Transient north-south anisotropies in the cosmic ray intensity

Planet.

Space Sci. 1972, Vol. 20, pp. 721 to 729.

Pcroamon

Press.

Printed

in Northern

Ireland

TRANSIENT NORTH-SOUTH ANISOTROPIES IN THE COSMIC RAY INTENSITY* J. B. MERCER, D. N. H. BARKER, W. E. GRIFFITHS and C. J. HATTON Department of Physics, University of Leeds, Leeds (Received in~~a~~5rrn 10 September 1971)

Abstract-Using data from four poIar neutron monitors, a search has been made for North-South anisotropies in the cosmic radiation during the period August 1966-November 1967 (solar rotations 1820-1837). It is found that (a) N-S anisotropies are a common feature of the cosmic radiation, occurring on >43 per cent of ail days, (b) al1 Forbush decreases are accompanied by such anisotropies in the onset phase, (c) a significant proportion of days outside Forbush decreases (-35 per cent of the days analysed) exhibit N-S anisotropies. The existence of anisotropies with large N-S components is discussed in relation to the study of anisotropies in the ecliptic plane. ~~~DUC~ON

Hitherto the majority of studies of cosmic ray anisotropies have been concerned with those whose maxima lie within the ecliptic plane and which give rise to the diurnal variation. In recent years however some observations have been reported of transient anisotropies, associated with Forbush decreases, which have displayed Iarge components perpendicular to the ecliptic plane (Kudo and Murakami (1965), Nagashima et al. (1968), Duggal and Pomerantz (1970)). It is appropriate therefore to question whether North-South (N-S) anisotropies are a feature of the cosmic ray intensity at all times-that is, not only during disturbed periods-and to determ~e their chara~teris~c features. To this end a study has been made of N-S anisotropies during the 17 month period from August 1966 to November 1967 (solar rotations 1820-1837) and the preliminary results of this study are presented in this paper. METHOD OF ANALYSIS

Although transient cosmic ray anisotropies may be arbitrarily directed in space, this paper will only be concerned with those anisotropies exhibiting large components perpendicular to the ecliptic plane. {The solar equatorial and ecliptic planes will be considered coplanar for the purpose of this paper.) Such a perpendicular component is, clearly, most effectively monitored by a pair of polar stations with viewing cones directed oppositely along the polar axes. However, because of the non-alignment of the terrestrial and ecliptic polar axes, fluctuations in the amplitude or phase of the ecliptic anisotropy will also produce an apparent N-S component. Furthermore, because in practice it is necessary to use stations having median latitudes of viewing 1x1 < 90”, the effects of the rotation of the Earth must also be considered. It is shown in the appendix, however, that according to the criteria used in this paper for selecting N-S anisotropies, these spurious effects are negligible for stations with median latitudes of viewing ll;il > 65”. In this analysis two pairs of stations, Afert (3 N +75”) and McMurdo (7f rrc -80’) and Thule (x *v _t 68”) and South Pole (A N - 65”) have been used. For each pair of oppositely directed stations the amplitude of the anisotropic component during the iti hour is defined as

*A report on this work was presented to the Cosmic Ray Conference at Hobart in September 1971. 721

722

J. B. MERCER,

D. N. H. BARKER,

W. K. GRIFFITHS

and C. J. HATTON

where Nd and Si are the hourly, pressure corrected, filtered amplitudes of the north and south viewing stations respectively. The station amplitudes are calculated with respect to their 27 day running means in order to minimise long-term instrumental drifts and uncorrected meteorological effects in the data. In addition these data are lightly smoothed by a running 3 h triangular filter. Similarly the isotropic component is defined as x.=

(N,t-Si)

I

2

(2)

*

At and Er have been computed and plotted for both pairs of stations over solar rotations 1820-1837 (August 1966-November 1967). By way of example, the results for both pairs of stations are displayed in Fig. 1 for solar rotation 1827. Several large, complex anisotropies are immediately seen in the A profile. That such anisotropies are not induced by short

I

I

I

I

,

I

5

10 DAY NUMBER

FIG.

1.

The A

15

20

25

OF ROTATION

A AND c PROFILES OF THE TWO PAIRS OF STATIONS, ibERTANDTHULESOUTHPOLE, FOR THE SOLAR ROTATION 1827. exhibit N-S anisotropies while the 2 profiles show isotropic cosmic ray

cOMFWUSON

OF THE

McMm~o

pro&s

variations. scale instrumental drifts, etc. is apparent from the excellent tracking between the two independent station pairs. The A profiles for the Alert-McMurdo pair are shown in Fig. 2 for the whole period analysed. In order to determine the frequency of occurrence of the N-S anisotropies it is necessary to establish a set of criteria for their identification. Because there exists a whole spectrum of fluctuations in the A profiles it is difficult to distinguish between small genuine anisotropies on the one hand and statistical fluctuations, instrumental drifts, etc. on the other. A set of criteria has therefore been adopted which, although conservative, unambiguously defines genuine anisotropies. The first condition imposed is that an anisotropy is not regarded as genuine unless it is present in the A profiles of both pairs of stations. Because South Pole is a mountain altitude station it responds to lower energy primaries than the other three (sea-level) stations and therefore displays larger amplitude variations during Forbush decreases etc. Therefore while the A profiles of the two pairs of stations are expected to be similar they will not be quantitatively identical. Three types of anisotropies were typical during the period studied and the following

TRANSIENT

NORTH-SOUTH

723

ANISOTROPIES

SOLAR

*OS% 45%

5

Fm,2.

10 15 DAY OF ROTATION

20

25

THE A PROFILTB FOR THB ALERT-MCMURDO PAIROF STATIONSFOR SOLAR ROTATIONS

1820-1837. The profiles contain in addition to genuine N-S anisotropies, ~n~butions from met~rolo~ effects and instrumental drifts. Genuine NS anisotropia were confirmed by comparing these profiles with those from the Thule-South Pole pair of stations.

724

J. B. MERCER,

D. N. H. BARKER,

W. K. GRIFFITHS

and C. J. HATTON

1, )

(2)

0

l&l b 0*25%, t L 2

FIG. 3. REPRE.SENTATIONOFTHETHREETYPICALCLASSESOFANISOTROPIES: (2) REVERSING, (3) PERSISTENT UNIDIRECTIONAL.

days

(~)UNIDIRECTIONAL,

criteria were used to classify them (see also Fig. 3): 1. Undirectional unisotropies: IAJ > 0.5 per cent lasting for a time t > 3 h. 2. Reversing anisotropies: IAp,, - A,,J za O-5 per cent with ADo3and Anes lasting for times t, > 3 h and t, > 3 h respectively, and with maxima separated by a time T Q 1 day. 3. Persistent unisotropies (undirectional): IA] > 0.25 per cent lasting for a time 22 days. A confirmed N-S anisotropy is one for which one of these sets of criteria has been simultaneously satisfied by both pairs of stations. The choice of these criteria was made from the following considerations of the observed ‘noise’ in the A profiles. In order to minimise the effect of real N-S anisotropies, a period amounting to nine solar rotations was chosen during which there were only small isotropic variations since it had been noted that the larger N-S anisotropies were associated with Forbush decreases. From the data of these rotations, those days which were considered to show large N-S anisotropies were removed. A histogram of the Alert-McMurdo A values from the remaining days was found to be similar to a Gaussian noise distribution having oA s O-2 per cent. A similar figure was obtained by calculating the Alert-Thule differences whose distribution should contain only noise information. It should be noted that from Poissonian statistics alone a noise level of 0.07 per cent would be expected : The higher figure is presumably due to meteorological effects and instrumental drifts in the station data. The probability that (Ai1> O-5 per cent by chance for three consecutive hours is negligible and therefore the above criterion, together with the requirement that any profile is confirmed by the second pair of stations, is considered to identify unambiguously the anisotropies of classes (1) and (2) above. Similar considerations apply to anisotropies in class (3) where due to their longer time scale the amplitude criterion can be reduced. RESULTS The A profiles obtained from the data covering the solar rotations 1820-l 837 have been examined for N-S anisotropies according to the above criteria and the results are summarised in Fig. 4. This is a Bartel’s type diagram showing the days on which confirmed anisotropies were observed. It should be noted that the shading indicates only that there was at least

TRANSIENT NORTH-SOUTH

725

ANISOTROPIES

1821 1822 1823 1824 1825 1826 1827 1828 1829

1 3 5 FE3.4.

7 9 Il 13 15 17 19 21 23 25 27 DAY OF ROTARON

A BM~L'S TYPEDIAGRAM GIVING THE DAYS ON WHICH

UNFIRED

b&-s AM~~OP~

OCCURRED.

Periods during which Forbush decreases occurred are indicated by single horizontal lines. Onset phases of these decreases are shown by the double lines. on a given day. The duration time of an anisotropy cannot be inferred from this diagram. The most significant fact emerging from Fig. 4 is that N-S anisotropies are a common feature of the cosmic radiation. Indeed 207 of the 486 days analysed (43 per cent) exhibit a N-S anisotropy. Of these more than two thirds are of the unidirectional type with equal numbers of north and south directed anisotropies, About one quarter of the events are reversing anisotropies. Because N-S anisotropies occur so frequently, there are many occasions when they are observed on consecutive days (see Fig. 2). It then becomes difficult to distinguish between a series of discrete short duration anisotropies and a single prolonged, ~ghly variable, anisotropy. Therefore it is thought justifiable to state only that isoIated, unidirectional anisotropies exist for periods - hours up to - 2 days and that complex trains of anisotropies may occur over intervals up to IO days. Similarly, it is not possible to determine u~ambiguously the distribution of amplitudes of discrete anisotropies at this stage. However, it is possible from the A profiles to determine this distribution for unidirectional anisotropies on a daily basis by defining the amplitude as the three hour average over the maximum of the A profile in a given day. The number distributions of such amplitudes for north and south directed aniso~opies are displayd in Fig. 5. Clearly the distributions have been forcibly truncated by the criterion adopted for the selection of anisotropies and the true distributions one anisotropy

726

J. B. MERCER, D. N. H. BARKER, W. K. GRIFFXTHS and C. J. HATTON

-North ---South

Directed Oirrrcied

FIG. 5, THE DISTRIIWTIoN OF AhfpLlTiJDEs(ON A DAILY BASIS) FOR THE cONFXRi+lED, CORONAL ~~~0~~s: (a) NORTHWARD A~SO~KW~DD~~~D PoR THE WHOLE PERIOD ANALYSED, (b} UNBEND mm FORBUSHDECRBASES,(C)C)CU~N~DURINO em ONSET PHASE OF Fomusi DECREASES, (d) OCCURRBio DURING TIiB RECOVeRY PHASE OF FORBUSH RECWES.

The amplitudes are taken from the Alert-McMwdo profiles to eliminate effects associated with rigidity dependence.

will contain amplitudes
As noted earlier the first reported N-§ anisotropies were associated with Forbush decreases (Kudo and Murakami, 1965; Nagashima et al., 1968; Duggal and Pomerantx, 1970)and therefore an examination of the anisotropies o~urring during Forbush decreases has been made over the 17 Mona period covered by this analysis. The general quiescence of solar cycle No. 20 has resulted in very few Forbush decreases of amplitude > 5 per cent as measured by high latitude neutron monitors. Nevertheless there exists in the period a large number of cosmic ray decreases which have the characteristic rapid onset followed by a slower recovery of the larger Forbush decreases. For the purpose of this analysis, decreases in the isotropic (E) profiles having amplitudes zb2 per cent have been defined as Forbush decreases provided that during the onset phase, a decrease of at least 2 per cent took place within 24 h. The recovery phase has been taken as complete when either a new quiescent level (not necessarily the pre-onset level) was attained or the recovery was inte~upted by a new onset, the latter occurring at least 24 h after the onset of the previous decrease. In the period examined there were 32 such decreases, the occurrences of which are shown in Fig. 4. Of these decreases, 28 were accompanied by N-S auisotropies during the onset phase. In the remaining three decreases anisotropies were observed by one of the pair of stations but not confirmed by the other pair. This suggests that as a general rule every Forbush decrease at the onset is a~ompanied by a N-S anisotropy.

TRANSIENT NORTH-SOUTH ANISOTROPIES

727

By

comparison, the recovery phases of the Forbush decreases were less anisotropic although 47 per cent of the recovery days were still accompanied by N-S anisotropies. The distribution of amplitudes of N-S anistropies during the recovery phases is compared with that for the onset days in Fig. 5. Clearly the largest observed N-S anisotropies occur during the onsets of Forbush decreases. As in the analysis of the whole period, equal numbers of the anisotropies associated with Forbush decreases were found to be directed northward and southward. Under the present method of selecting Forbush decreases, 350 days of the 486 included in this analysis are designated as being outside Forbush decreases. On 124 of these days (35 per cent), N-S anisotropies were observed indicating that they are not only associated with the major variations in the isotropic intensity changes observed at the earth. It can be seen in Fig. 5 that there is a close similarity between the amplitude distributions obtained from days outside Forbush decreases and those days during the recovery phases. The rank correlation coefficient between them is 0.8 and is significant at the 95 per cent level. This observation, together with the rather similar frequency of occurrence of anisotropies during these two periods (35 and 47 per cent) possibly suggests a common mechanism for such anisotropies. Of greater interest however is the striking dissimilarity of the amplitude distributions during the onset and recovery phases of Forbush decreases. Indeed, a rank test indicates no correlation between the distributions at the 95 per cent significance level. SUMMARY AND CONCLUSIONS

It has been shown that during the 18 solar rotations, 1820-1837 (August 1966-December 1967) N-S anisotropies were observed on > 43 per cent of the days. Equal numbers of north and south directed anisotropies were observed. While the largest of these anisotropies were found to be associated with the onsets of Forbush decreases they were also present during the recovery phases and during periods when there was little change in the isotropic intensity observed at the Earth. This clearly indicates that they are a more common feature of the cosmic ray intensity than had been previously suspected. Indeed, the above figure of 43 per cent is almost certainly a lower limit of the frequency of occurrence of N-S anisotropies resulting from the fact that in this analysis, in order to select unambiguously genuine anisotropies, conservative criteria have been adopted. It is anticipated that as an understanding of the physical processes underlying the anisotropies is acquired, a more objective set of criteria will ensue. The results of this study suggest that N-S anisotropies occur during the onset phase of all Forbush decreases. This is to be compared with the study of Duggal and Pomerantz (1970) who found that during 1964-66 approximately 1 in 4 Forbush decreases were accompanied by N-S anisotropies. However, this difference is attributable to the different selection criteria adopted as these authors were only concerned with the larger anisotropies having amplitudes > 1 per cent. The purpose of this note had been to draw attention to the common occurrence of N-S anisotropies. Despite the emphasis on these as independent entities it must be stressed that these results do not imply that such anisotropies are necessarily more than the projection onto the ecliptic poles of evolving anisotropies arbitrarily directed in space. The same considerations apply to transient anisotropies observed by stations viewing close to the ecliptic plane and determined by the ‘cosmogram’ analysis of Ables et al. (1967) or the ‘morphoplot’ analysis of Mercer ef al. (1968). With regard to these methods of analysis it

728

J. B. MERCER,

D. N. H. BARKER,

W. K. GRIFFITHS

and C. J. HATTON

should be noted that stations having significantly different median latitudes of viewing, x, will not observe the same component of the anisotropy and furthermore in the presence of a N-S component, a single ‘cos x’ correction will not be valid. Thus in the study of cosmic ray anisotropies it is considered that a three dimensional analysis of the type adopted by Nagashima et al. (1968), Mercer (1969) and Yoshida et al. (1971) is required. Such an analysis of anisotropies during the period covered by this note is currently being undertaken particularly with a view to establishing the connection with interplanetary features. Acknowfedgernents-We are grateful to those investigators who have made their data available to us either directly or through the World Data Centres. We also acknowledge with gratitude the assistance given by Miss M. E. Roden and Mrs. F. Khan of the Leeds neutron monitor group. J. B. M. wishes to thank the Leverhulme Trust for providing him with a visiting fellowship, and D. N. H. B. wishes to thank the Science Research Council for a maintenance grant. REFERENCES AXLES,J. G., BAROUCH,E. and MCCRACKEN,K. G. (1967). Planet. Space Sci. 15,547. DUGGAL, S. P. and POMERANTZ,M. A. (1970). Actaphys. hung. 29, Suppl. 2. KUDO, S. and MURAKAMI,K. (1966). Proc. Int. Conf. Cosmic Rays, London, p. 285. MCCRACKEN,K. G. RAO, U. R., FOWLER,B. C., SHEA, M. A. and SMART,D. F. (1965). IQSY Instruction Manual No. 10, London. MERCER,J. B. and WILSON,B. G. (1968). Can. J. Phys. 46, S849. MERCER,J. B. (1969) unpublished. NAGPSHIMA,K., DUGGAL, S. P. and POMERANTZ,M. A. (1968). Planet. Space Sci. 16,29. YOSHJDA,S., AKA~~PU, S.-I., OGITA, N. and Oun, A. (1971). J. geophys. Res. 76,1. APPENDIX Contributions To N-S Anisotropies From Anisotropies In The Ecl@tic Plane An anisotropy whose direction of maximum lies within the ecliptic plane will be observed terrestrial frame of reference as

from a

SJ(#, A)/J, = A{cos A [cos (4 + ,$ cos (p + r) + sin (4 + x) sin (@ + r) cos el + sin A sin @ + F) sin .s} -N A{cos E cos A cos (# - I’) + sin E sin A sin (p + F)}

(A.1)

where SJ(+, A)/JO is the fractional change in the primary differential spectrum observed in the direction (4, A), A is the amplitude of the anisotropy (possibly rigidity dependent), and the directional parameters A, +, x, /?, F and E are defined in Fig. 6. The term containing + is responsible for the diurnal variation observed at a latitude A and, recalling the definition of the N-S anisotropy (Equation l), will produce a diurnal contribution to A. An upper limit to the magnitude of this contribution may be readily determined by treating each of the stations as if it had an infinitesimally narrow acceptance cone and replacing # and A in Equation (A.1) by the median directions of viewing 4 and x, An anisotropy with an amplitude A N 0.5 per cent in free space is then found to produce a diurnal component in the A profiles for the Alert-McMurdo pair <0.06 per cent and for the Thule-S. Pole pair, <0.09 per cent. A more exact calculation using the variational coefficients of McCracken et al. (1965) which allows for the finite size of the acceptance cones reduces these values to 404 per cent and -0.06 per cent respectively. Therefore, even in the presence of enhanced diurnal amplitudes, no significant contribution will be made to the A profiles. There are also contributions to A‘ due to fluctuations in the ecliptic anisotropy about its 27-day running mean, the major contribution coming from the second term in Equation (A.l). The amplitude of one of the stations-a north polar station for example-can be approximated by 2

zJ~

[(F)

-

(E)]

W(R) dR

where W(R) is the coupling coefficient, and (SJ/JO) and (g/Jo) are the hourly and 27-day mean values respectively of the second term in Equation (A.l).

TRANSIENT NORTH-SOUTH

Ecliptic Pole

ANISOTROPIES

729

Eqwtoriat

line

Fm. 6. DWCTIC~NM, PARAMETERS

RELATTNQ AN ANWTROPYIN DIRECXION OF VIEWINQ OF A NEUTRON

THE ECLIP~C MONITOR.

PLANE

TO THE

The angles I’ and /3 are measured in the ecliptic plane @, x and A are measured with respect to the equatorial plane. Letting sin n z 1 and - 1 for north and south viewing stations respectively, the contribution these fluctuations is given by A; c+ sin a[& sin (/3 + I’,) - KCsin @I +- I’*)]

to A, from 64.3)

where & = jR A,W(R) dR and the barred quantity is a 27-day mean. Using values of K, and I’, obtained from the amplitude and phase of the diurnal variation observed by a station looking into the equatorial plane it is found that generally Ai ‘v Pl per cent. In particular, the Ah;were examined during days on which N-S anisotropies were observed and it was found that no spurious contribution >O.l per cent could have occurred. Values of K6 and I’* obtained in this manner are not appropriate for days containing Forbush decreases. However, with respect to the very dramatic N-S anisotropies that occur in association with Forbush decreases, it should be noted that circumstances in which fluctuations of the ecliptic anisotropy tend to suppress the observed A+are as likely as those which enhance it. It is unlikely, therefore, that the conclusions regarding frequency of observations of the N-S anisotropies would be altered by more approp~ate estimates of A:.