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Above the tachocline is the convective zone, which makes up the outer 30% of the solar sphere. The convective zone extends from a depth of about 200,000 km all the way to the visible surface, the photosphere. At the base of the convection zone the temperature is about 2 million degrees Celsius, but this decreases to about 6,000°C at the surface, where the density is only 0.0000002 gm/cm³ (about 1/10,000th the density of air at sea level).

As its name suggests, energy is transported within this zone by convection – the turbulent motion resulting from hot gas rising toward the surface and cooler gas sinking back toward the center.

Although, to us, the temperature in the lower convective zone seems incredibly high, gas in the convective zone is actually cool enough for the heavier ions (such as iron, carbon, nitrogen, calcium, and oxygen) to hold on to some of their electrons. This means that the material is more opaque, making it harder for radiation to travel through.

Heat trapped in the lower part of the convective zone makes the plasma unstable, so that it starts to boil or convect. The hotter, less dense, material starts to bubble up from below, and continues to rise as long as its temperature exceeds that of its surroundings. These convective motions carry heat quite rapidly to the surface, although the gas expands and cools as it rises. On the surface the convection can be seen in the form of granulation and supergranulation, creating a carpet of gaseous cells that rise and fall like bubbles in boiling liquid (Figure 2.14).

Helioseismic observations by the MDI instrument on the SOHO spacecraft have also revealed two huge circulation cells within the convective zone, probably extending to a depth of at least 100,000 km (4% of the solar radius).

Plasma near the surface flows from the equator to the poles at a speed of 32–64 km/h. There is a return flow lower down in the convection zone. Since the plasma density is much higher at depth, this flow toward the equator is much slower, about 5 km/h. Plasma can take anywhere from 30 to 50 years to complete the full circuit (see Figure 2.32).

Figure 2.12 Rotation rates of the plasma near the bottom of the convection zone (white line) – the level of the suspected magnetic dynamo – can change markedly over six months. Faster/slower rates are shown in red/blue. Meanwhile, near the surface (shown on the left of each cutaway) bands of faster (red) and slower (green) rotation move towards the equator.

(ESA/NASA)