Less rowdy than we realised

The flaming ball of plasma in the sky is calmer than we thought. A new analysis of decades of old observations of the sun shows that we previously overestimated its roiling.

As plasma churns within a star, it generates magnetic fields, which then wind around the star like pulled taffy as it rotates. The level of activity of these magnetic fields is constantly shifting: for our sun, it rises and falls in an 11-year cycle, but for other stars that cycle can be longer or shorter.

To compare stars’ magnetic activity, astrophysicists often use a scale called the S-index, which looks for the effects magnetic fields have on the wavelengths of light stars absorb and emit. This was developed by observing many stars using the Mount Wilson Observatory near Los Angeles, California, between 1966 and 2002.

It’s useful to compare other stars to the sun, because we can see processes up close in the sun that we can barely detect in other stars due to their distance. “It’s very important to get the sun correctly on this scale in order to understand its context, to make comparisons to something that we know very well,” says Ricky Egeland at the High Altitude Observatory of the National Center for Atmospheric Research in Boulder, Colorado.


But we can’t be sure our S-index value for the sun is ideal. The index was developed using a single set of instruments, so it’s hard to directly compare the values for any stars that were not included in the initial set because other telescope have their own idiosyncrasies. And it’s impossible to observe the sun with the same telescopes at the Mount Wilson Observatory.

“In general it’s really difficult to measure the sun as a star – you can’t use the same instruments, you can’t point a night-time telescope at the sun, it’ll break everything,” Egeland says.

The sun in context

When Egeland requested some data from Mount Wilson for another project, he was surprised to find hundreds of measurements of sunlight reflected off the moon.

He saw an opportunity to place the sun more accurately on the S-index: he could extract the necessary information about sunlight from those images of the moon. Then, he’d have solar observations from the same telescopes used for the original S-index.

“All of the common methods [for placing the sun on the S-index] involve extra steps that can introduce systematic errors, which aren’t necessary with these old measurements that I rediscovered,” says Egeland.

He found that, because of those systematic errors, nearly all of our previous estimates of the sun’s activity were inflated – the sun is 4 to 9 per cent less active than we thought. Knowing this will let us compare the sun more accurately with other sun-like stars and see which are similar to its past state and which might represent its future.

Journal reference: ArXiv, DOI: 1611.04540