Читать книгу Exploring the Solar System - Peter Bond - Страница 56

The corona emits a continuous stream of charged particles, known as the solar wind. This solar wind flows radially outward in all directions at an average speed of about 400 km/s. Almost a million tonnes of solar material is ejected into interplanetary space each second. This outward flow of particles creates an invisible bubble in space, known as the heliosphere, and is responsible for the anti‐sunward‐trailing ion tails of comets and the shape of the magnetospheres around the planets.

The solar wind comprises an equal number of protons and electrons, plus a smaller number of heavier ionized atoms. These particles originate in the corona where the temperature is so high that not even the Sun's powerful gravity can hold on to them. As the particles stream toward the outer reaches of the Solar System, they carry their own magnetic fields.

Figure 2.32 A diagram showing roughly east‐to‐west motion at a depth of about 7,000 km in the solar interior, as derived from helioseismology observations. The diagonal yellow bands are jet streams that are associated with the solar cycles. Flow cycle 23 is shown in yellow on the left side, while the flow for the next cycle (#24), which is notably slower, is shown in yellow on the right side. The jets move slowly toward the equator and when they reach 22° latitude, new sunspots begin to appear.

(National Solar Observatory/AURA/NSF)

Like winds on Earth, the solar wind is highly variable, changing with the state of the corona. During solar minimum, there is usually a fast, steady wind over the poles and a slow, variable wind at lower latitudes. At solar maximum, it is highly chaotic, with fast and slow streams of particles interrupted by more frequent coronal mass ejections at all solar latitudes.

The high‐speed wind (750–800 km/s) is derived from plasma leaking through the corners of a magnetic honeycomb of plasma cells, mainly in large coronal holes. These regions are most commonly located near the poles – where the solar atmosphere is less dense and cooler than surrounding areas. Their open magnetic field lines allow a constant flow of high‐density plasma to stream into space. This is particularly noticeable during solar minima, when coronal holes tend be larger and longer‐lasting.

In contrast, the slow solar wind (300 km/s) originates over lower latitudes, typically over active regions and streamers, where closed magnetic field lines trap the electrically charged coronal gases, except at the edges of bright, wedge‐shaped features called helmets. Many small mass ejections, driven by magnetic explosions, also contribute to the solar wind.

Massive compression and shock waves can result if a fast stream collides with a slow stream. As the Sun rotates, the various streams in the solar wind also rotate, producing a pattern similar to a rotating lawn sprinkler. If a slow stream is followed by a fast stream, the high speed material will catch up and plow into it.

When the high‐ and low‐speed streams interact with one another, they create dense regions known as co‐rotating interaction regions (CIRs) that trigger geomagnetic storms when they interact with Earth's atmosphere.