We agree with the view expressed in your News Feature “The dark side of the Sun” (Nature 441, 402–404; 2006) that solar magnetic activity is important. It leads to such dangerous events as solar flares and coronal mass ejections, as well as being associated with fascinating phenomena such as sunspots and the solar cycle.

The Sun's magnetic field is now believed to be generated by a hydromagnetic dynamo acting deep within its interior, where streams of highly ionized plasma generate electric currents and, in turn, magnetic fields. Progress in dynamo theory is extremely difficult, as it can be made only by understanding the interaction of turbulent plasma motions with magnetic fields. Indeed, the extreme conditions within the solar interior make this a formidable task; understanding even the much less turbulent environment of the Earth's atmosphere stretches current theories to their limits, and weather prediction is notoriously precarious.

It is in this context that current attempts to predict magnetic cycles on the Sun must be viewed. The model proposed by Mausumi Dikpati and her team, highlighted in your News Feature, relies on parametrization of many poorly understood effects. Although such parametrized models have been widely (and legitimately) used to explore specific features of dynamo processes, they have no detailed predictive power. Indeed, there is vociferous debate in the field, not just about the size of many of the effects included in Dikpati's (and many other people's) models but even their signs. Moreover, the dynamo equations are extremely nonlinear; the solar dynamo is believed to exist in a state of deterministic chaos, making prediction intrinsically yet more difficult. Any predictions made with such models should be treated with extreme caution (or perhaps disregarded), as they lack solid physical underpinnings.

Of course it is interesting to speculate on what direction solar magnetic activity might take in the future. Recent sunspot cycles have been exceptionally vigorous, as noted by S. K. Solanki and colleagues (Nature 431, 1084–1087; 2004). It is well known that, in the past, such episodes of high activity have tended to be followed by a dramatic crash into periods of severely reduced magnetic activity, termed Grand Minima. Although we would not presume to predict that this will happen soon, it would certainly be interesting to witness such a collapse.