The seasons of the Sun

Our star is subject not only to an eleven-year cyclical trend, but also a kind of seasonal variability, much shorter. This hypothesis is advanced by a team of researchers and published in the journal Nature Communications.
We have learned, starting from the observation of the number of sunspots and gradually, more and more in depth with other methods of investigation, to recognize that the Sun completes a cycle of activity in about eleven years. But things are not always so simple: sometimes this time turns out to be a bit shorter, sometimes a bit longer. To further complicate the situation there are cycles whose maximum and minimum are more or less intense than others and very violent events that develop during periods of rising or falling of the cycles.



To give an interpretation of these unexpected behaviors, from the columns of Nature Communications, it comes to a study led by Scott McIntosh, Director at the High Altitude Observatory of the National Center for Atmospheric Research in the United States. In the article the researchers suggest that at the base, there is an effect of variability ‘season’ of the Sun, which is manifested by a much shorter period – almost two years – and which interacts with the eleven-year. Seasonal variations seem to be induced by changes in the bands in which you have the intense magnetic fields in each solar hemisphere. These bands also determine the performance of the solar cycle to 11 years, which in turn is part of a periodic behavior longer, lasting approximately double. “The one on which we focused is the main culprit of solar storms,” says McIntosh. “Understanding better how these bands are formed in the Sun and as seasonal produce instability gives us the opportunity to greatly improve predictions of events related to space weather.” The study in fact follows the line of inquiry on the variability of the Sun due to the behavior of its magnetic stripes, which began last year with another article, published in The Astrophysical Journal and before signing more and McIntosh: on that article it is assumed that the eleven-year cycle of our star is driven by the behavior of two parallel bands of opposite magnetic polarity that migrate slowly, over the course of nearly 22 years, from high latitudes to the equator, where they meet and then cancel each other out.
In the new study, it is shown that the migration of the bands also produces seasonal variations in solar activity that can reach intensities similar to those that determine the modulation to 11 years. And this in each of the two hemispheres. “In analogy with the jet streams that propagate in the Earth, that have heavily influenced the behavior of meteorological regional scales over the past two years, the bands on the Sun generate waves that propagate very slowly but that can expand and deform “adds Robert Leamon, of Montana State University, who participated in the study. “Sometimes, this produces a mixing of the magnetic fields between two contiguous bands. In other cases, the dragging effect reveals magnetic fields next to tachocline, up towards the surface. “These ascents of highly magnetized plasma severely destabilize the crown and trigger the most violent solar storms: more than 95 percent of flares and coronal mass ejections of the most intense can be traced back to them. This scenario can also explain why the most energetic solar events usually focus a year or more after the maximum, calculated by the number of spots, a phenomenon which is called Gnevyshev Gap, named after the Soviet scientist who first, in the 40s of last century, pointed out this behavior: to produce it would be more seasonal variations in solar activity.
“The cycle of solar activity is the result of the evolution in time of a complex variety of phenomena that characterize it, but that have not yet been fully identified and quantified,” says Mauro Messerotti, expert in solar physics INAF. “This aspect, together with the fact that the sun has the characteristics of a complex physical system, makes it very difficult modeling, and then the prediction of the evolution of the same cycle and also the phenomena connected to it, such as solar storms. The work of analysis and interpretation of the authors adds another element to the mosaic, because it identifies one of the competing factors of variation that gives rise to a modulation on a time scale of nearly two years. The observations indicate that solar storms can occur at any time of the cycle of activity, or in the phase of ascent to the maximum, to the maximum and in the descent phase. Know that there is also a “season” of the phenomena with a period less than two years and being able to identify what “season”, more or less productive, the Sun is, increases the reliability of the forecasts of the phenomena at the origin of the “space weather “as flares and mass ejections from the solar corona (CME, Coronal Mass Ejections).”

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