This is a problem. To take one example, the level of solar activity controls the rate at which particles evaporate from the uppermost layers of Earth's atmosphere. These particles exert a tiny, but perceptible drag on spacecraft in low Earth orbit such as the
But it is in climate science where the existence of two rival sets of sunspot data has caused the most controversy. By grafting Hoyt and Schatten's series on to longer-term data inferred from tree rings and ice cores, it is possible to argue that solar activity has been steadily increasing, and indeed is higher today than at any time in the past 8000 years. That, rather than our own greenhouse-gas emissions, is the reason why the planet is warming, the argument goes.
For Svalgaard, this is a deeply unsatisfactory situation. "Why can't we provide a number that we can have some confidence in?" he asks. "That is something we as solar physicists should be ashamed of."
A few years back, he decided to do something about it. He wanted to get to the bottom of what was causing the inconsistencies and come up with a single, vetted number that everyone could agree on. He and his colleagues think they are now just about there.
The issue of the jump in Wolf's number in 1945 was a strange one. Daily variations in the magnetic field observed at Earth's surface are also influenced by solar activity, and Svalgaard noticed that these did not match up with the sunspot numbers as well as they should have after 1945. Something had gone awry with the counts. It turned out that, sometime after taking over the
One of the few he did tell was
Given the places where these numbers are hard-coded, they can't simply be changed. Because the aim is to ensure the consistency of the series over time, rather than establish an absolute number, Svalgaard's proposal, hammered out with colleagues from the
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