3.8 Solar magnetism

Observations of magnetic activity on the Sun reveal extremely complex behavior but systematic patterns also exist, at least some of which may be traced back to field generation in the convection zone and tachocline. Thus, a wide variety of magnetic activity is of relevance to solar interior dynamics; here we will only scratch the surface. More comprehensive reviews are given in these volumes by Fan (2004

) and Charbonneau (2005

), (see also Schrijver and Zwaan, 2000

; Ossendrijver, 2003

The most familiar and compelling magnetic activity pattern in the Sun is the sunspot cycle and the corresponding butterfly diagram (e.g., Stix, 2002 ). Sunspots and other manifestations of magnetic activity emerge in well-defined latitudinal bands which migrate toward the equator on a timescale of about 11 years. As these activity bands converge on the equator, the polarity of the global field reverses and the emergence pattern repeats, returning to its previous magnetic configuration after two reversals, yielding a net 22-year periodicity.

Sunspot groups are often separated into regions of outward and inward magnetic polarity which are aligned nearly east-west (meaning the neutral line is nearly north-south), but tilted somewhat relative to lines of constant longitude. The polarity of the leading (eastern) side is opposite in each hemisphere and reverses sign every 11 years with the activity cycle (known as Hale’s polarity rules) whereas the tilt angle increases approximately linearly with latitude (known as Joy’s law). These patterns suggest that bipolar active regions are made up of toroidal magnetic flux which has emerged as a loop from below the photosphere and may still be anchored there (Fan, 2004 ).

Figure 4:

Shown is a potential-field extrapolation of the radial magnetic field measured in the photosphere with the MDI instrument aboard the SOHO spacecraft (Schrijver and DeRosa, 2003

, ; see also

http://www.lmsal.com/forecast). White lines denote closed loops while green and magenta lines denote open fields of positive and negative polarity, respectively (courtesy M. DeRosa).

The loops which emerge are often twisted and many obey systematic rules for the sense of the twist as defined by the magnetic helicity or current helicity (e.g., Biskamp, 1993

). Helicity indicators in the photosphere, chromosphere, and corona are generally positive in the northern hemisphere and negative in the southern hemisphere (Pevtsov et al., 1994 , 1995 ; Zirker et al., 1997 ; Chae, 2000 ; Pevtsov, 2002 ). The pattern is most evident with relatively large-scale structures such as coronal loops.

Another pattern in magnetic activity which has particular relevance to solar interior dynamics is the presence of active nests or active longitudes: localized regions of the solar photosphere where magnetic flux appears to emerge preferentially and repeatedly over the course of multiple rotation periods (Bumba and Howard, 1965 ; Bogart, 1982 ; Brouwer and Zwaan, 1990 ). DeToma et al. (2000 ) chart a number of such regions during the rising phase of the current solar cycle. They find nests which persist for up to seven rotations, and the number of simultaneous nests increases progressively as the cycle proceeds from zero in late 1995 to three in 1998 (previous studies revealed up to six coexisting longitudinal bands of enhanced activity).

The global structure of the coronal magnetic field as inferred from white light observations can also provide insight into the nature of the solar dynamo operating in the interior, although it is strongly influenced by dynamical processes in the atmosphere as well, such as advection by the solar wind (Aschwanden et al., 2001 ). Potential-field extrapolations from photospheric measurements and more sophisticated coronal models yield a complex web of magnetic loops and open fields with a range of size scales and connectivity across the solar surface (e.g., Altschuler and Newkirk, 1969 ; Gibson et al., 1999 ; Aschwanden et al., 2001 ; Schrijver and DeRosa, 2003 ). On the largest scales, the axisymmetric component of the poloidal field is approximately dipolar during solar minimum with an amplitude at the solar photosphere of roughly 10 G. However, as the activity cycle progresses, the field becomes much more complicated and dynamic, with substantial contributions from higher-order multipoles. Figure 4 illustrates the coronal field structure near solar maximum. Note that a potential-field extrapolation as shown does not take into account dynamics occurring above the photosphere and thus may not in general be an accurate indicator of the actual field structure (Gibson et al., 1999 ; Aschwanden et al., 2001 ). However, it is a good first approximation and suffices for our purposes here, as a diagnostic of dynamo processes in the solar interior.