Cosmic ray intensity measured by Mariner  III AND by ground neutron monitors during Forbush decreas

Rao, U. R.

English

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STUDY OF COSMIC RAY INTENSITY MEASURED BY MARINER I1 AND BY GROUND NEUTRON 
MONITORS DURING THE FORBUSH DECREASE OF SEPTEMBER 30, 1962 
U.  R .  Rao 
Southwest Center f o r  Advanced Studies  
Dallas, Texas 75230 
P. 0. Box 30365 
A l e t t e r  
To be submitted t o  
JOURNAL OF GEOPHYSICAL RESEARCH 
ACKNOWLEDGEMENT - This research has  been supported by the  National 
Aeronautics and Space Administration undaer grant  NsG-269-62. 
GPO PRICE $ 
CFSTI PRICE(S) $ 1 
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1 
STUDY OF COSMIC R A Y  INTENSITY MEASURED BY MARINER I1 AND BY GROUND NEUTRON 
r 
MONITORS DURING THE FORBUSH DECREASE OF SEPTEMBER 30, 1962 
bY 
U. R. Rao 
Southwest Center f o r  Advanced Studies 
P. 0. Box 30365, Dal las ,  Texas 
The p l aae t a ry  probe Nariner  I1 which covered a r a d i a l  d i s t a n c e  of about 0.25 AU 
32 i t s  journey t o  the  o r b i t  of Venus contained ins t ruments  t o  measure cosmic r a y  
i n t e n s i t y  a s  w e l l  a s  a plasma probe to measure t h e  f l u x  and v e l o c i t y  of s o l a r  wind. 
T- A L , ~  cosziic r a y  i t e n s i t y  measurements were c a r r i e d  ou t  by an a rgon- f i l l ed  i n t e g r a t -  
i a g  ior. ization chamber, a halogen quenched, g l a s s  Geiger-Mueller counter  tube  with 
a macched s t a i n l e s s  steel  s h i e l d  and a second G.M. counter  with a beryl l ium sh ie ld .  
All t h e  t h r e e  cosmic r a y  instruments  had approximately t h e  same threshold  energy 
of 10 mev for protons.  The desc r ip t ion  of t h e  experiment and some of t h e  r e s u l t s  
of both t h e  cosinic r a y  measurements (Neher and Anderson, 1964; Anderson,, 1964) and 
t h e  plasma probe experiment (Snyder, Neugebauer, and Rao, 1964) have been publ ished 
elsewhere.  
The probe y ie lded  u s e f u l  d a t a  almost cont inuously from August 29 through t h e  
end of December, 1962. During t h i s  period, t h e r e  was  on ly  one d i s t i n c t  Forbush 
decrease,  which occurred on 30 September, 1962 (Day 273),  when Mariner I1 w a s  ap- 
proximately 0.1 AU from t h e  e a r t h  and o r i en ted  almost (within 50) along t h e  ea r th -  
sun l i n e .  I n  t h i s  no te  we examine the  d a t a  from va r ious  instruments  on Mariner I1 
and t h e  d a t a  from ground neut ron  monitors during t h e  above-mentioned Forbush decrease.  
F igure  1 shows t h e  time s e r i e s  of t he  hourly mean cosmic r a y  i n t e n s i t i e s  
aeasured  by Mariner I1 i o n  chamber; the hourly mean neut ron  i n t e n s i t i e s  a t  Deep 
River, E t .  Wellington, and Mawson; the  hourly mean s o l a r  wind v e l o c i t y ,  and t h e  
t h r e e  hour ly  Kp i n d i c e s  f o r  t h e  per iod 29 September (Day 272) thmugh 5 October, 
1962 (Day 278). The onse t  of t h e  Forbush decrease  occurs  a t  0830 U,T.as seen by 
t h e  <on chamber i n t e n s i t y  on Mariner 11. 
l a r g e  s t a t i s t i c a l  e r r o r s  i n  t h e  neutron monitor i n t e n s i t i e s ,  it i s  n o t - p o s s i b l e  
t o  i d e n t i f y  c l e a r l y  t h e  a n i s o t r o p i e s  and t h e  onse t  t i m e  d i f f e r e n c e s  a t  t h e  ground. 
However, t h e  c o r r e l a t i o n  between t h e  changes i n  neut ron  i n t e n s i t y  a t  ground s t a t i o n s  
Due t o  t h e  small decreases  (-3.0%) and 
I - 2- 
S L C ~ I  as Jsep R i v e r  and the changes i n  ion  chamber i n t e n s i t y  on Mariner I1 i s  very 
il..g11 (0.91 i 0.02). The r a t i o  of Forbush decrease observed by t h e  ior, chamber t o  
:ha: o j s e n e d  by  he neucron monitor, obtained from zhe r e g r e s s i o n  l i n e  i s  3.88 2 0.01. 
Sixz i\,aricer I1 i s  almosc along t h e  earth-sun l i n e  acd i s  less than 0 .1  AU from the  
esrri;,  ,h is  Yatio i s  t r u l y  r ep resen ta t ive  of t h e  r a t i o  of primary i n t e n s i t y  changes 
, - .  
LIl.3s~he:e ‘io t he  changes of  neutron i n t e n s i r y  a t  Deep River. - - tZe zk. eazzh’s =I+- - -  
Lack af o 5 s e w a t i o n  of any s p a t i a l  gradient  of i n t e n s i t y  (Anderson, 1964) during 
25.2 days 239 through 292 lends stroiig suyport  t o  t h e  abovz s t azeaen t .  
F r m  bsl loon obsezvations durin, t h e  same period (1959-1963), Nerurkar and 
Iie3ber (1’364) have S~OWLI  rilat che slope of t h e  long term and s h o r t  term v a r i a t i o n s  
o b s e i i ~ ~ d  a; bal loon altitudes mer Kir;nez?olis t o  t h e  corresponding neutron inten-  
s i t y  v a r t z z i o n s  tit Deep River is very much de?er,dent on t h e  level of observed inten-  
s l z y ;  t h e  s;o?e being l a r g e r  when t h e  level of i n t e n s i t y  a t  D e e ?  River i s  higher ,  
i n i i c s t i n g  :he presence of a d d i t i o n a l  low eae-rgy r a d i a t i o n .  
i n t e n s i t y  ar 3eep River p r e v a i l i n g  p r i o r  t o  t h e  Forbush dec res se  of  30 Septenber, 
i962, a s lope  of 3.75 i s  derived f o r  the long term v a r i a t i o n ,  u s ing  t h e  Nerurkar and 
W25Ser r e s u l r s .  
chamber i n t e n s i t y  on YarLner I1 and Deep River neutron i n t e n s i t y  during t h e  For3ush 
&crease 0, 30 September, 1962, we conclude Ehat t h e  speccrum of  v a r i a t i o n  during 
For t h e  level of neutron 
i 
CorrLparir,g t h r s  with the observed s lope  of 3.88 - .01 between ion  
t h e  A o r t  tern f l u c t u a t i o n s ,  such as Forbush decreases ,  i s  sirnila-r t o  t h e  specZrm 
of v a r i a t i o n  during long te;n changes. Anderson (1964), on t h e  o t h e r  hand, f i n d s  
t h a t  t h e  r a t i o  of v a r i z t i o n  of t he  d a i l y  average Mariner i o n  chamber rates t o  varia- 
t i o n s  of t he  dai;y average Deep River r a t e  f a l l s  between 2 and 3 d3rir.g d i f f e r e n t  
per iods of  observat ion.  It must, however, be rexembered chat  af’ier day 292, Xariner ii 
r a p l d l y  d r i f t e d  away from t h e  earch-sun l i n e ,  and away from t h e  e a r t h  toward Vems. .A 
systematic  i n t e n s i t y  g rad ien t  o f  Lpproximately 9”/,A’J has  been a s t ima tec  by Anderson 
(1964) from a ccnparison of ion chamber i n t e n s i t y  with Deep River nclutron i n t e n s i c y .  
Since t h e s e  f a c t o r s  w i l l  c l e a r l y  a f f e c t  t he  observed r a t i o s ,  t h e  r a t i o  oStained by 
- 3- 
* 
Anderson, even though less than t h a t  observed dur ing  t h e  Forbush decrease  o r  t h a t  
obtained by Nerurkar and Webbzr, is  not i n  c o n t r a d i c t i o n  with these  r e s u l t s .  
The percent  change i n  i n t e n s i t y  of a secondary component of type i, recorded 
a t  l a t i t u d e  A and he ight  h can be expressed as 00 
- wi(h,E) dE 
J E X  -'-' 100 6Nf(h) = 
b D  E 
100 L 
where E i  i s  the  v e r t i c a l  cu to f f  energy a t  l a t i t u d e  A , wft(h,E) i s  t h e  coupl ing 
cons tan t  f o r  secondary component. i defined as 
d i f f e r e n t i a l  energy spectrum of primaries  and mi(E,h) is the"mu1t ip l ic i ty  function. ' '  
D@) mf(E,h) where D(E) is t h e  
N p )  
S imi l a r ly ,  we can express  t h e  percent  change i n  i n t e n s i t y  of primary r a d i a t i o n  such 
as t h e  one measured by Mariner I1 i o n  chamber as 
00 
100 6 N  I(E) *E] D(E)dE 
P= (2 1 
NP I(E) D(E)dE 
where I ( E )  i s  t h e  i o n i z a t i o n  rate pe r  u n i t  omnid i rec t iona l  f l u x  of  p a r t i c l e s  wi th  
energy E.  [I(E) equals  u n i t y  f o r  Geiger counters .  e%in is t h e  threshold  of  t h e  
Mariner I1 inst ruments  (10 Mev f o r  protons).  
1 
Assuming t h e  spectrum of  v a r i a t i o n  
t o  be of t h e  form AE P and using t h e  coupling c o e f f i c i e n t s  f o r  sea level neutrons 
a t  50° l a t i t u d e  given by Webber and Quenby (1959) and t h e  r ecen t  IMP-1 r e s u l t s  on 
primary spectrum (McDonald and Ludwig, 1964), we have ca l cu la t ed  t h e  r a t i o  of per- 
c e n t  change of cosmic r a y  i n t e n s i t i e s  measured by Mariner I1 i o n  chamber and Geiger 
counters  t o  t h a t  observed by Deep River neut ron  monitor f o r  d i f f e r e n t  exponents of  
t h e  spectrum of v a r i a t i o n .  Figure 2 shows t h e  t h e o r e t i c a l l y  c a l c u l a t e d  r a t i o  of 
t h e  pe rcen t  change observed by Mariner I1 i o n  chamber and t h e  Geiger counters  t o  
t h e  pe rcen t  change of cosmic r a y  i n t e n s i t y  measured by Deep River neut ron  m o n i t o r  
f o r  v a r i o u s  va lues  of  t h e  exponent 8. 
. - 4- 
I n  Figure 3 i s  p l o t t e d  t h e  time s e r i e s  of t h e  6 hour ly  mean cosmic r ay  in t en -  
s i t i e s  neasilred by Hariner I1 i on  chamber, Be ,  and s teel  sh i e lded  G-M counters  
aboard ?iariner T I  and t h e  Deep River neutron i n t e n s i t y  f o r  t h e  period 29 September 
(272) through 13 October 1962 (286), 
as t h e  changes of i n t e n s i t y  i n  va r ious  instruments,  c o r r e l a t e  ve ry  w e l l  a s  seen 
The onse t  of t h e  Forbush decrease,  as w e l l  
from Figilre 3,  t h e  c o r r e l a t i o n  c o e f f i c i e n t  between any two i n t e n s i t i e s  being a t  
leas: 0.85 ? .05. However, t h e  s lopes  of  t h e  i n t e n s i t y  changes observed by t h e  
two G-X counters ,  both having similhr energy respozse t o  t h e  neutron i n t e n s i t y  
+ 
change a t  Deep River, do n o t  ag ree  very w e l l ,  t h e  s lopes  being 2.39 - .02 and 
3.05 $. .02. S i m i l a r  d i sc repanc ie s  have been noted by Anderson (1964). Hence, we 
have cunsicked t h e  mean of  t h e  two values, namely 2.72 2 .02, as t h e  proper r a t i o  
of  t h e  changes of  G-M counter  i n t e n s i t y  a t  Mariner I1 to t h e  changes of  neutron 
i n t e n s i t y  a t  Deep River. 
Using r h e  two observed ratios, one between t h e  ion chamber and Deep River 
+ neutron i n t e n s i t i e s  (3.88 - .01) and the o t h e r  between G M  counter  and Deep River 
+ neutron i n t e n s i t i e s  (2.72 - . O l ) ,  we obtain from Figure 2,two estimates f o r  0, 
namely -0.41 ? 0.01 and -0.48 
spectrum o f  v a r i a t i o n  during t h e  Forbush decrease w a s  of  t h e  fo-m AE 
.01 r e spec t ive ly .  W e  conclude t h a t  t h e  energy 
-0.45 -t .or, 
Due t o  l a c k  of good agreement between the two Geiger counters ,  we a r e ,  however, 
unable t o  determine whether t h e  i n t e n s i t y  changes during t h i s  Forbudh decrease was 
energy o r  r i g i d i t y  dependent. 
1~1 Figure 1 i s  a l s o  p l o t t e d  t h e  time series of  t h e  hour ly  mean s o l a r  win6 
v e l o c i t y  (reversed scale), measured by t h e  plasma probe i n  Mariner 11, and t h e  
t h r e e  hour ly  % i nd ices .  
plasma bulk v e l o c i t y ,  by t h e  v a l u e  f o r  the channel with t h e  l a r g e s t  measured c m -  
r e n t .  The e r r o r  a r i s i n g  from t h i s  appyoximation (Snyder, Neugebauer, and Rao) i s  
n e g l i g i b l e .  
wind v e l o c i t y  follows the  t i m e  series of i o n  chan5er cosmic r a y  i n t e n s i t y  ve ry  
c l o s e l y .  
For t h e  ana lys i s  presented he re  we have approxiinated t h e  
From t h e  f i g u r e  it i s  c l e a r  t h a t  t h e  i n v e r s e  time series of the s o l a r  
The inverse --relation between t h e  changes i n  plasma v e l o c i t y  and t h e  
- 5- 
char,ges iil ion chamber cosmic ray intensity is very high, being -0.98 2 .01. A 
cmqarlson of the plasma velocity chariges with the changes of ion intensity reveals 
char each decrease in ion chamber intensity is correlated with a corresponding 
Lacrease in plasma velocity, as if each individual burst of plasma cloud interacts 
i S 7 i t f ;  the  interplanetarv medium to produce a corresponding decrease of cosmic ray 
2- iALensitv. i 
tensity is very high. 
Similarly, .the correlztia between Kp indices and the ion chamber in- 
Even though the correlation of ion chamber intensity with 
plasma velocity, or 5, is very high during the Forbush decrease, the correlation 
is practicaliy nonexistent on a long term basis, (Snyder, Neugebauer, and Rao). 
Assming that the Forbush decrease has made a complete recovery when the intensity 
reaches wiihin 0.5 percent of the pre-evznt value, it can be seen from Figure 1 
that even though both $ and plasma velocity -recove, to their normal value in about 
two days (i.e, on day 276), the cosmic ray intensities continue to remain depressed. 
The xeutron intensities at various statlons recover on day 277. The ion chamber 
ixtensity and the G M  counter intensities on Yfriner 11, however, recover only on 
day 285 or 286 (Figure 3), showing that the recovery is energy dependent, the low 
energy cosmic rays recorded by the Mariner I1 ion chamber and G-M counters recover- 
ing last. 
In summary, we draw the following important conclusions: 
(1) The changes in ion chamber intensity measured in space during the Forbush 
decrease of 30 September 1962 are very well correlated with the changes of neutron 
intensity at Deep River and at other neutron monitor stations. The neutron inten- 
sity at the ground, however, recovers much faster than the ion chamber intensft.y, 
indicating that the intensity of low energy cosmic rays has still aot recovered 
to its pre-decrease value. 
(2) Both the short term variations, such as Forbush decreases, and the long 
term variations have a similar spectrurn of variation. 
(3) The spectrum of variation during the Forbush decrease may be represented 
-6- 
The changes i n  cosmic ray i n t e n s i t y  a r e  very w e l l  c o r r e l a t e d  withf l the changes 
i n  s o l a r  wind v e l o c i t y  during the  Forbush decrease.  The e f f e c t  i s  as though each 
ind iv idua l  b u r s t  of plasma cloud interacts with t h e  i n t e r p l a n e t a r y  medium t o  produce 
a corresponding decrease i n  the cosmic r ay  i n t e n s i t y .  
AC KNOWLED GMlZ NTS 
I am gra t e fu l  to D r s .  H.  Anderson, C .  W. Snyder, and M. Neugebauer of the  
Jet Propulsion Laboratory, Pas,adena, California,  f o r  permission t o  use the  
Mariner I1 data.  I am a l so  indebted to  D r .  K. G. McCracken f o r  he lpfu l  suggestions. 
The work reported here .was supported by the NASA Research Grant NSG269-62.  
REFERENCES 
Anderson, H.  R . ;  "Ionizing Radiation Measured Between Earth and Venus by Mariner 
11," paper presented a t  the  In te rna t iona l  Space Science Sympo;ium (COSPAR), 
Florence, 1964. (In Press) 
Fenton, A. G., K. G. McCracken, D .  C.  Rose, and B. G. Wilson; "The Onset Times of 
Forbush-type Cosmic Ray I n t e n s i t y  Decreases," Can. J. Phys; 3 7 ,  970-982, 1959.  
NcCracken, K. G.; "The Cosmic Ray Flare Effect," J. Geophys. Research, 6 7 ,  447- 
458,  1962.  
McDonald, F. B.,.and G. H. Ludwig; "Measurement of Low Energy Primary Cosmic Ray 
Protons on IMP-1 S a t e l l i t e , "  1964.  (In Press) 
Neher, H. V., and H. R. Anderson; "Cosmic Ray I n t e n s i t y  a t  Thule, Greenland, during 
1962 and 1963 and a Comparison with Data from Mariner 11," J. Geophys. Research, 
6 9 ,  807-814, 1964.  
Nerurkar, N. ,  and W. R. Webber; "Observations of Primary Cosmic Ray Variations 
Using Ion Chambers and Geiger Counters," J. Geophys. Research, 69 ,  815-830, 1964.  
Snyder, C .  W. ,  M. Neugebauer, and U. R. Rao; "The Solar  Wind Velocity and I ts  
Correlat ion with Cosmic Ray Variations and with Solar  and Geomagnetic Activity," 
J. Geophys. Research, 68 ,  6361-6370, 1963.  
CAPTIONS FOR FIGURES 
Figure 1 Time s e r i e s  showing t h e  ( a )  hourly mean cosmic r a y  
i n t e n s i t y  recorded by Mariner I1 ion chamber (b) 
hourly mean s o l a r  wind v e l o c i t y ,  (c) hourly mean 
neutron i n t e n s i t i e s  recorded a t  Deep River,  
M t .  Wellington and Mawson, and (d) t h e  3 hourly 
Kp ind ices  f o r  t he  per iod September 29 (272) 
through October 5,  1962 (Day 278). 
Figure 2 Theore t i ca l  curve showing the  r a t i o  of percent  change 
cosmic ray i n t e n s i t y  observed by Mariner I1 ion 
chamber and geiger  counters  t o  the  pe rcen t  change 
neutron i n t e n s i t y  a t  Deep River as a func t ion  of 0, 
f o r  a spectrum of the  type AE8. 
Figure 3 T i m e  s e r i e s  of hourly mean cosmic ray i n t e n s i t i e s  
measured by (a) Mariner I1 ion chamber, (b)  beryll ium 
and s t a i n l e s s  s t e e l  shielded ge ige r  coun te r s  aboard 
Mariner 11, and (c) t h e  neutron i n t e n s i t y  a t  Deep 
River f o r  t h e  period September 29 (272) through 
October 13, 1962 (286). 
' 1 '  I l l  1 1 '  I l l  276 277 2 78 272 273 274 2 75 ' 1 1  1 1 '  s I 
. . . . . . . . . . . . . . , . . . . . . . 
.... 
0 
MARINER E ION 
CHAMBER DATA - 2.0 
-4.0 
-6.0 
- 8.0 
' 10.0 
* 
I- 
0 I2 0 12 0 12 0 12 0 I2 0 I2 0 12 U.T. 
272 273 214 275 276 277 278 
DAY NUMBER 
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8.0 - 
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EXPONENT 
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272 213 214 215 276 277 270 279 280 281 282 203 284 285 286 
DAY NUMBER 


Study of cosmic ray intensity measured by mariner ii and by ground neutron monitors during the forbush decrease of september 30, 1962

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