A forecast of solar activity for the 21st solar cycle

Predicted values of some main indices of solar activity for the 21st solar cycle are given. The epoch of maximum of solar activity has been placed in 1980.8±0.1. The predicted peak values of the relative sunspot numbers published by other authors are also given.     

On the short term variations of solar activity during solar cycle 21

Short-term variations of the last solar activity cycle were studied by the flare and coronal indices using Gleissberg method. Systematic short-term variations are found from their course during the 21st solar activity cycle. Comparison of their autocorrelograms constructed by the new set of data obtained from the magnitude of the fluctuations showed us the existence of the phase shift between the temporal variations of the two indices.     

On the increase of solar activity in the southern hemisphere during solar cycle 21

The present paper investigates the north-south asymmetry for major flares (solar cycles 19 and 20), type II radio bursts (solar cycles 19,20 and 21), white light flares (solar cycle 19,20 and 21), and gamma ray bursts, hard X-ray bursts and coronal mass ejections (solar cycle 21). The results are compared with the found asymmetry in favour of the northern hemisphere during solar cycles 19 and 20 in favour of the southern hemisphere during solar cycle 21.     

On the solar rotation and activity

The interaction between differential rotation and magnetic fields in the solar convection zone was recently modelled by Brun (2004). One consequence of that model is that the Maxwell stresses can oppose the Reynolds stresses, and thus contribute to the transport of the angular momentum towards the solar poles, leading to a reduced differential rotation. So, when magnetic fields are weaker, a more pronounced differential rotation can be expected, yielding a higher rotation velocity at low latitudes taken on the average. This hypothesis is consistent with the behaviour of the solar rotation during the Maunder minimum. In this work we search for similar signatures of the relationship between the solar activity and rotation determined tracing sunspot groups and coronal bright points. We use the extended Greenwich data set (1878–1981) and a series of full-disc solar images taken at 28.4 nm with the EIT instrument on the SOHO spacecraft (1998–2000). We investigate the dependence of the solar rotation on the solar activity (described by the relative sunspot number) and the interplanetary magnetic field (calculated from the interdiurnal variability index). Possible rotational signatures of two weak solar activity cycles at the beginning of the 20th century (Gleissberg minimum) are discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface magnetic fields during the solar activity cycle

We examine magnetic field measurements from Mount Wilson that cover the solar surface over a 13 1/2 year interval, from 1967 to mid-1980. Seen in long-term averages, the sunspot latitudes are characterized by fields of preceding polarity, while the polar fields are built up by a few discrete flows of following polarity fields. These drift speeds average about 10 m s^-1 in latitude - slower early in the cycle and faster later in the cycle - and result from a large-scale poleward displacement of field lines, not diffusion. Weak field plots show essentially the same pattern as the stronger fields, and both data indicate that the large-scale field patterns result only from fields emerging at active region latitudes. The total magnetic flux over the solar surface varies only by a factor of about 3 from minimum to a very strong maximum (1979). Magnetic flux is highly concentrated toward the solar equator; only about 1% of the flux is at the poles. Magnetic flux appears at the solar surface at a rate which is sufficient to create all the flux that is seen at the solar surface within a period of only 10 days. Flux can spread relatively rapidly over the solar surface from outbreaks of activity. This is presumably caused by diffusion. In general, magnetic field lines at the photospheric level are nearly radial.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

A forecast of solar activity in cycle 21

Based on the similarity in the variation of sunspot number in the ascending branch between Cycle 21 and Cycles 3, 8, 18, we predict that the solar activity in Cycle 21 will probably be high, with a maximum smoothed monthly mean relative number of 149.4 and the maximum epoch will be 1979.7.     

Meridional drift of the large-scale solar magnetic fields in different phases of solar activity

An analysis is made of mean latitudinal profiles of the meridional drift of the large-scale solar magnetic fields. The previously detected equatorward migration of the drift pattern in the course of a cycle is confirmed. Evidence for the existence of a near-equatorial narrow zone ±7° with an equatorward drift with a rate of about 1 m s^-1 is obtained. The study revealed a significant difference in shapes and variations of average drift profiles for the large-scale magnetic field and small magnetic features (Komm, Howard, and Harvey, 1993).     

On the negative correlation between solar activity and solar rotation rate

An increase in solar activity is shown to be accompanied by a decrease in solar rotation rate. This effect has been established from various indices; it manifests itself as cyclic and secular variations in the global magnetic field, in the observations of the magnetic field of the Sun as a star, and in the observations of the solar corona. Some possible explanations of this effect are discussed.     

The solar differential rotation from the eightteenth solar cycle

The sidereal daily rotation of the Sun, (), depends on the data used. From an appropriate selection of the data — sunspots with regular motion — it is found that ()=14.31–2.70 sin² , where denotes the heliographic latitude. Moreover, it seems that there is a variation, of the order of 3%, with the solar activity.     

Differential rotation of solar magnetic fields

The connection of the differential rotation of solar magnetic fields with the field sign and strength is studied. The synoptic maps of magnetic fields over the last three solar cycles taken at the Kitt Peak Observatory served as input data for the study. The algorithm of magnetic field filtering over 14 chosen strengt intervals and successive 5-degree latitude zones was applied to these data. The Fourier transform of the time series obtained was then used. Analysis of the power spectra led to the conclusion that there are two types of magnetic fields. These differ in strength (0–50 and 50–700 G) and rotation characteristics. The rotation differentiality for strong magnetic field is almost twice as large as that for weak magnetic fields.     

On the 11-years cycle of solar activity

Each 11-years cycle of solar activity consists of two processes with different physical properties. The variety of shapes of the 11-years curves depends on the way these processes overlap.All events in the photosphere, chromosphere and corona, and all kinds of emissions like the radio- and corpuscular emissions take part in these two processes.Events taking place in the magnetosphere, ionosphere, troposphere and perhaps some chemical and biological events reflect the essential properties of the 11-years cycle.     

The large-scale velocity fields of the solar atmosphere

Magnetograph velocity data are studied for evidence of large-scale velocity fields. It is established that there exist on the surface of the sun regions of more or less coherent downward motion with dimensions of the order of a solar radius. Velocity amplitudes in these regions are in the range 50–75 m/sec. Downward-moving large-scale features are observed to live for at least several days in general and to rotate at least approximately with the solar rotation rate. Horizontal east-west motions appears to have lifetimes of at least many months. The extent in longitude of these horizontal features is about 25°. There is no evidence for meridional motions from these data, with an upper limit to the line-of-sight velocity of about 30 m/sec. Active regions, as reported previously, are areas of generally downward motion. Some features in the autocorrelation of the rotational velocity of the sun remain unexplained.     

Coronal rotation during solar cycle 20

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

Coronal rotation dependence on the solar cycle phase

A study of the green corona rotation rate, during the period 1970–1974, confirms that the differential rotation degree varies systematically through a solar cycle and that the corona rotates in an almost rigid manner before sunspot minimum. During the first two years, 1970–1971, the differential rotation degree, characteristic of high solar activity periods is detected. While during the years of declining activity, 1972–1974, a drastic decrease of the differential rotation degree occurs and the green corona rotates almost rigidly, as the coronal holes observed in the same period. These conclusions are valid only for the rotation of coronal features with lifetime of at least one solar rotation.     

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.     

Rotation of the large-scale solar magnetic fields in the equatorial region

A study is made of the rotation of large-scale magnetic fields using the synoptic maps from the Kitt Peak National Observatory for the time interval 1976–1985. The auto-correlation method and the mass-centers method of magnetic structures was applied to infer mean differential rotation profiles and rotation profiles separately for each magnetic field polarity. It has been found that in both hemispheres the leading polarity rotates faster than the following polarity at all latitudes by about 0.04° day^–1. The maximum rotation rate of the leading polarity is reached at about 6° latitude. In the mean profile for both polarities, this brings about two angular velocity maxima at 6° latitudes in both hemispheres. Such a profile appears as to have a dimple on the equator.     

Weak solar fields and their connection to the solar cycle

We discuss the weak solar magnetic fields as studied with the BBSO videomagnetograph (VMG). By weak fields we mean those outside active and unipolar regions. These are found everywhere on the Sun, even where there never have been sunspots. These fields consist of the network and intranetwork (IN) elements. The former move slowly and live a day or more; the latter move rapidly (typically 300 m s^–1) and live only hours. To all levels of sensitivity the flux is concentrated in discrete elements, and the background field has not been detected. The smallest detectable elements at present are 10¹⁶ Mx. The IN elements emerge in bipolar form but appear to flow in a random pattern rather than to the network edges; however, any expanding network element is constrained by geometry to move toward the edges.Because of the great number and short lifetime of the IN elements the total flux emerging in that form exceeds that emerging in the ER by two orders of magnitude and the flux in sunspots, by a factor 10⁴. However, the flux separation is small and there is no contribution to the overall field. In contrast with our earlier results, merging of IN fields is more important than the ephemeral regions as a source of new network elements.The conjecture that all solar magnetic fields are intrinsically strong is discussed and evidence pro and con presented. For the IN fields the evidence suggests they cannot exceed 100 G. For the network fields there is evidence on either side.Reconnection and merging of magnetic fields takes place continually in the conditions studied.Because there is a steady state distribution, the amout of new elements created by merging or emergence must balance that destroyed by reconnection or fission and diffusion of the stronger elements.Solar Cycle Workshop Paper.     

On large-scale solar convection

We examine the effects of rotation about a vertical axis on thermal convection with a simple model in which an inviscid, incompressible fluid of zero thermal conductivity and electrical resistivity is contained in a thin annulus of rectangular cross-section. The initial steady state assumed is one of no motion relative to the rotating frame with constant (unstable) vertical temperature gradient and uniform toroidal magnetic field. Small periodic disturbances are then introduced and the linearized perturbation equations solved. We also determine the second-order mean circulations and magnetic fields that are forced by non-zero Reynolds and thermal stresses and magnetic field transports.The solutions have several properties which are relevant to large-scale solar phenomena if giant long-lived convection cells exist on the sun. In particular, the convective cells are tilted in latitude in the same sense as bipolar magnetic regions, and induce vertical magnetic fields with the same tilt. They transport momentum across latitude circles through Reynolds stresses and induced meridional circulations thus setting up a differential rotation. Cells which grow slowly compared to the rotation rate and have comparable dimensions in latitude and longitude transport momentum toward the equator. The cells also form a poloidal magnetic field from initial toroidal field, in a manner similar to that put forth by Parker.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The dipole-quadrupole cycle of the background solar magnetic field

The evolution of the background magnetic field with the solar cycle has been studied using the dipole-quadrupole magnetic energy behaviour in a cycle. The combined energy of the axisymmetric dipole, non-axisymmetric quadrupole, and equatorial dipole is relatively lowly variable over the solar cycle. The dipole field changed sign when the quadrupole field was near a maximum, andvice versa. A conceptual picture involving four meridional magnetic polarity sectors proposed to explain these features may be in agreement with equatorial coronal hole observations. The rate of sector rotation is estimated to be 8 heliographic degrees per year faster than the Carrington rotation (P = 27.23^d synodic). Polarity boundaries of sectors located 180° apart show meridional migrations in one direction, while the boundaries of the other two sectors move in the opposite direction. A simple model of how the magnetic field energy varies, subject to specifying reasonable initial photospheric magnetic and velocity field patterns, follows the observed evolution of the dipole and quadrupole field energies quite nicely.     

Changes in solar rotation due to the solar energy generation cycle of 11 years

It is suggested that the observed differences in the periods of variation of some solar phenomena (solar brightness, appearance of sunspot maximum and interplanetary sector structure) occurring close to 27 days are due to differences in the rotation periods of the solar regions in which these phenomena are originated. Changes in periods during the solar cycle can be attributed to changes in the solar energy generation. On the basis of these considerations changes in the sign of the gradient of the Sun's angular velocity can be expected.