The quietly corrosive Sun

The most violent eruptions of particles from the surface of the Sun — known as coronal mass ejections — pose a radiation hazard to astronauts and jet passengers, and knock out power systems. These space weather events commonly occur when the sun is in the most active phase of its 11-year cycle. But scientists now think the effects of the Sun may be most damaging to our power grids during its quiet phase, its ‘solar minimum’.

The Earth is constantly bombarded by a stream of charged particles, or solar wind, emitted by the Sun. These particles can cause geomagnetic storms and aurorae on Earth if they arrive at fast enough speeds. Streams of high-speed particles are emitted from holes in the Sun’s corona where the magnetic field is open — that is, where the field lines do not loop back into the corona but extend virtually indefinitely into space. Coronal holes can persist for several months, and if they are at the Sun’s equator, which is most likely during solar minimum, they can cause repeated bouts of geomagnetic storms on Earth. This is because the Sun rotates every 27 days, so the stream of fast-moving particles barrages the Earth every time the hole points towards the Earth, like a lighthouse beam sweeping past an observer.

The geomagnetic storms initiated by fast solar wind streams tend to be less intense than those caused by coronal mass ejections, but they last for a week or so and are often recurrent. This means they can deposit more energy into the Earth’s magnetosphere and accumulate more charge in spacecraft structure and electronics. On the ground they induce currents in pipelines, railways and power grids, which can corrode metals, especially in wetter climates.

Find out more about space weather in other articles on this site (see links below).