the solar longitude dependence of proton event delay time

T H E S O L A R L O N G I T U D E D E P E N D E N C E O F

P R O T O N E V E N T D E L A Y T I M E

E. BAROUCH*, M. GROS, and P. MASSE* Service d'Electronique Physique, CEN Saclay, 91-Gif-Sur- Yvette, France

(Received 3 March, 1971)

Abstract. The relationship between heliographic longitude and the delay between flare occurrence and solar proton observation is studied using results obtained aboard HEOS A1 during 1969. The result obtained differs from previous findings. We ascribe this to the formation of a long-lived magnetic field configuration close to the Sun associated with a particular group of active regions.

1. Introduction

In this paper, we study several low-energy solar proton events observed in 1969. In most cases, we were able to associate an optically observed flare with the proton event. We have focussed our attention on the relationship between the longitude of the observed flare and the delay-time between optical observation and the initial arrival of the solar protons.

The time-intensity profile of solar proton events has been the object of many investigations. Several recent review articles describe the status of this question in detail (Anderson, 1969; McCracken and Rao, 1970). A great variety of such profiles has been observed and described: Figure 1 shows two examples observed by us aboard HEOS A1 and by neutron monitors at sea level. It is reasonable to expect that the spread in arrival times of the observed particles in a single event is mainly due to the propagation process. (A contribution due to a finite injection time of the solar particles cannot of course be excluded, but this aspect will not be discussed in the present paper). For instance, if the propagation is diffusive, particles arriving with a long delay are those which have been scattered many times along the way, and so on. Thus we see that the particles first detected are presumably those which have least interacted with the interplanetary medium, except if the propagation process has very unusual characteristics. Studying the initial protons gives us the opportunity to examine the less complicated part of the propagation process.

Early studies of the solar cosmic ray flare effect have provided evidence for the belief that the onset time is delayed further for east side flares than for west side flares. These early studies (McCracken and Palmeira, 1960) were based on neutron monitor data, which can detect statistically significant variations with a precision of about 5 min. Some feature of the flare is taken as corresponding to the proton injection time - initial optical observation, time of maximum brightness, onset of type IV radio emission, etc. The delay time is then defined as the difference between the assumed

* CNRS.

Solar Physics 19 (1971) 483-493. All Rights Reserved Copyright �9 1971 by D. Reidel Publishing Company, Dordrecht-Holland

484 E . B A R O U C H E T A L .

HEOS R1 oS72~

k..

-..,.

�9 .'...

7"

i i i i i i i i i i I i I

FFV '21 - HRB 7, 1969

I R Y

1~08 RI oS72o

1 - : ' ~ " ~ , ." .

. . . . .

1 . - . "

] - . . .

, 2 2 - DEC, I 1969

Fig. 1. The solar proton events of February 25th, 1969 and November 24th, 1969 as observed by HEOS A 1 in the following energy windows: 17- -6 .7 M e V < E < 2 . 4 MeV; 1 4 = 2 4 M e V < E < < 6 3 MeV; 1 0 = 6 3 M e V < E < 2 0 0 MeV; 0 6 = 2 0 0 M e V < E < 1500 MeV, and the Deep River

neutron monitor.

injection time and the observed counting rate enhancement time. It proved difficult to show that a definite relationship invariably associated the observed solar longitude o f the flare with a certain delay. Very few flares eject protons o f sufficient energy to cause a neutron monitor increase, and Yoshida and Akasofu (1965) in a careful study attempted to include PCA event observations in a similar way. A relationship fitting all events could not be ascertained, and Yoshida and Akasofu point out that this is partly due to the energy dependence o f the propagation process. PCA events are complex processes, and the energy of the particles causing these events cannot be determined with precision.

2. HEOS-A1 Observations in 1969

Satellite observations o f solar proton events permit the study o f these phenomena to be carried out at comparatively low energies in well-defined energy windows. The

THE SOLAR LONGITUDE DEPENDENCE OF PROTON EVENT DELAY TIME 485

Saclay detector aboard HEOS A1 (Figure 2) consists of a telescope of four lithium drifted silicon diodes and two gold absorbers in a coincidence arrangement, guarded by an anticoincidence shield (Barouch et al., 1969). Pulses caused by charged particles traversing the first diode are analysed into five energy loss channels before being shaped and processed by the coincidence logic. This arrangement permits a study of solar proton events in definite energy intervals. The satellite was launched on December 5th 1968 and a number of solar proton events has been detected since then. As the satellite orbi~ is highly eccentric, almost continuous data coverage can be achieved, thus

J1-J2 ~b =16 9,965 g/cm 2 " AU.

e=1.3

J3 ~ = 2 0 e =1-6 5,37g/cm 2 AU.

J4 ~b =26.5 e=2-2

H E O S A1 S 72 D E T E C T O R

Fig. 2. The Saclay proton spectrometer aboard HEOS A I.

TA

BL

E I

Ass

oci

atio

n o

f o

bse

rved

pro

ton

en

han

cem

ents

in t

he

6.7-

25 M

eV e

ner

gy

ran

ge

wit

h o

pti

cal

sola

r fl

ares

.

Opt

ical

fla

re

Rad

io-e

mis

sio

n

X-r

ays

Pro

ton

eve

nt

Dat

e T

ime

Imp

or-

M

acM

ath

C

oo

rdin

ates

S

tart

ing

T

ime

of

Ty

pe

Sta

rtin

g

Tim

e o

f O

nse

t ti

me

Del

ay t

ime

tan

ce

pla

ge

tim

e en

d

tim

e m

ax

-4- m

in

min

24.1

.69

0706

3N

98

79

N2

0 W

09

07

00

0850

est

IV

est

07

09

0730

08

54 •

15

10

8 24

.2.6

9 23

05

2B

9946

N

12

W31

23

08

2430

IV

23

06

2322

00

30 4

- 30

90

25

.2.6

9 09

00

2B

9946

N

12

W3

7

0904

11

30

IV

0856

09

18

930

• l0

30

27

.2.6

9 13

48

2B

9946

N

13 W

65

1405

14

58

IV

1356

14

12

15

00

4-?

72

12

.3.6

9 17

39

2B

99

66

N

12

W8

0

1731

17

48

II,

IV

1735

17

50

1900

4- 0

6 81

21

.3.6

9 01

38

2B

99

94

N

18 E

l5

0115

02

10 e

st

IV e

st

0133

02

08

0536

4-

30

210

30.3

.69

0100

es

tim

ated

(9

994)

W

100

0247

04

00 e

st

IV e

st

0246

03

02

0340

•

30

160

10.4

.69

0410

10

035

Nll

E90

03

52

0420

IV

est

II

0356

04

04

0530

(11.

4.69

) 15

16

2.5.

69

1745

1B

/2B

10

057

N08

W4

0

1748

18

34

IV

1746

17

58

1915

4-4

90

28

.5.6

9 12

41

2B

1010

9 N

10

W59

17

55

1800

est

IV

? V

12

54

1301

14

15 4

- 10

10

4 29

.5.6

9 00

20

lB

1010

9 N

12

W6

4

0018

00

25

V,

III

G

0019

00

27

146

• 6

86

29.5

.69

1939

1B

10

109

N1

0 W

76

19

38

1943

II

I G

G

1938

19

46

2114

4-

6 95

7.

6.69

09

45

2N

1013

4 N

12

E34

09

53

0959

IV

10

07

1845

•

10

540

25.9

.69

0658

3N

10

326

N1

4 W

13

0630

07

40 e

st

IV e

st

0642

est

07

20

0809

•

71

27.9

.69

0350

3B

10

333

N1

0 E

03

0359

04

15

IV

0330

est

04

30

09

00

4-

120

310

1028

02

.11.

69

1102

1N

10

385

N13

W9

0

1000

10

37

IVes

t 09

39

1050

11

59

90

07.1

1.69

03

24

2N

? I0

40

6?

N13

El0

? 08

30 •

30

30

6 24

.11.

69

0914

2B

10

432

N17

W31

09

13

1050

IV

est

09

15

0929

10

00 4

- 30

46

18

.12.

69

0745

1N

10

477

N15

E28

07

50

1105

II

I G

07

32

0800

16

22 4

- 6

517


permitting a fairly accurate determination of the proton onset time. The nominal

subframe duration of the telemetered data is 384 s. We have determined the onset

time of the solar proton event for a number of events in 1969. Using data obtained by astronomical means published in Solar Geophysical Data,

we have attempted to associate our observed proton enhancements in the 6.7-25 MeV energy range (which corresponds to one of our energy loss channels) with optical solar flares. Despite the very large number of optical flares observed, we think we have correctly identified the flares responsible for our proton observations, by requiring major radio and X-ray activity to be present for the flare to be considered a proton flare. Table I presents our results.

So as to clarify our understanding of the solar situation giving rise to the phenomena observed, we have plotted the Carrington coordinates of all flares occurring in the

northern hemisphere from January 1 1968 to March 30 1970 against time (Figure 3). As one can see in 1969 flare occurence followed a remarkably regular pattern. Almost

all the observed flares occur in two or three well-defined regions with a fairly small spread in longitude, over a period of several months. The most important proton flares of 1969 are associated with the region around Carrington longitude 80 ~ This fact has been used to improve our estimation of the longitude of the March 30th, April 10th and November 2nd flares, which occurred close to the limb.

The attribution of the March 30th event requires a little elucidation. Vernov et al. (1970) ascribe this proton increase to a flare 10~ 54~ Venkatarangan et al. (1970) on the other hand, claim that this event is due to an east-limb flare. We disagree with these statements and ascribe the event to an invisible flare at approximately 100~ ~

To support our assertion, we submit the following arguments: (1) The rise time of the March 30th event is much shorter than the rise-times of

the April l l th and June 7th solar proton events. These are both clearly eastern hemisphere events. On the other hand, the rise time of the February 25th, May 29th events are quite comparable with that of March 30th, and these events are definitely western hemisphere events.

(2) The events of March 21st, March 30th and April 1 l th are associated with Forbush decreases observed by the world-wide network of neutron monitor stations. In the case of the March 21st and April l l th events, no increase in the neutron monitor rate previous to the Forbush decrease (indicative of the arrival of high energy solar protons at Earth) is observed. On the contrary the increase on March 30th is so marked that the characteristic sharp Forbush leading edge is obliterated by the decay phase of the observed event. Similarly, satellite data obtained aboard the space probes Pioneer 8 and 9 (kindly communicated by K. G. McCracken) indicate that the solar particles are observed first by the space probe west of the Earth-Sun line, and that the proton onset is approximately 80 min before the onset observed by HEOS. Thus the type IV burst at 0247, which we have included in the table for the sake of completeness, is more indicative of the strong activity around this time than repre- sentative of the actual proton injection time, in this particular instance.

488 E . B A R + O U C H B T A L .

1 9 6 8

1 9 6 9

1 9 7 0

CARRINGTON LONGITUDE

0 ~ 1 2 0 ~ 2 4 0 ~ 3 6 0 ~ I I I I I I I I I I

. - - l b . " ": "

~ ,

t " . %

r +" ,k" q "~',

�9 4,q~ j p 1" ~

b � 9 1 4 9

"at -

~ � 9 1 7 6

. �9 -~ . Q. q % ~

�9 t � 9 b 'tlL q,

�9 . ".P:I. " "

~ , i ,

~ ' , ~ ' .~..~ , ..

...pp.:.;:,:+, �9 ~ . . . . ~

:~." ~ :

�9 + ,+

"+ ' ; i." .+ �9 ..

~

,+m. "~-'i

I l l ' t ' �9 �9

~+* i

- . . + t , ,J:

I " q.

. - T

.Mr ", r t ,~M ""

, �9 t

Fig . 3. C a r r i n g t o n l o n g i t u d e s o f n o r t h e r n h e m i s p h e r e f lares J a n u a r y I, 1 9 6 8 - M a r c h 30, 1970. A b s c i s s a - C a r r i n g t o n l on g i t u de in degrees . O r d i n a t e - t ime.

T H E S O L A R L O N G I T U D E D E P E N D E N C E OF P R O T O N E V E N T D E L A Y TIME 489

The situation described does not appear to be a common feature o f solar activity. Dodson and Hedeman (1967) have studied the succession of active regions on the sun over several years in the current solar cycle, and found a completely different picture, with no especially favoured zones of activity. We can thus expect peculiar phenomena to be evident due to the special configuration of solar activity.

Figure 4 shows the plot of observed delay times versus solar longitude. The sequence of points is remarkably regular, quite different from the results obtained by Yoshida and Akasofu (Figure 5).

,< 1000_

700 _ o

"6 30c _

>~ 2 o [

o ~ 10( _ o.

5 0 _

~ 3 0 _

go ~

E a s t

7.5 �9 �9 1812

711 ~

21.3

I 2"/,9

& 2t, .1 "[ �9 28.5

,t25.9

~.n

t ? 5.2

�9 e12.3 �9 29.5 29.5

A27.2

3o;,~

I I I I t I I I 70 ~ 50' 30 ~ 10" 0 ~ 10 ~ 3D" 50 ~ 70 ~ 90 ~

H e l i o g r a p h i c I o n g i?udr Wc..s t

Fig. 4. The proton onset delay time plotted against solar longitude for the main proton flares of 1969. Triangles represent flares from the active region around Carrington longitude 80 ~ Circles represent flares from the other active region, around Carrington longitude 240 ~ The flare of November

7th 1969, whose identification is doubtful, is represented by a square.

We have determined the delay time for the first particles in an 6-25 MeV proton window for each well-resolved event. The onset time depends on the initial energy spectrum and on the intensity of the event. For monoenergetic 25 MeV particles, the Sun-Earth transit time would be about 40 min, whereas for 6 MeV particles, it would be 80 min. Thus, for low intensity and soft spectrum events (for instance, when our 26-63 MeV window shows no enhancement) the observed delay may be increased because o f the preponderance of slower particles. For western hemisphere events, the delay is short, and the delay time will be increased noticeably over the expected value for such 'slow-protons' events.

490

Fig. 5.

Z

iz

Ld n,- < .._.1 lz.

r r Ld I,-- LL <

I - Z I i i > Ld

Z O I-- O r r (3..

rY < ._1 O (/)

h O W ~E I - >.- < -_1 Ld (:3

1 2 0 0

6 0 0

3 0 0

120

60

30

E.BAROUCH ETAL.

10

X x X

X

X

X

X X X

XX

X

X X

x x x x x

x x x

x x

x x x X x x

x x x x

x x x

x x x x x x

�9 x X x x

�9 X>~ X X X X X

x O x I o

o;

E A S T

�9 S O L A R C.R

x P C A

I I I I I

9 0 ~ 6 0 ~ 3 0 ~ 0 ~ 3 0 ~ 6 0 * 9 0 ~

H E L t O G R A P H I C L O N G I T U D E WEST

Aftel Yoshida and Akasofu (1965). The proton onset delay time plotted against the solar longitude for a large sample of PCA and ground level events.

3. Discussion

We must at tempt to explain (a) the regularity of our curve (b) the position of the minimum The flare-active regions shown in Figure 3 are probably associated with long-lived,

intense magnetic configurations on the Sun. We can assume that the three-dimensional large scale magnetic structures above these active regions also are somewhat perma- nent. For every proton event occurring within one of these regions, the particles will be guided in the same manner by the magnetic field to free interplanetary space, as in Burlaga's model of anisotropic diffusion (Burlaga, 1967), or to a diffusive layer close to the Sun as in Reid's model (Reid, 1964).

With these hypotheses, the delay will depend only on the distance from the point of injection to the Ear th-Sun line of force, and not on the injection process. Since we find a smooth delay time-longitude relationship, this means that the foot of the field line is relatively motionless with respect to the Earth-Sun line. The minimum


of the curve gives us its location, which ought to correspond to the 65 ~ solar longitude expected from the Parker theory. We observe in fact the minimum to occur for approximately at 45 ~

We can therefore assume that the large scale magnetic field configuration over the active regions distorts the particles trajectories to an appreciable degree, 20 ~ eastwards.

Figure 6 shows a possible magnetic field configuration achieving such a result.

l Guided protons

Fig. 6. An hypothetical magnetic field configuration in a plane parallel to the equatorial plane of the Sun.

The magnetic field configuration at a given time in a plane parallel to the equatorial plane of the Sun cannot as yet be observed. Nonetheless, eclipse observations of the corona may perhaps aid us in forming a picture of the possible magnetic field patterns. Figure 7 shows such a picture, which we might interpret as showing at least grossly the actual magnetic field configuration close to the Sun. There seems to be no flagrant contradiction between the observed pattern and the pattern we have assumed (Schatten, 1968).

Why then has such a regular pattern not been observed previously? On the one hand, the transient character of the active regions may generally preclude the establishment of a well-defined, stable magnetic field configuration guiding the fast particles generated in a flare to an injection point; on the other hand, even if such a configuration is achieved, the apparent point of injection will not be invariably in the same angular relationship to the flare point - for one region it might be west of the flare, for another above the flare, and for yet another east of the flare. For 1969, if our hypothesis is not invalid, we were fortunate in that almost all the proton flares came from one unique region.

492 E.BAROUCH ET AL.

It must be pointed out that our interpretation depends on the stability of the interplanetary magnetic field and of the solar wind velocity. Indeed, the point on the Sun connected with the Earth depends strongly on the tightness of the spiral loops, which in turn depends inversely upon the solar wind velocity.

Fig. 7.

I /v

:.i!i. '

- - s

S

I After Schatten (1968). Drawing of the solar corona during the

September 22nd 1968 solar eclipse.

We had hoped that the study of the delay longitude relationship would help us to understand the transverse propagation of solar particles between Sun and Earth. The conclusions to which we have been led set the further difficulty of first necessitating a theory of the magnetic field configurations above solar active regions and understanding the propagation of energetic particles in these large scale magnetic features.

Acknowledgements

We are pleased to acknowledge the interest and encouragement of Dr J. Labeyrie and Dr L. Koch in the planning and development of the Saclay HEOS experiment. Useful suggestions and discussions with Dr J. Engelmann, Professors K. G. McCracken and N. F. Ness are acknowledged with gratitude. An invaluable contribution to this work has been the access to the solar flare data carefully and painstakingly compiled by World Data Center No. 4 headed by Dr R. Servajean at Meudon.


References

Anderson, K. A. : 1969, XI Int. Conf. Cosmic Rays, Budapest. Barouch, E., Engelmann, J. J., Gros, M., Koch, L., and Masse, P. : 1969, X1 Int. Conf. Cosmic Rays,

Budapest. Burlaga, L. F.: 1967, Y. Geophys. Res. 72, 4449. Dodson, H. W. and Hedeman, E. R.: 1967, Solar Phys. 7, 278. McCracken, K. G. and Palmeira, R. A. R.: 1960, J. Geophys. Res. 65, 2673. McCracken, K. G. and Rao, U. R.: 1970, Space Sci. Rev. 11, 155o Reid, G. C.: 1964, J. Geophys. Res. 69, 2659. Schatten, K. H.: 1968, Nature 220, 1211. Venkatarangan, P., Venkatesan, P., and Van Allen, J. A. : 1970, Acta Physica Acad. Sci. Hungarian

29, 52, 409. Vernov, S. N., Chudakov, A. E., Vakulov, P. V., Gorchakov, E. V., Kontor, N. N., Logachev, Yu. I.,

Lyubimov, C. P., Pereslegina, N. V., and Timofeev, G. A. : 1970, in V. Manno and D. E. Page (eds.), 'Intercorrelated Satellite Observations Related to Solar Events', Reidel, Dordrecht, p. 53.

Yoshida, S. and Akasofu, S. I. : 1965, Planetary Space Sci. 13, 435.

the solar longitude dependence of proton event delay time

Documents