The Sun knows a Mini-Maximum


August 2014

At last! The solar maximum has arrived. The number of dark spots on the surface of our star is the highest since 2001, the date of the last peak of activity of the Sun. This 13 year wait is very long, knowing that the cycle normally lasts 11 years. Nothing out of the ordinary for Milan Maksimovic, astronomer at Lesia (Laboratoire d'études spatiales et d'instrumentation en astrophysique) at Paris Observatory : " The maximum is certainly two years late but since the solar cycle has been observed (across almost three centuries) its duration has varied between 9-14 years."

It must be admitted that this cycle no. 24 (astronomers number them from 1760) is rather feeble. Dean Presnell, of NASA, even speaks of a "mini-max" : "There was an initial peak in November 2011, when we counted 75 spots per month. At present, we are in a second peak where there are, on average, 90 of them". These figures are feeble, compared to the hundred or so spots of the maximum of 2001, which was itself already very much below that of 1958, when more than 200 spots crossed the surface of the Sun. According to Guillaume Aulanier of Paris Observatory this small maximum is not, in spite of all, an isolated phenomenon. " There has already been some maximums with a number of spots just as feeble - at the start of the 19th century and the start of the 20th century, for example. As to the fact that the peak is double, that has already been the case for several preceding cycles".

If there is so much interest around this maximum, that depends without doubt on the fact that in spite of all the efforts of researchers, the mechanism of the solar cycle remains, in great part, unexplained. There exist nice predictive models, the ideas of which are used by Dean Pesnell. But "the parameters are adjusted by hand to be consistent with the observations", explains Sacha Brun, an astrophysicist at CEA (Commissariat à l'énergie atomique et aux énergies alternatives). "Sometimes that works, but not necessarily for good reasons. Besides, when you apply these methods to other stars than the Sun, the results are again less accurate.

Therefore, many things still remain to be understood in the physics of the Sun. Our star is a ball of plasma; that is to say of electrically-charged particles, in rotation about itself. Now, an electric fluid in motion creates a magnetic field : that is the dynamo effect. This magnetic field is born in the depths of the Sun, where some form of 'friction' occurs at the interface between the radiative zone and the convection zone (see diagram at the side). These turbulent motions produce some pockets of magnetic gas which, just like bubbles, drift and rise to the surface. The magnetic field which then emerges shows itself in the form of arcs which carry away a part of the solar plasma with them. The base of these arcs are the sunspots.

Migration of Spots

The spots appear continually on the surface of the Sun, at first around the high latitudes, then becoming closer to the equator. They then migrate either to the poles or to the equatorial band, and participate with other processes connected with the magnetic poles. The North pole becomes the South pole, and vice-versa, every 11 years. "The fundamentals of the solar cycle are well understood", comments Sacha Brun. "But today we have not managed to reproduce the bobnne periodicite in our models. We obtain a cycle of one or two years in certain cases, 30 or 40 in other cases".

To know the ingredients does not suffice, the physicists must understand very subtly the manner of how all the different processes interact with each other. And is is impossible to treat the whole set of parameters at the same time. With her group at CEA, Sacha Brun is about to publish one of the most complete 3-D models of the Sun, which nevertheless does not take account of the magnetic field. "To simulate the Sun is very complex", continues the astrophysicist. "We have reduced our star numerically into a billion little cubes. In each of these cubes, the program determines the temperature, the density, the speed of the gas. That would have taken 1000 years on an office computer!".

The simulation describes with good precision the gravity waves which propagate in the radiative zone "the far cousins of the waves on the surface of water" and also the interaction between the different physical processes, which are not necessarily visible when the Sun is observed. "To understand the Sun, it is necessary to make the link between its physical interior and the events on the surface. The sunspots are not a key ingredient of the cycle of 11 years. They are only a manifestation", asserts Sacha Brun.

Magnetic Tribulations

"This approach to global simulation is interesting and similar to our own", notes Guillaume Aulanier. With his collaborators, he is attempting to simulate the solar eruptions. These outbursts of material result in the release of an enormous number of accelerated particles into space which have a considerable effect on the Earth : the aurora borealis, magnetic perturbations, or even the failure satellites and of systems of communication, or even power cuts. Now, the eruptions also follow the rhythm of 11 years. "At the time of minimum activity there is on average at least one eruption a day, at time of maximum activity there are three or four of them". The cycle 24 is feeble, the spots are smaller globally and the eruptions less intense. But there are exceptions, like that of 25 February 2014, one of the most violent eruptions of these past years. Happily, the gas was not expelled in the direction of the Earth.

It is the proof that these phenonomena are just as difficult to forecast. At the level of the regions which surround the spots, the plasma interacts with the electric currents which circulate in the solar corona. It happens therefore that the currents accumulate in sheets of size 1m, a miniscule size compared to that of sun spots, which surpass the Earth in diameter. Such a situation is unstable : the lines of the magnetic force terminate by changing their configuration on liberating their energy in an intense flash. This luminous burst is often accompanied by the expulsion of a gigantic quantity of material, up to 10 000 million tonnes, at several hundreds of kilometers per second. "What we come to understand via our simulations, is that these re-configurations of field lines are not brutal. They skim and glide, one over the other, and progressively exchange their connections", underlines G. Aulanier. But these exchanges propagate very quickly, in the space of some minutes, a scale of time which the simulation has not been able to reproduce. The researcher makes the comparison : "The solar physics of today resembles a little the meterology of the 19th century. We are still not at a stage of physical knowledge sufficiently advanced to be able to claim to make solid predictions for the cycles or the eruptions".

Sylvain Guilbaud