Читать книгу Exploring the Solar System - Peter Bond - Страница 42

Although the Sun is a giant ball made almost entirely of plasma, it is not uniform throughout. Models of the solar interior, based on a combination of theoretical studies and observations, show a layered structure, with two main spherical shells surrounding a high density core (Figure 2.11 and Table 2.2).

Each of the regions has different physical characteristics. The core, the innermost 25%, is where the Sun's energy is generated. This energy diffuses outward in the form of radiation (mostly gamma rays and X‐rays) through the radiative zone, then continues up toward the surface by convective fluid motion through the convection zone, the outermost 30%. A thin interface layer (the tachocline) between the radiative zone and the convection zone is where the Sun's magnetic field is thought to be generated.

Radiation takes over again in the thin, visible surface layer – the photosphere. Above the photosphere are the chromosphere and the corona. These two regions make up the Sun's sparse, but extremely hot atmosphere, which makes such spectacular viewing during total solar eclipses. This atmosphere is molded and modified by ever‐changing magnetic fields and electrical currents. The moving currents and the rotation of the Sun result in a complex magnetic structure, whose strength varies and whose polarity changes in roughly 11‐year cycles.

Relatively empty zones in the corona, known as coronal holes, often occur – particularly near the Sun's poles. Sometimes they spread towards the equator or even stretch all the way from pole to pole. They are particularly prominent at times of solar minimum. Since the magnetic fields in these holes are radially aligned to the surface, rather than curving back to the Sun, they offer an escape route for the charged particles in the high‐speed solar wind (see Figure 2.34).

Figure 2.11 At the center of the Sun is an extremely dense core, where the temperature reaches 15 million degrees Celsius. The core accounts for 2% of the Sun's volume but 60% of its mass. Energy generated during nuclear reactions in the core travels to the surface through the radiative and convective zones. The radiative zone lies between about 25% and 70% of the distance from the center. The convective zone extends to just below the surface. In this region, the reduction in temperature toward the surface is so rapid that the gas becomes unstable, allowing convection currents of rising and falling plasma to dominate.

(NASA)