Predictions from the past

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Visual records of aurorae offer a longer-term view of solar activity than the telescopic sunspot record.

he Sun is a very dynamic, ever-changing object. Solar activity – in terms of the strength of the solar wind and the number of sunspots, flares and coronal mass ejections – varies greatly over time. This solar activity and the ‘space weather’ it drives has important effects for life on Earth, especially now our civilisation is so reliant on electrical distribution grids.

Scientists trying to study the Sun, and better understand space weather and its influence on the Earth are faced with a problem. Solar activity varies  in a rhythm and not every one of these cycles is equivalent. Direct measurements of solar activity – such as counting sunspots – only date back to around the time of the invention of the telescope (see ‘What has happened to solar maximum?’ on page 32). To help understand the long-term variations in the Sun’s activity we need observations that go as far back in time as possible. In this new paper, Nicola Scafetta and Richard Willson present exactly that.

Scafetta and Willson have worked with the historical record of naked eye sightings of auroral activity over Hungary from 1523. These Northern Lights are caused by material streaming from the Sun that is deflected by our planet’s magnetic field to strike the atmosphere around the poles. Because Hungary  is fairly far south, aurora seen there relate to only the most intense outbursts, when the Sun is most active. Using a mathematical technique known as harmonic analysis to search complex variations for rhythmical patterns, the researchers found several longer-term cycles in this proxy of solar activity, with periods of roughly 43, 57, 86 and 171 years. These correspond closely to the periods of the ‘wobbles’ in the Sun’s position caused by the orbiting masses of Jupiter, Saturn, Uranus and Neptune.

But the power in this modelling comes not from simply trying to fit an equation to data already in existence, but extending the purported patterns into the future – to predict. The Hungarian aurora record only extends up to 1960 (when naked-eye observation was abandoned in favour of photography) and so Scafetta and Willson ran their model forwards  from this point to see if their modelled oscillations continued to successfully match reality. Their model predicts a particularly low solar minimum in the early 1970s and a significant maximum over the years 2000-2002, which was indeed what instruments saw. This validates the model and gives us confidence that it faithfully tracks actual solar behaviour.

So what about forecasting far into the future? Well, Scafetta and Willson’s model predicts a prolonged solar minimum around the 2030s, as is also predicted by other similar models. The important conclusion here is that the solar activity cycle is driven not just by internal dynamics deep within the Sun, but also by the gravitational influences of the larger planets 
in the Solar System.

This article first appeared in the May 2013 issue of Sky at Night Magazine.


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