Questo collage mostra i campi magnetici all'interno del Sole, secondo il modello della dinamo solare (al centro). A destra, la corona solare in fase quiesciente. A sinistra, la fase attiva che fa seguito alla precedente. Le simulazioni mostrano che si assiste a un minimo nell'attività solare quando le fasce del campo magnetico (le regioni in blu e in rosso a destra) si separano a causa di cambiamenti nel flusso del plasma. Questa separazione comporta l'assenza di macchie solari (e quindi di tempeste) tra due cicli successivi. Credit: NASA/Goddard/SDO-AIA/JAXA/Hinode-XRT.
Before returning to lead the show with magnetic storms, our star has spent an unusually long period of inactivity, characterized by few sunspots and a weak polar magnetic field. A group of researchers has provided an explanation of Nature. Alessandro Bemporad (INAF-OA Torino): "It's a bit 'as if the sun had consumed too quickly the magnetic field and had taken a break."
Our star has just emerged from a long sleep from which most did not seem to want to wake up. The last phase of quiescence of the Sun, in fact, was unusually prolonged, before he returned to show signs of life, with stains and subsequent geomagnetic storms. A collaboration between Indian and U.S. researchers, it turns out the reason for this strange "hibernation". How to write this week in Nature , Dibyendu Nandy and colleagues, all of the explanation lies in the variations of the flow velocity of hot and highly ionized plasma inside the Sun
The solar magnetic field varies periodically between high and low about every 11 years. "The phase of the cycle is characterized by counting the number of sunspots that appear on the disc: There are no data about at least since about 1610, practically since Galileo invented the telescope, "says Alessandro Bemporad, a researcher at the INAF-Osservatorio di Torino Astrom. "From these data we see that the solar cycle follows a regular, but irregular cycles were also observed particularly short of 8 years, and particularly long, up to 12 years, so to speak, then there is nothing unusual about the excessive length of the last minimum of activity. "
Towards the end of cycle 23, which has seen the peak in 2001, the Sun has fallen into a minimum phase characterized by a weak polar magnetic field and a very small number of sunspots. Only recently has run woke up.
There is no universally accepted explanation for these variations in the duration of the solar cycle. The Nature of proposed work on one. The researchers used a complex model of the "magnetic generator" to simulate how the solar sunspots vary at different speeds in the north-south movement of the plasma (the so-called movement meridiale). The result indicates that, at a very rapid flow in the first part of the cycle, followed by a slower flow in the second, the sun sinks into a period of minimum activity, longer than usual. Just what has happened.
"The problem the base is the periodic generation and subsequent cancellation of magnetic fields on the surface of the Sun, "says Bemporad. "The magnetic fields are created by turbulent plasma convection in the interior layers. The plasma goes back to the outer surface and moves in two main directions: the so-called differential rotation , faster at the equator than the poles, and the so-called meridian circulation, from the equator to the poles in the direction of the meridian. The acceleration in the meridian circulation in the first half of the cycle, followed by a deceleration in the second half of the cycle, would lead to the unusual features of the last phase of minimum. So to speak, is a bit 'as if the sun had consumed too quickly and its magnetic field had taken a break. "
quetso type of analysis are essential. Both to understand the workings of our star, is to monitor the flow of radiation which affects the Earth (and resulting geomagnetic storms). With the recent eruption, the Sun has launched its "awakening." But we can make some predictions that will be on stage? "No - replied the solar physicist. "At present we are unable to provide satisfactory predictions the behavior of our star. "
By Daniela Cipolloni
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