Charles Rotter
Last week I found out that Leif Svalgaard died on October 11, 2025, shortly after being diagnosed with cancer. He was 83. Leif spent the better part of two decades explaining the Sun to WUWT readers whether they wanted it explained or not, and when he died, more than a year ago now, this site said nothing, because none of us knew. I wish we had.
The details here come from the obituary in SolarNews, the newsletter of the American Astronomical Society’s Solar Physics Division, written by his Stanford colleague Todd Hoeksema with Ed Cliver, Frederic Clette, Hugh Hudson, Ken Schatten and Phil Scherrer and posted at the end of December 2025.
Leif was born in Copenhagen on May 12, 1942, studied physics and geophysics at the University of Copenhagen, and joined the Danish Meteorological Institute in 1966. The next year DMI sent him to its magnetometer station at Godhavn, Greenland, and there he worked out that the magnetic polarity of the solar wind could be read from ground-based magnetometer records. That meant the sector structure of the interplanetary magnetic field could be reconstructed for decades before anyone put a spacecraft up to measure it. S.M. Mansurov found the same thing at about the same time, which is why the textbooks call it the Svalgaard-Mansurov effect.
He came to Stanford in 1972 and spent six years on John Wilcox’s team, where, according to his colleagues, his skill at systems programming in ALGOL got the group’s computers working “in a time when memory was costly, operating systems were buggy, and many programs were written in assembly code.” He built a catalogue of interplanetary sector boundaries going back to 1926, and his work with the polar field measurements from the new Wilcox Solar Observatory helped establish the “precursor” relationship between the strength of the Sun’s polar fields at solar minimum and the size of the next cycle.
Then he left solar physics for twenty years and wrote software for a living. The people who worked with him in that life remember him too, from the IBM AS/400 world: “The man was a genius. Yet very human. You could sit down with him and have a sandwich and a beer. He reverse engineered the MI compiler. Which was not intended to be used by mere mortals.”
He came back to the Sun around 2000, and the second career may have been bigger than the first. He developed two new geomagnetic indices, IDV and IHV, that let him estimate the strength and speed of the solar wind back to about 1850 from old magnetometer records. In January 2005, with Ed Cliver and Yohsuke Kamide, he used the weak polar fields of the declining Cycle 23 to predict that Cycle 24 would peak at a smoothed sunspot number of 75 ± 8, “making it potentially the smallest cycle in the last 100 years.” At the time, as Anthony later noted, the best solar models predicted that Cycle 24 would be larger than Cycle 23, and he laid out how that played out, with Leif’s own chart of the method.
At the 2013 AGU Fall Meeting in San Francisco, he sat on a panel about the weak cycle, which WUWT covered under the line “None of us alive have ever seen such a weak cycle.”
His last big project was the sunspot record itself. Leif’s mantra, his colleagues wrote, was “Know thy data,” and his German was good enough to read Rudolf Wolf’s original Mittheilungen, where he found that most of Wolf’s observations had been made with small auxiliary telescopes rather than the standard Fraunhofer refractor everyone assumed. He identified Max Waldmeier’s habit of weighting spot counts by size, and the different ways Wolf and his successor Alfred Wolfer counted. That digging led to the community-wide recalibration that started in 2013, summarized in Clette, Svalgaard, Vaquero and Cliver 2014, and to the revised sunspot series the Royal Observatory of Belgium put in place on July 1, 2015. One effect, as a 2014 post here put it, was to “wipe out the so-called ‘Modern Maximum,'” which a fair number of our readers had grown attached to.
Anthony called him WUWT’s “resident solar expert,” and he earned the title one comment at a time, correcting anyone who got the Sun wrong regardless of which side of the climate argument they were on. Sceptics who wanted the Sun to explain all of the recent warming got the same treatment as alarmists who wanted it to explain none of the past. His 2018 paper reconstructing nine millennia of solar activity, which he sent to Anthony with “You are welcome to make of it what you want, if anything,” concluded “that there has been no secular up tick of activity the last three hundred years and that recent activity has not been out of the ordinary.” When Willie Soon took him to task at a 2021 conference, Leif answered in a guest post that set the two reconstructions side by side:
To my eye there is no difference between our reconstruction and theirs [based on Machine Learning], except that we have error bars and they don’t. It is amazing how people’s bias can cause them to draw contrary conclusions from [almost] identical data.
https://wattsupwiththat.com/2021/11/23/leif-svalgaard-responds-to-willie-soon/
He could be blunt in a thread, and plenty of commenters left bruised, but he kept showing up, kept posting his data and his papers on his own site, and kept answering questions from non-academics, which not many of his colleagues were willing to do on a blog full of sceptics.
In March 2010 I threw myself a 52nd birthday party in San Francisco and invited a few WUWT regulars, with acceptance “based on examining your comment history.” Leif’s application, in full, was “Where do I sign up?” He came, with his wife, to a party that ran from 10 pm to well past the hour when solar physicists in their late sixties are supposed to be asleep, with Steve Mosher tending bar. When I got home at 5:35 the next morning, I thanked Leif and his wife first. It was the best birthday I ever had.
His colleagues sent their condolences to his wife Vera and the rest of his family last winter. Ours are arriving a year behind theirs, and we send them all the same.
Leif Svalgaard, May 12, 1942 – October 11, 2025.
