Читать книгу Exploring the Solar System - Peter Bond - Страница 49

Images taken by spacecraft such as Hinode and IRIS have revealed fine structure in the hot, ionized gas which extends up from the surface granulation toward the corona. This escaping gas can also be seen in a huge swarm of jets known as spicules.

A snapshot of the Sun taken at any moment shows 10 million spicules erupting from the star's surface. These small jets of plasma are propelled upward from the photosphere at speeds of up to 150 km/s. They are less than 600 km in diameter, but soar to heights of 10,000 km, extending to the chromosphere's outer boundary in only a few minutes.

Although each jet is short‐lived, usually plunging back towards the surface, they often reform in the same spot every five minutes or so. This five‐minute periodicity is not a coincidence, since the surface of the Sun oscillates with the same period.

Figure 2.17 Taken by the Hinode satellite on January 12, 2007, this image reveals fine filaments of plasma in the chromosphere that extend outward from the top of the convection cells, or granulation, of the photosphere. Like a field of grass swaying in the wind, the structure results from the interaction of hot, ionized gas with constantly moving magnetic field lines.

(Hinode JAXA/NASA/PPARC)

Figure 2.18 Like a field of waving grass, small jets of superheated gas, known as spicules, soar upward from the Sun's chromosphere toward the corona. Millions of these plasma jets are thought to contribute to the heating of the corona. This Hinode image was taken in extreme ultraviolet light.

(JAXA/NASA)

Spicules almost always occur very close to swarms of small magnetic flux tubes, where the magnetic field is highly concentrated. If the flux tube is inclined, rather than vertical, the dominant five‐minute acoustic waves (see Box 2.2) can leak all the way up into the atmosphere. When they encounter lower densities of plasma, the waves grow exponentially with height and develop into shock waves. These shock waves then drive mass and energy upwards to create a supersonic jet of plasma: a spicule is formed.

The spicules may reach temperatures of more than 11,000°C and deliver huge amounts of energy and material into the corona, some of which is ejected into space in the form of the solar wind. They also generate Alfvén waves, a type of strong magnetic wave that scientists suspect is key to heating the Sun's atmosphere (see Coronal Heating).