The Sun's Magnetic Fields

    The entire solar atmosphere is permeated by magnetic fields that are generated by the solar dynamo mechanism at the base of the solar convection zone. Due to the magnetic buoyancy, they rise up through the photosphere into the overlying atmosphere. In its entirety, the Sun appears in magnetized dipole shape carrying a magnetic field intensity of about 1 Gauss (positive magnetic polarity in the northern hemisphere and negative magnetic polarity in the southern as shown in Figure 1). The magnetic flux of the Sun ranges in the interval of 1.5 - 2.5×1022 Maxwell. The mean magnetic field of the dipole periodically undergoes a reversal of polarity every 10 - 11 years. This reversal is evidenced by the appearance of new sunspots at latitudes of 38 - 40°.

    The solar magnetic fields in photosphere of the Sun can be directly diagnosed by measuring the Zeeman Effect of spectral lines formed in the photosphere. The so-called Zeeman Effect is the effect of splitting a spectral line into several components in the presence of a static magnetic field. The measured polarization of the split components of the spectral lines depends on the magnitude and direction of the magnetic field vector. By measuring the circular and linear polarization of the spectral line, one can derive the magnetic field vector in principle with some assumption of the solar atmosphere. So far, the relative reliable measurement of magnetic field of the Sun is just for the photosphere.

Figure 1 The Sun's magnetic fields show a dipolar shape.

    The magnetic fields of the Sun show significantly different characteristics in different regions.

1. Magnetic fields in Quiet Sun
    About 90% of the solar surface is covered by the so-called quiet Sun regions with weak field even in the solar maximum phase. Magnetic fluxes usually trace the convective cell boundaries at all scales, the smallest of which is the granulation network. Granulation network is well recognizable in the quiet sun magnetograms, where negative and positive magnetic flux tubes trace the granulation pattern and encircle the network. The network field has an intrinsic property. Its fundamental elements have likely typical field strengths of 1000 - 2000 Gausses and diameters of 100 km. Although the theoretical estimated minimum size is near the current spatial resolution limit of about 10 km. The boundaries of network field are prominent in the chromosphere in which the magnetic field lines fan out due to the decrease of the external plasma pressure. As a result, the magnetic field becomes more uniform in the upper chromosphere and corona.

2. Magnetic fields in active region
    As strong magnetic fluxes popping up from inside of the Sun, they form active regions on the photosphere. Thus, active regions are considered as 'tracers' of the magnetic processes undergoing in the interior of the Sun. The mean magnetic field of active regions is a few hundred gausses and their typical magnetic flux is 1022 Maxwell. Most active regions are bipolar with the flux well-ordered into two islands of opposite polarity, but occasionally a magnetically complex region forms as new flux emerges with a different orientation or as a new region appears within an existing one. Generally, flares are more favorably produced in active regions with intense magnetic field and complex field configuration. Left panel of Figure 2 shows a white light image of one complex active region with three main sunspots, of which each one had several umbrae. The right panel shows a light-of-sight magnetogram of the active region, where the white and black patches denote positive and negative polarities of magnetic fields, respectively.
    The magnetic field in the center of a sunspot umbra is more vertical and has strength of about 2000 to 3000 Gausses. Sometimes, it may reach as much as 4000 Gausses. The field strength decreases with distance from umbra center, and at the interface between the penumbra and the photosphere it is about 1000 - 1500 Gausses. On the contrary, the inclination to the vertical line increases with the distance across the penumbra from 40° to 90° and thereafter to point back towards the surface and become about 100° at the edge. The typical flux of sunspots is about 1021 Maxwell and can reach to 2×1022 Maxwell for a large sunspot.

Figure 2 white-light image and line-of-sight magnetic field of a complex active region.

3. Magnetic fields in high latitude area and polar region
    Usually, toward the end of the 11-year solar cycle active latitudes 'shrink' and remaining sunspots and sunspot groups migrate toward the equator, while the remnants of the previous solar cycle have already migrated toward the poles. The polar region field has a polarity of the ending cycle where small-scale magnetic elements occupy the polar and high altitude regions. At the turn of a solar cycle the opposite polarity sunspots and sunspot groups start to emerge at latitude of about ±40°. Newly emerged sunspots migrate toward the equator while their peripheral magnetic elements migrate toward the polar region. Therefore, in the first half of a new solar cycle, one can observe the merging of small-scale magnetic elements with opposite polarity of the previous cycle.





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