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Magnetism is the key to understanding the Sun. This is because it is mostly composed of plasma, an electrically conducting gas in which the atomic nuclei have been almost entirely stripped of their electrons. The flow of electrically charged ions and electrons in the plasma is readily deflected by local magnetic fields. This is particularly noticeable in the corona, where the gases are extremely rarefied and thus easily shaped by magnetism, rather than gravity.

Observations of the photosphere's magnetic field are made by measuring the splitting of spectral absorption lines (known as the Zeeman effect) and the polarization of light. Various techniques are then used to determine both the strength and the direction of the magnetic field. These magnetic field observations can then be compared with the observed structures in the Sun's outer regions.

The Sun's magnetic field, like Earth's, resembles that of a bar magnet surrounded by a dipole field (i.e. it has two magnetic poles). The field lines flow out of the Sun at the north pole, and re‐enter at the south pole. Usually, the magnetic axis roughly coincides with the rotation axis. During times of high activity, the Sun also features numerous dipoles with multiple bar magnets, each represented by an active region.

The complex “magnetic carpet” seen in the photosphere reaches the surface via the granular network of convection cells that are created as plasma wells up from below. Intense stirring causes magnetic dipoles to grow continually within the cells before being shed into the corona through magnetic reconnection.

Figure 2.21 Based on SOHO data, this image shows irregular magnetic fields (the “magnetic carpet”) on the photosphere. The carpet comprises a sprinkling of tens of thousands of magnetic concentrations which have both north and south magnetic poles. These are the bases of magnetic loops extending into the corona. Whiter areas represent more material at a temperature exceeding one million degrees Celsius, darker areas represent less. The black and white spots represent magnetic field concentrations with opposite polarities. Each spot is roughly 8,000 km across.

(Stanford‐Lockheed Institute for Space Research / NASA‐GSFC)

The Sun's magnetic variability waxes and wanes over the 11‐year sunspot cycle, changing in parallel with the number of active regions.¹⁰ At solar maximum, the magnetic field is very complicated, featuring numerous, relatively small, structures in the form of active regions. The field is weaker and concentrated at the poles around solar minimum, when the lack of turbulent activity does not favor the formation of sunspots or active regions.

The active regions arise in unpredictable locations, emerging from the deep interior and breaking through the photosphere into the corona. Features frequently (but not always) associated with magnetically active regions include sunspots, coronal loops, flares, and coronal mass ejections.

Sunspots occur where very intense magnetic lines of force break through the surface. The sunspot cycle results from the recycling of magnetic fields by the flow of material in the interior. Prominences that rise above the surface are supported by, and threaded through, with magnetic field lines.

Streamers and loops seen in the corona, sometimes reaching heights of several hundred thousand kilometers, are also shaped by magnetic fields (see The Corona). The arches comprise bright strands of plasma that connect two areas in the photosphere with opposite magnetic polarities. Bright blobs of hot plasma race up and down coronal loops at tremendous speeds along threads that follow the magnetic field lines.

The polarity of the Sun's magnetic dipole switches at the end of each 11‐year cycle, as the internal magnetic dynamo reorganizes itself (Figure 2.24). At solar minimum, the magnetic field resembles a simple dipole, with magnetic field lines running north–south. However, because the equator rotates much faster than the poles, the magnetic field becomes increasingly twisted as time goes by.

Figure 2.22 Active regions on the Sun are made up of many relatively small magnetic structures emerging from the surface at adjacent locations. In this EUV image from the Solar Dynamics Observatory, the closely spaced, bright active regions are linked by a tangle of magnetic loops. The base of each arch has a different polarity.

(NASA)

Over a period of 11 years, the twisted field lines wrap around the Sun, generating areas of intense magnetic fields that appear at the surface as sunspots. The process of tangling ends when the dynamo readjusts, recreating a dipole field, but with a reversal in polarity, i.e. the north magnetic pole switches to the south magnetic pole, and vice versa.

The change‐over is not always smooth. In March 2000, for example, the south magnetic pole faded and was replaced by a north pole. For nearly a month, the Sun had two north poles. The original south pole migrated north and, for a while, became a band of south magnetic flux smeared around the equator. By May 2000, it had returned to its usual location near the Sun's southern spin axis. Then, in 2001, the magnetic field completely flipped, so that the south and north magnetic poles swapped positions, which they retained throughout the rest of the cycle.

Figure 2.23 A magnetic butterfly diagram showing the distribution of the Sun's surface magnetic field (longitudinally averaged) over the last four solar cycles, i.e. since 1975. Sunspots appear in bands on either side of the equator. At the beginning of each cycle, the active regions emerge at latitudes of about 30 degrees. As the cycle progresses, the active regions emerge closer and closer to the equator. Cycles typically overlap by 2–3 years. The polarities reverse from one cycle to the next around the time of solar maximum.

(David H. Hathaway)

The Sun's magnetic field is carried out into the Solar System by the charged particles (electrons and protons) of the solar wind (see Solar Wind).