Structure and Cyclic Variations of Open Magnetic Fields in the sun

The structure and variations of open field regions (OFRs) are analyzed against the solar cycle for the time interval of 1970–1996. The cycle of the large-scale magnetic field (LSMF) begins in the vicinity of maximum Wolf numbers, i.e. during the polar field reversal. At the beginning of the LSMF cycle, the polar and mid-latitude magnetic field systems are connected by a narrow bridge, but later they evolve independently. The polar field at the latitudes above 60° has a completely open configuration and fills the whole area of the polar caps near the cycle minimum of local fields. At this time, essentially all of the open solar flux is from the polar caps. The mid-latitude open field regions (OFRs) occur at a latitude of 30–40° away from solar minimum and drift slowly towards the equator to form a typical 'butterfly diagram' at the periphery of the local field zone. This supports the concept of a single complex – 'large-scale magnetic field – active region – coronal hole'. The rotation characteristics of OFRs have been analyzed to reveal a near solid-body rotation, much more rigid than in the case of sunspots. The rotation characteristics are shown to depend on the phase of the solar cycle.     

Hale's attempts to determine the sun's general magnetic field

Hale's attempts to determine the sun's general magnetic field are reviewed. The field reported by Hale was an order of magnitude stronger than that presently measured with photoelectric techniques. The polarity was opposite to that expected from Babcock's theory of the solar cycle. Practically all the reduction work had been made by Van Maanen with a tipping-plate micrometer.To free the reductions from possible personal bias, a few hundred of the plates from the 1914 series were remeasured by the author with the digitized microphotometer at the Sacramento Peak Observatory. The line profiles were recorded on magnetic tape, and the computations of the Zeeman displacements were made using a CD 3600 at Uppsala.The same plates had been measured visually by Van Maanen. His results show a neat variation of field strength with heliographic latitude, with a maximum of about 11 G at latitudes + and –45 °. The solar equator forms a sharp demarcation line between the opposite polarities in the two hemispheres. In contrast, the computer reductions do not reveal any significant field at any latitude. An approximate upper limit for the observed field strength is 5 G. There is no correlation between the new results and the old values by Van Maanen.     

Spatial variations of background magnetic field polarity distribution on the Sun

Based on an analysis of the latitude and longitude regularities in the distribution of the sign of the background magnetic field (BMF) on the solar surface for 14 years (1969–1982), a classification of BMF distributions in the form of synoptic maps, is proposed. That the low- and high-latitude BMF distributions are differing in character appears to be due to the difference in the character of the BMF rotation at low and high latitudes. It is shown that as low-latitude BMF details make contact with the high-latitude field of the same polarity, the former grow in area. The low- and high-latitude fields come into contact through high-latitude field details protruding into lower latitudes as far as 10 to 15 ° below 40 ° latitude, and they are referred to as bulges. Bulges and low-latitude fields of like polarity are moving with respect to each other along the E-W line at a mean rate of 10 to 15 ° per rotation. Bulges of the same polarity in the same hemisphere are moving with respect to each other 3–5 ° per rotation, on the average. The above-mentioned properties of the structure, interaction and rotation of the low- and high-latitude magnetic field details, taken together, provide a qualitative scheme for the formation and variation of the longitude-regular (sectoral) BMF distribution in the latitude range 40 S-40 N.     

Unified theory of the interplanetary magnetic field

A simple model is used to present a unified picture of the polarity pattern of the interplanetary magnetic field observed during the solar cycle. Emphasis in this paper is on the field near solar maximum. The heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field is explained in terms of weak poloidal (dipolar) field sources in the sun's photosphere. Unlike the Babcock theory, the author hypothesizes that the dipolar field exists at equatorial latitudes (0–20°), too, (as well as in polar regions) and that the major source of the interplanetary magnetic field observed near the ecliptic plane is the dipolar field from equatorial latitudes. The polarity of the interplanetary field data taken in 1968 and in the first half of 1969 near solar maximum may possibly be explained in terms of a depression of the dipolar field boundary in space. The effect on the solar wind of the greater activity in the northern hemisphere of the sun that existed in 1968 and in the first half of 1969 is believed responsible for this hypothesized depression, especially near solar maximum, of the plane separating the + and - dipolar polarity below the solar equatorial plane in space. Predictions are made concerning the interplanetary field to be observed near the ecliptic plane in each portion of the next solar cycle.     

Variation of the solar atmospheric rotation over the 11 year cycle

The synodic rotation period and power spectra of solar microwave sources are investigated using accurate data in the interval 1956 to 1970. The variation of the approximate 27 day period is obtained over a complete solar cycle and is thought to be a result of the latitude change over the solar cycle of the origins of the radio emissions. High resolution power spectra have also been obtained and revealed the existence of a double peaked line near 160 day period. This line is attributed to changes in either the Eartn's heliographic latitudes or the Earth's inclination to the Earth-Sun line.     

Solar cycle variation of large-scale coronal structures

A green line intensity variation is associated with the interplanetary and photospheric magnetic sector structure. This effect depends on the solar cycle and occurs with the same amplitude in the latitude range 60° N–60° S. Extended longitudinal coronal structures are suggested, which indicate the existence of closed magnetic field lines over the neutral line, separating adjacent regions of opposite polarities on the photospheric surface.Supported by an ESRO/NASA fellowhip.On leave from Torino University, Italy; now at Istituto di Fisica, Universita di Torino, Italy.     

The Rotation Rates of Solar Magnetic Fields During Solar Cycles 21 – 23

Employing the synoptic maps of the photospheric magnetic fields from the beginning of solar cycle 21 to the end of 23, we first build up a time – longitude stackplot at each latitude between ±35°. On each stackplot there are many tilted magnetic structures clearly reflecting the rotation rates, and we adopt a cross-correlation technique to explore the rotation rates from these tilted structures. Our new method avoids artificially choosing magnetic tracers, and it is convenient for investigating the rotation rates of the positive and negative fields by omitting one kind of field on the stackplots. We have obtained the following results. i) The rotation rates of the positive and negative fields (or the leader and follower polarities, depending on the hemispheres and solar cycles) between latitudes ±35° during solar cycles 21–23 are derived. The reversal times of the leader and follower polarities are usually not consistent with the years of the solar minimum, nevertheless, at latitudes ±16°, the reversal times are almost simultaneous with them. ii) The rotation rates of the three solar cycles averaged over each cycle are calculated separately for the positive, negative and total fields. The latitude profiles of rotation of the positive and negative fields exhibit equatorial symmetries with each other, and those of the total fields lie between them. iii) The differences in rotation rates between the leader and follower polarities are obtained. They are very small near the equator, and increase as latitude increases. In the latitude range of 5° – 20°, these differences reach 0.05 deg day⁻¹, and the mean difference for solar cycle 22 is somewhat smaller than cycles 21 and 23 in these latitude regions. Then, the differences reduce again at latitudes higher than 20°.     

Characteristic size and diffusion of quiet sun magnetic patterns

We have previously studied large-scale motions using high-resolution magnetograms taken from 1978 to 1990 with the NSO Vacuum Telescope on Kitt Peak. Latitudinal and longitudinal motions were determined by a two-dimensional crosscorrelation analysis of pairs of consecutive daily observations using small magnetic features as tracers. Here we examine the shape and amplitude of the crosscorrelation functions. We find a characteristic length scale as indicated by the FWHM of the crosscorrelation functions of 16.6 ± 0.2 Mm. The length scale is constant within ±45° latitude and decreases by about 5% at 52.5° latitude; i.e., the characteristic size is almost latitude independent. The characteristic scale is within 3% of the average value during most times of the solar cycle, but it increases during cycle maximum at latitudes where active regions are present. For the time period 1978–1981 (solar cycle maximum), the length scale increases up to 1.7 Mm or 10% at 30° latitude. In addition, we derive the average amplitude of the crosscorrelation functions, which reflects the diffusion of magnetic elements and their evolutionary changes (including formation and decay). We find an average value of 0.091 ± 0.003 for the crosscorrelation amplitude at a time lag of one day, which we interpret as being caused by the combined effect of the lifetime of magnetic features and a diffusion process. Assuming a lifetime of one day, we find a value of 120 km² s^–1 for the diffusion constant, while a lifetime of two days leads to 230 km² s^–1.Operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation.     

Latitude-time distribution of 3 types of magnetic field activity in the global solar cycle

Large-scale solar activity is considered as a manifestation of 3 types of magnetic field activity which is demonstrated in the 22-year cycle (a) of small-scale flux emergence (polar faculae at latitudes > 40°), (b) of somewhat larger scale flux emergence (sunspots at latitudes < 40°), and (c) of the global magnetic neutral lines at all latitudes. The migration (poleward or equatorward) of the place of birth and/or of the phenomena themselves of these three types of manifestation of magnetic field is discussed. The poleward migration of the global field is explained in a phenomenological way.     

Meridional drift in the large-scale solar magnetic field pattern

A method of two-dimensional correlation functions has been applied to a sequence of synoptic maps of the large-scale magnetic field to obtain the meridional drift pattern of field structures. The meridional drift profile obtained is antisymmetric about the equator. The meridional drift is directed from the equator to the poles at latitudes below 45°. A maximum drift velocity of 11–13 m s^–1 is attained in the latitude range 30°. A picture of the space-time distribution of meridional drift is also obtained, which may be interpreted as resulting from the effect of azimuthal convective rolls (3 rolls per hemisphere) on the large-scale magnetic field. Rolls originate at high latitudes following the cycle maximum, and migrate equatorwards until the minimum of the next cycle. The picture in the equatorial region can correspond to convective rolls with lifetimes of about two years, or to the process of interaction of rolls from two hemispheres.     

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.     

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.     

Meridional motions of magnetic features in the solar photosphere

We cross-correlate pairs of Mt. Wilson magnetograms spaced at intervals of 24–38 days to investigate the meridional motions of small magnetic features in the photosphere. Our study spans the 26-yr period July 1967–August 1993, and the correlations determine longitude averages of these motions, as functions of latitude and time. The time-average of our results over the entire 26-yr period is, as expected, antisymmetric about the equator. It is poleward between 10° and 60°, with a maximum rate of 13 m s^–1, but for latitudes below ±10° it is markedly equatorward, and it is weakly equatorward for latitudes above 60°. A running 1-yr average shows that this complex latitude dependence of the long-term time average comes from a pattern of motions that changes dramatically during the course of the activity cycle. At low latitudes the motion is equatorward during the active phase of the cycle. It tends to increase as the zones of activity move toward the equator, but it reverses briefly to become poleward at solar minimum. On the poleward sides of the activity zones the motion is most strongly poleward when the activity is greatest. At high latitudes, where the results are more uncertain, the motion seems to be equatorward except around the times of polar field reversal. The difference-from-average meridional motions pattern is remarkably similar to the pattern of the magnetic rotation torsional oscillations. The correspondence is such that the zones in which the difference-from-average motion is poleward are the zones where the magnetic rotation is slower than average, and the zones in which it is equatorward are the zones where the rotation is faster.Our results suggest the following characterization: there is a constant and generally prevailing motion which is perhaps everywhere poleward and varies smoothly with latitude. On this is superimposed a cycle-dependent pattern of similar amplitude in which the meridional motions of the small magnetic features are directed away from regions of magnetic flux concentration. This is suggestive of simple diffusion, and of the models of Leighton (1964) and Sheeley, Nash, and Wang (1987). The correspondence between the meridional motions pattern and the torsional oscillations pattern in the magnetic rotation suggests that the latter may be an artifact of the combination of meridional motion and differential rotation.     

A large-scale pattern in the solar magnetic field

A clearly evident large-scale pattern in the interplanetary magnetic field during 1964 is used to search for a similar large-scale pattern in the solar magnetic field. It is found that such a pattern did exist in the photospheric field observations on both sides of the equator over a range of at least 40°N to 35°S. The pattern is basically similar at all these latitudes, and differs from that to be expected from solar differential rotation in three important respects. It is found that the solar magnetic pattern changed at all latitudes investigated within an interval of a few solar rotations.     

The Observed Long- and Short-Term Phase Relation between the Toroidal and Poloidal Magnetic Fields in Cycle 23

The observed phase relations between the weak background solar magnetic (poloidal) field and strong magnetic field associated with sunspots (toroidal field) measured at different latitudes are presented. For measurements of the solar magnetic field (SMF) the low-resolution images obtained from Wilcox Solar Observatory are used and the sunspot magnetic field was taken from the Solar Feature Catalogues utilizing the SOHO/MDI full-disk magnetograms. The quasi-3D latitudinal distributions of sunspot areas and magnetic fields obtained for 30 latitudinal bands (15 in the northern hemisphere and 15 in the southern hemisphere) within fixed longitudinal strips are correlated with those of the background SMF. The sunspot areas in all latitudinal zones (averaged with a sliding one-year filter) reveal a strong positive correlation with the absolute SMF in the same zone appearing first with a zero time lag and repeating with a two- to three-year lag through the whole period of observations. The residuals of the sunspot areas averaged over one year and those over four years are also shown to have a well defined periodic structure visible in every two – three years close to one-quarter cycle with the maxima occurring at − 40° and + 40° and drifts during this period either toward the equator or the poles depending on the latitude of sunspot occurrence. This phase relation between poloidal and toroidal field throughout the whole cycle is discussed in association with both the symmetric and asymmetric components of the background SMF and relevant predictions by the solar dynamo models.     

On the relationship between polar faculae and large-scale magnetic field

We investigate a latitude–time distribution of polar faculae observed at Ussuriysk Observatory in years 1966–1986. The distribution is compared with the longitude-averaged (zonal) magnetic field of the Sun calculated from the data obtained at Mount Wilson Observatory in the years 1966–1976, and at Kitt Peak National Observatory during the period from 1976 to 1985. We found that slow, poleward-directed migration of the polar faculae zones occurring during the course of the solar cycle is not a continuous process, but it contains several episodes of appearance and fast poleward drift of new zones of polar faculae. At the rising phase of the solar cycle, new zones of polar faculae appear at latitudes as low as 40°, but the ones observed during the declining phase of the solar cycle originate at higher latitudes of 50–55°. Such episodes of appearance and fast migration of the polar faculae zones are associated with the poleward-directed streams of magnetic field originated at low latitudes. Moreover, we found some evidence for existence of an additional component of the polar faculae activity that reveals an equatorward migration during the course of the solar cycle. We also investigated a relationship between the number of polar faculae, n, and absolute magnetic flux _z of the zonal mode of the solar magnetic field. We found that within the polar zones of the Sun, substantial correlation between temporal variations of n and _z takes place both on the time scale of the solar cycle and on a shorter time scale of 2–4 years. The relationship between the number of polar faculae and magnetic flux may be approximated by a linear dependence n=0.12_z (where _z is expressed in 10²¹ Mx), except for time interval 1977 through 1980 for which the factor of proportionality is found to have a systematically larger value of 0.20.     

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.     

Phase Relation Between Activities of Solar Active Prominences Separately at low and High Latitudes

In the present work, the phase relation between activities of solar active prominences respectively at low and high latitudes in the period 1957–1998 has been studied. We found that from the solar equator to the solar poles, the activity of the solar active prominences occurs earlier at higher latitudes, and that the cycle of the solar active prominences at high latitudes (larger than 50°) leads by 4 years both the sunspot cycle and the corresponding cycle of the solar active prominences at low latitudes (less than 40°).     

Radial variation of differential rotation in the solar electron corona

Long-lived brightness structures in the solar electron corona persist over many solar rotation periods and permit an observational determination of coronal magnetic tracer rotation as a function of latitude and height in the solar atmosphere. For observations over 1964–1976 spanning solar cycle 20, we compare the latitude dependence of rotation at two heights in the corona. Comparison of rotation rates from East and West limbs and from independent computational procedures is used to estimate uncertainty. Time-averaged rotation rates based on three methods of analysis demonstrate that, on average, coronal differential rotation decreases with height from 1.125 to 1.5 R _S. The observed radial variation of differential rotation implies a scale height of approximately 0.7 R _S for coronal differential rotation.Model calculations for a simple MHD loop show that magnetic connections between high and low latitudes may produce the observed radial variations of magnetic tracer rotation. If the observed tracer rotation represents the rotation of open magnetic field lines as well as that of closed loops, the small scale height for differential rotation suggests that the rotation of solar magnetic fields at the base of the solar wind may be only weakly latitude dependent. If, instead, closed loops account completely for the radial gradients of rotation, outward extrapolation of electron coronal rotation may not describe magnetic field rotation at the solar wind source. Inward extrapolations of observed rotation rates suggest that magnetic field and plasma are coupled a few hundredths of a solar radius beneath the photosphere.     

Prominences and the Green Corona Over the Solar Activity Cycle

Prominences, in contrast to other solar activity features, may appear at all heliographic latitudes. The position of zones where prominences are mainly concentrated depends on the cycle phase of solar activity. It is shown, for prominence observations made at Lomnický tít over the period 1967–1996, how the position of prominence zones changes over a solar cycle, and how these zones could be connected with other solar activity features. Our results obtained could be an additional source to do a better prediction of solar activity. Time-latitudinal distribution is also shown for the green corona (Fexiv, 530.3 nm). Distribution of the green coronal maxima shows that there are equator-migrating zones in the solar corona that migrate from latitudes of 45° (starting approximately 2–3 years after the cycle start) to higher latitudes 70°, and then turn (around the cycle maximum) towards the equator, reaching the equator in the next minimum (this duration lasts 18–19 years). Polar branches separate from these zones at the cycle minimum (2–3 years before above-mentioned zones) at latitudes of 50°, reaching the poles at the maximum of the present cycle. The picture becomes dim when more polar prominence zones are observed. Prominences show both the poleward and equatorward migration. Comparison between both solar activity features is also discussed.