In­stant ex­pert so­lar cy­cles

Ev­ery­thing you need to know about this phe­nom­e­non in five min­utes

All About Space - - Contents -

Sunspots, flares, the Sun’s hot corona and chro­mo­sphere, the so­lar wind and the Earth’s au­ro­rae all re­sult from mag­netic fields gen­er­ated by a dy­namo op­er­at­ing in­side the Sun. The dy­namo is cyclic, with a so­lar cy­cle pe­riod of about 11 years. To un­der­stand the cy­cle we adopt a strat­egy sim­i­lar to crime de­tec­tives, but in­stead of ‘fol­low the money’, we fol­low the en­ergy. Our story be­gins in the so­lar core where nu­clear fu­sion con­verts hy­dro­gen to he­lium, re­leas­ing en­ergy and heat­ing the core to about 15 mil­lion de­grees Kelvin. This en­ergy is car­ried out­wards by ra­di­a­tion for the first 70 per cent of the dis­tance to the sur­face, af­ter which it is car­ried by con­vec­tive mo­tions. These con­vec­tive mo­tions are driven by the tem­per­a­ture dif­fer­ence; be­cause the vis­cos­ity in­side the Sun is very low, the mo­tions are tur­bu­lent.

The con­vec­tive mo­tions in­ter­act with the ro­ta­tion of the Sun, which ro­tates once per 25 days at its equa­tor. Im­por­tantly this in­ter­ac­tion be­tween the small-scale con­vec­tive mo­tions and the large-scale ro­ta­tion leads to large-scale stresses and causes the poles to ro­tate slower than the equa­tor at a rate of about once ev­ery 33.5 days.

To un­der­stand the so­lar dy­namo we can think of a piece of string – a mag­netic field line – thread­ing from the north pole of the Sun through the con­vect­ing re­gion to the south pole. The piece of string at the poles will ro­tate once ev­ery 33.5 days, how­ever, the piece of string near the equa­tor will ro­tate once ev­ery 25 days, and the string will, over the course of sev­eral years, get wound up. This wind­ing up is half of the dy­namo process.

The sec­ond half hap­pens when the field be­comes suf­fi­ciently strong to erupt to the sur­face. Dur­ing the erup­tion process the ris­ing flux tube is af­fected by the ro­ta­tion of the Sun due to the Co­ri­o­lis force, and un­der­goes a ‘tilt’ so that it emerges with a small north-south sep­a­ra­tion be­tween its pos­i­tive and neg­a­tive mag­netic po­lar­ity. The com­bined north-south sep­a­ra­tion from many ac­tive re­gions builds up over an ad­di­tional sev­eral years to cre­ate a new polar field with the op­po­site po­lar­ity to that which was orig­i­nally there. This is one so­lar cy­cle, which lasts for about 11 years.

Cy­cle 23 min­i­mumAround the year 1997 the Sun had reached the min­i­mum ac­tiv­ity for the 23rd cy­cle. At this point the mag­netic fields are rel­a­tively set­tled, and this is shown vis­i­bly with a low num­ber of sunspots present of the face of the Sun. The mag­netic field will slowly be­come more tan­gled as the un­equal sur­face ro­ta­tion takes its toll.1999200020012002199820031997 Cy­cle 23 max­i­mumThe max­i­mum sunspot ac­tiv­ity for cy­cle 23, around the year 2001, was far greater than its suc­ceed­ing cy­cle, which is a strong in­di­ca­tion that the field lines are at their most tan­gled. This will sub­se­quently cause more sunspots and more so­lar ac­tiv­ity, for ex­am­ple more Coronal Mass Ejec­tions (CME) will oc­cur and the Sun will eject more highly en­er­getic par­ti­cles, pow­er­ing Earth’s aurora and cre­at­ing a mar­vel­lous spec­ta­cle.19962004

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