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3.1. Sun (~ 106 km; ~ 10-7 pc) (~ 10 Gauss)

Current magnetic fields in stars could have originated in pre-stellar matter (fossil theory), and be influenced later by (i) magnetic diffusion and (ii) breaking in the protostellar cloud as well as by (iii) plasma motions and (iv) dynamo effects inside stars (e.g., Shi-Hui 1994). The contraction of an initial protosolar cloud permeated by a magnetic field may have followed the equation involving diffusion, magnetic induction, as well as buoyancy and turbulence.

The internal structure of the Sun is reasonably well understood. The current internal structure of the Sun is understood to have a nuclear-burning core (out to ~ 0.2 solar radius), followed by a radiative zone or shell (out to ~ 0.6 solar radius), and by a convection zone or envelope (out to ~ 0.98 solar radius). The visible surface of the Sun is called the photosphere.

It is believed that the main energy source of the solar dynamo is the differential rotation inside the stable stratified 'overshoot layer' of thickness ~ 20000 km, sandwiched near 0.6 solar radius between the solar radiative zone and the solar convection zone (e.g., Zwaan 1978; Parker 1993). In this 'overshoot layer', the magnetic field needs to be ~ 100000 Gauss and the gas density ~ 1023 cm-3, and some of this magnetic field may then wind up later on at the solar surface with a 22-year complete solar cycle (e.g., Ossendrijver & Hoyng 1997). Such a thin-shell dynamo model for the Sun cannot generate a poloidal field fast enough to maintain a large scale field stretching across the entire sun; the field is therefore an oscillating dipole (e.g., Parker 1983).

The large scale solar magnetic field near the poles of the Sun is basically dipolar (with a strength of about 10 Gauss). The 11-year sunspot pattern occurs for each half of a 22-year complete solar cycle, and after each half the poles of the sun change again their large-scale magnetic sense. The polarity of the north geometric pole of the Sun changed in mid-1958, and in mid-1971, and again in mid-1980, etc. (e.g., chapter 7 in Shi-Hui 1994).

The data for the Sun's large scale magnetic field are consistent with some kind of active magnetic dipole, of strength ~ 3 × 1027 Gauss m3. The radius of Sun is ~ 700000 km. The magnetic field strength at the polar surface amounts to about 10 Gauss.

In practice, such a large-scale dipolar magnetic field shape will be distorted by environmental effects (solar rotation, solar wind) or by local surface effects (sunspots) or by tidal effects (combined effects or planets, nearby passing star).

There are small scale magnetic fields on the Sun. Magnetic activity on the scale of months or years appears away from the solar poles, usually as bipolar active regions (sunspots) covering small surface areas, where the magnetic field strength can reach about 2000 Gauss and sunspots tend to be paired by a localized dipolar magnetic field (small scale magnetic fields). Recent work by Westendorp Plaza et al. (1997) gave evidence for gas and magnetic field lines to rise near the center of a sunspot, and to turn horizontal in a penumbral canopy, and then to bend downward in the outer penumbral region at the outer edge of the sunspot. The gas density in the penumbral canopy is an order of magnitude smaller than in the inner penumbra and the canopy has 14 times smaller magnetic flux, implying a non-conservation of mass and field in the canopy, and explained by the field bending and downward mass motion at the outer edge of the sunspot. Over the course of the first half of a 22-year complete solar cycle, the mean sunspot latitude starts around 40° and then slowly tends to decrease toward the solar equator, after which (~ 11 years) the poles of the Sun change their magnetic sense (large-scale dipole).

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