Sun's Polar Field Research Articles

The structure of the heliospheric current sheet: 1978–1982

The structure of the heliospheric magnetic field changes substantially during the 11‐year sunspot cycle. We have calculated its configuration for the period 1976–1982 by using a potential field model, continuing our earlier study near solar minimum in 1976–1977 (Hoeksema et al., 1982). In this paper we concentrate on the structure during the rising phase, maximum, and early decline of sunspot cycle 21, from 1978 to 1982. Early in this interval there are four warps in the current sheet (the boundary between interplanetary magnetic field toward and away from the sun) giving rise to a four‐sector structure in the interplanetary magnetic field observed at earth. The location of the current sheet changes slowly and extends to a heliographic latitude of approximately 50°. Near maximum the structure is much more complex, with the current sheet extending nearly to the poles. Often there are multiple current sheets. As solar activity decreases, the structure simplifies until, in most of 1982, there is a single, simply shaped current sheet corresponding to a two‐sector interplanetary magnetic field structure in the ecliptic plane. The sun's polar fields, not fully measured by magnetographs such as that at the Stanford Solar Observatory, substantially influence the calculated position of the current sheet near sunspot minimum. We have determined the strength of the polar field correction throughout this period and include it in our model calculations. The lower latitude magnetic fields become much stronger as the polar fields weaken and reverse polarity near maximum, decreasing the influence of the polar field correction. The major model parameter is the radius of the source surface, the spherical surface at which the field lines become radial. Correlations of interplanetary magnetic field polarity observed by spacecraft with that predicted by the model calculated at various source surface radii indicate that the optimum source surface radius is not significantly different from 2.5 RS during this part of the solar cycle.

The strength of the Sun's polar fields

The magnetic field strength within the polar caps of the Sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field. Measurements of the line-of-sight component of the magnetic field generally yield 0.1 to 0.2 mT near times of sunspot minimum. In this paper we report measurements of the polar fields made at the Stanford Solar Observatory using the Fe i line λ 525.02 nm. We find that the average flux density poleward of 55° latitude is about 0.6 mT peaking to more than 1 mT at the pole and decreasing to 0.2 mT at the polar cap boundary. The total open flux through either polar cap thus becomes about 3 × 1014 Wb. We also show that observed magnetic field strengths vary as the line-of-sight component of nearly radial fields.

Using Dynamo Theory to predict the sunspot number during Solar Cycle 21

On physical grounds it is suggested that the sun's polar field strength near a solar minimum is closely related to the following cycle's solar activity. Four methods of estimating the sun's polar magnetic field strength near solar minimum are employed to provide an estimate of cycle 21's yearly mean sunspot number at solar maximum of 140 ± 20. We think of this estimate as a first order attempt to predict the cycle's activity using one parameter of physical importance based upon dynamo theory.

Large‐scale properties of the interplanetary magnetic field

Our knowledge of the large‐scale properties of the interplanetary magnetic field is reviewed. The early theoretical work of Parker is presented, along with the observational evidence supporting his Archimedes spiral model. The variations present in the interplanetary magnetic field from the spiral angle are related to structures in the solar wind. The causes of these structures are found to be either nonuniform radial solar wind flow or the time evolution of the photospheric field. The coronal magnetic models are related to the connection between the solar magnetic field and the interplanetary magnetic field. The direct extension of solar field‐magnetic nozzle controversy is discussed, along with the coronal magnetic models. The effect of active regions on the interplanetary magnetic field is discussed with particular reference to the evolution of interplanetary sectors. The variation of the interplanetary magnetic field magnitude is shown throughout the solar cycle. The percentage of time the field magnitude is greater than 10 γ is shown to closely parallel sunspot number. The suggested influence of the sun's polar field on the interplanetary field and alternative views of the magnetic field structure out of the ecliptic plane are presented. In addition, a variety of significantly different interplanetary field structures are discussed.

The second harmonic of the cosmic ray daily variation

Since on the Parker model for the quiet interplanetary magnetic field the spirals are less tightly wound at high solar latitudes, cosmic rays arriving along the Sun's polar field lines may suffer less modulation than those arriving in the ecliptic plane. Hence a rising cosmic ray density gradient is expected away from the solar equatorial plane which gives rise to a second harmonic variation with maxima at right angles to the spiral field direction. A model for the density gradient is considered in which particle diffusion is important only along the field direction and the resulting second harmonic varies as the first positive power of magnetic rigidity in the range 1 → 15 GV. This is shown to be in reasonable accord with experimental results over the solar cycle from 1957 to 1964.

The daily-variation second harmonic and a cosmic-ray intensity gradient perpendicular to the ecliptic

Since the spiral interplanetary magnetic field is expected to be much less tightly wound at high solar latitudes, galactic particles arriving along the sun's polar field lines may suffer much less modulation than those arriving in the ecliptic plane. Hence a rising cosmic-ray density gradient is expected away from the ecliptic which will give rise to a second harmonic in the cosmic-ray daily variation with maxima at right angles to the spiral field direction. Two extreme models for the gradient are considered: one for predominantly particle diffusion along the field lines and the other when the diffusion is all perpendicular to the lines. Both models give a first positive power of rigidity dependence to the second harmonic in the 1 to 15 GV range with a double amplitude of about 0.05% at 10 GV. This is in rough accord with experimental results, which also can be shown to favor the parallel diffusion model at higher rigidities.

The Zeeman effect in astrophysics

George E. Hale discovered the Zeeman effect in sunspot spectra in 1908, and derived the laws of sunspot polarity. Recent observations of the much weaker (1 to 50 gauss) magnetic fields of the solar photosphere, made with the solar magnetograph, are described. The bearing of these results on circulation in the sun and their consequences for the topology of the sun's magnetic field are mentioned, with reference to the solar cycle and to reversal of the sun's polar field. The problem of measuring the magnetic field of stars is outlined, with reference to instrumental requirements. Spectrographs and crystals are described, together with techniques of measurement of magnetic fields in sharp-line stellar spectra. Results of a systematic program of observations with the 100-inch and 200-inch telescopes are reviewed. All stellar magnetic fields are variable; some are periodic. The strongest yet measured is 34 000 gauss. The magnetic variations are usually related to variations in light, color, line width, and line intensity. Chemical abundance anomalies are common in magnetic stars, and this has led to much work on nuclear reactions in stellar atmospheres, but the phenomena have not been satisfactorily explained. The possible importance of element concentration through paramagnetic migration in strong magnetic field gradients, enhanced by optical pumping, is pointed out.

A Note on the Origin of the Sun's Polar Field

A Note on the Origin of the Sun's Polar Field

The Sun's Polar Magnetic Field.

view Abstract Citations (285) References (2) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Sun's Polar Magnetic Field. Babcock, Harold D. Abstract The polarity of the high-latitude dipolar magnetic field of the sun was opposite to that of the earth's field from 1953 to 1957. About the middle of 1957, the polarity of the magnetic field near the south heliographic pole was reversed; reversal of the field near the north pole was not observed until November, 1958. At present, the sun's polar field is parallel to that of the earth. Publication: The Astrophysical Journal Pub Date: September 1959 DOI: 10.1086/146726 Bibcode: 1959ApJ...130..364B full text sources ADS |