Cosmic Ray Density Gradient & Its Dependence on the North-South Asymmetry in Solar Activity

Indian Journal of Radio & Space Physics, Vol. 14, December 1985, pp. 151-156

Cosmic Ray Density Gradient & Its Dependence on the North-South Asymmetry in Solar A~tivity

BADRUDDIN, R S Y ADA V & N R Y ADA V

Department of Physics, Aligarh Muslim University, Aligarh 202001 Received 16 April 1984; revised received 25 July 1985

A systematic and detailed analysis of the diurnal anisotropy on geomagnetically quiet days has been performed using the cosmic ray inttnsity data for the period 1964-1979 for neutron monitors at Deep River, Leeds, Rome and Tokyo. The days have been separated according to the polarity of the IMF on that day. Appreciable difference in the amplitude and phase is found for towards and away polarity days, particularly, during the years when the solar activity is high and the north-south asymmetry is also quite large. These results on geomagnetically quiet days show some better relationship with the expected results, especially, the behaviour of the time of maximum; however, these did not show any good relationship with the expected result when all days in a year are considered. The results of the study indicate that the north-south asymmetry in solar activity is influential in determining the latitude gradient of cosmic ray flux.

1 Introduction amplitudes and phases of the away and towards

The existence of perpendicular density gradient of polarity days was found by Kananen et a/.3 and cosmic rays has been studied by separating the cosmic Swinson and Kananen4 in 1969-71 at neutron monitor ray diurnal vectors into groups corresponding to the energies.

direction of interplanetary magnetic field (IMF). In the In order to explain the observed semi-diurnal case of southward gradient and if the field is away from anisotropy Subramanian and Sarabhai 5 and Lietti and the sun, the resultant diurnal variation should have a Quenby6 postulated the existence of a cosmic ray larger amplitude and the time of maximum should density gradient perpendicular to the ecliptic plane.

occur earlier, whereas, if the field is towards the sun the The proposed gradient required a minimum density of resultant diurnal variation should have a smaller cosmic rays in the ecliptic plane and rising amplitude and the time of maximum should occur symmetrically both above and below the plane. Later, later. Swinson1 and Hashim and Bercovitch2 have Subramanian 7 obtained results indicating an increase applied this method for the years 1967 and 1968. Their of cosmic ray density with distance below the ecliptic results are consistent with the southward gradient plane and decreasing above it during the period 1962- during these years. The same method has been further 65. Using the ground based observations, Pathak and applied, using the 'neutron and meson monitor data for Agrawal8 have recently shown that the symmetrical the period 1965-75, for examining t,he nature of the cosmic ray gradients above and below the solar perpendicular gradient before and after the reversal of equatorial plane is not observed in the latitude range the sun's magnetic pole3.4 in 1969-71. Kananen et a/.3 :1:7.250 from equator, 'during the period 1973-75..and

Swinson and Kananen4 found that the amplitude The north-south asymmetries (in the solar activity)

\ of the diurnal anisotropy on away days exceed those of may ha ve implications in the structure and evolution of towards days during the year 1965-68 and that the the heliosphere, its current sheet and the cosmic ray reverse is generally the case after the reversal of sun's propagation 9. The possibility of the deflection or polar magnetic field in 1969-71. These data point to a deformation of the current sheet near the sun by cosmic ray gradient, perpendicular to the ecliptic processes occurring on the solar surface was speculated plane, pointing southwards prior to 1969, and by Thomas and Smith 1°. The bending of the current changing to northward pointing gradient after the sheet arises as a result of north-south asymmetry in reversal of sun's polar magnetic field in 1969-71 except solar activity and the resulting predominance of one in 1974. However, the phase does not show the effect particular sector polarity arising from this asymmetry expected from the amplitude behaviour. The difference can lead to seasonal variation in cosmic ray diurnal in phase between the data for the two sets of IMF anisotropy 1 1,12. Erdos and Kota13 have suggested that polarities is much less marked than in the case of the difference in inward and outward daily vectors of amplitude3,4. No significant difference in the cosmic ray anisotropy may originate in possible north- 151

INDIAN J RADIO & SPACE PHYS, VOL 14, DECEMBER 1985

south asymmetry of the heliosphere. polarity to obtain the average for different years from Earlier Agrawal et al.14 found that on a day-to-day 1964 to 1979.

basis the coherence for the cosmic ray anisotropy is The averagt;' daily vectors are studied by' better for days of low to average solar wind speed, appropriately grouping these years according to the which also is related to geomagnetically quiet days. level of the solar activity, its north-south asymmetry on Therefore, it would be more meaningful to use only the solar surface and magnetic field and plasma these days for determining the annual averages of the characteristics of the heliosphere. The frequency cosmic ray daily variation, particularly, for sunspot

maximum activity period, during which large number A-Amplitude(%) P-Phase ( rs) of cases of time varying isotropic changes in cosmic ray 0.5

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intensity are observed. In this paper, we have analyzed A. "', ' ' the neutron monitor data for the period 1964-79 for 0.2 ' DEEP

four stations, by selecting sixty most quiet days in a 19

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year and determined the amplitude and time of P " , J' , ...

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separately. On the basis of these results the 15

perpendicular density gradients in relation to north- O'6~ M ~).. ..

south asymmetry in solar activity are discussed. Some A ", ," "e', ,8 ~.~

indirect information about the three-dimensional '--' ,-

structure of the heliosphere especially the current sheet, 0.2

that may be inferred from these results, are outlined. 22

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The hourly pressure corrected data of neutron ..., "'f monitors at four stations, namely, Deep River (cut-off 16

rigidity R,,-1.02GV), Leeds (R,,-2.20GV), Rome(R" -6.32 GV) and Tokyo (R,,-11.61 GV), have been 0.5

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a year. The location and threshold rigidity of the 0,1t

neutr.on monitor stations ~re pre.se~ted in Tabl~ 1. The 20

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amplitude and phase of dally VarIatIon are obtamed for', I

each individual day after correcting the hourly data for P "'~, ~ ~

long term trend by subtracting the 24-hr moving 5 '...

average. The values of amplitude and phase of the 1 diurnal anisotropy at the minitor, thus obtained, are 0."

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transformed in interplanetary space by applying A '. --~, ,,' corrections for geomagnetic bendingls.16. The diurnal 0.1

cosmic ray variation vectors have been separated into 19

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groups corresponding to towards and away IMF P \ ~ .-,

polarities using IMF data given by Svalgaard 17 and Sheeley and Harvey 18. The daily vectors are 15 ',.,' " ..

subsequently averaged for each year and each IMF

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Station Geographic location Threshold Period of YEAR rigidity analysis

Lat. Long. GV Fig. I-Observed yearly average solar diurnal amplitudes and deg deg phases on quiet days for away polarity (solid line) and towards

Deep River 46.ION 77.50W 1.02 1964-79 polarity days (broken line) calculated from Deep River, Leeds, Rome 'j- Leeds 53.80N 1.55W 2.20 1965-74 and Tokyo neutron monitor data [The number of major solar flares

Rome 41.91N 12.52E 6.32 1967- 75 in northern (crosses) and southern (circles) hemispheres in respective Tokyo 35.75N 139.72E 11.61 1970- 78 years are also shown. The :t 10' standard error for amplitudes of

)16 individual years comes between -0.015 and -0.045 ~".] t

152 t

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BADRUDDIN et al.: COSMIC RAY DENSITY GRADIENT & N-S ASYMMETRY IN SOLAR ACTIVITY

distribution of amplitudes and phases for these days representation of the diurnal amplitude and phase on has also been discussed for each year from 1964 to 1979 day-to-day basis for these days is given in Fig. 3, which by using Deep River super neutron monitor data. shows the frequency distribution of diurnal amplitude

and phase in different years.

3 Results and Discussion Since the- vector representation only shows the Fig. 1 shows the yearly average amplitude and the average of all the individual values, it is the frequency time of maximum for each year, on geomagnetically plot which shows the peaks of the individual values quiet days, during away and towards polarity days. that lie in the particular interval. The frequency The neutron monitor data from four neutron diagram also shows whether the average values monitoring stations, well distributed in latitude and are of a shallow distribution or sharp peaked longitude, are used. Fig. 1 shows that during 1964-68 distribution. The difference in amplitudes and phases the amplitude on away polarity days is, in general, of away and towards polarity days is also seen from higher than on towards polarity days, and the phase on frequency distribution plot (Fig. 3) similar to those towards polarity days is higher than the away polarity shown in Figs 1 and 2.

days. The differences in the amplitude and phase on Kananen et a/.3 and Swinson and Kananen4 did not away and towards polarity days are more pronounced find any significant difference in the amplitudes and during the higher solar activity periods (1967 and phases of away and towards polarity days during 1969- 1968). These results for 1964-68 are consistent with 71. Their results imply no ";:ensity gradient (pointing those suggested by Swinson 1, Hashim and Be- northwards or southwards) during this period of rcovitch2, Kananen et a/. 3 and Swinson and Kananen4. reversal of sun's polar magnetic field. From 1966 to However, the results of Kananen et a/.3 for the time of 1970, the solar activity was hig11 and the north-south maximum do not show the right type of effect as asymmetry was also quite large and positive expected. The reason advanced by Kananen et a/.3 for throughout this period19.2O. In cosmic ray modulation unexpected results for the time of maximum of diurnal studies the epoch consideration in relation to the solar anisotropy is that this might be due to separation of magnetic field polarity reversal is somewhat variable days into only two groups which allows the direction of especially during 1969-71, the period of polarity the IMF to fluctuate in wide limits within them. Using reversal of solar polar magnetic field. The cosmic ray the data for three neutron monitoring stations, we variations, depending on the polarity of the IMF show found that the results for the time of maximum on somewhat different characteristics among different geomagnetically quiet days are also, generally, in periods, say between 1964-68 and 1969-79 (Refs 13,21, agreement with those expected from the amplitude 22), 1964-69 and 1970-79 (Refs 23-25), 1964-70 and.

behaviour during 1964-68, especially in 1967 and 1968. 1971- 79 (Ref. 26), 1964-70 and 1972-79 (Ref. 27). These

\ To give a better representation, we have plotted the results point towards the transition in cosmic ray average vectors for the period 1964-68, separately, for behaviour in one or the other year during 1969-71.

towards and away polarity days (Fig. 2). A microscopic Thus there might be a significant difference in

1964-68 1969 1970 1971-7) 1974 1975-76 1977 1978-79

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Fig. 2-A verage solar diurnal vectors on quiet days for IMF away from (dots. solid line) and towards (crosses, broken line) the sun for years indicated

153

INDIAN J RADIO & SPACE PHYS, VOL 14, DECEMBER 1985

amplitude and phase on towards and away polarity days during the years 1964-68, changed to higher value days, at least, in 1969 and 1970. Since the north-south for towards polarity days than away polarity days in asymmetry during the year 1969 and 1970 is of the 1969 though at Rome similar change has been- same sign as during 1967 and 1968 (years of high solar observed in 1970. However, as far as the phase is activity), our results may also provide some indication concerned, it has changed to later hours for towards towards selecting the transition year for cosmic ray polarity days than the away polarity days in 1969 at all studies as regards the polarity of the IMF. Fig. 1 shows the three stations, i.e. Deep River, Leeds and Rome. In that the amplitude at Deep River and Leeds, which was 1970, the amplitude was much higher for towards higher for away polarity days than for towards polarity polarity days than away polarity days and the time of

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Fig. 3-Amplitude and phase distribution of the diurnal vector on quiet days for each year from 1964 to 1979 calculated from Deep River neutron monitor data, for away (solid line) and towards (broken line) polarity days 154

BADRUDDIN et al.: COSMIC RAY DENSITY GRADIENT & N-S' ASYMMETRY IN SOLAR ACTIVITY

maximum was quite significantly towards later hours modulation process responsible for the sharp fall in for away polarity days than towards polarity days. cosmic ray intensity in 1974 and then its recovery by , These results are also shown in the vector diagram of the end of the year might have also disturbed the i Fig. 2. In many earlier studies13.2l.22, it has been prevailing perpendicular gradient, particularly at

assumed that the IMF in northern and southern lower rigidities. Ahluwalia and Riker29 have found a hemispheres has reverse configurations during 1964-68 transient shift in the diurnal time of maximum to later and 1969-70. The north-south asymmetry in solar hours. To explain this they suggested that the abrupt activity during 1969-70 was of the same sign (positive) rearrangement of the solar field in 1974 is not very as it was during 1967-68. Thus if the current sheet was conducive to the coronal transport of off-ecliptic displaced downwards due to positive north-south cosmic rays; it nearly chokes off the field aligned flow of asymmetry during 1967-68, it will be the same case the particles, thereby permitting the diurnal time of during 1969-70 too. But since the IMF polarity may be maximum to shift to later hours. It is to be mentioned different in northern and southern hemispheres during' here that the north-south asymmetry in the solar 1967-68 and 1969-70, the reverse behaviour in diurnal activity was somewhat negative as evidenced from amplitude and phase on away and towards polarity solar flares and, moreover, during 1973-75 the very days during the same periods may be also due to this intense solar wind streams were prominently observed.

reason. However, in 1971, when the solar activity was In the solar activity minimum period of 1975 and quite low as compared to that in 1969 and 1970 (the 1976, there is no appreciable difference in the north-south asymmetry was also small in this year), no amplitude and phase values for towards and away significant difference in the amplitude and time of polarity days. Fig. 1 shows these values yearwise, while maximum for towards and away polarity days is the average vector is plotted in Fig. 2 and the frequency observed at all the four stations. Only the amplitude at distribution for the amplitude and time of maximum in

Deep River shows some difference for towards and Fig. 3. The solar activity was quite low during these away days. Similar difference at Deep River was also years and also the north-south asymmetry was very reported by Swinson and and Kananen4 also. small. However, in 1977, there is some evidence for a

During the period 1971-73 there was some difference northward pointing gradient as seen from the in the amplitude of towards and away polarity days amplitude at Deep River and the amplitude and phase only at Deep River ne:utron monitoring station. Fig. 2 at Tokyo for towards and away polarity days. The solar shows the average vectors for the period 1971-73 for all activity in 1977 was not high and the north-south the four stations. During this period there was no asymmetry was also small. However, there was an marked north-south asymmetry as evidenced from accelerated increase in the solar activity after mid-, solar flares. Hundhausen et al.28 have reported the' 1977.

[ correlation between cosmic ray intensity and the size of In .the present solar cycle, the north polar field has the polar coronal holes during the period 1965-76. reversed in mid-1980 and the south polar field in mid- They attributed the solar cycle modulation of cosmic 1981 (Ref. 30). Prior to this polarity reversal, the solar rays to the three-dimensional global characteristics of activity in 1978 and 1979 was very high. However, the the IMF. Ahluwalia and Riker29 have suggested that north-south asymmetry was small. We see from the after 1971 .the electromagnetic condition in solar results for amplitude and phase on away and towards corona made it easier for off-ecliptic cosmic rays to be polarity days that there is no appreciable difference in transported from high heliolatitudes to low amplitude and phase for the two type of days both at heliolatitude locations in the solar corona. A rapid Deep River and Tokyo.

~' rebirth of polar coronal holes, after their absence in From the above results we see that there is an

~ 1969 and 1970 was observed in late 1970 (in southern appreciable difference in the amplitude and/or phase hemisphere) and in early 1971 (in northern hemisphere) on towards and away polarity days during the years of and a slower average growth was observed28 during high solar activity and large north-south asymmetry.

descending phase of the solar cycle. Swinson and During the years of lower solar activity and/or small Kananen4 did not find a consistent northward gradient north-south asymmetry this differences is not at the meson monitor energies during 1971-73, though appreciable. These results indicate that the north- their results point to a northward gradient in 1972 and south asymmetry in solar activity is influential in 1973 for Deep River and Oulu neutron monitors. determining the latitude gradient of cosmic ray flux.

An interesting behaviour is observed in 1974 when These results support the view that, at least during the the amplitude of away polarity days is higher than the period of high solar activity and appreciable north- l towards polarity days. Similar result was also found by south asymmetry, either the density gradient pointing Swinson and Kananen4. The argument advanced for away from the solar equatorial plane is not symmetric this result by Swinson and Kananen4 is that the or the current sheet might have been displaced from the 155

INDIAN J RADIO & SPACE PHYS, VOL 14, DECEMBER 1985

ecliptic plane. Thus it seems right to suggest3i that in 13 Erods G & Kota J, A.~troph}'s & Space Ph}'s (Netherlands), 67 order to detect any B x V N panisotropy, the position of (1980) 45.

the neutral sheet (on average) would have to be 14 Agrawal S P,.Mishra B L, Pathak S P, Yadav R S, Kumar S &

displaced from the ecliptic plane due to asymmetric Badr~ddm, Proceedings o! the seventeenth international

..cosmic ray conference, PariS, France, Vol. 4, 1981, 119.

actIvity on the sun. 15 McCrackenKG,Rao U R,Fowler BC, Shea MA & Smart D F,

IQS Y mannual No. 10, 1965.

Acknowledgement 16 Shea M A, Smart D F, McCracken K G & Rao U R, Supplement,

Th th f I .. I IQSY mannual No. 10, 1968.

e au ors are grate u to various experlmenta 17 S I_{va gaar,} d L SUIPR R_eport N_o. 648_,nstltute I .,," _{oJ PIasma} groups (from research instItutIons In Canada, England, Research, Stanford University, California, USA, 1976.

Italy and Japan) who supplied the neutron monitor 18 SheeleyNR(Jr)&HarveyJW,SoIPh}'s(Netherlands),70(1981) data. Two of them (B and NRY) are thankful to CSIR, 237.

New Delhi and University Grants Commission, New '\/f9 Yadav R S,Badruddin & Kumar S,lndianJ Radio & Space Phys,

D Ih. . I .. ^fi

.

e I, respectIve y, lor Inancla assIstance. v~ "' 0 B d dda ru m, ' Y da av R S & Ya av d N R Ind,Ian ' J Rad.10 " & Space '

Phys, 12 (1983) 124.

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8845. ray conference, Bangalore, India, Vol. 10, 1983, 55. ~

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