The earth coasts within the sun's outer atmosphere, bathing in its stream of sunlight, invisible radiation, magnetic field, and charged particles — not close enough to be fried; not distant enough to freeze.

T h e sun ' s benef icent rays are the very

sou rce o f ou r weather ; o f that there is n o

doubt . T h e sun 's energy drives the earth's

a tmospher i c heat e n g i n e , g iv ing u s the

pressure sys tems, w inds , and c l o u d s that

w e familiarly ca l l weather . M a y it l o n g

c o n t i n u e to d o so .

H o w e v e r , what w e , in ou r cur ios i ty ,

w o u l d l ike to k n o w is whether variations

o f the sun — sunspots , flares, and the l ike

— cause the earth's weather m a c h i n e to

falter or h i c c u p in a c k n o w l e d g e m e n t o f

the master sou rce ' s b l emishes . N o w that's

a far b igge r ques t ion . I f unpred ic tab le var­

iat ions have a s ignif icant impac t o n the

weather , then weather fo recas t ing c a n

never be anything but an inexac t s c i e n c e .

A s k a pract i t ioner o f s c i e n c e the ques ­

t ion , and the answer wi l l p robab ly d e p e n d

o n the scient is t ' s spec ia l iza t ion . A s k a

m e t e o r o l o g i s t and h e w i l l a lmos t invari­

ably g ive y o u a negat ive answer (or s o m e ­

times an agnos t i c o n e ) . I o n o s p h e r i c and

solar phys ic i s t s o n the other h a n d are

m u c h m o r e l ikely to be bel ievers . In the

cou r se o f this article w e wi l l m e e t adhe­

rents f rom bo th c a m p s .

I f the sun drives the weather , and the

w e a t h e r is s o c u s s e d l y va r i ab l e , t h e n

( lack ing any other apparent cause) surely

the sun is to b lame? Perhaps those dark

b lo t ches w e ca l l sunspots are the culpr i ts .

T h e largest o f these strange visi tat ions

u p o n the sun ' s d i sc can b e seen by the

naked eye. ( A s p e o p l e are warned before

solar ec l ipses , it i s , o f cou r se , m o s t u n w i s e

to l o o k d i r e c t l y at the s u n . ) A n c i e n t

C h i n e s e as t ronomers observed sunspots

filtered by jade , mis t , or the a tmosphere at

sunset . Be fo re the t e l e scope was invented ,

m o s t Eu ropeans failed to see , or at any rate

record , these manifestat ions, perhaps b e ­

cause o f the pervading t h e o l o g i c a l v i e w

that it was heresy to perce ive b l emi shes

u p o n G o d ' s perfect sun.

B u t u s i n g h i s t o r i c a l r e c o r d s f r o m

Ch ina , Arabia , and Central A m e r i c a , w e

can bu i ld u p a fairly c o n t i n u o u s p ic ture o f

sunspot activity for the last 2 0 0 0 years.

T h e d iagram o n page 14 s h o w s the p ic ture

s ince 1600.

T h e m o s t no tab le feature is the r o u g h l y

1 1 - y e a r c y c l e i n the a p p e a r a n c e o f

sunspots . Unpred ic tab ly , s o m e t i m e s it's

as short as 7 years or as l o n g as 16 years.

N o general ly accepted sc ient i f ic theory

yet exists to expla in it, or i ndeed w h y

sunspots appear at all . Never the less , the

w a x i n g and w a n i n g o f the sunspo t c y c l e

has p rompted many p e o p l e to search for

We can build up a fairly continuous picture of sunspot activity for the last 2000 years.

s i m i l a r p e r i o d s i n the w e a t h e r ' s b e ­

haviour . W e shall l o o k at s o m e o f their

efforts before l o n g .

A solar flare captured by a rocket-borne camera.

31

Two views, from earth orbit, showing ; very large solar eruption. The picture below was taken in the extreme ultravk In the colour density rendition on the right, colour varies with the thickness c the eruption.

Y e t solar phys ic s , as currently under­s tood , indicates that the effect o f sunspots shou ld be pract ical ly impercep t ib le . R e ­cen t measurements m a d e by D r R o b e r t Bray, a solar phys ic is t at the C S I R O D i v i ­s ion o f A p p l i e d Phys i c s , s h o w that even large sunspots are un l ike ly to alter the sun 's output o f energy (the 'solar c o n ­stant') by m o r e than 0 • 1 %.

Solar physics, as currently understood, indicates that the effect of sunspots should be practically imperceptible.

Far larger c h a n g e s w o u l d s e e m to b e needed to in f luence our weather . M o s t Sydneys iders , for e x a m p l e , c a n tell sum­m e r f rom winter w i thou t asking a sc ien­tist. H o w e v e r , i f they d id , the scient is t w o u l d p o i n t o u t tha t t he i n s o l a t i o n (number o f sun 's watts fal l ing o n each square metre o f Sydney) at the he igh t o f s u m m e r is tw ice that w a rm ing the city at the depth o f winter . N o w that's pure ly a result o f the geome t ry o f the e a r t h - s u n s y s t e m , b u t n e v e r t h e l e s s the w e a t h e r responds as if the sun were that m u c h d i m m e r .

S imi lar ly , a c h a n g e i n solar input of, to c h o o s e a typical f igure , 2 5 % , p r o d u c e d i n l o c a l i z e d areas by dus t f r o m v o l c a n i c erupt ions l ike the Krakatoa e x p l o s i o n o f 1883 and this year 's M t St H e l e n s erup­t ion, can cause measureable loca l tempera­ture and weather effects. Other features o f as t ronomica l geomet ry , l ike the el l ipt ical orbit o f the earth a round the sun , m e a n that f rom an earthl ing 's po in t o f v i e w the sun 's output is forever c h a n g i n g .

This N A S A satellite, powered by solar cells, was designed to help unravel the sun's mysteries.

T h e el l ipt ici ty o f the earth's orbi t g ives a 3 - 4 % annua l var ia t ion ( so S o u t h e r n H e m i s p h e r e s u m m e r s are hot te r than Nor the rn o n e s ) and longer - te rm celest ia l wander ings p r o d u c e in their turn no t in­c o n s e q u e n t i a l c h a n g e s o v e r t e n s o f thousands o f years. F r o m this v i e w p o i n t , the 0 • 1 % c h a n g e attributable to sunspots s h o u l d b e s w a m p e d by far larger inf lu­ences .

Keeping tabs on the sun

Great impor t ance has been attached o f late to m o n i t o r i n g f l icker ings i n the sun 's energy output . T h i s is best d o n e by satel­lites above the perturbing in f luence o f our a t m o s p h e r e . T h e S o l a r M a x i m u m M i s s i o n ( S M M ) satel l i te , l a u n c h e d i n February this year by N A S A , shou ld g ive defini te ind ica t ions o f what? s g o i n g o n . Its radiometer is de s igned to accurate ly mea­sure the sun 's total radiat ion output .

I n this year o f an expec ted m a x i m u m in the 11-year sunspo t c y c l e , the satellite,

wi th its array o f specia l instruments f o ­cused o n the sun , is expec ted to p rov ide m o r e in format ion o n our star than all pre­v i o u s satellite observat ions c o m b i n e d . It provides data about the c o r o n a th rough its ar t i f ic ia l -ecl ips ing co ronag raph , and is carrying s ix other inst ruments that are l o o k i n g at the sun 's ul traviolet , soft and hard X-ray , and gamma-ray e m i s s i o n s .

Desp i t e difficult ies i m p o s e d by the at­m o s p h e r e , g r o u n d - b a s e d i n s t r u m e n t s p rov ide a great deal o f in format ion . Data ga the red b y t h e m s e e m to p o i n t to a gradual rise in the sun ' s intr insic output s i n c e about 1920. But i f an increase in fact has occur red , it has b e e n o f less than 1 %. S igni f icant ly , dur ing that t ime , u p till 1960, the peaks i n the sunspot c y c l e stead­i ly i n c r e a s e d . T h i s s u g g e s t s that the n u m b e r o f sunspots o c c u r r i n g — unden i ­ably a g o o d indicator o f solar activity — may also b e an express ion o f the sun 's total energy output .

A s further c i rcumstant ia l e v i d e n c e , o l d records indica te that the pe r iod b e t w e e n 1650 and 1700 was remarkably free o f sunspo ts . C o n t e m p o r a r y r eco rds o f au­roras and s ight ings o f the sun 's c o r o n a dur ing ec l ipses a lso attest the sun 's quiet­ness then.

K n o w n as the M a u n d e r m i n i m u m , this pe r iod was a lso the t ime o f a little ice-age in E u r o p e , wi th temperatures depressed by an average o f abou t 1 . 5 ° C . Scient is ts ca lcula te that a 1-5% decrease in solar output w o u l d p r o d u c e this effect.

W e can d e d u c e that this pe r iod also saw an increased inf lux o f c o s m i c rays, an effect n o w k n o w n to b e d u e to r educed solar activity ( the sun 's weaker m a g n e t i c f ie ld at s u c h t imes a l lows m o r e c o s m i c radiat ion to r each the earth). T h e evi­d e n c e res ides in tree r ings , w h e r e the a m o u n t o f radioact ive ca rbon-14 in r ings laid d o w n in the 17th Century s h o w s an

The number of sunspots fluctuates in a cycle of approximately 11 years. According to most records, there were few or no sunspots in the second half of the 17th Century.

14

A coronagraph image of the sun. The glowing solar atmosphere (the corona) becomes visible when the bright light of the sun is masked, as it also is during a total eclipse.

excess indicat ive o f m o r e plentiful c o s m i c rays.

V e r y recent ly , h o w e v e r , the reality o f the M a u n d e r m i n i m u m has b e e n chal ­l e n g e d by t w o C h i n e s e as t ronomers work ­i n g at the N a n k i n g P u r p l e M o u n t a i n Observatory. T h e y have c o l l e c t e d obser­v a t i o n s o f s u n s p o t s f r o m p r e v i o u s l y untapped C h i n e s e records , and they c l a i m these demonstra te that solar activity, c o n ­t inued unabated dur ing this pe r iod .

Never the less , taken together , the avail­able e v i d e n c e does indica te a r educ t ion in the l e v e l o f so l a r ac t iv i ty d u r i n g the s e c o n d hal f o f the 17th Century. Its extent remains a matter o f his tor ical d ispute .

Sunspots reinstated

W h i l e ephemera l solar activity, s u c h as sunspots and solar flares, p r o d u c e s little c h a n g e in the total a m o u n t o f energy emit ­ted by the sun, m a n y investigators have d o c u m e n t e d its effect o n the earth's upper a tmosphere , part icularly the e lectr ical ly charged r e g i o n above 80 k m labe l led the i o n o s p h e r e . Solar flares c a n cause m a g n e ­t ic s torms, withsresulting auroral d isplays , rad io b lack-outs , or disruptive currents i n d u c e d i n p o w e r l ines , p ipe l ines , or rail­way s igna l l ing c i rcui t s . T h e effects o f a magne t i c s torm c a n even penetrate b e l o w the i onosphe re to the m e s o s p h e r e at a he igh t o f 6 0 - 8 0 k m .

H o w e v e r , invest igators a t tempt ing to u n c o v e r a l inkage f rom the sun to the t r o p o s p h e r e — the l o w e r a t m o s p h e r e whe re in ou r weather patterns u n f o l d — n e e d to take a c c o u n t o f the fact that e v e n l a r g e s o l a r e v e n t s s u p p l y n e g l i g i b l e energy c o m p a r e d wi th the weather sys­tems that they supposed ly affect. A run-o f - t h e - m i l l s t o r m r e l e a s e s a b o u t 1 0 s

ki lowat t -hours , whereas a solar flare may n o t perturb the w h o l e a tmosphere by as m u c h .

Temperatures continue to increase above 150 km altitude, and by 600 km have reached about 1200°C.

Undaun ted , bel ievers have postula ted a trigger effect, whe reby a solar i n f luence h i g h u p magnif ies its ac t ion by setting of f a cha in o f other effects l o w e r d o w n . S o m e k n o w n a tmospher i c interact ions g ive this plausibi l i ty.

Large ly , t h o u g h , phys ica l m e c h a n i s m s have b e e n igno red , and scientists have b e e n con ten t to seek , p e r c h a n c e to f ind, statistical corre la t ions be tween the sun 's behav iour and the weather ' s .

A long-held idea

' S i n c e expe r i ence has already c o n v i n c e d us , that ou r seasons are some t imes very severe, and at other t imes very m i l d , it remains o n l y to be cons ide red whe ther w e s h o u l d a s c r i b e t h i s d i f f e r e n c e i m ­media te ly to a m o r e or less c o p i o u s emis ­s ion o f the solar beams . '

D a t i n g f r o m 1801 , this idea o f the as­t ronomer W i l l i a m H e r s c h e l ga ined w i d e ­spread cur rency . B y 1978 a reviewer c o u l d c o u n t m o r e than a thousand papers o n sun—weather l inks pub l i shed dur ing the in te rven ing years . F a m o u s n a m e s l ike G a u s s , Sab ine , Faraday, Ai ry , L o c k y e r , and K e l v i n cont r ibute to the list.

Believers have postulated a trigger effect, whereby a solar influence sets off a chain of other effects.

H e r s c h e l at tempted to find s o m e sys­temat ic variat ion i n terrestrial weather that c o u l d b e l inked wi th c h a n g e s o n the sun. T h e o n l y l o n g stretch o f data avail­able to h i m was statistics o n wheat pr ices . H e assumed these w o u l d reflect the aver­age weather c o n d i t i o n s o f each year, and l o ! the f luctuat ions they s h o w e d s e e m e d attributable to solar variat ion, a c c o r d i n g to H e r s c h e l .

O n e investigator in 1874 i ngen ious ly co r re l a t ed the s u n s p o t c y c l e w i t h the n u m b e r o f vessels put t ing in to Maur i t ius for repair after damage caused by heavy seas . H i s s ta t is t ics , f r o m of f i c i a l po r t r ecords , make l ivelier reading than m o s t : ' S h i p Berar, in 2 4 ° S and 6 8 ° E , experi­e n c e d a heavy ga l e , w i th thunder and l igh tn ing ; ship h o v e o n her b e a m ends ; cut away top gallant mast ; every plate, all c lo th ing , and every b o o k washed away. Barometer 2 9 . 3 0 inches . '

M o d e r n attempts at r oo t i ng ou t correla­t ions differ little f rom last century ' s , ex­cept that the a m o u n t o f data i nvo lved has

b e c o m e a g o o d deal larger. O n e recent attempt m a d e use o f s o m e ten m i l l i o n measurements o f air temperature.

D r R o b e r t V i n e s o f the C S I R O D i v i s i o n o f C h e m i c a l T e c h n o l o g y is o n e o f the latter-day seekers after sun—weather co r ­relat ions. Or ig ina l ly a scep t ic , D r V i n e s was persuaded o f the probabi l i ty o f a l ink i n 1964 after no t ing the c h a n c e remark o f foresters that bushfires s e e m to recur i n cyc les (he was then i n v o l v e d in bushfire research) . W h e n h e sub jec ted rainfall data to analysis — bad forest fires are usual ly associa ted wi th d rough t c o n d i t i o n s — h e found , m u c h to his surprise, quas i -cyc l i c behav iour sugges t ing c o n n e c t i o n s wi th the sunspot c y c l e .

H e b e g a n to l o o k for further conf i rma­t ions o f the sun 's i n f l u e n c e , unt i l what b e g a n as a s ide-l ine n o w o c c u p i e s h i m in full- t ime research.

D r V i n e s uses a c o m p u t e r p r o g r a m de­vised b y D r E . G . B o w e n , former C h i e f o f the C S I R O D i v i s i o n o f R a d i o p h y s i c s and h i m s e l f i n v o l v e d i n e l u c i d a t i n g s u n -weather l inks . T h e p r o g r a m sifts series o f rainfall (or other) data th rough four filters, e a c h i so la t ing a b a n d o f pe r iod i c i t i e s . T h u s , the first filter u n c o v e r s f luctuat ions c l o s e to 7 years ; the s e c o n d , abou t 10 years; the third, r o u g h l y 2 0 years; and the four th g ives the l o n g - t e r m trend. A n y c y c l e in the data ( in the range 5 to 50 years) w i l l s h o w itself as an osc i l l a t ion in o n e o f the filters.

F r o m analysis o f V ic to r i a ' s rainfall, D r V i n e s c o n c l u d e d that the genera l trend c o u l d b e a p p r o x i m a t e l y d e s c r i b e d i n t e r m s o f t h r e e d i f f e r e n t l o n g - t e r m pe r iod ic i t i e s , the peaks and t roughs o f w h i c h co r r e spond wi th years o f e x c e p ­t ional ly h i g h or l o w rainfall. H i s results s h o w o n e o f them, a 10- to 12-year o sc i l ­la t ion, a c c o u n t i n g for abou t 1 0 % o f the rainfall f luctuat ions , and h e suggests it is p robab ly related to the sunspot c y c l e .

S u b s e q u e n t l y , D r V i n e s has f o u n d similar regularit ies i n rainfall data f rom o the r p l a c e s , s o m e s t r e t c h i n g b a c k a hundred years. H e has a l so tried to p i n d o w n what s eems to b e , for any o n e l oca ­t ion , a fairly cons tan t phase difference b e ­tween the sunspo t c y c l e and the output o f the s e c o n d filter. T h e c o m p l i c a t i o n is that the phase difference varies f rom o n e l oca ­t ion to another .

T h u s , there is an apparent c o n n e c t i o n b e t w e e n s u n s p o t m a x i m a and rainfall m a x i m a in T a s m a n i a and N e w Zea l and . I n V i c t o r i a and no r the rn Q u e e n s l a n d , h o w e v e r , rainfall m i n i m a are obse rved c l o s e to sunspo t m a x i m a . Simi lar differ­ences i n phase b e t w e e n sunspo t and rain-

15

fa l l c y c l e s are a p p a r e n t i n d i f f e r e n t

r eg ions o f Sou th Afr ica .

D r V i n e s h o p e s to find further correla­

t ions that m a y th row l ight o n the reasons

for these d i f ferences . T h i s w o u l d h e l p

dispel the c r i t i c i sm that the per iodic i t ies

are fortui tous p roduc ts o f the filter p ro ­

g r a m itself, rather than true ref lec t ions o f

s o m e phys ica l p r o c e s s .

Other correlations

A n assoc ia t ion b e t w e e n thunders torms

and sunspots is an idea i n h e r e n d y m o r e

attractive than m a n y o f the other pur­

por ted m e t e o r o l o g i c a l l inks , and m a n y

p e o p l e have sough t it. T h e reason ing be -

Solar flares certainly affect the earth's ionosphere, and one result is auroras like this one photographed at the Mawson base in Antarctica.

This huge group of sunspots, covering an area about 20 times that of the earth, was photographed in 1946.

gins by n o t i n g the u n q u e s t i o n e d effect o f

solar emis s ions o n the i o n o s p h e r e . It then

g o e s a step further by a s s u m i n g a l ink wi th

thunders torms via the altered electr ical

potentials generated by the part icles. In­

deed , what was o n c e regarded as o n e o f the

m o s t c o n v i n c i n g examples o f an influ­

e n c e o f solar cyc l e s o n the weather relates

to the f r e q u e n c y o f t h u n d e r s t o r m s i n

Siberia.

Data were or ig ina l ly pub l i shed in 1926

by a Russ i an me teo ro log i s t , E . Septer,

f r o m 2 2 9 s t a t ions c o v e r i n g the years

1 8 8 8 — 1 9 2 4 . T h e y w e r e s u b s e q u e n t l y

analysed in 1934 b y C.E.P . B r o o k s , an

Eng l i sh me teo ro log i s t . H e found that the

average number s o f thunders torms near

sunspot m a x i m a and m i n i m a s e e m e d to

f o l l o w a pattern paral lel ing the sunspo t

c y c l e ; there s e e m e d to b e o n l y a 1 i n

10 000 poss ib i l i ty o f the pattern o c c u r r i n g

by c h a n c e . Unfor tunate ly , invest igat ions

i n 1967 (by Z . P . K l e i m e n o v a ) c o u l d o n l y

u n c o v e r records for 167 stations, and her

analysis o f t h e m s h o w e d the c l a i m e d c o r ­

relat ion to b e 'no t i n a c c o r d wi th f a c f .

Autosuggestion?

D e s p i t e d i sappoin tments l ike this, work ­

ers in the field have s tuck by c l a ims o f an

in f luence , albeit weaker , o f the 11-year

sunspot c y c l e o n the weather . T h e y have

also main ta ined that short-term solar in­

f luences , over a matter o f days, and l o n g -

t e r m o n e s , o v e r a c e n t u r y o r s o , are

d i scern ib le in s o m e weather patterns.

S o m e Russ i an workers have c l a i m e d

that solar activity exerts 'a substantial in­

f luence o n a tmospher i c p rocesses . A l l o w ­

ance for (it) is o f great impor t ance in pre­

paring weather forecasts. ' A g a i n s t this ,

A . S . M o n i n , a s c e p t i c a l f e l l o w -

coun t ryman , has d i smissed the c o l l e c t e d

e v i d e n c e for the in f luence o f solar activity

o n weather, w h i c h h e said ' p roduces o n l y

an impres s ion o f success fu l exper iments

i n au tosugges t ion ' .

A c c o r d i n g to D r Barrie P i t tock , o f the

C S I R O D i v i s i o n o f A t m o s p h e r i c Phys i c s ,

the c r u c i a l d i f f icu l ty i n c la r i fy ing the

ques t ion o n e way or the other is separat­

i n g the smal l so la r - induced ' s ignal ' f rom

the h i g h b a c k g r o u n d n o i s e o f weather var­

iability and c l ima te c h a n g e s . D r P i t tock

has p u b l i s h e d a c r i t i ca l r e v i e w o f the

w h o l e sun—weather ques t ion .

I n t h e c o u r s e o f p r e p a r i n g i t , h e

e x a m i n e d at leas t 140 r e c e n t , w i d e l y

quoted , or part icularly relevant papers o n

the general sub jec t o f solar cyc les and the

weather or c l imate . H e says that o n e th ing

that stands ou t is the l ack o f any c o n v i n c ­

i n g e v i d e n c e o f statistically s ignif icant o r

16

Australia through a break in the c louds .

pract ical ly useful corre la t ions . C o u p l i n g this wi th the f requency wi th w h i c h fal­l a c i e s , i l l - f o u n d e d c o n c l u s i o n s , c o n ­tradict ions, errors, and biases appear in the literature, h e c o n c l u d e s that c o n c e r n

. . one thing that stands out is the lack of any convincing evidence of statistically significant or practically useful correlations.'

mus t be expressed as to the 'prevai l ing s t a n d a r d s o f o b j e c t i v i t y a n d c r i t i c a l analysis ' .

Statistics present m a n y traps for the unwary and, in the f ield w e are d i scuss ing , p robab ly all o f t h e m have been sprung by s o m e b o d y or other. Statistical too ls c a n never lead to absolu te certainty, on l y to a probabi l i ty . Whe the r this c o n v i n c e s any­b o d y depends a lo t o n their degree o f s cep ­t ic i sm. D r P i t tock ' s f ind ing is that there is n o t a s i n g l e c o r r e l a t i o n w i t h e n o u g h statistical p u n c h to c o n v i n c e a seasoned cr i t ic . Indeed , h e suggests that, w h e n the latest statistical t echn iques are appl ied to the m o s t c o m p r e h e n s i v e r e g i o n a l and g loba l m e t e o r o l o g i c a l data sets, the re­sul t ing nega t ive f ind ings (often g l o s s e d over) are m o r e l ikely to turn an agnos t i c in to a d isbel iever .

Statistical tools can never lead to absolute certainty, only to a probability.

O v e r l o o k e d , h e s a y s , h a s b e e n t he s imple fact that o n e result i n 100 wi l l s h o w , by pure c h a n c e , statistical signifi­c a n c e at t he 9 9 % c o n f i d e n c e l e v e l i f v i ewed i n i so la t ion . T h i s o n e result is l ikely to b e reported, whereas the others wi l l not . A n o t h e r w a y o f s ee ing signif i­c a n c e w h e n there is n o n e has b e e n to elaborate the o r ig ina l hypo thes i s unt i l hypothes is and data fit. F o r e x a m p l e , D r Pi t tock ci tes a paper w h e r e the general hypothes is that the sunspo t c y c l e m o d ­ulates rainfall is c h a n g e d to o n e that says on ly i n certain r eg ions d o e s solar activity affect rainfall. O n l y i f the n e w hypothes i s is tested o n n e w data, or u sed to make successful p red ic t ions , c a n w e g ive it any c redence .

The magnetosphere extends from about 100 km above the earth to the magnetopause. The solar wind makes it asymmetrical.

E v e n s m o o t h i n g o f data, a c o m m o n prac t ice , general ly in t roduces extra o p ­portunit ies for f ind ing corre la t ions , espe­c i a l l y s i n c e c l i m a t i c da ta s e r i e s are no to r ious ly unstable ( 'badly behaved ' ) in their statistical propert ies .

D r Pi t tock suggests that a g o o d w o r k i n g rule is to p l ace little c o n f i d e n c e i n the ex is tence o f a c y c l e unt i l the f igures s h o w at least five or s ix repeti t ions o f the pat­tern. W i t h a sunspo t pe r iod o f abou t 11 years, m a n y a tmospher ic data sets are re­grettably short.

Short-term responses

P r o s p e c t s m a y b e br igh ter for f i n d i n g s t a t i s t i c a l l y s i g n i f i c a n t t i es b e t w e e n short-term responses o f the a tmosphere and s o m e types o f solar activity. In 1973 a co r r e l a t i on was repor ted by A m e r i c a n workers , first by W . O . Rober t s and RJH. O l s o n and later by J .M. W i l c o x and c o l ­l eagues . T h e y correlated o c c a s i o n s w h e n the solar w i n d changes its magne t i c polar­ity (registered o n earth) w i th an unl ike ly-s o u n d i n g quant i ty c a l l e d the 'vor t ic i ty area index". T h i s i n d e x is a measure o f the intensity and area, po le -ward o f 2 0 ° N , o f l o w pressure t roughs at the 3 0 0 mi l l ibar level i n the a tmosphere , w h i c h g ive r ise, i n the ma in , to 'bad weather".

It s eems that the i ndex is 10'% l o w e r about o n e day after the earth enters a r e g i o n o f the solar w i n d o f reversed polar­ity (or as the scientists say, a 'solar sector b o u n d a r y sweeps across us ) . T h e effect is on ly seen i n winter .

T h e relatively short t ime scale (days) o f this c l a imed effect, and the comparat ive ly l o n g t ime over w h i c h solar w i n d polari ty changes are k n o w n (many years) have al­l o w e d deta i led test ing o f its statistical s ign i f icance .

T h e credibi l i ty o f the cor re la t ion rose marked ly w h e n a o n e - t i m e cr i t ic o f it, C O . H i n e s , c o m p i l e d a list o f extra t imes w h e n the earth passed th rough solar sec ­tor boundar i e s . O n l y i f the co r re la t ion were real w o u l d y o u expec t the r e sponse to b e dupl ica ted in the n e w data. It was . H i n e s was fo rced to c o n c l u d e in 1977 that the effect ' s hou ld be accep ted as a physi ­ca l ly m e a n i n g f u l s ignal w h o s e o r ig ins shou ld n o w b e sought".

Bu t let it b e said that n o t all the cr i t ics are satisfied, i n c l u d i n g D r Pi t tock . M o r e recent data have tended to g o against the corre la t ion , and the i n d e x itself, s o m e ­what specia l i n its def in i t ion, strangely c o n f i n e s i tself to winter , nor th o f 2 0 ° N , and above a certain he igh t i n the a tmos­phere .

Seeking a mechanism

T h e crucia l ques t ion , for this corre la t ion or for any o the r , is the mat ter o f the m e c h a n i s m i n v o l v e d . H o w e v e r m a n y n u m b e r s are c o l l e c t e d , the o ld doub t s wi l l no t d ie so l o n g as all that c a n b e offered are statistical regulari t ies.

Professor K e i t h C o l e , H e a d o f the D i v i ­s ion o f T h e o r e t i c a l and S p a c e Phys i c s at L a T r o b e Univers i ty , h o p e s data c o l l e c t e d d u r i n g the r e c e n t In t e rna t iona l M a g -ne tospher ic S tudy ( I M S ) and the u p c o m ­ing M i d d l e A t m o s p h e r i c P r o g r a m ( M A P ) may th row l igh t o n the matter. H e be ­l ieves t w o types o f p h e n o m e n a c o u l d be i nvo lved in any sun—weather assoc ia t ion .

O n e invo lves weather systems i n po ia r r eg ions that c a n l a u n c h 'planetary-scale ' waves u p to the i o n o s p h e r e . T h e r e the waves p r o d u c e e lec t r i c f ie lds that m a y affect the capture o f solar w i n d part icles by the earth's magne t i c f ie ld.

T h e s e c o n d type invo lves c h a n g e s to m i n o r cons t i t uen t s o f the a t m o s p h e r e caused by variations in the sun 's ultra­vio le t radiat ion and by m a g n e t i c s torms. T h e s e m a y l e a d t o c h a n g e s i n t h e dynamics o f the m e s o s p h e r e and strato­sphere.

T h e end o f last year saw the c o n c l u s i o n o f the 3-year- long da ta -co l lec t ion phase o f the I M S . M o r e than $ 1 0 0 m i l l i o n was spent i n the first really concent ra ted l o o k at t he m a g n e t o s p h e r e — the c o m e t -s h a p e d r e g i o n tha t e n c o m p a s s e s t he earth's magne t i c f ie ld. O n the sun-ward s ide , the magne tosphe re rises to 60 0 0 0 k m ; away f rom the sun, the earth's mag­ne t ic f ie ld is d rawn ou t in to space by the solar w i n d to f o r m a tail hundreds o f thousands o f k i lometres l o n g .

continued on page 19

17

The sun — what we do know

Close study by many generations of scien­tists has permitted a fascinating picture of the sun to be built-up.

The sun is a star, a thermonuclear-fired ball of gas — the only one close enough (150 million km or 8 light-minutes) for its disc to be visible.

It has a mass of 2 x 10 2 7 tonnes, about 75% of which is hydrogen and the rest mostly helium, although other elements appear too. Gravity holds it all together, and the weight of the outer layers causes the density and temperature to increase inwards, until at the centre the tempera­ture is 15 million °C and the density 160 times that of water. Under such condi­tions, hydrogen nuclei fuse into helium, releasing the energy upon which we de­pend.

The disc that we see, from which most of the heat and light is emitted, is called the photosphere. It measures 1 .4 million

km across and has a temperature of about 6000°C. The photosphere appears mot­tled with millions of 'granules', a sign of the sun's bubbling and seething. Each granule, some 1000 km across, is the top of a convection cell and lasts, on average, about 10 minutes. Somet imes the granules give way to much larger sunspots, which are accompanied by strong magnetic fields.

Like a moulded jelly, areas of the sun's surface thousands of kilometres across are seen to gently pulsate. Over 5 minutes, they oscillate through a range of about 200 km.

Outwards from the photosphere, we en­counter the chromosphere, a thin layer visible only at solar eclipse (as a beautiful rosy arc) or with special filters. Surpris­ingly, it has a higher temperature than the layers below. This is due to heating by supersonic shock waves, which start off as ordinary sound waves in the granulation layer. As in a jet engine exhaust, the sound is generated by turbulence.

The extended outermost portion of the

sun's atmosphere is called the corona. Its structure reveals the presence of enorm­ous loops of magnetic field, which are an­chored to sunspots. Shock waves here produce temperatures of more than a mill­ion degrees. The outward streaming of hot coronal material gives us the solar wind.

Of all the various aspects of solar activ­ity, flares are the most intensively studied, mainly because of their effects on the outer layers of our atmosphere. Flares are vast explosive events in the chromosphere and corona, and are thought to be caused by sudden changes in the magnetic fields of sunspots. Flares produce e lec­tromagnetic radiation of all types and streams of energetic particles, mostly pro­tons and electrons (our atmospheric blan­ket stops most of the flare's emission from reaching the ground). However, through their effect on the ionosphere, we observe radio black-outs, auroras, magnetic field changes, and currents induced in cables, power lines, pipe-lines, and railway sig­nalling circuits.

Spying on the sun

Many mysteries still surround the sun. Why sunspots occur (in cycles) and why the sun rotates more slowly at its poles (31-day period) than at its equator (25 days) are at present only answerable by speculative theories about the behaviour of its magnetic field.

But Dr Ralph Loughhead, a solar astronomer at the CSIRO Divis ion of Applied Physics, feels that scientists are making such steady progress in finding out the workings of the sun that answers to the above questions may be forthcom­ing in a matter of a few years.

Continued observations are the key,

and satellite-borne instruments help a lot in this regard, but the core of solar data will continue to be derived from ground-based observations.

In Australia, the focus of sun-watching is at Culgoora, 600 km north-west of Sydney. Here, where clear skies are com­mon, lies the CSIRO Solar Observatory operated by the Divis ion of Applied Physics and the Division of Radiophysics. The Ionospheric Prediction Service of the Department of Science and the Envi­ronment also operates a solar patrol tele­scope at the site; this monitors the sun for signs of flares.

The observatory looks for radio bursts with a spectograph, a spectropolarimeter, and a unique radioheliograph. This last instrument, with its 96 aerials arranged in a 3-km-diameter circle, produces radio images at three separate frequencies. N o other solar radio telescope can give two-dimensional images at such a long wavelength or so rapidly.

A specialized 30-cm refractor, with a very narrow-band optical filter, observes the sun at wavelengths in the visible part of the spectrum. When Culgoora obser­vers see interesting solar activity, they notify N A S A in America so that instru­ments aboard the S M M satellite can be set to record it.

The University of Sydney's radio ob­servatory at Fleurs, 50 km west of Sydney, uses a synthesis telescope to scan the sun daily. The University of Tasmania is also involved in solar radio astronomy.

The United States Air Force, in associ­ation with the Ionospheric Prediction Service, operates a solar observatory at Learmonth in Western Australia. The observatory has both optical and radio in­struments, and a high-resolution mag-netograph to look at the sun's magnetic fields is being installed.

Part of the l)6-dish raclioheliograph at the c s i R O Solar Observator-v,-.('.ulgooni. .

Solar granulation. Each granule, some 1000 km across, is the top of a rising column of hot gas.

M o r e than 10 0 0 0 scientists f rom 50 nat ions e n g a g e d in m o r e than 1000 ex­per iments — us ing satellites, rocke t s , bal­l o o n s , and g r o u n d stations — in an effort to t ho rough ly p robe the magne tosphe re . P rofessor Cole , is the p res iden t o f the S c i e n t i f i c C o m m i t t e e o n S o l a r -terrestrial Phys i c s that is o rgan iz ing the I M S , and the nex t task,.he says, is to un­derstand what is con t a ined in the formid­able data base n o w buil t u p .

P re l imina ry p r o g r a m s i n M A P have b e g u n , and dur ing the nex t 7 years an effort s imilar to that m a d e dur ing I M S w i l l b e p u t i n t o s tudy o f the ear th ' s stratosphere and m e s o s p h e r e .

A computer model

M r Barrie H u n t o f the Austra l ian N u m e r ­ical M e t e o r o l o g y R e s e a r c h Cent re works wi th a c o m p u t e r m o d e l o f the a tmosphere . ( S o m e o f his c o m p u t e r ' exper iments ' o n the a tmosphere are desc r ibed inEcos 10.)

R e c e n t l y , h e ran an e x p e r i m e n t that suggests a phys ica l causal l ink b e t w e e n the sun (or , at least, s o m e aspects o f its variability) and events in the t roposphere . H e r e m o v e d a por t ion o f the o z o n e layer in his m o d e l (to s imulate the effect observed by satellites where p ro tons f rom a solar flare b o m b a r d ou r a tmospher ic mant le , fo rm nitr ic o x i d e , and destroy o z o n e ) and l o o k e d for an effect l o w d o w n i n the m o d e l ' s a tmosphere .

T h e beauty o f the c o m p u t e r m o d e l is that it a l l o w e d M r H u n t to obse rve what happened in the l o w e r a tmosphere wi th and wi thou t the deple ted o z o n e layer pre­sent. T h e m o d e l was started o f f o n bo th o c c a s i o n s wi th the same weather pattern; after it had run for a n u m b e r o f 'days ' , and d i f f e r e n c e s h o w i n g u p i n the a t m o s ­

phere 's behav iour c o u l d b e attributed to

the differences in the o z o n e layer.

T h e m o d e l ' s o z o n e layer was deple ted at h i g h latitudes (around 70° ) to ma tch the way solar p ro tons d ive in near the p o l e s , on ly the o z o n e reduc t ion was exaggerated so that a clearer a tmosphe r i c r e sponse m i g h t b e d i sce rned . O n the o ther hand , the o z o n e defici t was main ta ined for on ly several days, whereas satellites s h o w that the actual recovery t ime c a n be 20 days or m o r e .

A n d what happened after let t ing the c o m p u t e r r u n o n f o r ' w e e k s ' ? N e a r latitude 4 5 ° , after abou t 24 days, the sub­tropical jet stream ( runn ing west to east at an altitude o f about 10 k m ) was e n h a n c e d signif icantly. Its m e a n speed in the ' c o n ­trol ' m o d e l was 18 metres per s e c o n d ; in the ozone-dep le ted m o d e l its speed had increased s o m e 2 5 % . After several m o r e m o d e l days, the w i n d reverted to its nor­mal value.

T h e s ign i f i cance o f this in terms o f weather is difficult to w o r k out , because o f the 'no is iness ' o f the m o d e l (and o f the a t m o s p h e r e ) . T h e n a t u r e o f t he m e c h a n i s m i n v o l v e d in t ransferr ing a stratospheric effect d o w n to the t ropo­s p h e r e a l s o c a n n o t b e p i n n e d d o w n exactly.

M o d e l s are inevitably overs impl i f ica­t ions o f the real wor ld , l ack ing innumer ­able subtleties (perhaps s o m e cruc ia l to a sun—weather m e c h a n i s m ) , and they exis t o n l y as c o m p l e x sets o f m a t h e m a t i c a l equa t ions . A l l that c a n b e said is that o z o n e i m b a l a n c e pe r tu rbs the a t m o s ­phere 's radiation e x c h a n g e , thereby m o d ­i f y i n g s t r a to sphe r i c t empera tu res and winds . I n turn, these effects alter the f l o w o f m e t e o r o l o g i c a l energy b e t w e e n l o w and h i g h reg ions o f the a tmosphere .

H o w e v e r , the pe r fo rmance o f M r H u n f s m o d e l suggests that the large 'p lanetary w a v e s p r o p a g a t i n g u p w a r d s a r o u n d latitude 7 0 ° may play an important ro le . T h e r e a lmos t s eems to b e a wavegu ide at this latitude channe l l i ng waves be tween the top and b o t t o m o f the a tmosphere .

W h e n M r H u n t repeated the exper i ­m e n t w i t h the o z o n e d e p l e t e d at 4 5 ° latitude, there was virtually n o disturb­ance l o w e r d o w n in the a tmosphere .

E n c o u r a g e d by the results, M r H u n t is n o w trying to s imulate what happened d u r i n g the M a u n d e r m i n i m u m , w h e n sunspots appear to have b e e n largely ab­sent. H e thinks o z o n e shou ld have b e e n c o n t i n u o u s l y deple ted then because o f the extra c o s m i c rays a l l owed to reach the earth by the r educed solar activity.

I f the o z o n e ' ho l e ' leads to a l o w e r i n g o f

This photograph of the sun was taken at the CSIRO Solar Observatory in hydrogen light.

t empera tures akin to that e x p e r i e n c e d dur ing the li t t le i ce -age , w e may have found a m e c h a n i s m for the interact ion be ­t w e e n the s u n and o u r w e a t h e r . T h e w h o l e con t rovers i a l f ie ld may then b e t ransformed in to respectabil i ty.

A final catch

But i f solar activity is s h o w n to have sig­nif icant effects o n our weather , w e are l ed to the i n e s c a p a b l e c o n c l u s i o n that, i n order to p r o d u c e accurate weather fore­casts cons is ten t ly , w e wi l l n e e d to pred ic t the sun ' s behav iour — 'a lmos t a tragedy for m e t e o r o l o g y ' , as o n e scientist , A . S . M o n i n , sees it.

Desp i t e the ex i s t ence o f the sunspot c y c l e , the types o f p r e d i c t i o n o f so lar activity that w o u l d be needed canno t b e m a d e yet, and many scientists d o u b t that they w i l l ever b e poss ib le . S o m e progress has b e e n m a d e in pred ic t ing flares, but there is a l o n g way to g o . It wi l l certainly b e s o m e t i m e b e f o r e w o r d s s u c h as ' s unspo t ' and ' f lare ' enter the m e t e o r ­o log i s t s ' vocabulary — i f they ever d o .

Andrew Bell

More about the topic

A crit ical l o o k at l ong - t e rm sun—weather re la t ionships . A . B . Pi t tock. Reviews of Geophysics and Space Physics, 1978 , 16, 4 0 0 - 2 0 .

' S u n , Weather , and Cl imate . ' J.R. Her ­m a n a n d R . A . G o l d b e r g . ( N A S A : W a s h i n g t o n D . C . 1979.)

A n e v a l u a t i o n o f a s u n — w e a t h e r m e c h a n i s m us ing a general c i rcu la t ion m o d e l o f the a tmosphere . B . G . Hun t . Journal of Geophysical Research, 1980 , 85, ( in press) .

Pos s ib l e re la t ionships b e t w e e n rainfall, c r o p yie lds and the sunspot c y c l e . R . G . V i n e s . Journal of the Australian Insti­tute of Agricultural Science, 1977 , 43>, 3 - 1 3 .

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