LETTERE AL NUOVO CIMENTO VOL. 33, -~. 1 2 Genna io 1982

Cosmic Rays From Jupiter.

B. ~'~ITRA, S. K. BOS~: a n d S. R. GA~.-(~ULy

D e p a r t m e n t o f P h y s i c s , J a d a v p u r U n i v e r s i t y - Ca lcu t t a -700 032, I n d i a

( r icevuto il 27 Agos to 1981)

I t was s u g g e s t e d b y PIZZELLA a n d 3*rENDITTI (1) a n d PIZZELLA (2) t h a t J u p i t e r m a y be a source of cosmic r ays o b s e r v e d a t t h e E a r t h . A c c o r d i n g to t h i s sugges t ion , pro- tons are acce le ra ted to h i g h energ ies in J u p i t e r ' s m a g n e t o s p h e r e w h i c h t h e n l e ak out a long i n t e r p l a n e t a r y magne t ic - f i e ld l ines e v e n t u a l l y r e a c h i n g t h e E a r t h , p r o v i d e d i t is a p p r o p i a t e l y l oca t ed on t h e s ame field l ine. I)IZZELLA (2) e x a m i n e d n e u t r o n mon- i to r d a t a for t h e yea r s 1961-65 for seve ra l s t a t i o n s w i t h h i g h cut-off energies . He f o u n d ev idence t h a t t h e cosmic - ray flux is g r e a t e r for a c e r t a i n E a r t h l o n g i t u d e in a r e fe rence sys tem w i t h cen t r e a t t h e Sun a n d co - r o t a t i ng w i t h J u p i t e r .

T h e e jec t ion of h i g h - e n e r g y e lec t rons a n d also p r o t o n s b y J u p i t e r ' s m a g n e t o s p h e r e in to t he i n t e r p l a n e t a r y space h a s been o b s e r v e d (2). Quie t t i m e inc reases of h i g h - e n e r g y e lec t rons h a v e been r e c o r d e d b y I M P a n d the se inc reases show a 13 m o n t h pe r iod i c i t y , i nd i ca t i ng t h e i r J o v i a n o r ig in fa l l on t h e p r e d i c t e d t i m e of cross ing of J u p i t e r ' s mag- net ic-f ie ld l ine b y t he E a r t h (a).

SwL~s0.~" (s) p o i n t e d ou t t h a t if cosmic r ays do r e a c h t h e E a r t h f r o m J u p i t e r , t h e pe r iod w h e n E a r t h a n d J u p i t e r a re close to t he same field l ine shou ld show an e n h a n c e d so lar d i u r n a l v a r i a t i o n w i t h t h e p h a s e of m a x i m u m occur ing s o m e w h a t l a t e r as com- p a r e d to t h e o t h e r pe r iods . SwI~'SO_~ a n a l y s e d t en yea r s {1965-1975) of d a t a f r o m E m b u d o u n d e r g r o u n d rouen te lescope to e x a m i n e t h i s effect , a s s u m i n g a v a l u e of t h e E a r t h - S u n - J u p i t e r angle of 125 ~ acco rd ing to t h e r e su l t s of P izzc l la , w h e n t h e two p l a n e t s are on t h e s ame i n t e r p l a n e t a r y - m a g n e t i c - f i e l d l ine. Hi s ana lys i s of t he d a t a d id no t show dif ference in d i u r n a l v a r i a t i o n of t h e e x p e c t e d n a t u r e , w h e n c e he conc luded t h a t the effect of J u p i t e r on cosmic r a d i a t i o n a t t he E a r t h is m in ima l .

T h e p r e s e n t work is a n a t t e m p t to t h r o w some l i g h t on t h i s c o n t r o v e r s i a l i ssue of w h e t h e r a n y p a r t of t h e h i g h - e n e r g y cosmic rays r ece ived a t t h e E a r t h o r ig ina t e a t Jup i t e r .

(1) G. PIZZELLA and. G. V'E.~DITTI." Proceedings o/ the X I I I International Con/erence on Cosmic Rays, Vol. 2 (1973), p. 1129. (9) G. PIZZELLA: The Magnetosphere o/ the Earlh and Japiler, edited by V. FOR~drlSANO (Dordrecht, 1975), p. 425. (s) J .A . SIMPSON and R. B. MCKIBBE~r: J~piter, edited by T, GEHREL~ (Tucson, Ariz., 1976), p. 738. (4) B.J . TEEGARDEN', F. I3. McDoI~'ALD, J. H. TRAIt'OIl, ")V. R. WEBBER and E. C. I~OELOF : J. Geophys. Res., 79, 3615 (1974). (~) ]). B. SwI.'cSO.~: Proceedings of the X I V Internationa2 ~on/erence on Cosmic Rays, Vol. 4 (1975), p. 1486.

10 B. MITRA, 8. 1s BOS:E and s. R. GANGULY

We have analysed cosmic-ray intensity data of the multi-directional meson telescope operated by Nagoya University at Nagoya, Japan (35 ~ 09' N, 136 ~ 58' E, vertical cut- off rigidity 11.5 GV) for the years January 1972- March 1976, this being a period of decreasing solar activity similar to that examined by PIZZ~LLA, but in the succeeding solar cycle. We did not include the remaining months of 1976 as there were changes in the base level of the telescope twice in the later months owing to the adjustments in the instruments. We also omitted a few days with unusual disturbances including Forbush decreases. (The idea behind selecting a high-cut-off station is that the high- energy particles would be less disturbed by magnetic irregularities in the interplane- tary space.) The data used by us are hourly values of the meson intensi ty corrected for barometer effect. Being meson intensities, these are subject to the influence of the atmos- pheric-temperature effect, the correction for which is uncertain, due to the nonavailabil i ty of proper atmospheric-temperature data. We have eliminated this atnmspherie effect by taking the second-order difference (G-component) between the data of pairs of com- ponents (e).

(1) G = (30 ~ ~ + (30 ~ ~

If cosmic rays are reaching Ear th from Jupiter when both are on the same mag- netic-field line, then it is expected that the cosmic-ray intensi ty recorded on Ear th would show a 13 month periodicity (the synodic period of Jupiter). The monthly averages of intensi ty of the G-component are calculated for the period under study. A mean straight line is drawn for these data by the least-square fit method and the differences (A2') of monthly averages from this fitted line are subjected to Chree analysis. The resulting curve is sho~-n in fig. 1. I t is seen that a definite 13 month periodicity is exhibited by the cosmic-ray data. This shows that cosmic rays may be reaching Earth from Jupiter under favourable Earth-Jupiter positions.

0.30 13 26

o

0 x o o �9

o I I 1

--0"300" 10 20 30 40 PI

Fig. 1. - C h r e e analys is of m o n t h l y a v e r a g e cosmic - r ay in tens i ty . A F ~ difference f r o m m e a n , ~* ~ m o n t h s .

We have next examined daily averages of the cosmic-ray intensi ty as a function of the relative positions of Ear th and Jupiter. ~[n order to correct for the solar-cycle rood-

(6) K . NAGASHIMA, • . FUJIMOTO, Z. FUJI , H . L'I~NO a n d I . HONDO: l~ep. Ionos. ,Space Res. Jp•, 26, 31 (1972).

COSMIC RAYS FROM J U P I T E R l l

ula t ion, we, fol lowing the m e t h o d of P izze l la (2), define the q u a n t i t y

I ( t ) - - Fi t (t) F(t) -- -- ,

F i t (t)

where l ( t ) is the dai ly average va lue of the in tens i ty of the G-component ca lcula ted f rom relat ion (1), t being t i m e in days and P i t (t) is the l inear leas t -square fit of the da i ly average. A re la t ion be tween the quan t i t y ~'(t) and the posi t ion of E a r t h and J u p i t e r in relat ion to Sun, is now sought for. To do this , we have ca lcula ted the angle 0s$ = = 0 s - 0~, where Or. and 0j are the be l iographic longi tudes of the E a r t h and Jup i t e r , respect ively, for each day. Thus 0EZ gives the E a r t h longi tude in a reference sys tem with centre in the Sun and co-ro ta t ing wi th Jup i t e r . We now plot _~'(t) ave raged over an angle i n t e rva l of A0 = -4- 40 ~ as a funct ion of 0F.Z, the resul t ing curve being shown in fig. 2. ( I t is found tha t there is no effect of the par t i cu la r angle in t e rva l (A0) on tho nature of the curve excep t ing some smooth ing for a larger in te rva l . )

0.I

0 I I

S " - - 0 . 1 I I ! I I I 0 50 I00 150 200 250 300 360

8 E J (d.egrees)

Fig . 2. - V a r i a t i o n of i n t e n s i t y w i t h E a r t h - J u p i t e r p o s i t i o n s . ( F o r e x p l a n a t i o n s of F ( t ) a n d eE$ see t e x t . ) �9 A 0 = • ~ , o A 0 = 4- 40 ~ .

From the curve we m a y perhaps , say tha t there is an increase in t im cosmic-ray inten- sity rece ived on E a r t h when 0sa is be tween 150 ~ and 290 ~ the peak in tens i ty be ing at about 0E j - 220 ~ �9

According to Pa rke r ' s spiral-f ield mode l of in te rp lane ta ry magnet ic field, the va lue of 0~j when the E a r t h and J u p i t e r are on the same tield line is 220 ~ if the ve loc i ty of the solar wind is 445 kin/s, which is near ly the average solar-wind ve loc i ty dur ing the per iod under inves t iga t ion (as measured by ,NASA Ames Research Centre and r epor t ed in Solar Geophysica l Data) . Our resul t there fore seems to indica te (ha t a pa r t of cosmic rays or iginates on J u p i t e r and reaches the E a r t h along the in t e rp lane ta ry -mag- netic-field l ine which is g iven by Pa rke r ' s model .

The curve ob ta ined by PIZZEI.LA (~) shows a b road peak be tween 0Ea = (60 : 180) ~ with the m a x i m u m at about 0r:a = 130 ~ This is in te rp re ted by h im as due to J o v i a n cosmic rays reaching the E a r t h along the magnet ic-f ie ld l ine which connects the E a r t h wi th J u p i t e r when OF.Z = 130 ~ Besides Pa rke r ' s spiral field, the in t e rp l ane ta ry field beyond the E a r t h m a y be affected by o ther processes (like shock f ronts etc.) as is

12 r . MITRA~ S. K. BOSE and s. R. GANGULY

po in t ed out by ])IZZELLA. T h i s migh t perhaps be responsible for the different va lues of 0rz at m a x i m u m in the two different per iods s tud ied by PlZZELLA and by us.

I f J u p i t e r is a source of cosmic rays, t he ampl i t ude of solar d iurna l va r i a t ion may also be inf luenced by the r e l a t ive posi t ions of E a r t h and Jupi te r �9 W e have calcu- l a t ed the average ampl i tude of solar d iurnal va r i a t ion of the G-component (averaged over A0 -- • 24 ~ ) as a func t ion of 0Ej and the resul t ing curve is shown in fig. 3. I t is seen t h a t the d iurna l ampl i tude shows a defini te increase dur ing the in t e rva l 0 ~ = (160 --260) ~

0.15

"~- 0.10

.__.

F

c 050 =r 100 150 200 ~O 300 360 �9 . ~ 2~. OE% (d-e,3rces)

F i g . 3. - V a r i a t i o n of t h e a m p l i t u d e of d i u r n a l w a v e w i t h E a r t h - J u p i t e r p o s i t i o n s .

To examine the effect more closely, we selected the per iod for which 0s~ lies be tween 170 ~ and 270 ~ and also an equal pe r iod 180 ~ apart . The former includes 0r= = 220 ~ for which we ob ta ined a m a x i m u m in the _,v(t) vs. O~ curve in fig. 2 and which m a y be t aken to be the average E a r t h - S u n - J u p i t e r angle when F~arth and J u p i t e r are on the same mag~mtic-field line. Th is pe r iod m a y therefore be considered most favourah le for cosmic rays f rom J u p i t e r causing enhanced d iurnal var ia t ion , and the o ther per iod most unfavourable . We made a ha rmonic analysis of t he hour ly va lues of the G-component and ca lcu la ted the average ampl i tude and phase of the m a x i m u m of d iurna l wave re fe r red to the local t ime for bo th the favourab lc (F) and unfavourab le (U) periods. T h e y are p lo t t ed in a ha rmon ic dial in fig. 4. I t is seen tha t the ampl i tude for the favourab le per iod (0.072%) is la rger t han those for the unfavourab le per iod (0.056%). Th is suppor ts the idea tha t cosmic rays arc reaching the E a r t h f rom Jupi te r �9

0 h

0.i~

18h

U

I I I I 6h O .'I ~

'I2h(LT)

F i g . 4. - S o l a r d i u r n a l v e c t o r s f o r t h e f a v o u r a b l e (F ) a n d t h e u n f a v o u r a b l e ( U ) p e r i o d .

COSMIC RAYS FRO]~I J U P I T E R 13

The present work may be said to lend support to the view that Jupi ter is a som'ce of high-energy cosmic rays which travel along the magnetic-field line and are received by the Earth when it is on this field line. During the period under study, this happens when the angle between Sun-Earth and Sun-Jupiter lines is about 220 ~ which cor- responds to the field in accordance with Parker 's model.

The authors express their sincere thanks to the Director of the Cosmic-Ray Research Laboratory, Nagoya University, Nagoya, Japan for sending the Multidirectional Meson Telescope data. They also wish to thank the Director of the Positional Astronomy Centre, Calcutta for some useful discussions and SR* M. D~B RoY, Reader of the department of Physics, Jadavpur University for valuable help throughout the progress of the work. The work was carried out under a research project sponsored and funded by the Indian Spac6 Research Organization, Government of India, Department of Space. One of us (BM) is thankful to ISRO for granting a Research Fellowship.