A monthly Forbush decrease index (Fd-I) is generated and it is compared with the observed long term chnges in the cosmic ray intensity near earth at energies greater than or equal to 1 Gev over 1976-83. Significant correlation is observed between the two except for 1978. Such an effect is also seen in the correlation plot between the solar flare index (SFI) and Fd-I

Agrawal, S. P.

Jain, A. K.

Mishra, B. L.

English

NASA Technical Reports Server

392
SH4.i-i0
CUMULATIVE EFFECT OF FORBUSH DECREASES IN THE HELIO-
SPHERIC MDDULATION DURING THE PRESENT SOLAR CYCLE
S. P. AgrawalI, B. L. MishraI, A. K. Jainl,2
iphysics Department (Vikram Space Physics Centre)
A.P.S. University, Rewa (M.P.) 486003, India
2Government Engineering College
Rewa (M.P.) 486002, India
ABSTRACT
We generate a monthly Forbush decrease index (Fd-l) and
compare it with the observed long-term changes in the cosmic
ray intensity near earth at energies > 1 Gev over 1976-83.
Significant correlation is observed between the two except
for 1978. Such an effect is also seen in the correlation
plot between the solar flare index (SFI) and Fd-l.
I. Introduction. Characteristics of the long term variation of cosmic
ray intensity have been obtained in the past using neutron monitor data
(e.g. Lockwood and Webber, 1984). Recent studies from spacecraft have
determined the radial gradient of cosmic ray intensity to a distance of
up to 30 AU (e.g. Venkatesan et al., 1984; Van Allen and Randall,
1985). Furthermore specific cosmic ray intensity changes such as
Forbush decreases have also been studied to large heliospheric radial
distances (Van Allen, 1979; and references therein). The purpose of
this paper is to explore the role of the short-term cosmic ray intensity
decreases in generating the profile of the cosmic ray long-term
variation.
2. Analysis and Results. The percent deviation has been calculated for
each month for the interval 1976-83, using the cosmic ray intensity
maximum for the months of Sept.-Nov. 1976 as 100%. The Forbush decrease
index (Fd-l) is computed from the hourly data of Deep River neutron
monitor after examining the plots of a number of stations as well. The
magnitude of each Fd is added to form a cumulative total of all the Fd's
observed in a month; such a total is the index, Fd-l. Similarly, the
solar flare index (SFI) has been computed by using all solar flares
(importance > I) and by assigning a weight according to the numerical
importance of the flare and the relative brightness of each solar
flare. Such objective criteria give proper weight to the energetic
flares, which generally produce much larger decreases in cosmic ray
intensity (Shukla et al., 1978).
The long-term modulation has been reported to be a direct
consequence of large number of solar flares (e.g. Hatton, 1980; Agrawal,
1983; and references therein). Similarly, the Fd effects are also the
consequence of energetic solar flares (Shukla et al., 1978). We
therefore suggest that the long term modulation is the cumulative effect
of large number of small and big Fd's, the validity for which is
examined in Figure 1. Note that the cosmic ray intensity reaches a
maximum in 1976, and a minimum in 1982. However, the Fd-I changes very
2 
VE FECT  ORBUSH ES IN HE lO-
H C O ION NG HE RESENT OLAR CLE 
S. P. grawal 1 , B. L. lshra l , . . ain l ,2 
Iphysics epartment (Vikram Space Physics entre) 
. . . ni ersity, ewa ( . .) 03, I ia 
ernment gin ering llege 
 . .) , n a 
 
4.l l0 
e erat   onthl    in  I   
co pare it ith the observed long-ter  changes in the cos ic 
r  i t it  r rt  t ergies)   r - . 
Significant correlation is observed between the two except 
 .   t s   n h  r l  
t t  th  l   i  I   -I. 
1. cti . haracteristics   l  term ri t   i  
r  i t sit    t i  i  th  st i  tr  onit r t  
(e.g. Lock ood and ebber, 1984). Recent studies froll) spacecraft have 
deter ined the radial gradient of cos ic ray intensity to a distance of 
up to 30 AU (e.g. Venkatesan et al., 1984; Van Allen and Randall, 
1985). urther ore s ecific cos ic ra  i t sit  changes such as 
r ush ecreases have als  been st ie  t  large eli s eric r i l 
distances ( an llen, 1979; and references t erei ). he purpose of 
this paper is to explore the role of the short-ter  cos ic ray intensity 
decreases i  generating the r file of the cos ic ray long-ter  
ri t . 
. nalysis and esults. he ercent e iati  has been calc late  f r 
each month for the interval 1976-83, using the cosmic ray intensity 
axi  f r t  onths f pt.- v.  s . e r s  r s  
index (Fd-I) is co puted fro  the hourly data of Deep River neutron 
onitor after exa ining the l ts of a nu ber of statio s as ell. he 
agnitude f        ulati e t l  l   d's 
observed in a onth; such a total is the index, Fd-I. i ilarly, the 
s l r fl r  i e  ( I) as ee  c pute  by si  ll s l r fl r s 
(i portance) 1) and by assigning a eight according to the nu erical 
i portance of the flare and the relative brightness of each solar 
flare. Such objective criteria give proper eight to the energetic 
flares, which generally produce III.1ch larger decreases in cos ic ray 
i t sit  ( kl  t l., 8). 
he l -ter  odulation has been re rte  t  be a ir ct 
consequence of large number of solar flares (e.g. atton, 1980; Agrawal, 
1983; and references therein). i ilarly, the Fd effects are also the 
consequence of energetic s lar flares ( hukla et l., 1978). e 
t eref re suggest t at the long ter  odulation is t e cu ulative effect 
of large nu ber of s all and big Fd's, the validity for hich is 
exa ined i  i ure 1. ote t at t e cos ic ray i t sit  reac es a 
axi u  in 1976, and a ini u  in 1982. However, the Fd-I changes very 
https://ntrs.nasa.gov/search.jsp?R=19850026520 2020-03-20T17:01:24+00:00Z
393
SH4.1-10
abruptly, with a very large peak in 1978, and with a conspicuous minimum
in 1980. The anomalous behavlour of low Fd activity during 1980 is
being investigated separately in a paper (SH 5.1-15). Moreover, the
peak value of Fd-I in 1978, is of considerable interest, and will be
discussed further.
Note from Fig. I that both Rz and SFI are still increasing during
1978, while Fd-I shows a very large peak in 1978. Geomagnetic
disturbance index Ap also shows moderately low values till 1981, after
which it increases, (probably associated with the dominating effect of
coronal holes).
The correlation between the long term cosmic ray variations and the
Forbush decrease index, is investigated by a cross-plot (Fig. 2) using
monthly values for the two quantities during 1976-83. Note that in
spite of significant correlation, many points lle close to X-axls.
Opposite is true for the year 1978, when most of the points lie close to
y-axls, indicating large Fd-I without any associated cosmic ray
decreases. Nevertheless, correlated changes observed during other
periods show that the long term modulation of cosmic ray intensity can
be understood in terms of the _Jmulative effect of Fd's. We believe
that the exception during 1978 of Fd-I on %-deviation of cosmic rays is
perhaps attributable to the faster recovery of cosmic rays during
1978. This needs further investigation as also why during certain
months of high solar activity no Forbush decreases are seen.
The correlation between Fd-I and percent cosmic ray deviations,
with SFI during 1976-83, is shown in Figure 3(a) and (b). Note again
that large Fd-I's are associated with small SFI during 1978. Reverse is
true for certain months in 1980. The correlation is poor in 1980 and
during the recovery of cosmic ray intensity in 1983, when large cosmic
ray intensity deviations are observed for low values of SFI. Neverthe-
less, Figure 3(a) indicates a correlated variation in the cosmic ray
intensity with the observed changes in SFI. Thus, from Fig. 3(a) and
3(b), we observe the effect of SFI on both Fd-I and cosmic ray intensity
(except for certain specific periods mentioned earlier), which also in
turn Justify the correlation shown in Fig. 2. It is therefore, reason-
able to presume that the long term cosmic ray variation is generated by
the cumulative effect of Forbush decreases, and the departure is quite
significant during 1978 and 1980, in the present sunspot cycle.
4. References.
Agrawal, S. P., (1983), Space Sci. Rev., 34, 127.
Hatton, C. J., (1980), Solar Phys.66___,159.
Lockwood, J. A., and W. R. Webber, (1984), J. Geophys. Res., 89, 17.
Shukla, A. K., J. P. Shukla, S. M. Sharma, R. L. Singh, and S. P.
Agrawal (1978), Ind. J. Radio & Space Phys.7, 179.
Van Allen, J. A., (1979), Ge0phys. Res. Lett., 6, 566.
Van Allen, J. A. and B. A. Randall, (1985), J. Geophys. Res. 90, 1399.
Venkatesan, D., R. B. Decker and S. M. Krlmigis, (1984), J. Geophys.
Re__._[s.89, 3735.
3 
l-l0 
r tly, ith a ery large eak in , and ith a co spicuous inimuM 
in . e no lous iour f low d tivity ring 980 is 
being investigated separately in a paper (SH 5.1-15). oreover, the 
peak alue of d-I in 1978, is of c siderable interest, and ill be 
scussed ther. 
Note from Fig. 1 that both R  and SFI are still increasing during 1978, hile d-I shows a very large peak in 1978. eo agnetic 
disturbance index Ap also shows oderately low values till 1981, after 
hich it in reases, (probably sso iated ith the i ating ffect f 
 l . 
The correlation between the long term cos ic ray variations and the 
Forbush decrease index, is investigated by a cross-plot (Fig. 2) using 
onthly l es f r the two antities ri g 6-83. te that in 
s ite of significant correlation, any points lie close to -axis. 
posite is true f r the ear 1978, hen ost of t e ints li  cl se to 
y-axis, indicating large Fd-I ithout any associated cos ic ray 
decreases. evertheless, c rrelated changes observed during other 
periods show that the long term odulation of cos ic ray intensity can 
 tood  erms  he cu ul ive e   ' s.  ieve 
that the exception during 1978 of Fd-I on -deviation of cos ic rays is 
perhaps attributable to the faster recovery of cos ic rays during 
. is ee s f rther i stigation as lso hy ring rtain 
ont s f igh l r tivity  r sh r ses re . 
The correlation between Fd-I and percent cos ic ray deviations, 
with SFI during 1976-83, is shown in Figure 3(a) and (b). Note again 
that large Fd-I's are associated ith s all SFI during 1978. Reverse is 
tru  f r rt in onths i  . he rr lation is r in 1980 and 
during the recovery of cos ic ray i te sity i  1983, hen large cos ic 
 e ty i ions  ved  low s  . ever
1 ess, Figure 3(a) indicates a correlated variation in the cos ic ray 
intensity ith the observed changes in SF!. Thus, from Fig. 3(a) and 
3(b), we observe the effect of SFI on both Fd-I and cosmic ray intensity 
(except for certain specific periods entioned earlier), which also in 
turn justify the correlation shown in Fig. 2. It is therefore, reason-
able to presume that the long term cosmic ray variation is generated by 
the cu ulative effect of Forbush decreases, and the departure is quite 
significant during 1978 and 1980, in the present sunspot cycle. 
. ef r  
Agrawal, S. P., (1983), Space Sci. Rev., li, 127. 
atton, C. J., (1980), Solar Phys.~ 159. 
Lockwood, J. A., and W. R. Webber, (1984), J. Geophys. Res., 89, 17. 
hukla, . ., J. P. Shukla, S. . Shar a, . L. i h,and S. P. 
Agrawal (1978), Ind. J. Radio & Space Phys.~, 179. 
Van Allen, J. A., (1979), Geophys. Res. Lett., 6, 566. 
Van Allen, J. A. and B. A. Randall, (1985), J. Cleophys. Res. 90, 1399. 
Venkatesan, D., R. B. Decker and S. M. Krimigis, (1984), J:- Geophys. 
s ~  3735. 
394
SH4.I-i0
Figure 1 Yearly means of the solar flare index, sunspot number, Ap
index, Forbush decrease index (Fd-I) and the cosmic ray
intensity deviations from its maximum value, for 1976-84.
The annual average values for 1984 are derived from limited
number of monthly values. A larKe peak in Fd-I in 1978 is
clearlyevident.
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Figure 1 Yearly means of the solar flare index, sunspot number, Ap 
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intensity deviations from its maximum value, for 1976-84. 
The annual average values for 1984 are derived from limited 
number of monthly values. A large peak in Fd-I in 1978 is 
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$ COSMIC RAY INTENSITY CHANGES 
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( x )  DEVIATION (b.R) SOLAR FLARE INDEX 
Figure 2 Cross-plot of monthly values of % deviation of cosmic ray 
tntens i ty  and Fd-I during 1976-83 with various symbols f o r  
d i f ferent  years. 
Figure 3 (a ,b)  Crossplot of monthly values of SF1 and (Fd-I)/percent 
deviat ion of cosmic rays during 1976-83. Symbols are the 
same as  used i n  Figure 2.  
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Figure 2 Cross-plot of monthly values of % deviation of cosmic ray 
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Cumulative effect of Forbush decreases in the heliospheric modulation during the present solar cycle

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