2.1 Observations

Presently, three solar cycles of continuous data have been collected by a variety of ground- and space-based observatories (Mount Wilson Observatory, Wilcox Solar Observatory, Kitt Peak, SoHO/MDI, SOLIS) mapping the distribution and evolution of the Sun’s normal magnetic field component at the level of the photosphere. An illustration of this can be seen in Figure 1View Imagea (from Hathaway, 2010Jump To The Next Citation Point). The image is known as the solar magnetic “butterfly diagram” and illustrates the longitude-averaged radial magnetic field as a function of time (horizontal axis) versus sine-latitude (vertical axis). Yellow represents positive flux and blue negative flux, where the field saturates at ± 10 G. The main features in the long-term evolution of the global magnetic field are:
View Image

Figure 1: (a) The Solar Butterfly Diagram (reproduced from Hathaway, 2010). Yellow represents positive flux and blue negative flux where the field saturates at ± 10 G. (b) Example of a typical radial magnetic field distribution for AB Dor taken through Zeeman Doppler Imaging (ZDI, data from Donati et al., 2003) where the image saturates at ± 300 G. White/black denotes positive/negative flux. Due to the tilt angle of AB Dor, measurements can only be made in its northern hemisphere. Image reproduced by permission from Mackay et al. (2004Jump To The Next Citation Point), copyright by RAS.

While continuous global measurements of magnetic activity only exist from around 1975, observations of the numbers of sunspots may be used to provide a long running data set of solar activity back to 1611 (Hoyt and Schatten, 1998Jump To The Next Citation Point; Vaquero et al., 2011). These show that on top of the approximate 11 or 22-year activity cycle there are strong modulations in the number of sunspots (or magnetic flux emergence rate) over periods of centuries. It is also possible for large-scale magnetic activity to disappear. Such an event occurred between 1645 and 1715 where it is known as the Maunder minimum (Eddy, 1976; Ribes and Nesme-Ribes, 1993). Before 1611 indicators of magnetic activity on the Sun may be found through the use of proxies such as 14C (Stuiver and Quay, 1980Jump To The Next Citation Point) and 10Be isotopes (Beer et al., 1990Jump To The Next Citation Point). Through this, reconstructions of the level of magnetic activity over the past 10,000 years may be made (Usoskin, 2008) and show that many such “grand minima” have occurred over the last 10,000 years.

While our present day knowledge of solar magnetic fields is vast, the majority of this knowledge comes from observing the line-of-sight component at the level of the photosphere. To gain a much fuller understanding of the Sun’s magnetic field, vector field measurements are required (Lites et al., 1994). However, these measurements are complicated to make, with problems including low signal-to-noise ratios and resolving the 180° ambiguity. In addition, in the past such measurements have not been regularly made until the launch of Hinode (ca. 2006) which can make vector magnetic field measurements over localised areas of the Sun. However, with the new space mission of SDO (launched in 2010) and the availability of ground based vector magnetic field measurements (SOLIS), such measurements should now be made regularly over the full solar disk in strong field regions. This – combined with modeling techniques – should significantly enhance our understanding of solar magnetic fields in years to come. While current vector magnetic field measurements should increase our knowledge, to fully understand the Sun’s magnetic field vector, measurements are also required in weak field regions over the entire Sun. This poses a significant technical challenge, but is something that future instrument designers must consider.

While our understanding of magnetic fields on stars other than the Sun is at an early stage, significant progress has been made over the last 10 years. For young, rapidly rotating solar-like stars, very different magnetic field distributions may be found compared to the Sun. An example of this can be seen in Figure 1View Imageb, where a typical radial magnetic field distribution for AB Dor taken through Zeeman Doppler Imaging (ZDI, Semel, 1989) is shown. AB Dor has a rotation period of around 1/2 day, which is significantly shorter than that of the Sun (27 days). Compared to the Sun, key differences include kilogauss polar fields covering a large area of the pole and the mixing of both positive and negative polarities at the poles. While this is an illustration of a single star at a single time, many such observations have been made across a wide range of spectral classes. In Figure 3 of Donati et al. (2009) the varying form of morphology and strength of the magnetic fields for a number of stars ranging in spectral class from early F to late M can be seen compared to that of the Sun. The plot covers stars with a rotation period ranging from 0.4 to 30 days and masses from (0.09 to 2M ⊙). While ZDI magnetic field data sets are generally too short to show cyclic variations, recent observations of the planet-hosting star, τ Bootis, have shown that it may have a magnetic cycle with period of only 2 years (Fares et al., 2009). Indirect evidence for cyclic magnetic field variations on other stars can also be seen from the Mt. Wilson Ca II H+K observations, which use chromospheric observations as a proxy for photospheric magnetic activity (Baliunas et al., 1995). These show that magnetic activity on stars of spectral types G2 to K5V has three main forms of variation. These are (i) moderate activity and regular oscillations similar to the Sun, (ii) high activity and irregular variations (mainly seen on young stars), and finally (iii) stars with flat levels of activity. The final set are assumed to be in a Maunder like state. In the next section magnetic flux transport models used to simulate the evolution of the radial magnetic field at the level of the photosphere on the Sun and other stars are discussed.

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