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.     

Variations of the Solar Differential Rotation Associated with Polarity Reversal

Variations of solar differential rotation have been studied using observations of solar quiescent Hα filaments obtained during 1965–1993 at the Abastumani Astrophysical Observatory. In both hemispheres of the Sun, propagation of a quasi-biennial pulse of residual rotation velocities of filaments was found. There is a pulse drift from high latitudes to the equator in the northern hemisphere in 1968–1970, 1979–1981, 1988–1990 and in the southern one in 1969–1971, 1979–1981, 1989–1991. Propagation of a pulse starts near the time of the polarity reversal of the circumpolar regions of the Sun. High-latitude double peaks of rapid motion were found in the northern hemisphere for cycle 20 and in the southern hemisphere for cycle 22. The relation of the appearance of suggested double pulse peaks of residual velocities with the threefold polarity changing of the circumpolar areas is suggested.     

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.     

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.     

Large-Scale Structure of the Sunspot Activity and the Background Magnetic Field Formation

Large-scale distribution of the sunspot activity of the Sun has been analyzed by using a technique worked out previously (Erofeev, 1997) to study long-lived, non-axisymmetric magnetic structures with different periods of rotation. Results of the analysis have been compared with those obtained by analyzing both the solar large-scale magnetic field and large-scale magnetic field simulated by means of the well-known flux transport equation using the sunspot groups as a sole source of new magnetic flux in the photosphere. A 21-year period (1964–1985) has been examined.The rotation spectra calculated for the total time interval of two 11-year cycles indicate that sunspot activity consists of a series of discrete components (modes) with different periods of rotation. The largest-scale component of the sunspot activity reveals modes with 27-day and 28-day periods of rotation situated, correspondingly, in the northern and southern hemispheres of the Sun, and two modes with rotation periods of about 29.7 days situated in both hemispheres. Such a modal structure of the sunspot activity agrees well with that of the large-scale solar magnetic field. Moreover, the magnetic field distribution simulated with the flux transport equation also reveals the same modal structure. However, such an agreement between the large-scale solar magnetic field and both the sunspot activity and simulated magnetic field is unstable in time; so, it is absent in the northern hemisphere of the Sun during solar cycle No. 20. Thus the sources of magnetic flux responsible for formation of the large-scale, rigidly rotating magnetic patterns appear to be closely connected, but are not identical with the discrete modes of the sunspot activity.     

Torsional oscillations of the Sun

A series of digitized synoptic observations of solar magnetic and velocity fields has been carried out at the Mount Wilson Observatory since 1967. In recent studies (Howard and LaBonte, 1980; LaBonte and Howard, 1981), the existence of slow, large-scale torsional (toroidal) oscillations of the Sun has been demonstrated. Two modes have been identified. The first is a travelling wave, symmetric about the equator, with wave number 2 per hemisphere. The pattern-alternately slower and faster than the average rotation-starts at the poles and drifts to the equator in an interval of 22 years. At any one latitude on the Sun, the period of the oscillation is 11 years, and the amplitude is 3 m s^-1. The magnetic flux emergence that is seen as the solar cycle occurs on average at the latitude of one shear zone of this oscillation. The amplitude of the shear is quite constant from the polar latitudes to the equator. The other mode of torsional oscillation, superposed on the first mode, is a wave number 1 per hemisphere pattern consisting of faster than average rotation at high latitudes around solar maximum and faster than average rotation at low latitudes near solar minimum. The amplitude of the effect is about 5 m s^-1. For the first mode, the close relationship in latitude between the activity-related magnetic flux eruption and the torsional shear zone suggests strongly that there is a close connection between these motions and the cycle mechanism. It has been suggested (Yoshimura, 1981; Schüssler, 1981) that the effect is caused by a subsurface Lorentz force wave resulting from the dynamo action of magnetic flux ropes. But, this seems unlikely because of the high latitudes at which the shear wave is seen to originate and the constancy of the magnitude of the shear throughout the life time of the wave.     

Long term evolution of solar sector structure

The large-scale structure of the solar magnetic field during the past five sunspot cycles (representing by implication a much longer interval of time) has been investigated using the polarity (toward or away from the Sun) of the interplanetary magnetic field as inferred from polar geomagnetic observations. The polarity of the interplanetary magnetic field has previously been shown to be closely related to the polarity (into or out of the Sun) of the large-scale solar magnetic field. It appears that a solar structure with four sectors per rotation persisted through the past five sunspot cycles with a synodic rotation period near 27.0 days, and a small relative westward drift during the first half of each sunspot cycle and a relative eastward drift during the second half of each cycle. Superposed on this four-sector structure there is another structure with inward field polarity, a width in solar longitude of about 100° and a synodic rotation period of about 28 to 29 days. This 28.5 day structure is usually most prominent during a few years near sunspot maximum. Some preliminary comparisons of these observed solar structures with theoretical considerations are given.     

On the rotation of large-scale background fields in the 21st cycle of solar activity

We study some peculiarities of the time variation of dipole components in the longitudinal field distribution in individual low-latitude belts of the Sun. For analyzing the horizontal dipole rotation and variations of amplitudes we used magnetic and H data.From 1979 to 1981 the rotation of the dipoles of the northern and southern low-latitude belts (0°–30° N and 10°–40° S) occurs with periods of about 26.8 days (N) and 28.2 days (S), in agreement with the results reported by Antonucci, Hoeksema, and Scherrer (1990) and Hoeksema and Scherrer (1987). A uniform rotation of the low-latitude dipoles of these belts continued until the end of 1981. Following the next coincidence of the magnetic poles in longitude the dipoles change in their rotation character. During about 15–20 rotations the low-latitude dipoles co-rotate with a new period close to the Carrington period. This is followed by a rapid (in 3–5 rotations) transition of the poles to a new stable state, also with the Carrington rotation period. The change in rotation and dynamics of the low-latitude dipoles at the end of 1981-beginning of 1982 can be explained either by a mutual penetration of the fields of different hemispheres to the opposite hemisphere or by the onset of the formation of relatively shortlived (15–20 rotations) structures which cover the entire low-latitude belt.Unlike the trajectories of the poles, the dipole amplitudes of the low-latitude belts showed a significant variability. However, simultaneous increases of the amplitudes in both hemispheres correlated with times at which the dipole poles coincide in longitude, and the greatest increase corresponded to the moment of merging of the dipole poles early in 1982. This suggests that sources of large-scale structures of the background field in the low-latitude belts of the Sun or the related fields interacted when the dipole poles coincided.     

Interplanetary Magnetic Flux Ropes as Agents Connecting Solar Eruptions and Geomagnetic Activities

We investigate the solar wind structure for 11 cases that were selected for the campaign study promoted by the International Study of Earth-affecting Solar Transients (ISEST) MiniMax24 Working Group 4. We can identify clear flux rope signatures in nine cases. The geometries of the nine interplanetary magnetic flux ropes (IFRs) are examined with a model-fitting analysis with cylindrical and toroidal force-free flux rope models. For seven cases in which magnetic fields in the solar source regions were observed, we compare the IFR geometries with magnetic structures in their solar source regions. As a result, we can confirm the coincidence between the IFR orientation and the orientation of the magnetic polarity inversion line (PIL) for six cases, as well as the so-called helicity rule as regards the handedness of the magnetic chirality of the IFR, depending on which hemisphere of the Sun the IFR originated from, the northern or southern hemisphere; namely, the IFR has right-handed (left-handed) magnetic chirality when it is formed in the southern (northern) hemisphere of the Sun. The relationship between the orientation of IFRs and PILs can be taken as evidence that the flux rope structure created in the corona is in most cases carried through interplanetary space with its orientation maintained. In order to predict magnetic field variations on Earth from observations of solar eruptions, further studies are needed about the propagation of IFRs because magnetic fields observed at Earth significantly change depending on which part of the IFR hits the Earth.     

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.     

Photospheric Magnetic Field: Relationship Between North–South Asymmetry and Flux Imbalance

Photospheric magnetic fields were studied using the Kitt Peak synoptic maps for 1976?–?2003. Only strong magnetic fields (B>100 G) of the equatorial region were taken into account. The north–south asymmetry of the magnetic fluxes was considered as well as the imbalance between positive and negative fluxes. The north–south asymmetry displays a regular alternation of the dominant hemisphere during the solar cycle: the northern hemisphere dominated in the ascending phase, the southern one in the descending phase during Solar Cycles 21?–?23. The sign of the imbalance did not change during the 11 years from one polar-field reversal to the next and always coincided with the sign of the Sun’s polar magnetic field in the northern hemisphere. The dominant sign of leading sunspots in one of the hemispheres determines the sign of the magnetic-flux imbalance. The sign of the north–south asymmetry of the magnetic fluxes and the sign of the imbalance of the positive and the negative fluxes are related to the quarter of the 22-year magnetic cycle where the magnetic configuration of the Sun remains constant (from the minimum where the sunspot sign changes according to Hale’s law to the magnetic-field reversal and from the reversal to the minimum). The sign of the north–south asymmetry for the time interval considered was determined by the phase of the 11-year cycle (before or after the reversal); the sign of the imbalance of the positive and the negative fluxes depends on both the phase of the 11-year cycle and on the parity of the solar cycle. The results obtained demonstrate the connection of the magnetic fields in active regions with the Sun’s polar magnetic field in the northern hemisphere.     

Dynamo action in simulations of penetrative solar convection with an imposed tachocline

We summarize new and continuing three-dimensional spherical shell simulations of dynamo action by convection allowed to penetrate downward into a tachocline of rotational shear. The inclusion of an imposed tachocline allows us to examine several processes believed to be essential in the operation of the global solar dynamo, including differential rotation, magnetic pumping, and the stretching and organization of fields within the tachocline. In the stably stratified core, our simulations reveal that strong axisymmetric magnetic fields (of ∼ 3000 G strength) can be built, and that those fields generally exhibit a striking antisymmetric parity, with fields in the northern hemisphere largely of opposite polarity to those in the southern hemisphere. In the convection zone above, fluctuating fields dominate over weaker mean fields. New calculations indicate that the tendency toward toroidal fields of antisymmetric parity is relatively insensitive to initial magnetic field configurations; they also reveal that on decade-long timescales, the magnetic fields can briefly enter (and subsequently emerge from) states of symmetric parity.We have not yet observed any overall reversals of the field polarity, nor systematic latitudinal propagation. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

North–South Asymmetry in the Distribution of Solar Background Magnetic Field

The aim of this article is to investigate how the background magnetic field of the Sun behaves in different hemispheres. We used SOHO/MDI data obtained during a period of eight years from 2003 to 2011 to analyze the intensity distribution of the background magnetic field over the solar surface. We find that the background fields of both polarities (signs) are more intense in the southern than in the northern hemisphere. Mixed polarities are observed in the vicinity of the equator. In addition to the main field, a weaker field of opposite polarity is always present in the polar regions. In the declining phase of the cycle, the main field dominates, but at the minimum and in the rising phase of the cycle, it is gradually replaced by the growing stronger secondary field.     

Electric current helicity in the solar atmosphere

In the theories of solar magnetism, kinetic and magnetic helicities, which arise as a consequence of the rotation of the Sun, play a key role. The dynamo for the main field is assumed to operate in the convection zone. The solar rotation also may be the ultimate cause for the generation of dc electric currents in the atmosphere, needed as the energy source for flares. Then in the atmosphere the electric current helicity, H _C = B · × B, which is a pseudo-scalar quantity, should be antisymmetric about the equatorial plane. An inspection of 16 active regions, for which H _C has been estimated by using extrapolation of measured photospheric magnetic fields, leads to the result that the electric current helicity is predominantly negative in the northern and positive in the southern hemisphere. The helicity of the large-scale currents generated according to standard dynamo theory by the alpha effect in the convection zone is just opposite in sign. Current generation due to rotational motions of sunspots and other magnetic elements in accordance with the global differential rotation, i.e., counter-clockwise in the northern and clockwise in the southern hemisphere, however, can explain the rule found. Also in some alternative dynamo models for the global field, in which the dynamo operates at the base of the convection zone, the large-scale current helicity generated by the alpha effect has the sign needed.     

<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.     

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.     

Simulation of the apparent anomalies of the galactic structure

In the maps of the galactic structure based on the kinematical method, several systematic heliocentric anomalies are found: in the northern galactic hemisphere the spiral arms are more tightly wound and the extent of neutral hydrogen is smaller than in the southern hemisphere; with separate rotation curves for the north and the south the arms become anomalously circular with a consequent discrepancy to the stellar distribution; there are straight portions in the arms pointing towards the Sun, as well as systematic strong curvatures and knee-like features; the inner arms affect the structure of the outer arms; with the northern rotation model, Hii-regions and Hi avoid the southern tangential circle; in the rear of the Galaxy, at symmetric longitudes, enhanced Hi-densities are found; the Perseus arm is displaced atl=180°. All of these anomalies can be explained with a simple model involving a non-velocity redshift field within the Galaxy, with an enhancement within the spiral arms. This is demonstrated by numerical simulations of the structural anomalies. Reducing the redshift effect from the kinematic data, the Galaxy's structure and kinematics appear symmetric. The significance of the result for the redshift problem is discussed.     

The Hale solar sector boundary

A Hale solar sector boundary is defined as the half (northern hemisphere or southern hemisphere) of a sector boundary in which the change of sector magnetic field polarity is the same as the change of polarity from a preceding spot to a following spot. Above a Hale sector boundary the green corona has maximum brightness, while above a non-Hale boundary the green corona has a minimum brightness. The Hale portion of a photospheric sector boundary tends to have maximum magnetic field strength, while the non-Hale portion has minimum field strength.     

Non-Axisymmetric Magnetic Fields and Flip-Flops on the Sun and Cool Stars

The modulation of solar activity closely follows the solar rotation period suggesting the existence of long-lived active regions at preferred longitudes. For instance, two preferred active longitudes in both southern and northern hemispheres are found to be persistent at the century time scale. These regions migrate with differential rotation and periodically alternate their activity levels showing a flip-flop cycle. The pattern and behaviour of active longitudes on the Sun is similar to that on cool, rapidly rotating stars with outer convective envelopes. This suggests that the magnetic dynamo, including non-axisymmetric magnetic fields and flip-flop cycles, is also similar in these stars. This allows us to overview the phenomenon of stellar magnetic activity and to study it in detail on the Sun.     

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.