Recurrence of solar activity: Evidence for active longitudes

The autocorrelation coefficients of the daily Wolf sunspot numbers over a period of 128 years reveal a number of interesting features of the variability of solar activity. In addition to establishing periodicities for the solar rotation, the solar activity cycle, and perhaps the Gleissberg Cycle, they suggest that active longitudes do exist, but with much greater strength and persistence in some solar cycles than in others. There is evidence for a variation in the solar rotation period, as measured by sunspot number, of as much as two days between different solar cycles.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Rotation cycles of the sector structure of the solar magnetic field and its activity

We study the rotation of the sector structure of the solar magnetic field by using Stanford magnetographic observations from 1975 until 2000 and magnetic synoptic Hα-maps obtained from 1904 until 2000. The two independent series of observations yielded the same rotation periods of the two-sector (26.86 days) and four-sector (13.64 days) structures. We introduce a new index of the solar rotation, SSPM(t). The spectral power density of the sector structure of the magnetic field is shown to exhibit a 22-year cyclicity. The two-and four-sector structures of the magnetic field rotate faster at the maxima of even 11-year sunspot cycles. This phenomenon may be called the Gnevyshev-Ohl rule for the solar rotation. The 11-year sector-structure activity cycles are shown to lead the 11-year sunspot cycles (Wolf numbers) by 5.5 years. A 55-year component with the slowest rotation in the 18th cycle (1945–1955) was distinguished in the sector-structure rotation.     

‘Bartels' active longitudes’, sector boundaries and flare activity

The flare activity and especially the proton-flare activity is concentrated in the zones of Bartels' active longitudes and in the neighbourhood closest to the sector boundaries of the interplanetary magnetic field. This concentration seems to be greater if the importance of the event increases.On leave from the Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Academy of Sciences, U.S.S.R.     

Effects of activity complexes and active longitudes on variations in total solar irradiance

A numerical technique of time-longitude analysis has been developed by studying the fine structure of temporal variations in total solar irradiance (TSI). This analysis produces maps of large-scale thermal inhomogeneities on the Sun and reveals corresponding patterns of radiative excess and deficit relative to the unperturbed solar photosphere. These patterns are organized in two-and four-sector structures and exhibit the effects of both activity complexes and the active longitudes. Large-scale patterns with radiative excess show a facular macrostructure caused by the relaxation of large-scale thermo-magnetic perturbations and/or energy output due to very large-scale solar convection. These thermal patterns are related to long-lived magnetic fields that are characterized by rigid rotation. The patterns with radiative excess tend to concentrate around the active longitudes and are centered at 103° and 277° in the Carrington system when averaged over the time-longitude distribution of thermal inhomogeneities during activity cycles 21–23.     

The XUV structure of solar active regions

XUV spectroheliograms of 2 active regions are studied. The images are due to lines emitted at temperatures between 8 x 10⁴ K and 2 x 10⁶ K and thus are indicative of transition region and coronal structures. The hot coronal lines are formed solely in loop structures which connect regions of opposite photospheric magnetic polarity but are not observed over sunspots. Transition region lines are emitted in plages overlying regions of intense photospheric magnetic field and in loops or loop-segments connecting such regions. The hot coronal loops are supported hydrostatically while only some of the transition zone loops are. The coronal and transition zone loops are distinctly separated and are not coaxial. A comparison of direct measurements of electron densities using density sensitive line ratios with indirect measurements using emission measures and path lengths shows the existence of fine structures of less than a second of arc in transition region loops. From a similar analysis, hot coronal loops do not have any fine structure below about 2 seconds of arc.     

Green corona and solar sector structure

Analysis of the green line corona for the interval 1947–1970 suggests the existence of largescale organization of the emission. The green line emission at high northern latitudes (≈ 40°–60°) is correlated with the emission at high southern latitudes 6, 15 and 24 days later, while the low latitude green corona seems to be correlated on both sides of the equator with no time lag. These coronal features are recurrent with a 27-day period at all latitudes between ± 60 °, and we associate these large-scale structures with the solar magnetic sector structure. The high correlation between northern and southern high-latitude emission at 15 days time lag is explained as a signature of a two-sector structure, while four sectors are associated with the 6 and 24 day peaks.     

Observation of solar particle fluxes over extended solar longitudes

Detailed particle observations from various Pioneer Spacecrafts located at different heliolongitudes during the complex solar flare events of March 30–April 10, 1969 have been utilised to investigate the energy dependence of azimuthal gradients of cosmic ray particles and its effect on the decay of the flare intensity. For an observer located to the east of the centroid of the population, the azimuthal corotation term and the convection term will be additive, resulting in a short decay time constant. An observer located to the west of the centroid of the population will experience a much longer decay time constant, the corotation term partially or completely compensating the loss of particles due to convection. At very low energies, the azimuthal corotation term may even be more than the convection term, thus resulting in a rise in intensity instead of decay during late in the event. Using the relationship showing the dependence of the spectral exponent of the cosmic ray flux late in a flare event on the azimuth from the centroid of the population given by McCracken et al., the energy dependence of the decay time constant and the cross-over energy at which the azimuthal gradient term equals the convection term are investigated. The experimental observations are shown to be generally consistent with the theoretical picture, confirming the importance of convection and the azimuthal gradient in determining the decay profile of flare events.On leave from Physical Research Laboratory, Ahmedabad, India.Now at CSIRO, G.P.O. Box 136, North Ryde, N.S.W., Australia.     

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.     

Relation of the observed far ultraviolet solar irradiance to the solar magnetic sector structure

Comparison of the observed solar far ultraviolet irradiance and the observed solar sector structure during 1969 through 1972 shows a tendency for EUV maxima to be located near sector boundaries.     

Coronal magnetic structure at a solar sector boundary

The persistent large-scale coronal magnetic structure associated with a sector boundary appears to consist of a magnetic arcade loop structure extending from one solar polar region to the other in approximately the north-south direction. This structure was inferred from computed coronal magnetic field maps for days on which a stable magnetic sector boundary was near central meridian, based on an interplanetary sector boundary observed to recur during much of 1968 and 1969.     

The properties of solar active regions responsible for ground level enhancements during solar cycles 22 and 23

This is a study designed to analyze the relationship between ground level enhancements（GLEs）and their associated solar active regions during solar cycles 22and 23.Results show that 90.3%of the GLE events that are investigated are accompanied by X-class flares,and that 77.4%of the GLE events originate from super active regions.It is found that the intensity of a GLE event is strongly associated with the specific position of an active region where the GLE event occurs.As a consequence,the GLE events having a peak increase rate exceeding 50%occur in a longitudinal range from W20 to W100.Moreover,the largest GLE events occur in a heliographic longitude at roughly W60.Additionally,an analysis is made to understand the distributional pattern of the Carrington longitude of the active regions that have generated the GLE events.     

Differential rotation and sector structure of solar magnetic fields

The differential rotation and sector structure of solar magnetic fields has been studied using digitized data on photospheric magnetic fields recorded at the Mount Wilson Observatory during the period August 1959–May 1970. The power spectra show considerable power in high-frequency peaks, corresponding to harmonic components with wavelengths less than 1/10 solar rotation. Calculations for a series of shorter time intervals show how the distribution of power over the various harmonic components in the sector pattern varies strongly with the solar cycle. The equatorial rotation rate of solar magnetic fields is about 0.1 km s^-1 faster than that of the photospheric plasma determined from Doppler shifts. It is shown that the Doppler measurements mainly refer to the non-network regions. The differential flow of 0.1 km s^-1 forms streamlines around the magnetic fine structures. The different rotation rates of various solar features can be explained in terms of the rotation rates of magnetic and non-magnetic regions. The rotation rates of the magnetic fields in active and quiet regions agree at the equator. At higher latitudes, however, the background fields deviate less from solid-body rotation, indicating that their source is below the deepest layers to which the sunspot magnetic fields penetrate. This suggests that turbulent diffusion of the field in old active regions may not be the main source for the background magnetic field, but that the source is located close to a rigidly rotating solar core with a synodic rotation period of 26.87 days.     

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.     

On the problem of active longitudes of sunspots and flares

The active longitudes of indices for sunspot activity and solar flares were detected and investigated by the method of isoline for the period July 1, 1957 to December 31, 1962. In the most active hemisphere of the sun the active longitudes of sunspot and flares appear to coincide. It is shown that the rate of concentration in the active longitudes is the highest for more important formations. Arguments for the reality of the active longitudes of sunspot areas are advanced. In conclusion the question of the influence of the uncertainty of the solar rotation period on the detection of active longitudes of flares is considered.     

The periodicity of solar activity cycles

On the basis of published Wolf Numbers and Schove Row, solar activity cycles in the interval of 11 to hundreds of years have been investigated. In this case the method of investigation of pulsating stars showing the Blazhko effect was applied. The elements of cycles and O-C were calculated and compared with results of solar activity parameters determined by classical methods.     

Evolution of latitude zonal structure of the large-scale magnetic field in solar cycles

Properties of a latitude zonal component of the large-scale solar magnetic field are analyzed on the basis of H charts for 1905–1982. Poleward migration of prominences is used to determine the time of reversal of the polar magnetic field for 1870–1905. It is shown that in each hemisphere the polar, middle latitude and equatorial zones of the predominant polarity of large-scale magnetic field can be detected by calculating the average latitude of prominence samples referred to one boundary of the large-scale magnetic field. The cases of a single and three-fold polar magnetic field reversal are investigated. It is shown that prominence samples referred to one boundary of the large-scale magnetic field do not have any regular equatorward drift. They manifest a poleward migration with a variable velocity up to 30 m s^-1 depending on the phase of the cycle. The direction of migration is the same for both low-latitude and high-latitude zones. Two different time intervals of poleward migration are found. One lasts from the beginning of the cycle to the time of polar magnetic field reversal and the other lasts from the time of reversal to the time of minimum activity. The velocity of poleward migration of prominences during the first period is from 5 m s^-1 to 30 m s^-1 and the second period is devoid of regular latitude drift.     

Magnetic and microwave structure in solar active regions

The structure of six active regions observed at 2.8 cm with the Stanford interferometer is compared with the configuration of the underlying photospheric magnetic fields, as given by the Kitt Peak magnetograph.The similar resolution and accuracy on the measured positions of both instruments allowed us to establish a more detailed spatial correspondence between radio and magnetic features than previously reached.The radio features which correspond to the cores of the active sources are always found to overlay regions of enhanced magnetic fields. Different spatial associations have been found depending on the brightness temperature of the sources. The possibility that this effect might be due to the development of the active region is also considered.     

The relationship between solar flares and solar sector boundaries

A superposed epoch analysis of 1964–1970 solar flares shows a marked increase in flare occurrence within a day (13° of longitude) of (- +) solar sector boundaries as well as a local minimum in flare occurrence near (+ -)sector boundaries. This preference for (- +) boundaries is more noticeable for northern hemisphere flares, where these polarities match the Hale polarity law, but is not reversed in the south. Plage regions do not show such a preference.     

The large-scale build-up of solar magnetic cycles

The purpose of the present article is to analyze the solar cycles from the point of view of the large-scale surface magnetic field (LSMF) polarity distributions. Using synoptic charts of the LSMF for the 1870–1991 time interval at maxima and minima and the spherical harmonic analysis of the polarity distributions, a connection between magnetic cycles has been found. The weight of the large-scale sectoral mode (m = 1) in the common LSMF polarity distribution at minima of the sunspot cycle is the source of sunspot activity at maxima after 16–18 years. The connections found suggest that surface LSMFs have a dual nature - the main source below the convective zone and a secondary source (sunspot production). The sunspot production has no visible influence on the LSMF cycles.     

Dependence of inferred magnetic sector structure upon geomagnetic and solar activity

Inferred solar sector polarity given by the AC index of Svalgaard, has been intensively studied as a single time series and as a time series correlated with geomagnetic and solar activity. Power auto-spectra of the AC index yield a highly significant harmonic series with fundamental at 27 days period and possessing clear harmonics up to the sixth; and a very prominent peak at a period of 1 yr. The 27 day harmonic series clearly indicates the solar control of the index while the 1-yr period might be taken as confirmation of the work of Rosenberg and Coleman to the effect that the sector pattern observed on Earth depends upon Earth's heliographic latitude which has a 1-yr period.Cross correlation analysis and superposed epoch analysis are used to show that sectors inferred to be positive or away are associated with low geomagnetic and solar activity whereas sectors inferred to be negative or toward exhibit significantly enhanced geomagnetic and solar activity.These results appear to be in conflict with superposed epoch analyses by Wilcox and Ness using satellite observed sector polarities which showed that geomagnetic activity increased after passage of a sector boundary, independent of the nature, whether + ? or ? + of the boundary.The conflict is resolved here by noting that the yearly correlation coefficient, at zero time lag, between inferred sector structure and geomagnetic activity averaged about 0·5 for the year 1927–1958, dropped to low values by 1960, recovered by 1962 and then dropped sharply in 1963 by an order to magnitude; the correlation has remained essentially zero ever since. Thus, the satellite results, all obtained post 1963, would not show increased activity during either sector sign.The results cast doubt upon the accuracy of the early ‘inferred’ sector polarities because it is felt that the only simple explanation for the strange behavior of the correlation coefficient lies in some artifact of the data.