Читать книгу Exploring the Solar System - Peter Bond - Страница 61

Flares are often associated with huge eruptions of ionized material, known as coronal mass ejections (CMEs), that can propel up to 10 billion tons of matter into space. These massive bubbles of plasma, threaded with magnetic field lines, are ejected from the Sun over the course of several hours.

We now know that CMEs are very common. During solar maximum, the Sun averages five of these outbursts per day, with about 100 of them heading Earthward per year. During solar minimum, the annual total may drop to about 180, with a strong CME every two days and about 10–15 per year directed at Earth.

Like all solar activity, CMEs are associated with releases of magnetic energy in active regions, particularly the concentrated magnetic fields close to sunspots. However, although they are often associated with flares and prominence eruptions, they can also occur without either of these events taking place.

Forecasting CMEs is not easy. One possible sign of something stirring is the emergence of an inverse‐S shape (sigmoid structure) in the X‐ray corona. This happens when a magnetic flux tube begins to twist, due to the rotation of one or both of the footprints of a cluster of loops located above a sunspot group.

In 2008, Hinode observed a CME in which one end of the tube spun clockwise, whilst the other rotated counterclockwise. This unfurling action caused the field lines to rupture and realign – a process called magnetic reconnection – producing a huge explosion that heated a huge cloud of material and propelled it away from the Sun in the form of a CME.

Although CMEs are so large, they are very sparse and spread out, containing only a few particles per cubic centimeter. Much of their ability to disrupt the flow of the solar wind and disturb planetary environs comes from their magnetic fields, particularly if their polarity is aligned in the opposite direction to a planet's field, leading to favorable conditions for magnetic reconnection to take place.

CMEs travel outward from the Sun at speeds ranging from less 250 km/s to around 3,000 km/s. The fastest CMEs can reach Earth in only 15–18 hours, though it usually takes two to four days to cross the 150‐million‐km gap.

During their journey, they expand in size as they move away from the Sun. Larger CMEs can reach a size comprising nearly a quarter of the space between Earth and the Sun by the time they reach our planet.