You may have come across reports that the solar wind – that stream of particles that blows unceasingly from the surface of our neighbourhood star – is particularly weak this year. And you may have been a little confused by the stated consequences of this stellar breeziness. If so, fear not; the sun is not dying. And if you hear anthropogenic climate change deniers claiming that this means we are due for global cooling, you can tell them where to get off, as that’s not how it works.
First of all, it’s worth briefly explaining what the solar wind is. This is a sparse stream of charged particles – mostly protons and electrons – moving outwards in a garden hose-like pattern as the sun rotates with a period of roughly 27 days. The solar wind speed varies greatly, as do the particle energy and density, and the variations follow a roughly 11-year cycle, which is known as the Schwabe or sunspot cycle. In addition to the Schwabe cycle there are a number of longer-term cycles: e.g., 22 years (Hale); 70–90 years (Gleissberg); 210 years (Suess/de Vries); and 2,300 years (Hallstatt).
The strongest of these – the 11-year solar cycle – arises from magnetic fields on the sun’s surface becoming knotted over time as the star rotates at different rates at different latitudes (the sun is a ball of incandescent gas, not a solid body). Where these knots of magnetic field lines become particularly tight we see sunspots and small regions that belch powerful flares of hot, fast-moving particles, some of which impact on the earth, firing up the northern and southern lights, interrupting radio communications and damaging satellite electronics.
Sporadic events such as solar flares are embedded in the solar wind, and are extreme manifestations of natural variations that cause the wind to move with a typical quiet-time speed of around 400 kilometres per second. However, this figure is not particularly meaningful given the wide spread of observed speeds, which can be less than 300 km/s and greater than 800 km/s. If the solar wind now is at its weakest for 50 years, this should be seen in the context of the current solar cycle minimum, in which quiet conditions are to be expected when it comes to magnetic activity in the space environment. The variation in the sun’s visible and ultraviolet radiation, on the other hand, is negligible, and so too therefore is the effect of solar cycles on terrestrial climate.
A 20–25% fall in solar wind speed as compared with the preceding solar minimum may seem highly significant, but we have only been measuring solar wind speed directly since the beginning of the space age, and that gives us only a handful of solar cycles to work with. Note that the NASA and ESA scientists reporting on wind-speed measurements from their Ulysses spacecraft stress that, based on ground-based measurements of solar activity over a much longer period, the sun’s current behaviour is well within long-term variations.
Possible confusion for those unfamiliar with space plasma physics arises from statements in recent news reports that a low solar wind speed could be harmful to astronauts and spacecraft.
The solar wind consists of charged particles the energy of which is greater than that carried by the magnetic field generated in the sun. That means that the wind drags solar magnetic field lines out into space, forming the Interplanetary Magnetic Field (IMF). As the sun travels through interstellar space, the field lines form a magnetic bubble that diverts charged particles from exploding stars and other exotic astrophysical phenomena. This is the heliosphere.
A weak solar wind will have a major effect on the heliosphere, allowing through more cosmic ray particles of galactic origin. But as far as the earth is concerned, we need to consider also the interaction of the IMF and the planet’s own magnetic field, which forms a protective bubble known as the magnetosphere. With a weaker solar wind the magnetosphere swells in size on the sunward side, and spacecraft flying within the magnetosphere are to a large degree protected from the effects of galactic cosmic rays.
It is solar cosmic rays such as those associated with solar flares that space mission planners are most concerned with, and these are fewer in number and intensity at solar minimum. Paint jobs and gutter repairs on the International Space Station are tasks to be avoided at solar maximum, not when the sun is at its quietest.