Differential rotation of photospheric magnetic fields associated with coronal holes

An interesting aspect of solar rotation is the fact that coronal holes seem to exhibit little or no differential rotation. We set out to investigate the question of whether or not the photospheric magnetic fields underlying coronal holes also exhibit reduced differential rotation. In order to accomplish this we measured the daily positions of filaments and plages surrounding a large coronal hole that lasted for several disk passages. The resulting differential rotation curve was considerably flatter than the standard curve for long-lived filaments and was in remarkably good agreement with the curve found for the overlying coronal hole itself.     

Comparison of the mean photospheric magnetic field and the interplanetary magnetic field

The mean photospheric magnetic field of the sun seen as a star has been compared with the interplanetary magnetic field observed with spacecraft near the earth. Each change in polarity of the mean solar field is followed about 4 1/2 days later by a change in polarity of the interplanetary field (sector boundary). The scaling of the field magnitude from sun to near earth is within a factor of two of the theoretical value, indicating that large areas on the sun have the same predominant polarity as that of the interplanetary sector pattern. An independent determination of the zero level of the solar magnetograph has yielded a value of 0.1±0.05 G. An effect attributed to a delay of approximately one solar rotation between the appearance of a new photospheric magnetic feature and the resulting change in the interplanetary field is observed.     

Short period variation of the photospheric magnetic field

The photospheric magnetic data recorded from August 12, 1959 to September 29, 1967 and averaged over Bartels rotation periods are treated as zonal terms of the solar magnetic field which is expanded in a series of spherical harmonics. Numerical analysis of the reduced data gives seven periods. Three of these seem to be essential in the superposed variation of the solar magnetic field. The first of them (17.74 yr) is thought to be a contribution from the magnetic cycle for the determination of which the data covering only 8 yr interval are of insufficient accurity. For this reason, a 22.2 yr period is favoured by the computations. The numerical values of the two shorter periods are deduced as 2.557 yr and 4.194 yr. The amplitudes and phase angles of the periodic terms in question are determined.     

On possible correlations in the photospheric magnetic field

We have searched for correlations between photospheric magnetic field changes in the north and south hemispheres of the Sun. Both active region logs and analysis of Mount Wilson magnetograms were employed. No correlations were found, and we infer that local convective turbulence is more important than dynamo processes with regard to the appearance of individual active regions.     

Evolution of photospheric magnetic field patterns during skylab

The evolution of the photospheric magnetic field pattern over eleven solar rotations preceding a minimum of the activity cycle is shown to be characterized by abrupt changes of the dominant geometrical patterns of the field. These changes are associated with the onset and end of a sudden increase in the calculated total energy content of the field, which is otherwise decreasing through the period. The calculated geometrical rearrangements correspond in time to observed restructurings of the corona, the interplanetary field, and the solar rotation pattern.     

Changes in the photospheric magnetic field associated with solar flares

The flare-related, persistent and abrupt changes in the photospheric magnetic field have been reported by many authors during recent years. These bewildering observational results pose a challenge to the current flare theories in which the photospheric magnetic field usually remains unchanged in the eruption. In this paper, changes in the photosphere magnetic field during the solar eruption are investigated based on the catastrophe model. The results indicate that the projection effect is an important source that yields the change in the observed photospheric magnetic field in the line-of-sight. Furthermore one may observe the change in the normal component of magnetic field if the spectrum line used to measure the photospheric magnetic field does not exactly come from the photospheric surface. Our results also show that the significance of selecting the correct spectral lines to study the photospheric field becomes more apparent for the magnetic configurations with complex boundary condition (or background field).     

Measurements of magnetic fluxes and field strengths in the photospheric network

We present digital pictures of an active region network cell in five quantities, measured simultaneously: continuum intensity, line-center intensity, equivalent width, magnetogram signal, and magnetic field strength. These maps are derived from computer analysis of circularly polarized line profiles of FeI 5250.2; spectral and spatial resolution are 1/40 Å and 1.5, respectively. Measured Zeeman splittings show the existence of strong magnetic fields (1000–1800 G) at nearly all points with a magnetogram signal exceeding 125 G. The mean and rms deviation of the field strengths change by less than 20% over a factor-of-four range of fluxes. From the significant disparity between measured fluxes and field strengths, we conclude that large flux patches (up to 4 across) consist of closely-packed unresolved filaments. The smallest filaments must be less than 0.7 in diameter. We also observe the dark component of the photospheric network, which appears to contain sizable transverse fields.     

The evolution of a coronal streamer and the photospheric magnetic field

A large equatorial coronal streamer observed in the outer corona (3R ) grew in brightness and size during successive limb passages between October 6, 1973 and January 10, 1974 (solar rotations 1606–1611). Unlike previous studies of streamers and their photospheric associations, no definite surface feature could be identified in the present case. This suggests that the streamer is associated with the large scale photospheric magnetic field. Comparison of the streamer growth with observed underlying photospheric magnetic flux changes indicated that as the streamer increased in brightness, areal extent, and density, the photospheric magnetic flux decreased. Three possible explanations for the streamer's growth are presented; the conceptually simplest being that the decrease in photospheric field results in an opening of the flux tubes under the streamer which permits an increased mass flux through the streamer.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The equatorial photospheric rotation rate

The equatorial photospheric rotation rate has been observed on 14 days in 1978–1980. The resulting rotation rate, = 14.14±0.04°/day, is 2% slower than the rate as observed for long-lived sunspots.Stationed at Kitt Peak National Observatory.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

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.     

The role of photospheric magnetic field in the development of solar flares

A statistical investigation has been made about the flare-process in relation to the photospheric magnetic field and configuration. It is understood from the analysis that the flare energy bears a linear relationship with the rate of change of flux of the longitudinal component of photospheric magnetic field.     

Large-scale photospheric magnetic field: The diffusion of active region fields

The large-scale photospheric magnetic field has been computed by allowing observed active region fields to diffuse and to be sheared by differential rotation in accordance with the Leighton (1969) magnetokinematic model of the solar cycle. The differential rotation of the computed field patterns as determined by autocorrelation curves is similar to that of the observed photospheric field, and poleward of 20° latitude both are significantly different from the differential rotation of the long-lived sunspots (Newton and Nunn, 1951) used as an input into the computations.Now at Department of Physics, Victoria University of Wellington, Wellington, New Zealand.     

The importance of photospheric magnetic field complexity for coronal energy storage

Possibilities for the storage of energy in coronal electric currents in different magnetic background field configurations are investigated in the framework of the solar flare energy build-up model of Van Tend and Kuperus (1978). The results are compared to characteristics of filaments and X-ray loops. Empirical flare predictors are interpreted theoretically.     

Evolution of the photospheric magnetic field and coronal null points before solar flares

Based on a topological model for the magnetic field of a solar active region (AR), we suggest a criterion for the existence of magnetic null points on the separators in the corona. With the problem of predicting solar flares in mind, we have revealed a model parameter whose decrease means that the AR evolves toward a major eruptive flare. We analyze the magnetic field evolution for AR 9077 within two days before the Bastille Day flare on July 14, 2000. The coronal conditions are shown to have become more favorable for magnetic reconnection, which led to a 3B/X5.7 eruptive flare.     

Average photospheric poloidal and toroidal magnetic field components near solar minimum

Average (over longitude and time) photospheric magnetic field components are derived from 3 Stanford magnetograms made near the solar minimum of cycle 21. The average magnetograph signal is found to behave as the projection of a vector for measurements made across the disk. The poloidal field exhibits the familiar dipolar structure near the poles, with a measured signal in the line Fe i 5250 Å of 1 G. At low latitudes the poloidal field has the polarity of the poles, but is of reduced magnitude ( 0.1 G). A net photospheric toroidal field with a broad latitudinal extent is found. The polarity of the toroidal field is opposite in the nothern and southern hemispheres and has the same sense as subsurface flux tubes giving rise to active regions of solar cycle 21.These observations are used to discusse large-scale electric currents crossing the photosphere and angular momentum loss to the solar wind.Now at Kitt Peak National Observatory, Tucson, Ariz. 85726, U.S.A.     

The age dependence of photospheric tracer rotation

From the analysis of the motions of sunspot groups recorded at Catania Astrophysical Observatory over a 7-years period from 1972 to 1978 the mean angular velocity as a function of latitude and age is calculated. The results suggest that the age of photospheric tracer (sunspot groups) affects the rotation curve slope. The implications of this result are discussed.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

On the role of the motion of the photospheric footpoints of the magnetic field lines

By means of a simple relation between the velocity v of the fluid particle and the velocity v_f of the photospheric footpoint of the magnetic field line v_z and B_z being respectively the components of v and the magnetic field B normal to the photospheric surface, it is shown formally that through the phtospheric surface the transport of all the quantities attributed to the magnetic field, such as the magnetic flux, the magnetic energy and the helicity, is independent of v_z, and v_f is the only kinematical quantity on which the transport depends. In addition, in the neighborhood of the neutral line the velocity v_l of the moving curve of constant B_z is found to be equal approximately to the component of v or v_f in the direction of v_l. Since v_l can be measured or extimated, so can the components of v and v_f near the neutral line.     

Solar rotation: The photospheric height gradient

For selected pairs of Fraunhofer lines the height of formation has been calculated corresponding to that portion of the profile intercepted by the magnetograph exit slits. A photospheric height discrimination of 150–300 km is realized. In 1971 simultaneous measurements of equatorial angular velocity from spectroscopic displacements of these line pairs indicate no height gradient in excess of 1%.The disturbing influence of telluric line blends is analyzed. It is proposed that undetected telluric lines could account, at least in part, for the photospheric height gradient which has been found in a number of photographic investigations.Operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.     

Semi-regularities in the photospheric magnetic fields

Using Stanford large-scale magnetic field synoptic charts of rotation 1676 to 1739 and by delineating LLUMR, i.e., long-lived unipolar magnetic regions of both polarities surviving at least for four solar rotations, the semi-regular nature of their photospheric magnetic field pattern and their rotational properties have been examined. The investigation demonstrates the existence of regularities in the background field patterns as shown from the regular patterns of LLUMR rows and streams. This confirms the results of Bumba and Howard concerning regularities in large-scale photospheric magnetic field patterns. LLUMR streams seem to be arranged in a wave pattern of alternating polarities. Coronal holes and associated sections of photospheric field patterns suffer differential rotation. The rotation rates of the background field patterns which are not associated with the coronal holes are different from those which are.