift) between northern and southern hemispheric solar activity in a cycle. About an eight-cycle period is inferred to exist in such phase shifts. The activity on the Sun may be governed by two different and coupled processes, not by a single process.     

Latitude Dependence of the “Gnevyshev” Peaks and Gaps

The occurrence of double peaks near the maximum of sunspot activity was first emphasized by Gnevyshev (Solar Phys. 1, 107, 1967) for the peak years of solar cycle 19 (1954 – 1964). In the present analysis, it is shown that double peaks in sunspot numbers were clearly visible in solar latitudes 10 – 30° N but almost absent in the southern latitudes, where some single peaks were observed out of phase by several months from any of the peaks in the northern latitudes. The spacing between the double peaks increased from higher to lower northern latitudes, hinting at latitudinal migration. In the next cycle 20 (1965 – 1976), which was of about half the strength of cycle 19, no clear-cut double peaks were seen, and the prominent peak in the early part of 1967 in the northern latitudes was seen a few months later in the southern latitudes. A direct relationship of Gnevyshev peaks with changes in the solar polar magnetic fields seems to be dubious. The commencements do not match.     

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.     

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.     

Statistical study of the north–south asymmetry of the solar differential rotation based on various solar structures during 1966–1985

The time variation and latitude dependence of the solar rotation are found using observational data on Hα filaments and compact magnetic features with different polarities during solar activity cycles 20 and 21 (1966–1985). Statistical analysis of the observational data shows that there is a north–south asymmetry in the rotation, both for the Hα filaments and for compact magnetic features (structures) with negative and positive polarities. The N-S asymmetry in the differential rotation of the Hα filaments and the compact magnetic features with both polarities shows up quite distinctly in solar activity cycles 20 and 21, but the asymmetry for the compact magnetic features with positive polarity is comparatively lower in cycle 21. The confidence level is lower the compact magnetic features with positive polarity than for the compact magnetic features with negative polarity.     

<Emphasis Type="Italic">In Situ</Emphasis> Signatures of Interchange Reconnection between Magnetic Clouds and Open Magnetic Fields: A Mechanism for the Erosion of Polar Coronal Holes?

We outline a method to determine the direction of solar open flux transport that results from the opening of magnetic clouds (MCs) by interchange reconnection at the Sun based solely on in-situ observations. This method uses established findings about i) the locations and magnetic polarities of emerging MC footpoints, ii) the hemispheric dependence of the helicity of MCs, and iii) the occurrence of interchange reconnection at the Sun being signaled by uni-directional suprathermal electrons inside MCs. Combining those observational facts in a statistical analysis of MCs during solar cycle 23 (period 1995 – 2007), we show that the time of disappearance of the northern polar coronal hole (1998 – 1999), permeated by an outward-pointing magnetic field, is associated with a peak in the number of MCs originating from the northern hemisphere and connected to the Sun by outward-pointing magnetic field lines. A similar peak is observed in the number of MCs originating from the southern hemisphere and connected to the Sun by inward-pointing magnetic field lines. This pattern is interpreted as the result of interchange reconnection occurring between MCs and the open field lines of nearby polar coronal holes. This reconnection process closes down polar coronal hole open field lines and transports these open field lines equatorward, thus contributing to the global coronal magnetic field reversal process. These results will be further constrainable with the rising phase of solar cycle 24.     

ON THE POSSIBLE CHANGES OF THE SOLAR DIFFERENTIAL ROTATION DURING THE ACTIVITY CYCLE DETERMINED USING MICROWAVE LOW-BRIGHTNESS-TEMPERATURE REGIONS AND Hα FILAMENTS AS TRACERS

The solar rotation rate obtained using the microwave Low-brightness-Temperature Regions (LTRs) as tracers in the heliographic range ± 55° from the years 1979–1980, 1981–1982, 1987–1988, and 1989–1991 varied from 3% to 4% in medium latitudes, and below 1% at the equator. Using H filaments as tracers at higher latitudes from the years 1979, 1980, 1982, 1984, and 1987, the solar rotation rate variation was between 2% and 8%. This represents an upper limit on the rotation rate variation during the solar activity cycle. Such changes could be caused by short-lived, large-scale velocity patterns on the solar surface. The Sun revealed a higher rotation rate on the average during the maxima of the solar activity cycles 21 and 22, i.e., in the periods 1979–1980 and 1989–1991, respectively, which differs from the rotation rates (lower on the average) in some years, 1981–1982 and 1987–1988, between the activity maximum and minimum (LTR data). Simultaneous comparison of rotation rates from LTRs and H filament tracings was possible in very limited time intervals and latitude bands only, and no systematic relationship was found, although the rotation rates determined by LTRs were mostly smaller than the rotation rates determined by H filaments. The errors obtained by applying different fitting procedures of the LTR data were analyzed, as well as the influence of the height correction. Finally, the north–south asymmetry in the rotation rate investigated by LTRs indicates that the southern solar hemisphere rotated slower in the periods under consideration, the difference being about 1%. The reliability of all obtained results is discussed and a comparison with other related studies was performed.     

Solar-Cycle-Related Variations in the Solar Differential Rotation and Meridional Flow: A Comparison

We have analysed a large set of sunspot group data (1874 – 2004) and find that the meridional flow strongly varies with the phase of the solar cycle, and the variation is quite different in the northern and the southern hemispheres. We also find the existence of considerable cycle-to-cycle variation in the meridional velocity, and about a 11-year difference between the phases of the corresponding variations in the northern and the southern hemispheres. In addition, our analysis also indicates the following: (i) the existence of a considerable difference (about 180^°) between the phases of the solar-cycle variations in the latitude-gradient terms of the northern and the southern hemispheres’ rotations; (ii) the existence of correlation (good in the northern hemisphere and weak in the southern hemisphere) between the mean solar-cycle variations of meridional flow and the latitude-gradient term of solar rotation; (iii) in the northern hemisphere, the cycle-to-cycle variation of the mean meridional velocity leads that of the equatorial rotation rate by about 11 years, and the corresponding variations have approximately the same phase in the southern hemisphere; and (iv) the directions of the mean meridional velocity is largely toward the pole in the longer sunspot cycles and largely toward the equator in the shorter cycles.     

Solar-Cycle Characteristics Examined in Separate Hemispheres: Phase,Gnevyshev Gap,and Length of Minimum

According to research results from solar-dynamo models, the northern and southern hemispheres may evolve separately throughout the solar cycle. The observed phase lag between the northern and southern hemispheres provides information regarding how strongly the hemispheres are coupled. Using hemispheric sunspot-area and sunspot-number data from Cycles 12 – 23, we determine how out of phase the separate hemispheres are during the rising, maximum, and declining period of each solar cycle. Hemispheric phase differences range from 0 – 11, 0 – 14, and 2 – 19 months for the rising, maximum, and declining periods, respectively. The phases appear randomly distributed between zero months (in phase) and half of the rise (or decline) time of the solar cycle. An analysis of the sunspot cycle double peak, or Gnevyshev gap, is conducted to determine if the double-peak is caused by the averaging of two hemispheres that are out of phase. We confirm previous findings that the Gnevyshev gap is a phenomenon that occurs in the separate hemispheres and is not due to a superposition of sunspot indices from hemispheres slightly out of phase. Cross hemispheric coupling could be strongest at solar minimum, when there are large quantities of magnetic flux at the Equator. We search for a correlation between the hemispheric phase difference near the end of the solar cycle and the length of solar-cycle minimum, but found none. Because magnetic flux diffusion across the Equator is a mechanism by which the hemispheres couple, we measured the magnetic flux crossing the Equator by examining Kitt Peak Vacuum Telescope and SOLIS magnetograms for Solar Cycles 21 – 23. We find, on average, a surplus of northern hemisphere magnetic flux crossing during the mid-declining phase of each solar cycle. However, we find no correlation between magnitude of magnetic flux crossing the Equator, length of solar minima, and phase lag between the hemispheres.     

The Long-term Behavior of the North – South Asymmetry of Sunspot Activity

A new index, the cumulative difference of sunspot activity in the northern and southern hemispheres, respectively, is proposed to describe the long-term behavior of the North – South asymmetry of sunspot activity and to show the balance (or bias) of sunspot activity in the two solar hemispheres on a long-term scale. Sunspot groups and sunspot areas from June 1874 to January 2007 are used to show the advantage of the index. The index clearly shows a long-term characteristic time scale of about 12 cycles in the North – South asymmetry of sunspot activity. Sunspot activity is found to dominate in the southern hemisphere in cycle 23, and in cycle 24 it is predicted to dominate still in the southern hemisphere. A comparison of the new index with other similar indexes is also given.     

Structure of the maximum phase of solar cycles 21 and 22

A distinctive peak and gap structure in a number of solar indices was observed in the maximum phase of solar cycles 21 and 22. The effect became even more prominent after separating the northern and southern solar hemispheres. In cycle 21 the multi-peaked structures observed in the two solar hemispheres were not synchronous and their sum resulted in the rather shallow two-peaked solar maximum for the parameters taken over the whole solar disk. In cycle 22 there were only double peaks in each hemisphere which were rather synchronous. Examination of solar activity in the northern and southern hemispheres has shown that the structured maximum appears to be due to the superposition of two quasi-oscillating processes with characteristic time-scales of 11 years and of 1–3 years (quasi-biennial oscillations). The absolute amplitude of the quasi-biennial oscillations depends on the 11-year cycle phase and reaches its maximum at the maximum of the 11-year cycle. This explains the occurrence of a double- or triple-peak structure in the solar maximum phase.     

N–S asymmetry in the differential rotation of the sun and its variation with the solar activity cycles

Peculiarities in the characteristics of the solar differential rotation are investigated using hydrogen filaments as tracers. The existence of North–South (N–S) asymmetry in hydrogen 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 (Cycles 20 and 22) while the rotation of southern hemisphere dominates in odd one (Cycle 21). The mechanism of the solar activity should be responsible for the N–S asymmetry of the solar differential rotation.     

Solar Rotation Rate during Solar Activity Cycle 23

We studied the solar rotation rate and its temporal change, using the sunspot data obtained during activity cycle 23 (1996 – 2006). The equatorial rotation rate is nearly the same as in the former cycle 22, while the latitudinal gradient of differential rotation considerably increased. Comparison of our results with others indicates the existence of a long-term periodicity of about eight cycles in differential rotation. In addition, no significant asymmetry in differential rotation between the northern and southern hemispheres during cycle 23 was found. The equatorial rotation rate and the latitudinal gradient of the differential rotation in the period of cycle 23 are approximately constant, except for the initial and final phases in the cycle.     

Return Meridional Flow in the Convection Zone from Latitudinal Motions of Umbrae of Sunspot Groups

We have derived the velocities of meridional flows by measuring the latitudinal motions (or drifts) of umbrae of spot groups classified into three categories of area: 0 – 5 μ, 5 – 10 μ, and >10 μ (μ area in millionths of the solar hemisphere). The latitudinal drifts (or the meridional flows) in all three categories are directed equatorward in both the northern and southern hemispheres. By sorting the spot groups into three area classes, we are able to relate the respective latitudinal drifts with the three depths in the convection zone where the footpoints of the flux loops of the spot groups of each area class are anchored. We obtain estimates of the anchor depths through a comparison of the rotation rates of the spot groups of each area class with the rotation-rate profiles from helioseismic inversions. The equatorward drifts obtained provide estimates of the meridional flows at the three depths in the convection zone and thereby suggest the presence of return meridional flows as envisaged in the flux-transport dynamo models, which have remained undetected so far. The data sources for this study are measurements of positions and areas of umbrae of sunspots from the photographic white-light images of the Sun of the Kodaikanal Observatory archives for the period 1906 – 1987 and a very similar, but independent, data set from the Mt. Wilson Observatory archives for the period 1917 – 1985.     

Large-Scale Magnetic Field and Sunspot Cycles

Hα magnetic synoptic charts of the Sun are processed for 1915–1999 and the spherical harmonics are calculated. It is shown that the polarity distribution of the magnetic field on Hα charts is similar to the polarity distribution of the Stanford magnetic field observations during 1975–1999. The index of activity of the large-scale magnetic field A(t), representing the sum of the intensities of dipole and octupole components, is introduced. It is shown that the cycle of the large-scale magnetic field of the Sun precedes on the average by 5.5 years the sunspot activity cycle, W(t). This means that the weak large-scale magnetic fields of the Sun do not result from decay and diffusion of strong fields from active regions as it is supposed in all modern theories of the solar cycle. On the basis of the new data the intensity of the current solar cycle 23 is predicted and some aspects of the theory of the solar cycle are discussed.     

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.