Nigel Weiss, who has died aged 83, was a world authority on the physics of sunspots and the mechanisms driving a cycle of solar activity that lasts for 11 years. Sunspots appear as darker, cooler patches on the sun’s surface that may last from a few days to a few months. They were noticed by ancient Chinese and Greek astronomers and came to prominence following telescopic observations by Galileo and others. The number of spots on the sun at any time follows the 11-year cycle and the sun is slightly brighter during maximum activity.
In the early 20th century it was realised that sunspots are associated with strong magnetic fields. Sunspots and other solar activity such as flares are the result of interaction of the sun’s rotating gas with the global magnetic field that is generated in the sun’s interior through what is known as the solar dynamo.
Nigel’s contribution lay in studying the detailed interaction between the rotation and magnetic field of the sun, and its convective motions.
In 1966 he was the first to calculate the mechanism of “flux expulsion” by rotating eddies, by which a conducting fluid undergoing rotation acts to expel the magnetic field from the region of motion, a process now known to occur in the photosphere – the visible surface – of the sun and other stars. He also demonstrated how the magnetic field in the sun’s convective zone can be concentrated into “ropes” from which fluid motion is excluded, a process important for sunspots and solar flares.
As part of his wide-ranging studies of how the sun’s convective motions interact with the magnetic field, he showed how the oscillation in the magnetic field responsible for the solar activity cycle can become chaotic. Nigel’s view, in contradiction to that of some other experts, was that this means it is not possible to predict reliably the nature of the next solar cycle.
Born in Johannesburg, Nigel was the son of Oscar Weiss and his wife, Molly (nee Kisch). Oscar, a geophysicist, travelled extensively before settling in South Africa. Many members of his large Jewish-Hungarian family were killed at Auschwitz. Molly was one of the few female opposition MPs in South Africa in the 1960s.
From Hilton college, Natal, Nigel went to Britain to study for two years at Rugby school, and then natural sciences at Clare College, Cambridge. From researching seismic tremors he switched to the study of magnetohydrodynamics, the behaviour of electrically conducting fluid in the presence of electric and magnetic fields, completing his PhD in 1961.
During a spell at Culham Laboratory (now Culham Science Centre), in Oxfordshire, he pioneered the use of stable numerical codes for solving magnetohydrodynamic problems, and used these for his much celebrated flux expulsion work.
In 1965 he moved back to Cambridge as a lecturer in the department of applied mathematics and theoretical physics, and a fellow of Clare College. There he remained throughout his academic life, becoming professor of mathematical astrophysics in 1987, and emeritus professor in 2001. He was elected fellow of the Royal Society in 1992. As a member of the Royal Astronomical Society he served as president (2000-02), and in 2007 I had the pleasure of presenting him with the society’s Gold Medal.
The textbooks Sunspots and Starspots (with John Thomas, 2008) and Magnetoconvection (with Mike Proctor, 2014) were followed by further papers during his retirement, on the solar and stellar dynamos, on convection, on sunspots, and on the solar cycle, with the last appearing in 2018.
In 1968 Nigel married Judy Martin, an expert on Anglo-Norman literature, and they had three children, Catherine, Tim and Naomi. He was something of a polymath, able to discuss anything from 16th-century history to 20th-century opera and able to get to grips with unfamiliar languages in order to communicate in far-flung countries. He explored occupied Vienna by himself aged 15 and two years later worked for the Hudson Bay Company in northern Canada.
In his 60s and 70s he went several times to India, to work and travel extensively. From an early age he had a keen interest in 20th-century art and encouraged his father to build up a collection; in later life he was a member of the National Gallery scientific consultative group (1996-2012).
Nigel deplored apartheid, but wanted to keep up his close ties with South Africa. He and Judy set up an educational trust in Cape Town to provide school and university bursaries each year for young South Africans from townships to access more education, and kept in touch with them as they progressed.
His university webpage carries a striking statement on solar activity and global warming. A grand maximum of solar magnetic activity lasting about 80 years has come to an end. The sun might be entering a grand minimum, which would be expected to have a mild cooling effect on the Earth’s climate.
But he had always maintained that “any temperature changes caused by variations in solar activity are small compared to the global warming that we are already experiencing, and very small compared to what will happen if we continue to burn fossil fuel at the present rate”.
He is survived by Judy, their children and five grandchildren.
• Nigel Oscar Weiss, mathematician and astrophysicist, born 16 December 1936; died 24 June 2020