SUNSPOTS
A N D
YOU
INTRODUCTION
B e f o r e we can u n d e r s t a n d t h e n a t u r e o f sunspots and how t h e y a f f e c t shortwave
r e c e p t i o n , we must know something about o u r sun. A t t h e end o f t h i s a r t i c l e we w i l l
g i v e a b i b l i o g r a p h y o f t h e books used as r e f e r e n c e s , so you can pursue t h e s u b j e c t
f u r t h e r on y o u r own. Our sun i s u n i q u e i n one r e s p e c t a t l e a s t . I t i s t h e one s t a r
i n t h e whole u n i v e r s e whose s u r f a c e we can see. A l l t h e o t h e r s t a r s , even t h e
n e a r e s t and l a r g e s t , a r e so remote t h a t even t h e g i a n t 200 i n c h t e l e s c o p e a t
Mount Palomar i n C a l i f o r n i a can see them o n l y as d i s t a n t p o i n t s o f l i g h t . Radio
p h y s i c i s t s and shortwave e n g i n e e r s , as wel 1 as a s t r o n o m e r s , s t u d y t h e sun because
some s o l a r d i s t u r b a n c e s have a p r o f o u n d e f f e c t on shortwave Communications, and
sunspots a r e one t y p e o f s o l a r d i s t u r b a n c e .
To t h e a s t r o n o m e r o u r sun i s a s t a r o f s p e c t r a l t y p e G, w h i c h i s i n t h e m i d d l e
of t h e range o f t h e s e t y p e s o f s t a r s . l t s l u m i n o s i t y i s n e i t h e r v e r y h i g h nor v e r y
low. I t doesn't have marked p e c u l i a r i t i e s . I n f a c t , o u r sun i s a v e r y o r d i n a r y s t a r
i n t h e M i l k y Way system. I t i s n o t even c e n t r a l l y p l a c e d i n t h e d i s k o f 100,000
mi 11 i o n s t a r s . The sun i s l o c a t e d about two t h i r d s o f t h e d i s t a n c e f r o m t h e c e n t r e
of o u r g a l a x y t o w a r d s t h e r i m , i n one o f t h e o u t e r s p i r a l arms. Astronomers have
measured t h e sun as b e i n g 149.5 m i l l i o n km ( a b o u t 93 m i l l i o n m i l e s ) d i s t a n t from
the e a r t h . T h i s measurement i s b e l e i v e d t o be c o r r e c t t o about 10,000 m i l e s . A r a y
of l i g h t o r s u n s h i n e t a k e s 500 seconds o r 8.3 m i n u t e s t o reach t h e e a r t h . The sun
i t s e l f i s made up o f v a r i o u s l e v e l s , s o l a r l e v e l s , and a l t h o u g h i t i s a s p h e r i c a l
b a l l o f m a t t e r h e l d t o g e t h e r by g r a v i t y , i t i s n o t e n t i r e l y f e a t u r e l e s s . The
t e m p e r a t u r e o f t h e l o w e s t l e v e l i s e s t i m a t e d t o be i n t h e o r d e r o f 24 m i l l i o n
degrees K e i v i n . ( L o r d K e i v i n was a B r i t i s h p h y s i c i s t who d i e d about t h e t u r n o f t h e
c e n t u r y . He i n v e n t e d t h e system o f t h e r m a l measurement named a f t e r h i m . )
The n e x t l e v e l t r a v e l l i n g o u t w a r d f r o m t h e c e n t r e i s t h e p h o t o s p h e r e , and t h a t
i s about 700,000 k i l o m e t e r s ( a b o u t 430,000 m i l e s ) f r o m t h e sun's c e n t r e . The
photosphere i s p r o b a b i y t h e most i m p o r t a n t l e v e l t o us s i nee i t i s f r o m i t t h a t we
get t h e g r e a t e r p a r t o f o u r l i g h t ^ a n d h e a t . The t e m p e r a t u r e o f t h i s r e g i o n o f t h e
sun i s e s t i m a t e d t o be about 6000° K e i v i n . The p h o t o s p h e r e i s a v e r y t h i n l a y e r o r
l e v e l , perhaps no more t h a n 100 t o 200 k i l o m e t e r s i n d e p t h . Because t h e photosphere
i s an e x t r e m e l y t h i n l a y e r , i t g i v e s t h e sun i t s sharp o u t l i n e , somewhat s i m i l a r t o
a s o l i d s u r f a c e . I t i s a t t h i s l e v e l i n t h e p h o t o s p h e r e t h a t s u n s p o t s o c c u r . We
w i l l come back t o t h e p h o t o s p h e r e when we s t a r t t o t a l k about s u n s p o t s i n more
d e t a i l . S t i l l t r a v e l l i n g outwards f r o m t h e c e n t r e o f t h e sun, t h e n e x t l a y e r o r
l e v e l we come t o i s known as t h e r e v e r s i n g l a y e r , s o - c a l l e d because i t i s a l a y e r
of c o o l e r g a s , about 1000 k i l o m e t e r s deep ( a b o u t 620 m i l e s ) . The l a y e r above t h e
r e v e r s i n g l a y e r i s t h e chromosphere. I t i s about 12,000 k i l o m e t e r s above t h e
photosphere and s o l a r p h y s i c i s t s have measured a t e m p e r a t u r e o f about 30,000
degrees Kei v i n f o r t h i s r e g i o n .
The l e v e l above t h e chromosphere i s t h e c o r o n a , t h e extended o u t e r atmosphere
of t h e sun. I t i s i m p o s s i b l e t o see t h e corona i n o r d i n a r y l i g h t because i t i s so
nebulous. However, d u r i n g a t o t a l e c l i p s e , photographs have been t a k e n o f t h i s h a l o
around t h e sun. The corona t e m p e r a t u r e i s e s t i m a t e d t o be i n t h e r e g i o n o f one
m i l l i o n degrees K e i v i n . So t h e r e we have t h e v a r i o u s s o l a r l e v e l s .
HlSTORY OF SUNSPOTS
Sunspots were f i r s t observed by t h e Chinese b e f o r e G a l i l e o and h i s i n v e n t i o n o f
the t e l e s c o p e i n 1601. I n f a c t , i t i s known t h a t t h e Chinese observed sunspots many
years b e f o r e C h r i s t was b o r n . I n 1612 G a l i l e o used t h e t e l e s c o p e t o make sketches
of t h e s o l a r d i s c showing many l a r g e s u n s p o t s . However, i t was n o t u n t i l t h e m i d d l e
of t h e 1 8 t h C e n t u r y t h a t a number o f astronomers i n Europe began t o keep r e g u l a r
r e c o r d s o f s u n s p o t s . H e n d r i c k Schwaber o f Germany, a chemist and an amateur
astronomer, d i s c o v e r e d t h e e l e v e n - y e a r c y c l e o f sunspot a c t i v i t y . Schwaber recorded
his d a i l y c o u n t o f sunspots u s i n g a small t e l e s c o p e and, a f t e r s e v e r a l y e a r s , he
found t h a t t h e number o f s p o t s v a r i e d i n a r e g u l a r c y c l e .
A f t e r w a t c h i n g t h e sun d a i l y f o r a l m o s t 20 y e a r s , Schwaber came t o t h e
c o n c l u s i o n t h a t sunspot a c t i v i t y reached a peak, d e c l i n e d t o a minimum and t h e n
c l i m b e d t o a n o t h e r peak o f maximun a c t i v i t y , o v e r a t i m e span o f about 10-1/2
years. Wolf, t h e D i r e c t o r o f t h e Z u r i c h Observatory a t t h e t i m e , r e a l i z e d t h e
n e c e s s i t y f o r a r r i v i n g a t some Standard means o f counting sunspots and he produced
the f o r m u l a s t i l l i n use t o d a y , which i s known as t h e sunspot number o r t h e "Wolf"
number "R".
STRUCTURE AND DEVELOPMENT
.
o
A c a r e f u l e x a m i n a t i o n o f t h e s o l a r s u r f a c e shows t h e r e i s a l a r g e amount o f
f i n e s t r u c t u r e . T h i s s t r u c t u r e , i n appearance has been compared t o r i c e g r a i n s o r
w i l l o w l e a v e s . We w i l l s t a y w i t h t h e r i c e g r a i n s d e s c r i p t i o n because t h e t e r m
" g r a n u l a t i o n " i s now used by astronomers t o d e s c r i b e t h e sun's v i s i b l e s u r f a c e , t h e
photosphere. Each o f t h e s e g r a n u l e s o r g r a i n s i s r o u g h l y c i r c u l a r i n shape and has
a d i a m e t e r o f about 1500 km ( a p p r o x i m a t e l y 930 m i l e s ) and a l i f e o f o n l y a few
m i n u t e s . The s u r f a c e b r i g h t n e s s o f any g r a n u l e may be as much as 10 p e r c e n t
g r e a t e r t h a n t h a t o f t h e s u r r o u n d i n g s o l a r s u r f a c e . As one g r a n u l e fades away,
a n o t h e r t a k e s i t s p l a c e . Rather l i k e r i c e b o i l i n g i n a p o t o f w a t e r . Slow m o t i o n
f i l m s o f t h e sun, t a k e n a t t h e P i c - d u - M i d i O b s e r v a t o r y i n France by t h e l a t e
Dr. Bernard L y o t , g i v e a v i v i d i m p r e s s i o n o f t h i s b o i l i n g a c t i o n , The g r a n u l e s a r e
b e l i e v e d t o be t h e t o p s o f c o n v e c t i o n c u r r e n t s w h i c h o r i g i n a t e i n t h e lower l a y e r s
of t h e sun, ( t h a t i s , below t h e p h o t o s p h e r e ) w h i c h t r a n s p o r t t h e sun's heat t o t h e
s u r f a c e t h r o u g h t h e chromosphere and t h e c o r o n a , i n t o o u t e r space. T h i s upward
motion o f t h e g r a n u l e s has been c a l c u l a t e d a t a v e l o c i t y i n t h e o r d e r o f one km per
second. I t i s a k i n d o f t u r b u l e n t m o t i o n w i t h h o t b r i g h t gases moving r a p i d l y
upward and c o o l e r d a r k e r gas downward. Sometimes, t h e s e g r a n u l e s s e p a r a t e t o form a
dark area between them, and t h i s d a r k area i s known as a " p o r e " . When s e v e r a l o f
these pores u n i t e , a sunspot i s b o r n . O b s e r v a t i o n o f sunspots has r e v e a l e d t h a t t h e
photosphere i s r o t a t i n g , b u t t h i s r o t a t i o n i s n o t l i k e t h a t o f a s o l i d body, s i n c e
the sun's r o t a t i o n i s f a s t e r a t t h e s o l a r e q u a t o r t h a n i t i s a t h i g h o r low s o l a r
l a t i t u d e s . S c i e n t i s t s who have observed t h e movement o f sunspots a c r o s s t h e s o l a r
d i s c , always from e a s t t o w e s t , have found t h a t t h e p e r i o d o f r o t a t i o n o f t h e sun
i s 26.89 days, o r , i n round f i g u r e s , 27 days. T h i s i s known as t h e 27-day r o t a t i o n
p e r i o d . Thus, a s p o t born on t h e extreme e a s t edge o f t h e sun, o r as t h e
astronomers c a l l i t , t h e e a s t l i m b , appears t o d r i f t westward a c r o s s t h e c e n t r e o f
the sun, t o t h e extreme west l i m b , and t h e n d i s a p p e a r s as t h e sun r o t a t e s . I f i t i s
a l a r g e s p o t , o r groups o f s p o t s , i t w i l l reappear about 13 days l a t e r , a g a i n a t
the sun's extreme e a s t l i m b where i t was f i r s t o b s e r v e d . T h i s complete r o t a r y
movement t a k e s about 27 days. For t h i s r e a s o n , a number o f n a t u r a l phenomena here
on e a r t h b e l i e v e d t o be i n f l u e n c e d by sunspots have a p e r i o d i c i t y o f 27 days. The
l i f e t i m e o f a sunspot can v a r y from a few hours t o many weeks. Each f u l l y developed
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spot c o n s i s t s o f two d i s t i n c t r e g i o n s . The dark i n n e r r e g i o n , t h e umbra, i s
surrounded by a more luminous r e g i o n , t h e penumbra. Rather l i k e a f r i e d egg! Except
t h a t t h e yoke i s b l a c k i n s t e a d o f y e l l o w . The b o u n d a r i e s between t h e umbra and t h e
penumbra and between t h e penumbra and t h e s u r r o u n d i n g photosphere a r e s u r p r i s i n g l y
sharp. When observed under t h e b e s t c o n d i t i o n s , t h e penumbra has t h e appearance o f
white hot threads.
Large sunspots a r e t o some e x t e n t h o l l o w s i n t h e p h o t o s p h e r e , and i t has been
suggested by some s c i e n t i s t s t h a t t h e s e penumbra ( o r t h r e a d s ) a r e a s e c t i o n a l view
of t h e c o n v e c t i o n c u r r e n t s whose t o p s were r e f e r r e d t o e a r l i e r as g r a n u l a t i o n . .»
Astronomers say t h a t t h e t e m p e r a t u r e i n s i d e a s p o t i s about 1000° l o w e r t h a n
o u t s i d e i t . A l l sunspots have s t r o n g magnetic f i e l d s whose f i e l d s t r e n g t h decreases
from t h e c e n t r e t o a v e r y small v a l u e n o t f a r o u t s i d e t h e penumbra. The h i s t o r y o f
a sunspot group i s r e a l l y o n l y one f a c e t o f a s t o r y i n v o l v i n g a number o f known
phenomena.
In t h e e a r l y s t a g e s o f t h e development o f a s u n s p o t , t h e pores t e n d t o c l u s t e r
around 2 c e n t r e s o f a c t i v i t y known as t h e " l e a d i n g " and t h e " f o l l o w i n g " s p o t s .
Since t h e sun r o t a t e s on i t s a x i s from e a s t t o w e s t , t h e l e a d i n g spot i s t h e most
w e s t e r l y o f t h e p a i r . D u r i n g t h e f i r s t few days t h e s e two s p o t s move r a p i d l y a p a r t ,
almost as i f t h e y were r e p e l l i n g one a n o t h e r , and a t t h e same t i m e each grows i n
s i z e . The l e a d i n g s p o t has an average l i f e f o u r t i m e s t h a t o f i t s f o l l o w e r , which
i s u s u a l l y i r r e g u l a r i n shape and decays q u i c k l y a f t e r r e a c h i n g i t s maximum s i z e .
The l e a d i n g s p o t i s more compact and r e g u l a r i n shape and d w i n d l e s s l o w l y . A f t e r
about t h e l O t h day, t h e l e a d e r and f o l l o w e r approach each o t h e r . The f o l l o w e r
d i s a p p e a r s and t h e l e a d e r t a k e s on a rounder shape and may p e r s i s t f o r weeks b e f o r e
g r a d u a l l y d i s a p p e a r i n g . The s i z e o f a sunspot can v a r y from a small one o f a few
thousand m i l e s t o huge s p o t s many t i m e s t h e s i z e o f o u r own p l a n e t e a r t h .
The cause o f sunspots i s s t i l l unknown, b u t one o f t h e most i m p o r t a n t
d i s c o v e r i e s about them o c c u r r e d i n 1908 when Dr. George H a l e , o f t h e Mount W i l s o n
O b s e r v a t o r y i n C a l i f o r n i a , photographed sunspots i n enough d e t a i l t o show t h a t
l a r g e s p o t s a r e o f t e n surrounded by g r e a t w h i r l i n g masses o f gas. I n 1914, Dr. Hale
made a n o t h e r i m p o r t a n t d i s c o v e r y when he proved t h a t sunspots o f t e n have magnetic
f i e l d s much more p o w e r f u l t h a n t h e magnetic f i e l d s u r r o u n d i n g o u r e a r t h . Working
from t h e s e two d i s c o v e r i e s , p h y s i c i s t s have developed what i s perhaps a p a r t i a l
e x p l a n a t i o n o r t h e o r y f o r s u n s p o t s . Many o f them b e l i e v e t h a t t h e y a r e caused by
s t r o n g magnetic f i e l d s l y i n g deep w i t h i n t h e i n t e r i o r o f t h e sun. Huge e n e r g i e s
generated by t h e s e magnetic f i e l d s sometimes break t h r o u g h t o t h e sun's v i s i b l e
s u r f a c e , t h e p h o t o s p h e r e . These magnetic e r u p t i o n s on r e a c h i n g t h e photosphere have
a s t r o n g magnetic f i e l d i n t h e i r c e n t r e s . These, i n t u r n , cause c e r t a i n changes i n
t h i s e l e c t r i f i e d gas. The t e m p e r a t u r e drops below t h a t o f t h e s u r r o u n d i n g s u r f a c e
of t h e sun w h i c h r e s u l t s i n a v i s i b l e c o n t r a s t , a d a r k area w h i c h i s i d e n t i f i e d as
a sunspot.
SUNSPOT COUNT
Now, as t o t h e a c t u a l sunspot c o u n t , t h e degree o f t h e sun's s p o t t e d n e s s a t any
t i m e can be measured by c o u n t i n g t h e s p o t s o r by n o t i n g t h e i r t o t a l a r e a . We know
from past r e c o r d s and c u r r e n t o b s e r v a t i o n s t h a t t h e number o f sunspots v a r i e s from
day t o day and from y e a r t o y e a r . A l s o , t h e number o f s p o t s counted and t h e i r area
depends, t o some e x t e n t , upon t h e o b s e r v e r and h i s t e l e s c o p e . A c c o r d i n g t o
P r o f . Dr. M. Waldmeier o f t h e Z u r i c h O b s e r v a t o r y , i n t h e y e a r 1848 Rudof W o l f , then
the D i r e c t o r o f t h e O b s e r v a t o r y , i n t r o d u c e d t h e r e l a t i v e sunspot numbers as a
4
measure o f sunspot a c t i v i t y . They a r e based on t h e s t r i k i n g f a c t t h a t sunspots v e r y
o f t e n appear i n g r o u p s . I t i s seldom t h a t t h e sun's f a c e shows o n l y one s p o t ,
u n l e s s i t i s a g i a n t p r o b a b l y formed from a number o f s m a l l e r s p o t s . The Wolf
f o r m u l a i s R=k ( l O g + f ) ; where " g " i s t h e number o f sunspot groups and " f " i s t h e
number o f a c t u a l s p o t s , whether i n groups o r n o t . The c o n s t a n t " k " has a v a l u e
which depends upon t h e t y p e o f t e l e s c o p e used and t h e v i e w i n g c o n d i t i o n s .
Dr. Waldmeier says t h a t Wolf c a r r i e d o u t h i s o b s e r v a t i o n s w i t h a F r a u n h o f e r
r e f r a c t o r t e l e s c o p e o f 8 cm. a p e r t u r e and UO cm. f o c a l l e n g t h , w i t h a
m a g n i f i c a t i o n o f 64 g i v i n g t h e " k " c o n s t a n t a v a l u e o f 1 . Wolf c o n s i d e r e d t h e "g"
number a l o n e d i d n o t r e p r e s e n t a s a t i s f a c t o r y measure o f sunspot a c t i v i t y because
t h e s i z e s o f t h e sunspot groups d i f f e r e d g r e a t l y . C o n s i d e r i n g t h i s and o t h e r
c i r c u m s t a n c e s , he showed t h a t t h e m u l t i p l i e r 10 w h i c h m u l t i p l i e s " g " , t h e observed
number o f sunspot g r o u p s , gave g r e a t e r w e i g h t t o t h e l a r g e g r o u p s , w h i c h he
b e l i e v e d t o be a more i m p o r t a n t i n d i c a t o r o f s o l a r a c t i v i t y t h a n s h o r l i v e d small
s p o t s . The F r a u n h o f e r r e f r a c t o r t e l e s c o p e i s s t i l l i n use t o d a y a t t h e Z u r i c h
O b s e r v a t o r y . O b s e r v a t i o n s f r o m around t h e w o r l d a r e c o l l e c t e d a t t h e O b s e r v a t o r y i n
Z u r i c h and examined b e f o r e t h e f i n a l sunspot number i s c o m p i l e d f o r t h a t day.
A l t h o u g h i n t h e W o l f f o r m u l a t h e R number may seem t o be a r a t h e r a r b i t r a r y method
o f d e t e r m i n i n g t h e sunspot c o u n t , i t has proved t o be a s a t i s f a c t o r y measure o f
s o l a r a c t i v i t y . Over t h e y e a r s i t has shown a h i g h degree o f c o r r e l a t i o n w i t h o t h e r
r e l a t e d phenomena, f o r example, t h e e l e c t r o n d e n s i t y i n t h e i o n o s p h e r e .
SIZE AND TEMPERATURE
As s t a t e d e a r l i e r , some sunspots were v e r y l a r g e and o t h e r s were, i n
comparison, r a t h e r s m a l l . The l a r g e s t group on r e c o r d c r o s s e d t h e sun's c e n t r a l :
m e r i d i a n on 7 A p r i l 1947. I t s maximum area was o v e r 7,000 m i l l i o n square m i l e s .
Another huge sunspot group was observed a t i t s maximum s i z e on F e b r u a r y 6, 1946
when i t exceeded 6,000 m i l l i o n square m i l e s . By t h e way, a w a r n i n g t o budding
a s t r o n o m e r s , never s t u d y sunspots d i r e c t l y , e i t h e r w i t h t h e naked eye o r t h r o u g h a
t e l e s c o p e p o i n t e d a t t h e sun. P r o f e s s i o n a l a s t r o n o m e r s use a t e l e s c o p e a r r a n g e d so
t h a t t h e sun's image i s p r o j e c t e d o n t o a c i r c l e o f paper o r opaque p l a s t i c about 18
inches i n d i a m e t e r . I t i s t h i s p r o j e c t i o n w h i c h i s s t u d i e d , and n o t t h e d i r e c t sun.
T h i s i s necessary t o p r o t e c t t h e eyes. A l t h o u g h t h e umbra, o r c e n t r e o f t h e
sunspot, l o o k s b l a c k , i t i s i n f a c t q u i t e luminous and has a t e m p e r a t u r e o f about
4500° K. The umbra l o o k s dark because o f t h e c o n t r a s t between i t and t h e
p h o t o s p h e r e , whose s u r f a c e i s about 1500° h i g h e r i n t e m p e r a t u r e (6000° K ) . We c o u l d
say t h a t a sunspot i s a t y p e o f r e f r i g e r a t i n g mechanism c a p a b l e o f h o l d i n g a l a r g e
r e g i o n o f t h e p h o t o s p h e r e a t a l o w e r t e m p e r a t u r e t h a n normal f o r many days and
sometimes weeks.
SMOOTHED SUNSPOT NUMBER
Under r e a s o n a b l y normal c o n d i t i o n s t h e sunspot number o v e r t h e c o u r s e o f an
U - y e a r c y c l e ranges f r o m "O" ( s u n s p o t minimum c o n d i t i o n s ) t o about 200 ( s u n s p o t
maximum c o n d i t i o n s ) . However, e x p e r i e n c e has shown t h a t a 12-month r u n n i n g average
sunspot numbers i s t h e b e s t i n d i c a t o r o f i o n o s p h e r i c p r o p a g a t i o n c o n d i t i o n s . T h i s
smoothed number t a k e s i n t o account t h e Z u r i c h m o n t h l y sunspot numbers f o r a
12-month p e r i o d . T h i s i s n e c e s s a r y because t h e d a i l y numbers show g r e a t v a r i a t i o n s
as do t h e m o n t h l y numbers. For example, by a c t u a l o b s e r v a t i o n , t h e Z u r i c h observed
sunspot number f o r J a n u a r y 1970 was 115, f o r F e b r u a r y i t was 130, i n March i t had
dropped t o 102, i n A p r i l i t had r i s e n s l i g h t l y t o 109 and f o r May i t had shot up t o
131. Thus, i t i s easy t o see t h a t even t h e m o n t h l y average number shows pronounced
5
v a r i a t i o n s . I t i s t r u e t h a t t h e d a i l y v a r i a t i o n s have been g r e a t l y e l i m i n a t e d , b u t
the m o n t h l y v a r i a t i o n s s t i l l show t o o s t r o n g a f l u c t u a t i o n t o be r e a l l y u s e f u l i n
f o r e c a s t i n g . The e l i m i n a t i o n o f t h e s e v a r i a t i o n s i s necessary f o r many purposes,
and e s p e c i a l l y so i f we w i s h t o o b t a i n a sunspot a c t i v i t y c u r v e w h i c h ascends from
minimum t o maximum and descends from maximum t o minimum a c t i v i t y as smoothly as
p o s s i b l e . Such a smoothed sunspot c y c l e c u r v e g i v e s a t r u e r p i c t u r e o f l o n g - t e r m
s o l a r a c t i v i t y . I t i s t h i s smoothed sunspot number p l o t t e d o v e r many y e a r s which
c l e a r l y shows t h e famous U - y e a r c y c l e . T h i s s o l a r c y c l e has a number o f
t e r r e s t r i a l e f f e c t s , the most important o f which i s on the ionosphere, which i n
turn a f f e c t s r a d i o Communications.
CONCLUSION
' '
'
I t would be i m p o r t a n t e c o n o m i c a l l y and s c i e n t i f i c a l l y i f i t were p o s s i b l e t o
f o r e c a s t f u t u r e sunspot numbers o r sunspot a c t i v i t y w i t h some degree o f accuracy.
The 11-year c y c l e p r o v i d e s some g u i d a n c e , b u t even w i t h t h i s t h e i r r e g u l a r i t i e s
between c y c l e s reduce i t s u s e f u l n e s s . Some c y c l e s have a sunspot h i g h o f over 200
w h i l e o t h e r s h a r d l y reach 130, - t h a t i s t h e maximum count o f each c y c l e . Some
e m p i r i c a l knowledge has been gained i n t h e l a s t few y e a r s and i t i s p o s s i b l e t o
make f a i r p r e d i c t i o n s o f average t r e n d s i n s o l a r a c t i v i t y a few y e a r s i n advance,
but t h e r e i s as y e t p r a c t i c a l l y no knowledge a v a i l a b l e r e l a t i n g t o t h e p r e d i c t i o n
of t h e o c c u r r e n c e o f t h e i n d i v i d u a l s p o t s . D a i l y and m o n t h l y sunspot numbers a r e
a v a i l a b l e from many s o u r c e s . A complete l i s t i n g can be o b t a i n e d m o n t h l y from t h e
D i r e c t o r , t h e Zurich Observatory, Steinwarte, Zurich, Switzerland.
The sunspot c y c l e which began i n 1755 i s known as Cycle I . The l a s t complete
c y c l e , number 2 1 , began i n mid 1976 and ended i n l a t e 1986. Sunspot a c t i v i t y i s
expected t o i n c r e a s e g r a d u a l l y over t h e n e x t f o u r t o s i x y e a r s , u n t i l sunspot
maximum i s reached. A c t i v i t y w i l l t h e n g r a d u a l l y decrease u n t i l t h e p r e s e n t sunspot
c y c l e reaches t h e minimum p o s i t i o n once a g a i n i n a p r o x i m a t e l y 1997.
The o b s e r v a t i o n a l knowledge c o n c e r n i n g sunspots i s d e t a i l e d and p r e c i s e . I n
c o n t r a s t t h e t h e o r e t i c a l e x p l a n a t i o n o f t h e cause and c o n s t r u c t i o n o f sunspots i s
s p e c u l a t i v e and c o n t r o v e r s i a l . One f a c t which s t i l l r e q u i r e s e x p l a n a t i o n i s t h e
c o m p a r a t i v e l y low t e m p e r a t u r e o f a sunspot (4500° K) compared w i t h t h e s u r r o u n d i n g
s u r f a c e o f t h e photosphere (6000° K ) . Why t h i s 1500° d i f f e r e n c e ? I t i s , o f c o u r s e ,
t h i s c o n t r a s t i n t e m p e r a t u r e s which makes t h e sunspot v i s i b l e as a dark area i n t h e
l i g h t o f t h e sun's v i s i b l e s u r f a c e . What causes i t and how does i t work? The
sunspot c y c l e i s t h e most r e l i a b l e index o f t h e i o n o s p h e r e ' s r e f r a c t i v e e f f i c i e n c y ,
t h a t i s i t s c a p a b i l i t y f o r r e f l e c t i n g s h o r t r a d i o waves e a r t h w a r d s . That i s why
p r e d i c t i o n s o f sunspot a c t i v i t y a r e used i n t h e p r e p a r a t i o n o f i o n o s p h e r i c
p r o p a g a t i o n p r e d i c i t i o n s used f o r s c h e d u l i n g shortwave f r e q u e n c i e s . For more t h a n
35 y e a r s , i o n o s p h e r i c p r e d i c t i o n s o f h i g h f r e q u e n c y r a d i o p r o p a g a t i o n have been
issued by t h e I n s t i t u t e f o r Telecommunication Sciences a t B o u l d e r , C o l o r a d o , and
o t h e r i o n o s p h e r i c r a d i o p r o p a g a t i o n groups around t h e w o r l d . The s i g n i f i c a n c e o f
the r e l a t i v e sunspot numbers l i e s i n t h e f a c t t h a t t h e y p r o v i d e t h e l o n g e s t
c o n t i n u o u s s o l a r index on r e c o r d . They a r e a v a i l a b l e i m m e d i a t e l y a f t e r t h e i r
o b s e r v a t i o n and t h i s i s i m p o r t a n t f o r t h e p r e d i c t i o n o f t e r r e s t r i a l phenomena which
are i n f l u e n c e d by s o l a r a c t i v i t y . Since computers a r e now commonly used t o produce
i o n o s p h e r i c p r e d i c t i o n s , i t i s d e s i r a b l e t o adapt o l d methods o f f o r e c a s t i n g s o l a r
a c t i v i t y t o computer o p e r a t i o n .
6
BIBLIOGRAPHY
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vm" . r.;n
-s^'
We have come a l o n g way i n t h i s d i s c u s s i o n o f sunspots and t h e i r importance t o
many aspects o f l i f e on e a r t h . Much i s known about t h e i r b e h a v i o u r b u t l i t t l e i s
known about t h e i r cause. They a r e a f a s c i n a t i n g s u b j e c t f o r s t u d y and we hope t h a t
t h i s a r t i c l e w i l l have i n t e r e s t e d you and perhaps encouraged some o f you t o read
more about them. I f so, here a r e some books which have been c o n s u l t e d i n t h e
preparation o f t h i s a r t i c l e :
"THE SUN and i t s INFLUENCE" by M.A. E l l i s o n p u b l i s h e d by Routledge and Kegan Paul
L t d . , o f London, England.
"RADIO ASTRONOMY" by J.L. Pawsey and R.N. B r a c e w e l l p u b l i s h e d by t h e Clarendon
Press, O x f o r d , England.
"THE SUNSPOT ACTIVITY IN THE YEARS 1610-1960" by P r o f e s s o r M. Waldmeier, p u b l i s h e d
by Z u r i c h Schulhess & Co. AG o f Z u r i c h , S w i t z e r l a n d . ( P r o f e s s o r Waldmeier i s
the D i r e c t o r o f t h e Swiss Federal O b s e r v a t o r y i n Z u r i c h ) .
.
The annual p u b l i c a t i o n "WORLD RADIO-TV HANDBOOK" c o n t a i n s a b r i e f s e c t i o n on
sunspot a c t i v i t y i n each e d i t i o n .
A l l o f t h e s e books s h o u l d be a v a i l a b l e t h r o u g h y o u r l o c a l
bookstore.
.
; ^:.f
The Shortwave L i s t e n e r s ' D i g e s t a-;
Radio Canada I n t e r n a t i o n a l
^fm
P.O. Box 6000
M o n t r e a l , Quebec
CANADA H3C 3A8