Are sunspots re­ally in de­cline?

Lu­cie Green re­veals how the sci­ence of the Sun’s mag­netic heart­beat ex­plains a drop in so­lar ac­tiv­ity

Sky at Night Magazine - - CONTENTS -

Our re­cent so­lar ac­tiv­ity has seen a tail­ing off of sunspot num­bers. Is this the start of a new so­lar cy­cle?

The num­ber of sunspots ob­served on the so­lar sur­face fol­lows a roughly 11-year pat­tern of peaks and troughs known as the so­lar cy­cle. I started my ca­reer in so­lar physics in cy­cle 23 and we are cur­rently com­ing to the end of cy­cle 24. While we still have a lot to learn to ac­cu­rately es­ti­mate the pat­terns of fu­ture so­lar cy­cles, pre­dic­tions over the past decade sug­gested that our cur­rent cy­cle would be smaller than av­er­age. Does this mean an over­all wan­ing of so­lar ac­tiv­ity? New re­search over the com­ing years may help us get a bet­ter idea of what the fu­ture holds for our host star.

Sunspots have been ob­served for thou­sands of years, even be­fore the de­vel­op­ment of the so­lar tele­scope pro­vided a way to safely view our daz­zling lo­cal star. Sys­tem­atic records of sunspot num­bers and their po­si­tions didn’t be­gin, though, un­til the mid-1800s. Still, these records rep­re­sent the long­est run­ning dataset of any cos­mic phe­nom­e­non and re­veal an in­trigu­ing side to the Sun’s char­ac­ter.

That the so­lar cy­cle was ever no­ticed is thanks to as­tronomers spurred on by the pos­si­bil­ity of dis­cov­er­ing a new planet. In 1846 Nep­tune had been dis­cov­ered in the search to un­der­stand whether ir­reg­u­lar­i­ties in the or­bit of Uranus were due to prob­lems with New­ton’s the­ory of grav­ity, or the grav­i­ta­tional ef­fect of a then undis­cov­ered planet. The lat­ter idea won. So when ir­reg­u­lar­i­ties were dis­cov­ered in Mer­cury’s or­bit, it seemed only sen­si­ble to take the same ap­proach and the ex­is­tence of

the planet Vul­can or­bit­ing close to the Sun was pro­posed. The the­ory could be tested ob­ser­va­tion­ally by look­ing for the planet as it tran­sited the Sun.

One per­son who took up this chal­lenge was Ger­man phar­ma­cist-turned- as­tronomer Hein­rich Sch­wabe, who ob­served the Sun ev­ery clear day from 1826 to 1843. Although he found no ev­i­dence for the hy­po­thet­i­cal planet Vul­can, his sunspot records did re­veal a 10-year pe­riod or so dur­ing which their num­bers rose. A repet­i­tive process seemed to be at work. Soon as­tronomers across Europe were be­ing en­cour­aged by the di­rec­tor of Bern Ob­ser­va­tory – Ru­dolf Wolf – to make reg­u­lar ob­ser­va­tions of the Sun and record their fnd­ings. Wolf com­bined the new sunspot records with those taken in the cen­turies be­fore and was able to re­con­struct sunspot cy­cles back to 1755. He called this ‘cy­cle 1’ and num­bered all the fol­low­ing cy­cles con­sec­u­tively.

What causes sunspots?

So­lar cy­cle stud­ies fo­cused for many years on data col­lec­tion, such as sunspot num­ber, lo­ca­tion and shape, un­til the true ori­gin of sunspots was shown by Ge­orge Ellery Hale in 1908. Hale dis­cov­ered that these dark spots on the Sun are ac­tu­ally the in­ter­sec­tion of colos­sal tubes of mag­netism pen­e­trat­ing the pho­to­sphere. This dis­cov­ery started a rev­o­lu­tion in so­lar physics and showed us that the so­lar cy­cle is ac­tu­ally a mag­netic one.

To­day we un­der­stand that fows of elec­tri­cally charged gas in­side the Sun both sus­tain and evolve the global mag­netic feld. The so­lar cy­cle cap­tures all the stages of this evo­lu­tion. At the start of the cy­cle, the Sun’s global feld is aligned in a north-south di­rec­tion. The fows then drag out this feld so it be­comes more aligned with the east-west di­rec­tion. When por­tions of the in­te­rior feld grow too strong, they be­come buoy­ant and a loop rises to pen­e­trate the Sun’s sur­face. Sunspots form at the two foot points of the loop: one sunspot with a south mag­netic pole next to a spot with a north pole. Just like the con­fgu­ra­tion of a mag­net.

The fuid na­ture of the Sun means sunspots don’t live for­ever. The so­lar cy­cle pro­gresses be­cause fows at the sur­face of the Sun start to tear apart the mag­netic felds of the sunspots, spread­ing the feld over ever-larger ar­eas. As the feld dis­perses, the sunspots dis­ap­pear. Even­tu­ally, more fuid fows take the mag­netic feld up to­wards both poles of the Sun where it gets re­pro­cessed, ready to feed into the next cy­cle.

Over­all, the evo­lu­tion of the Sun’s mag­netic feld that drives the so­lar cy­cle might seem straight­for­ward.

But con­sider this: cy­cles can be large or small, and cy­cles of dif­fer­ent sizes do not fol­low each other at ran­dom. In­stead, a few large cy­cles will be fol­lowed by a few small cy­cles. Analysing this trend has also re­vealed an 80-year so-called ‘Gleiss­berg cy­cle’ on top of the 11-year cy­cle. And on top of this are the 200-year de Vries and the 2,300-year Hall­stat cy­cles too. De­vel­op­ing a model that can ex­plain all these de­tails is an im­por­tant area in mod­ern so­lar physics.

The dis­cov­ery of the Sun’s mag­netism was also key to un­der­stand­ing the vi­o­lent side of the Sun’s char­ac­ter: so­lar fares and coro­nal mass ejec­tions (CMEs) driven by colos­sal re­leases of en­ergy stored in the so­lar mag­netic feld. The fre­quency of so­lar fares and CMEs peaks at the apex of each so­lar cy­cle, as does the amount of light that the Sun emits. At cy­cle max­i­mum, with the most sunspots, the Sun is frac­tion of a per cent brighter.

Cer­tain rules are obeyed dur­ing a so­lar cy­cle. Sunspots ap­pear at high lat­i­tudes at the start, with the sunspots that form dur­ing sub­se­quent years ap­pear­ing at pro­gres­sively lower lat­i­tudes. This pat­tern is shown won­der­fully clearly in the so- called ‘but­terfy’ di­a­gram pub­lished by as­tronomers An­nie and Wal­ter Maun­der in 1904 (see page 34). When pairs of sunspots ap­pear, one with a north pole and one with a south, sunspots in each hemi­sphere will nearly al­ways ap­pear in the same order as the Sun ro­tates. For cy­cle 24, pairs of sunspots in the north­ern hemi­sphere that came into view as the Sun ro­tated had the south pole spot ap­pear slightly ahead of the north. In the south­ern hemi­sphere, it was the op­po­site way round. This swaps ev­ery so­lar cy­cle, mean­ing you can use the mag­netic feld of sunspots to de­ter­mine which cy­cle they formed in. The rules ap­ply to the over­all global feld of the Sun too, and at the peak of each cy­cle the po­lar mag­netic felds swap their mag­netic po­lar­ity.

All this prompted the Amer­i­can Na­tional Oceanic and At­mo­spheric Ad­min­is­tra­tion and NASA to form

This mon­tage of 365 images from ESA’s Proba-2 satel­lite shows the low ac­tiv­ity of our Sun through­out 2018. At the ex­treme ul­travi­o­let wave­lengths pic­tured, ac­tive re­gions are shown as bright spots

Prof Lu­cie Green is a Pro­fes­sor of Physics at Mullard Space Sci­ence Lab­o­ra­tory. She is au­thor of 15 Mil­lion De­grees: Jour­ney to the Cen­tre of the Sun

Look­ing back at re­cent so­lar cy­cles re­veals a de­cline in sunspot num­bers. Early in­di­ca­tions sug­gest this trend is likely to con­tinue in cy­cle 25

So­lar cy­cle 23, cap­tured in ex­treme ul­travi­o­let light by NASA’s SOHO space­craft, re­veals max­i­mum ac­tiv­ity in 2001

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.