Relationships between Forbush decreases and associated geomagnetic storm characteristic

Akasofu, S.-I.

Yoshida, S.

English

NASA Technical Reports Server

The Development of the Forbush Decrease 
and t h e  Geomagnetic Storm Fields+ 
* 
S. Yoshida and S . - I .  akasofu 
December 1965 
Department o f  Physics and Astronomy 
University of  Iowa 
Iowa City, Iowa 
* 
Geophysical In s t i t u t e ,  University o f  Alaska, College, Alaska. 
-f- Research supported i n  par t  by grants from t h e  National Aeronautics 
and Space Administration t o  the University of Alaska ( N s G  201-62) 
and t o  the University of Iowa ( N s G  233-62). 
https://ntrs.nasa.gov/search.jsp?R=19680002145 2020-03-12T06:21:56+00:00Z
r 
C 
ABSTRACT 
Relationships between the development of Forbush decreases 
and character is t ics  of associated geomagnetic storm f i e l d s  are  
examined. 
decrease i s  more closely related t o  t,he development of successive 
sudden impulses o r  world-wide geomagiietic micropulsations during 
geomagnetic storms than t o  a simple s tep function-like storm sudden 
commencement. 
obvious re la t ion  with the magnitude of the Forbush decrease. 
It is  shown tha t  the  onset of a s ignif icant  Forbush 
The magnitude of the main phase decrease shows no 
3 
1. INTRODUCTION 
A considerable amount of work has been done i n  the pas t  on 
the charac te r i s t ics  of the  Forbush decrease during geomagnetic 
storms [c f  . Dorman; Sandstrbm; (2) R ~ e d e r e r ' ~ ) ]  . They have 
confirmed t h a t  the Forbush decrease begins a t  about the onset time 
of a geomagnetic storm (namely, of the storm sudden commencement). 
Unfortunately, however, l i t t l e  a t ten t ion  has been paid t o  the problem 
as t o  how the development of the Forbush decrease i s  r e l a t ed  t o  the 
geomagnetic storm f i e l d s  t h a t  develop a f t e r  the sudden commencement. 
Since charac te r i s t ics  of geomagnetic storms d i f f e r  g rea t ly  from one 
t o  the other,(4) it i s  worthwhile t o  examine i n  d e t a i l  t h i s  problem 
fo r  individual cases. 
4 
2. THE FORBUSH DECREASE AND INTENSE 
ACCIVITY OF SUDDEN IMPULSES 
I n  t h i s  section, we demonstrate t h a t  a s ign i f icant  Forbush 
decrease develops i f  geomagnetic storms are  accompanied by intense 
a c t i v i t y  of sudden impulses (abbreviated by s i ) .  For t h i s  purpose, 
we have chosen here geomagnetic storms whose onsets are  characterized 
 by a simple s tep function-like storm sudden commencement (abbreviated 
by s sc )  and are  then followed by intense s i  ac t iv i ty .  
a.  The Forbush decreases during the  
geomagnetic storm of  April 23/24, 1959 
h The storm began w i t h  a f a i r l y  simple ssc  a t  10 36m UT on 
April  23 and reached the maximum epoch a t  about 21-22 UT. 
h before the maximum epoch, 19 
A l i t t l e  
30m UT, intense s i  a c t i v i t y  began; 
t h e i r  world-wide occurrence can be seen i n  Figure l a  which shows a 
co l lec t ion  of magnetic records (the H component) from seven s t a t ions  
widely separated i n  longitude. Figure l b  shaws the var ia t ion  of the 
cosmic r ay  (neutron) in tens i ty  during the storm a t  f i v e  s ta t ions  from 
the  polar  region, Thule, Resolute Bay, Murchison Bay, Mawson, and 
Hiffiss Is ; the  reason f o r  choosing the polar  s t a t ions  i s  t h a t  t h e i r  
records a r e  l e s s  ser iously affected by the deformation of the 
5 
magneto sphere due t o  the  growth of the r ing  current.  ( 5 )  Further, 
i n  order t o  eliminate the effects  of the abnormally enhanced d a i l y  
variation, ( 6 )  t h e i r  average var ia t ion i s  obtained t o  examine only 
i t s  world-wide var ia t ions.  
The average var ia t ion  of the  cosmic ray in t ens i ty  thus 
obtained indicates  t h a t  the Forbush decrease associated with the  
ssc  a t  10 
enhanced a f t e r  19 
h 36m UT was very weak. However, the decrease was notably 
h 30m UT, when the intense s i  a c t i v i t y  began. 
b. The Forbush decrease during the  
geomagnetic storm of June 27/28, 1960 
The geomagnetic storm of June 27/28, 1960, was a l so  acco’mpanied 
The var ia t ion  by intense s i  a c t i v i t y  well a f t e r  i t s  onset (Figure 2). 
of the cosmic ray  in t ens i ty  corresponding t o  the same period i s  
obtained by averaging the neutron in t ens i ty  measured by Deep River (7 1 
(Pc - 1.3 BV) and Uppsala(8) (Pc - 1.3 BV). The Forbush decrease 
associated w i t h  the  ssc  ( a t  01 
in t ens i ty  ( .. 8) a t  about loh - llh UT on June 27 and began t o  
recover a t  12  UT. However, a new Forbush decrease began a t  about 
16 lom UT, simultaneous w i t h  the onset of intense s i  ac t iv i ty ;  it 
became m o l  I - 1 1 1  Pnse a f t e r  21 UT, reaching i t s  maximum in t ens i ty  a t  
h 45m UT on June 27) reached the maximum 
h 
h 
h 
6 
about 02h UT on June 28. 
after the s i  a c t i v i t y  was much greater  (6.5%) than tha t  associated 
w i t h  the very clear  ssc (24). 
The magnitude of  the  maximum decrease 
There are a number o f  other excellent examples of t h i s  type; 
among them are the geomagnetic storm of January 21, 1957; September 2, 
1957; September ~ 8 ,  1957; February 10, 1959; January 11, 1960; 
May 8, 1960; Ju ly  14, 1960; and December 1, 1961. 
We note t h a t  i n  most o f t h e  earlier studies of the Forbush 
decrease only the onset time of ssc  was noted, so t h a t  the Forbush 
decrease during geomagnetic storms was simply associated w i t h  the 
storm sudden commencement.; f o r  example, i n  t h e  second example, t he  
6-55 decrease w a s  considered t o  be associate? with t h e  csc  at 01 45m UT, 
and no consideration has been made t o  examine how the various geomagnetic 
storm f i e lds  t h a t  follow the ssc are related t o  the  decrease. 
h 
7 
30 THE FORBUSH DECREASE AID INTENSE 
GM)MAGNETIC PULSATIONS 
In  t h i s  section, we are  concerned with much more rapid 
changes of t he  geomagnetic f i e l d  than si's. When quick-run 
magnetic records a re  examined, such changes appear t o  be osc i l la -  
t o ry  (rather than a succession of si 's), so t h a t  they a r e  ca l led ,  
geomagnetic micropulsations. 
l 
a. The Forbush decreases during the 
geomagnetic storms of September 21/22, 1946 
Figure 3 shows three successive geomagnetic storms which 
occurred on September 21/2:, 1946, and associated changes of the 
cosmic ray intensi ty .  (9) The f i r s t  storm was characterized by a 
f a i r l y  simple ssc  (17 
phase, but without any significant main phase; the second storm 
developed the main phase a f t e r  the ssc (04h 23m UT on September 22). 
The th i rd  storm began a t  10 
h 13m UT on September 21) and an i n i t i a l '  
h 12m UT on September 22, a l i t t l e  a f t e r  
the  maximum epoch of the second storm. 
characterized by intense geomagnetic micropulsations; note tha t  i n  
the  San Juan record the H magnetic t race between 13 h 30m and 15 h 00" 
The th i rd  storm f i e l d  was 
is not c lear ly  seen because of extremely intense pulsations. The 
8 
same micropulsation activity was recorded at other stations, such 
as Greenwich and Kakidka (Japan), so that it had a world-wide extent. 
The cosmic ray variation during the storms shows that the 
first two storms were not associated with a clear-cut Forbush 
decrease, but a significant decrease occurred during the third storm. 
b. The Forbush decrease during the 
geomagnetic storm of May 12, 1949 
The geomagnetic storm of May 12, 1949, was accompanied.by 
intense fluctuations of the geomagnetic field at about the maximum 
epoch of the main phase, 15 h 50" UT (Figure 4). The simultaneous 
cosmic ray intensity variation (meson)(9) shows that the decrease 
associated with the micropulsations is more significant than that 
associated with the ssc (06h 40m UT). 
July 17, 1959, is also an excellent example of this type; it was 
accompanied by intense micropulsations of period of order 4 minutes. 
The geomagnetic storm of 
9 
4. THE FORBUSH DECREASE AND THE 
MAIN F'HASE DECREASE 
A s  ea r ly  as  1938, Forbush (lo) noted t h a t  the in t ens i ty  of 
the decrease has no r e l a t ion  with the  magnitude of the main phase. 
This can be demonstrated i n  two ways. F i r s t  of all, it i s  not 
d i f f i c u l t  t o  show t h a t  geomagnetic storms without any s igni f icant  
main phase decrease are  of ten associated with a large Forbush 
decrease (the geomagnetic storms of May 11, July 11, and August 20, 
1959; t.he f i r s t  storm w i l l  be discussed i n  the  next sec t ion) .  
Secondly, there are  a number of geomagnetic storms which a re  not 
preceded by ssc  but have a s ignif icant  main phase; thus, they can 
be used t o  examine relationships between the  main phase decrease 
and the  Forbush decrease without 'contamination' of the ssc ef fec t .  
For t h i s  purpose, twelve well developed Sg storms (the main phase 
decrease of more than 50 y )  during the IGY were examined, and none of 
(11 1 
them were found t o  be associated w i t h  any appreciable Forbush decrease. 
It may be noted also t h a t  one of t he  most intense Sg storms, the 
storm of April 23, 1960, was no t  associated w i t h  the Forbush decrease, 
although %he magnitude of the main phase decrease was as la rge  as 
200 y .  The following table  shows the charac te r i s t ics  of the twelve 
Sg storms. 
10 
Storm Onset Maximum Dst Decrease 
02 UT ’ August 13, 1957 73 7 
18 October 1, 1957 51 
04 October 10, 1957 57 
18 October 20, 195 53 
09 November 3, 1957 50 
00 March 12, 1958 79 
12 s March 30, 1958 66 
S L  April 4, 1958 71 
11 May 26, 1958 52 
19 June 29, 1958 83 
12 November 2, 1958 82 
09 November 25, 1958 79 
11 
5 .  DISCUSSION 
It i s  not d i f f i cu l t  t o  infer from the above study tha t  
when the  onset time of a large Forbush decrease coincides approxi- 
mately with the  ssc of t he  associated geomagnetic storm, the  ssc  
i s  orten followed by ei ther  intense s i  a c t i v i t y  o r  micropulsations, 
o r  by both. The May 11, 1959, storm provides an interest ing 
example; the Forbush decrease began at about t he  time of the  ssc 
and reached the  maximum decrease of order 12% (Figure 5) .  F i r s t  
of all, t he  storm was caused by an intense f l a r e  at 20 
on May 10 a t  47" E, 23" N on the solar  disk.  
h 55m UT 
As Sinno(12) and 
Haurwitz, Yoshida, and Akasofu (13) have shown, the  decreases of 
more than 10% are mostly caused by eastern f l a r e s  and centra3 
flaxes,  but not by fa r  western f l a r e s .  Secondly, t h e  ssc was 
soon followed by s i  and micropulsation a c t i v i t i e s  and was not a 
s tep f'unction l i k e  ssc .  Thirdly, t h e  storm w a s  not associated 
with a s ignif icant  main phase decrease, so t h a t  t h i s  is another 
way t o  prove tha t  the  main phase decrease i s  not re la ted t o  the  
Forbush decrease. Finally, the Forbush decrease las ted  un t i l  as 
l a t e  as May 23 or for  about 10 days, although the  magnetic a c t i v i t y  
w a s  much reduced at the  end of May 12. 
12 
Intense s i  a c t i v i t y  contains frequent and rapid world-wide 
changes (both pos i t ive  and negative) of the horizontal  component of 
geomagnetic f i e l d ;  Nishida and Jacobs (14.) . have shown t h a t  they have 
e s s e n t i a l l y  the same charac te r i s t ics  as those of ssc .  Since s s c  i s  
now in te rpre ted  as a sudden compression of the  magnetosphere by an 
enhanced so la r  plasma flow, intense s i  a c t i v i t y  may be in te rpre ted  
as an ind ica t ion  of ' turbulent '  flow; embedded i n  such a turbulent  
flow, the magnetospheric cavity expands or  contracts ,  causing 
f luc tua t ions  of the  magnetic f i e l d  ins ide  the cavi ty .  
magnetic pulsat ions discussed i n  t h i s  paper may be in te rpre ted  as 
Intense geo- 
an indicat ion of 'v ibrat ion '  of the magnetosphere by a ' turbulence ' .  (15 1 
However, t he  f a c t  t h a t  the Forbush decrease l a s t s  much longer 
than the  duration of s i  a c t i v i t y  or  micropulsation suggests t h a t  the 
turbulent '  region of the plasma flow a c t s  a s  a b a r r i e r  and reduces 
the  cosmic ray  i n t e n s i t y  behind the  b a r r i e r  f o r  a long period a f t e r  
? 
it passes the v i c i n i t y  of the ear th .  
f i e l d  discussed o r ig ina l ly  by Parker 
Yoshida, and Akasofu (13) may sometimes contain ' turbulence'  which 
The kind of in te rp lane tary  
and l a t e r  by Haurwitz 
reduces t h e  cosmic r ay  in t ens i ty  mpre e f f i c i e n t l y  than a simple 
b a r r i e r .  
. .  
13 
We would l i k e  'to thank D r .  S. Chapman and 
D r .  J. A. Van Allen for  t h e i r  encouragement during t h e  prepara- 
t i o n  of t h i s  paper. The work presented here was  supported i n  pa r t  
by grants from t h e  National Aeronautics and Space Administration 
t o  t h e  University of Alaska (NsG 201-62) and to t he  University of 
Iowa (NSG 233-62). 
REFERENCES 
1. L. I. Dorman, Progress i n  Elementary Pa r t i c l e s  and Cosmic Ray 
Physics 1, ed. by J. G. Wilson and S. A. Wouthuysen, North- 
Holland Pub? Co., Amsterdam (1963). 
A .  E. Sandstr'bm, Cosmic Ray Physics, North-Holland Pub. Co., 
Amsterdam (1965). 
2. 
3 .  J. G. Roederer, Research i n  Geophysics, ed. by H. Odishaw, 
The MIT Press (1965). 
4 .  
5 .  
6. 
7. H. Carmichael, Weekly Report, Deep River, Canada. 
8. A. E. SandstrGm, Pub. Uppsala Univ., I n s t i t u t e  of Physics (1961). 
9 -  I. Lange and S. E. Forbush, Cosmic Ray Results, Dept. Terr. Mag., 
S.-I. Akasofu and S. Chapman, J. Geophys. Res. 68, 125 (1963). 
S. Yoshida and M. Wada, Nature - 183, 381 (1959). 
S. Yoshida, Nuovo C i m .  - 4 ,  1410 (1956). 
-
Carnegie I n s t i t u t i o n  of Washington, Pub. No. 175, Washington, D.C. 
(1957) ' 
10. 
11. 
12. 
S. E. Forbush, Terr.  Mag. - 4.3, 203 (1938). 
S.-I. Akasofu, Planet. Space Sci.  - 13, 297 (1965). 
K. Sinno, J. Phys. SOC. Japan JJ, Suppl. MI, 396 (1962). 
13. M. W. Haurwitz, S. Yoshida, and S.-I. Akasofu, J. Geophys. Res. 
- 70, 2977 (1965) 
14. A. Nishida and J. A. Jacobs, J. Geophys. Res. 9, 525 (1962). 
15. J. W. Dungey, Pub. Ionosphere Res. Lab., Pennsylvania State  Univ., 
No. 69 (1954). 
16. E. N. Parker, Interplanetary Dyn amical Processes, Interscience 
Pub., 1963. 
16 
FIGURE CAPTIONS 
Figure la .  The horizontal component magnetic records on 
April 23/24, 1959, from magnetic s ta t ions widely dis- 
t r ibuted i n  longitude. 
the  recovery phase o f  the storm. 
Note intense s i  ac t iv i ty  during 
Figure lb .  The magnetic record (H, Z, D) on April 23/24 
from Honolulu and the  variation of t he  cosmic ray 
(neutron) in tens i ty  at  Thule, Resolute Bay, Murchison 
Bay, Mawson, and Hif iss  Is during the  same period; the  
average (world-wide) variation of the cosmic ray 
in tens i ty  i s  shown a t  t h e  bottom. 
Figure 2. "he Honolulu (H, Z, D) and San Juan (D, 2, H) 
magnetic records of t he  July 27/28, 1960, magnetic storm 
(note tha t  i n  the Honolulu record, a downward c h q e  i s  
posi t ive i n  t h e  horizontal component) and the  average 
var ia t ion of t he  neutron in tens i ty  (Deep River and 
Uppsala) during the  same period. 
Figure 3 .  The San Juan magnetic record (Z, D, H) of the  
September 21/22, 1946 magnetic storms and the  average 
var ia t ion of the  meson intensi ty  (Huancayo and Cheltenham) 
during the  same period. 
Figure 4. The San Juan magnetic record of May 12, 1949, 
storm and the  average variation of the  meaon in tens i ty  
(huancayo and Cheltenham) during the same period. 
Figure 5. The San Juan magnetic record (Z, D, H) of the  
May 11/12, 1959, magnetic storm and the  average var ia t ion 
of the neutron intensi ty  (Deep River and Uppsala) during 
t h e  same period. 
50Y 
I 
50 7 
I 
5 07 
I 
5 0 7  
I 
i \  ~ i 
I I 
GUAM 
HONOLULU 
...._.._ - 
I 
507 I 1 I 
I 
1 I I 8  
10 12 la 0 6 I 2  GMT 
APRIL 23  1959 APRIL 2 4  
Figlure ,a 
LT=Oh 
mz/L Pc = 0.0 BY//;,; 
1 
1 
AVERAGE 
I I I I I I I I 1 1 
04 08 12 16 20 24 04 08 12 16 
Figure ib 
APRIL 23 HOUR UT APRIL 24 1959 
. . 
. .... __ 
-~ 
. ~. 
I 
M 
, 
0 -1 - 
0 -  
t 
-7 
-1 I \ 


The Development of the Forbush Decrease and the Geomagnetic Storm Fields

Forbush decreases and development of geomagnetic storm field

. 
? *  
. *  . * . ~  u. of l0wui~5-41 
I i 
I 
I I 
Department of Physics and Astronomy 
THE UNIVERSITY OF IOWA 
Iowa City, Iowa 
https://ntrs.nasa.gov/search.jsp?R=19660007096 2020-03-16T23:12:57+00:00Z
' .  
The Development of t h e  Forbush Decrease 
and t h e  Geomagnetic Storm Fields+ 
* 
S.  Yoshida and S. - I .  Akasofu 
December 1965 
Depaytment of Physics and Astronomy 
University of Iowa 
Iowa City, Iowa 
* 
Geophysical In s t i t u t e ,  University o f  Alaska, College, Alaska. 
-F Research supported i n  part by grants from t h e  National Aeronautics 
and Space Administration t o  t h e  University of Alaska (NsG 201-62) 
and t o  the  University of Iowa (NsG 233-62). 
ABSTRACT 
Relationships between the development of Forbush decreases 
and character is t ics  of associated geomagnetic storm f i e l d s  are  
examined. It i s  shown tha t  the onset of a s ignif icant  Forbush 
decrease i s  more closely related t o  t,he development of successive 
sudden impulses or world-wide geomagiletic micropulsations during 
geomagnetic storms than t o  a simple s tep function-like storm sudden 
commencement. The magnitude of the main phase decrease shows no 
obvious re la t ion  with the magnitude of the Forbush decrease. 
. 
3 
1. INTRODUCTION 
A considerable amount of work has been done i n  the pas t  on 
the charac te r i s t ics  o f  t h e  Forbush decrease during geomagnetic 
storms [ cf . Dorman; 
confirmed t h a t  t he  Forbush decrease begins a t  about the onset time 
of a geomagnetic s t o r m  (namely, of the storm sudden commencement). 
Unfortunately, however, l i t t l e  a t ten t ion  has been paid t o  the problem 
as t o  how the development of t he  Forbush decrease i s  re la ted  t o  the  
geomagnetic storm f i e l d s  tha t  develop after the  sudden commencement. 
Since charac te r i s t ics  of geomagnetic storms d i f f e r  g rea t ly  from one 
t o  the  other,(4) it is  worthwhile t o  examine i n  d e t a i l  t h i s  problem 
f o r  individual cases. 
Sandstrom; (2) R ~ e d e r e r ' ~ ) ]  . They have 
4 
2. THE FORBUSH DECREASE AND INTENSE 
ACTIVITY OF SUDDEN IMPULSES 
I n  t h i s  section, we demonstrate t h a t  a s ign i f icant  Forbush 
decrease develops if geomagnetic storms a re  accompanied by intense 
a c t i v i t y  of sudden impulses (abbreviated by s i ) .  
we have chosen here geomagnetic storms whose onsets a re  characterized 
by a simple s tep  function-like storm sudden commencement (abbreviated 
by ssc)  and are  then followed by intense s i  ac t iv i ty .  
For t h i s  purpose, 
a .  The Forbush decreases during the 
geomagnetic storm of April 23/24, 1959 
h The storm began w i t h  a f a i r l y  simple ssc  a t  10 36m UT on 
April  23 and reached the maximum epoch a t  about 21-22 UT. 
before the maximum epoch, 19 30m UT, intense s i  a c t i v i t y  began; 
t h e i r  world-wide occurrence can be seen i n  Figure l a  which shows a 
col lect ion of magnetic records (the H component) from seven s t a t ions  
A l i t t l e  
h 
widely separated i n  longitude. 
cosmic ray  (neutron) in tens i ty  during the storm a t  f i ve  s ta t ions  from 
the polar region, Thule, Resolute Bay, Murchison Bay, Mawson, and 
Hiffiss I s  ; the  reason for choosing the polar  s ta t ions  i s  t h a t  t h e i r  
records a r e  l e s s  seriously affected by the deformation of the 
Figure l b  shows the var ia t ion  of the 
5 
magnetosphere due t o  the  growth of the r ing  current.  ( 5 )  Further, 
i n  order t o  eliminate the e f fec ts  of the abnormally enhanced d a i l y  
var ia t ion,  (6) t h e i r  average var ia t ion i s  obtained t o  examine only 
i t s  world-wide var ia t ions.  
The average var ia t ion of  the cosmic ray in t ens i ty  thus 
obtained indicates t h a t  the Forbush decrease associated with the 
ssc  a t  10 
enhanced a f t e r  19 
h 36m UT was very weak. However, the decrease was notably 
h 30m UT, when t h e  intense s i  a c t i v i t y  began. 
b. The Forbush decrease during the 
geomagnetic storm of June 27/28, 1960 
The geomagnetic storm of June 27/28, 1960, was a l so  acco'qanied 
The var ia t ion  by intense s i  a c t i v i t y  well  a f t e r  i t s  onset (Figure 2). 
of the  cosmic ray  in t ens i ty  corresponding t o  the same period i s  
obtained by averaging the neutron in t ens i ty  measured by Deep River (7) 
(Pc - 1.3 BV) and Uppsala(8) (Pc - 1.3 BV). The Forbush decrease 
associated w i t h  the ssc  (a t  01 
in t ens i ty  ( - 6)  a t  about loh - llh UT on June 2'7 and began t o  
recover a t  1 2  UT. However, a new Forbush decrease began a t  about 
16 lom UT, simultaneous wi th  the onset of intense s i  ac t iv i ty ;  it 
became IMJJ 1 I 1 1 1  Pnse a f t e r  21 UT, reaching i t s  maximum in t ens i ty  a t  
h 45m UT on June 27) reached the maximum 
h 
h 
h 
6 
h about 02 UT on June 28. 
a f t e r  the s i  ac t iv i ty  was much greater  (6.59) than tha t  associated 
with the very clear  ssc  ( Z & ) .  
The magnitude of the maximum decrease 
There are  a number of other excellent examples of t h i s  type; 
among them are the geomagnetic storm of January 21, 1957; September 2, 
1957; September Z8, 1957; February 10, 1959; January 11, 1960; 
May 8, 1960; July 1 4 ,  1960; and December 1, 1961. 
We note tha t  i n  most of the e a r l i e r  studies of the Forbush 
decrease only the onset t ime of ssc was noted, so tha t  the Forbush 
decrease during geomagnetic storms was simply associated with the 
storm sudden commencement.; for example, i n  t h e  second example, the 
6.59 decrease w a s  considered t o  be associate? w i t h  t h e  G S O  a t  O l h  45m UT, 
and no consideration has been m a d e  t o  examine how the various geomagnetic 
storm f i e l d s  t h a t  follow the ssc are  re la ted t o  the  decrease. 
7 
3s THE FORBUM DECREASE AND INTENSE 
GEXblAGNEIIC PULSATIONS 
I n  t h i s  section, we are concerned with much more rapid 
changes of t he  geomagnetic f i e l d  than si's. 
magnetic records a re  examined, such changes appear t o  be osc i l la -  
t o ry  (rather  than a succession of si's), so t h a t  they are cal led 
geomagnetic micropulsations. 
When quick-run 
I 
a.  The Forbush decreases during the  
geomagnetic storms of September 21/22, 1946 
Figure 3 shows three successive geomagnetic storms which 
occurred on September 21/2c', 1946, and associated changes of the  
cosmic ray  intensi ty .  (9) The first storm was characterized by a 
f a i r l y  simple ssc  (17 
phase, but  without any s ignif icant  main phase; t h e  second storm 
developed the main phase a f t e r  the ssc  (04h 23m UT on September 22). 
The t h i r d  storm began a t  10 12m UT on September 2E, a l i t t l e  after 
the max imum epoch of the  second storm. 
characterized by intense geomagnetic micropulsations; note tha t  i n  
h m  the  San Juan record the H magnetic t race  between 13h 30m and 15 00 
h 13m UT on September 21) and an i n i t i a l  
h 
The th i rd  storm f ie ld  was 
i s  not c l ea r ly  seen because of extremely intense pulsations.  The 
8 
same micropulsation activity was recorded at other stations, such 
as Greenwich and Kakidka (Japan), so that it had a world-wide extent. 
The cosmic ray variation during the storms shows that the 
first two storms were not associated with a clear-cut Forbush 
decrease, but a significant decrease occurred during the third storm. 
b. The Forbush decrease during the 
geomagnetic storm of May 12, 1949 
The geomagnetic storm of May 12, 1949, was accompanied by 
intense fluctuations of the geomagnetic field at about the maximum 
epoch of the main phase, 15 The simultaneous 
cosmic ray intensity variation (meson)(9) shows that the decrease 
associated with the micropulsations is more significant than that 
associated with the ssc (06h 40m UT). 
July 17, 1959, is also an excellent example of this type; it was 
accompanied by intense micropulsations of period of order 4 minutes. 
h 50m UT (Figure 4). 
The geomagnetic storm of 
9 
4.  THE FORBUSH DECREASE AND THE 
MAIN PHASE DECREASE 
A s  ea r ly  as 1938, Forbush noted t h a t  the  in t ens i ty  of 
the decrease has no r e l a t ion  with the  magnitude of the  main phase. 
This can be demonstrated i n  two ways. F i r s t  of all, it i s  not 
d i f f i c u l t  t o  show tha t  geomagnetic storms without any s igni f icant  
main phase decrease are often associated with a la rge  Forbush 
decrease (the geomagnetic storms of May 11, July 11, and August 20, 
1959; the f i rs t  storm w i l l  be discussed i n  the next section).  
Secondly, there  are  a number of geomagnetic storms which a re  not 
preceded by ssc  but  have a s ignif icant  main phase; thus, they can 
be used t o  examine relationships between the  main phase decrease 
and the  Forbush decrease without 'contamination' of the ssc e f fec t .  (11) 
For t h i s  purpose, twelve well developed Sg storms (the main phase 
decrease of more than 50 y )  during the IGY were examined, and none of 
them were found t o  be associated with any appreciable Forbush decrease. 
It may be noted a l so  t h a t  one of t he  most intense Sg storms, the 
storm of April 23, 1960, was not associated with the Forbush decrease, 
although the magnitude of t h e  main phase decrease was as large as 
200 y .  (") The following tab le  shows the charac te r i s t ics  of the twelve 
Sg storms - 
. 
10 
Storm Onset Maximum D s t  Decrease 
02 UT ' August 13, 1957 73 7 
18 October 1, 1957 5 1  
04 October 10, 1957 57 
18 October 20, 195 53 
09 November 3, 1957 50 
00 March 12, 1958 79 
12 . March 30, 1958 66 
11 April 4 ,  1958 71 
11 May 26, 1958 52 
19 June 29, 1958 83 
12 November 2, 1958 82 
09 November 25, 1958 79 
i 
11 
5. DISCUSSION 
i 
. .  
It i s  not d i f f i c u l t  t o  i n fe r  from the  above study t h a t  
when the onset time of a large Forbush decrease coincides approxi- 
mately with the  ssc  of the  associated geomagnetic storm, the  ssc  
i s  often followed by e i ther  intense s i  a c t i v i t y  or  micropulsations, 
or  by both. The May 11, 1959, storm provides an interest ing 
example; the  Forbush decrease began at about t he  t h e  of the  ssc 
and reached the  maximum decrease of order 12% (Figure 5) .  F i r s t  
h of all, the  storm was caused by an intense f l a r e  a t  20 55m UT 
on May 10 a t  47" E, 23' N on t he  solar disk.  As Sinno (12) and 
Haurwitz, Yoshida, and Akasofu (13) have shown, the  decreases of 
more than 10% are  mostly caused by eastern f l a r e s  and cent ra l  
f lares ,  but not by f a r  western f l a r e s .  Secondly, t h e  ssc  w a s  
soon followed by s i  and micropulsation a c t i v i t i e s  and w a s  not a 
s tep function l i k e  ssc. Thirdly, t h e  storm was not associated 
with a s ignif icant  main phase decrease, so t h a t  t h i s  i s  another 
way t o  prove t h a t  t he  main phase decrease i s  not re la ted t o  the 
Forbush decrease. Finally, t h e  Forbush decrease las ted  unt i l  as 
l a t e  as May 23 or  for  about 10 days, although the  magnetic a c t i v i t y  
was much reduced a t  the end of May 12. 
12 
Intense s i  a c t i v i t y  contains frequent and rapid world-wide 
changes (both pos i t ive  and negative) of the horizontal  component of 
geomagnetic f i e l d ;  Nishida and Jacobs (14') have shown t h a t  they have 
e s sen t i a l ly  the  same cha rac t e r i s t i c s  as those of ssc .  Since s sc  is  
now interpreted as a sudden compression of the  magnetosphere by an 
enhanced so lar  plasma flow, intense s i  a c t i v i t y  may be in te rpre ted  
as  an indicat ion of ' turbulent '  flow; embedded i n  such a turbulent  
flow, the  magnetospheric cavi ty  expands or contracts ,  causing 
f luc tua t ions  of the  magnetic f i e l d  inside the  cavi ty .  Intense geo- 
magnetic pulsat ions discussed i n  t h i s  paper may be in te rpre ted  as  
an indicat ion of 'v ibra t ion '  of the magnetosphere by a ' turbulence' . (15 1 
However, the  f a c t  tha t  the  Forbush decrease l a s t s  much longer 
than the  duration of s i  a c t i v i t y  o r  micropulsation sugges ts . tha t  the 
' turbulent '  region of the plasma flow a c t s  as a b a r r i e r  and reduces 
the cosmic ray  i n t e n s i t y  behind t h e  b a r r i e r  f o r  a long period a f t e r  
it passes the v i c i n i t y  of  the ear th .  
f i e l d  discussed o r ig ina l ly  by Pmker (16) and l a t e r  by Haurwitz 
The kind of in te rp lane tary  
Yoshida, and Akasofu (13) may sometimes contain ' turbulence'  which 
reduces t h e  cosmic r ay  in t ens i ty  mpre e f f i c i e n t l y  than a simple 
b a r r i e r .  
13 
ACKNOWLEDGEMENTS 
We would l i k e  'to thank Dr. S. Chapman and 
Dr. J. A. Van Allen fo r  t h e i r  encouragement during t h e  prepwa- 
t i o n  of t h i s  paper. The work presented here w a s  supported i n  pa r t  
by grants from the  National Aeronautics and Space Administration 
t o  the  University of Alaska (NsG 201'62) and t o  the  University of 
Iowa (NSG 233-62). 
\ 
i .  
14 
REFERENCES 
i 
1. 
2. 
3. 
4 .  
5. 
6. 
7 .  
8. 
9. 
L. I. Dorman, Progress i n  Elementary Par t ic les  and Cosmic Ray 
Phmics 1, ed. by J. G. Wilson and S. A. Wouthuysen, North- 
Holland Pub * Coo , Amsterdam (1963). 
A. E. Sandstrom, Cosmic Ray Physics, North-Holland Pub. Co., 
Amsterdam (1965). 
J. G .  Roederer, Research i n  Geophysics, ed. by H. Odishaw, 
The MIT Press (1965). 
So-I. Akasofu and S. Chapman, J. Geophys. Res. - 68, 125 (1963). 
S. Yoshida and M. Wada, Nature - 183, 381 (1959). 
S. Yoshida, Nuovo Cim.  - 4 ,  1410 (1956). 
H. Carmichael, Weekly Report, Deep River, Canada. 
A .  E. SandstrEm, Pub. Uppsala Univ., I n s t i t u t e  of Physics (1961). 
I. Lange and S. E. Forbush, Cosmic Ray Results, Dept. Terr. Mag., 
Carnegie Ins t i t u t ion  of Washington, Pub. No. 175, Washington, D.C. 
(1957) 
10. 
11. 
12. 
S. E. Forbush, Terr. Mag. - 4.3, 203 (1938). 
S.-I. Akasofu, Planet. Space Sci .  - 13, 297 (1965). 
K. Sinno, J. Phys. SOC. Japan - 17, Suppl. AII, 396 (1962). 
13. M. W. Haurwitz, S. Yoshida, and S.-I. Akasofu, J. Geophys. Res. 
- 70, 2977 (1965) 
1 4 .  A. Nishida and J. A. Jacobs, J. Geophys. Res. - 67, 525 (1962). 
15. J. W. Dungey, Pub. Ionosphere Res. Lab., Pennsylvania S ta te  Univ., 
No. 69 (1954). , 
! 
16. E. N. Parker, Interplanetary Dyn amicaJ. Processes, Interscience 
Pub., 1963. 
i 
16 
FIGURE CAPTIONS 
Figure l a .  The horizontal component magnetic records on 
April 23/24, 1959, from magnetic s ta t ions widely dis-  
t r ibu ted  i n  longitude. 
the  recovery phase of t he  storm. 
Note intense s i  ac t iv i ty  during 
Figure lb .  The magnetic record (H, Z, D) on April 23/24 
from Honolulu and the var ia t ion of t he  cosmic ray 
(neutron) in tens i ty  at Thule, Resolute Bay, Murchison 
Bay, Mawson, and Hifiss Is during the  same period; the 
average (world-wide) variation of t h e  cosmic ray 
in tens i ty  i s  shown at the  bottom. 
Figure 2.  The Honolulu (H, Z, D) and San Juan (D, Z, H) 
magnetic records of the Ju ly  27/28, 1960, magnetic storm 
(note tha t  i n  the Honolulu record, a downward change i s  
posi t ive i n  t h e  horizontal component) and the  average 
var ia t ion of the  neutron in tens i ty  (Deep River and 
Uppsala) during the  same period. 
Figure 3. The San Juan magnetic record (Z, D, H) of the 
September 21/22, 1946 magnetic storms and t he  average 
var ia t ion of the meson in t ens i ty  (Huancayo and Cheltenham) 
during t h e  same period. 
! .  
i ,  
I !  
17 
Figure 4. The San Juan magnetic record of May 12, 1949, 
storm and t h e  average var ia t ion of  the  meaon in t ens i ty  
(huancayo and Cheltenham) during the  same period. 
Figure 5. The San Juan magnetic record (Z, D, H) of the  
May 11/12, 1959, magnetic s torm and the  average var ia t ion 
of the  neutron in tens i ty  (Deep River and Uppsala) during 
the  same period. 
507 
507 
I 
I 
50 7 
5 07 
T 
' I  
, 
KAKIOKA 
GUAM 
HUANCAY: h A  1- 
507 d V  I-' 1 
i 
IO I2 18 0 6 
APRIL 23 1959 APRIL 24 
Figare La 
I2 G M T  
LT=Oh 
.- 
YLLI 1 
AVERAGE 
J 
I 1 I I 1 I I 1 1 I 
04 08 12 16 20 24 04 08 12 16 
APRIL 23 HOUR U T  APRIL 24 1959 
Figure l b  

M 




The Development of the Forbush Decrease and the Geomagnetic Storm Fields

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