Indian Journal of Radio & Space PhysicsVol. 27, October 1998, pp. 236-240

Effect of solar poloidal magnetic field reversal on diurnal anisotropy ofcosmic ray intensity on quiet days

Santosh Kumar', S K Shrivastava', S K Dubey', M K Richharia' & Umesh Gulati''Department of Post Graduate Studies and Research in Physics and Electronics

Rani Durgavati University, Jabalpur 4820012Department of Physics, Govt. Auto. Science College, Jabalpur 482001

3Departmentof Physics, Govt. Tilak P G College, Katni 483 501

Received 27 Apri11998; revised received 5 August 1998

Using the Deep River neutron monitor (NM) data for the period 1964-95 covering three solar cycles, namely,20,21 and 22, the long term trend and the effect of reversal of polarity of the solar magnetic field (PSMF) on thediurnal anisotropy of cosmic ray (CR) intensity on quiet days (QDs) have been investigated. The phase of thediurnal anisotropy on QD has started regaining its usual corotational direction after 1976. aquiring it in 1985 andremains statistically invariant till 1991. The phase of the diurnal anisotropy on QD has shown a significant shift toearly hours when the PSMF in the northern hemisphere (NH) is positive during the periods 1971-79 and 1992-95 ascompared to that during the periods 1964-70 and 1980-89 when the PSMF in the NH is negative, confirming againthe earlier results of 22-yr periodicity.

1 IntroductionGalactic cosmic rays when enter the heliosphere,

are SUbjected to solar modulation. The magneticfield frozen in the radially expanding solar windconvects the charged particles outward. Thesecharged particles undergo adiabatic decelerationdue to the expansion of the solar wind plasma. ·Theenergy loss of cosmic rays and the reduction ofintensity are very closely connected. The solardaily vector (diurnal, semi-diurnal and furtherhigher harmonics) is the most interesting parameteramong the various components of the cosmic ray(CR) anisotropy and it has also the longest andmost extensive history.

A 22-yr solar magnetic cycle has been suggestedto be connected with the diurnal anisotropy incosmic ray intensity' in early 1950s. Later,Forbush'', from the analysis of data for more thanthree decades, concluded that the diurnalanisotropy contains two components - one calledW, having its maximum in a direction 1280 east ofsun-earth line and the other called V having itsmaximum in the asymptotic direction (900 east ofsun-earth line). Quenby and Hashim3 have alsoarrived at the conclusion similar to that ofForbush", but with different reasoning that the CRmay enter the heliosphere in solar polar regionsand be convected out by the solar wind in the

equatorial plane (neutral sheet). They3 haveinferred that this scenario would produce a radialflow and also set up a symmetric gradient withintensity rising in helio-altitudes.

The existence of 22-yr variation of CR intensityrelated to solar magnetic field cycle is confirmedby Ahluwalia"; thus, the existence of 20-yr wave inthe diurnal anisotropy of CR intensity has. beendisputed by Ahluwalia'. It is shown that during theperiod 1957-70, the diurnal anisotropy isunidirectional with direction along 1800 hrs LT;whereas during 1971-79, the diurnal anisotropyconsists of two components - one in east-westdirection and the other radial component in 1200hrs LT direction. However, no existence of 20-yrwave in the diurnal anisotropy of CR intensity isobserved.

Ballif et al.6 correlated the sums Kp and Ap withthe mean fluctuations in amplitude ofinterplanetary magnetic field (IMF) which, in turn,is related to diffusion component of convection-diffusion theory. The Ap is also found to be relatedwith solar wind velocity, which is related toconvective component of convection-diffusiontheory. Days with low values of Kp and Ap sums arealso called geomagnetic ally quiet .days (QDs).Different workers 7-9 have. examined the neutronmonitor (NM) data on geomagnetically quietest

SANTOSH KUMAR et a/: SOLAK. MAGN. FIELD REVERSAL EFFECf ON COSMIC RAY ANISOTROPY 237

days (Qes,Ds) in a year. Further, long term variationof CR diurnal anisotropy has been examined usingneutron and muon monitor data for all days in ayearlO•l1• It is plausible that ehe diurnal anisotropyof one modulation component is related to 22-yrsolar magnetic cycle and the other to ll-yr solaractivity cycle'".

The observed phase shift with PSMF reversalduring 1971 is interpreted theoretically byinvoking drift effect12-14• According to drift theory,during 1960s and 1980s, when the direction of IMFis inward above the heliospheric current sheet, CRsenter and diffuse predominantly in inner helio-sphere mainly through the ecliptic plane. Thisleads to an azimuthal diurnal variation, and theconvection-diffusion model provides the satis-factory explanation for the observed anisotropyduring this epoch. During the 1950s and 1970swhen the direction of IMF is outward above theheliospheric current sheet, these particles enter theinner heliosphere mainly from the polar region.The net inflow of CR balances the net outflow inthe ecliptic plane and this results in a relativeincrease in the radial component, which leads to ashift in the phase to earlier hours.

The present work first justifies the study on 60quietest days (Qes,Ds) as compared to 120 QDs andall days in a year by performing comparison duringthe period 1985-95. Using the Deep River NM dataon QestDs, the effect of reversal of PSMF on thediurnal anisotropy of CR intensity on Qes,Ds hasbeen looked into.

2 Experimental data and analysisThe pressure corrected data for the Deep River

NM (cutoff rigidity: 1.02 GV; lat. 46.1°' N, long.282.5°E; alt. 145m) have been used for the solarcycles 20-22; i.e., 1964-95 period. The long termeffect has been taken care of by applying trendcorrection". Data obtained in this way have beensubjected to Fourier analysis for obtaining theamplitude and phase of diurnal anisotropy in C~intensity. Based on values of sum Kp and Ap, 60Qes,Ds and 120 QDs (i.e., choosing first five quietdays and first ten quiet days in a month,respectively) are selected in a year. Annual data forthe minimum period of solar activity during 21stsolar cycle (1985-87) and declining period of 22ndsolar cycle (1992-95) are investigated on a day-to-day basis on 60 Qes,Ds and 120 QDs and all days in

a year. Finally, the study of diurnal anisotropy for60 Qes,Ds in a year has been performed with thedata for the period 1964-1995. The days withextra-ordinary amplitude, if any, have not beenconsidered. Furthermore, the variation in thediurnal anisotropy with the reversal of PSMF hasalso been investigated.

3 Results and discussionThe amplitude and phase of the diurnal

anisotropy of CR intensity are obtained on a day-to-day basis for 60 Qes,Ds, 120 QDs and all days ina year. Values of amplitude (%) and phase (hrs) atground are grouped for different intervals andpercentage of occurrence of days for amplitude andphase in different intervals for all the three sets ofdays during the year 1986 have been shown in Fig.1[(a)-(c)]. It is observed that maximum percentageof occurrence of days lies in same interval, forthese three sets in a year, for amplitude and phaseseparately. However, the peak becomes wider asone moves from 60 QestDs to 120 QDs and itwidens further in case of all days. Similarly,histograms have been plotted for the differentepoches of one solar cycle during the period 1985-95 and likewise results have been observed for the

Ieo QutO 6OOntO

8 I I~ (a);: .~.>

238 INDIAN J RADIO & SPACE PHYSICS, OCTOBER 1998

transinon periods and maximum as well asminimum activity periods of solar cycle.

The annual average values of amplitude andphase, as calculated for these three sets of days, areplotted in Fig. 2 and are observed to be almostequal within the statistical errors for all the threecases in respective years. These observed resultslead to the inference that for studying the annuallong or short term behaviour of the diurnalanisotropy in CR intensity, QestDs are better suitedas compared to 120 QDs or all days in a year. Thisis due to the reason that the interplanetaryconditions are quieter and more stable during thesedays.

Thus, the diurnal anisotropy of CR intensity hasbeen investigated on QestDs during the period1964-95 for the Deep River NM station. Theamplitude and phase of the diurnal anisotropyalong with average values of Ap indices on QestDsand PSMF in the northern and southernhemisphere of the sun have been plotted in Fig. 3.It is observed that the diurnal phase remainsconstant during the period from 1964-70 and startsshifting towards early hours in the year 1971 andcontinues till 1976, except the year 1974. Thus,during 1976, which is the year of minimum solaractivity, the shift in phase towards early hours ismaximum (= 3 h). Later to 1976, the diurnal phaseon QestDS gradually regains its usual corotationaldirection and it increases gradually till 1985-87

o 00 ClJJElESTDAa 120 ~T DAYS

I- 0 ~ ~ ....--' ;;!'" • Q ~~Q'.c

UJ @S

SANTOSH KUMAR et al: SOLAR MAGN. FIELD REVERSAL EFFECf ON COSMIC RAY ANISOTROPY 239

DEE P RI V ERNEUTRONMONITOR

lIhrs--r.;......-+~"':"':II ••

12hrs

Fig. 4 - Vector addition diagram for the diurnal anisotropy of CR intensity on Q",Ds at ground in LT for Deep River NMstation on harmonic dial for the period 1964-95

be looked into so as to fmd some more appropriateexplanation for it.

The values of diurnal anisotropy vectors areplotted on a harmonic dial for the period 1964-95in Fig. 4. It also shows that the diurnal vectors areconstantly in the same direction = 16 hrs LT atground during the period 1964-70 and then showsa wave like nature. The vectors are almost in 12hrs direction during 1975-76 and in 1995 andpossibly onwards, till the minimum level of solaractivity is reached, which is in aF,eement with thefinding of Bieber and Evenson' for Newark NMstation.

On the basis of PSMF, the entire period of studyhas been divided in four groups, i.e., 1964-70 and1981-90, when the. PSMF in the northernhemisphere (NH) is negative and 1972-79 and1992-95 when the PSMF in the NH is positive. Theaverage diurnal anisotropy vectors have beenplotted on a harmonic dial for these four groups ofobservations in Fig.· 5. The average diurnalanisotropy vector for the entire period of study

DEEP RIVER

18hrs

12hrs

Fig. 5 - Diurnal anisotropy vectors on Qe&lDsat ground in LTfor the negative polarity periods 1964-70 and 1981-90 and forthe positive polarity periods 1972-79 and 1992-95 of the solarmagnetic field in the northern hemisphere and also the averagevector for the period 1964-95 on harmonic dial

240 INDIAN J RADIO & SPACE PHYSICS, OCTOBER 1998

(1964-95) has also been shown. It has beenobserved that during the period of negative PSMF,the diurnal anisotropy vectors have shifted to laterhours, whereas during the periods of positivePSMF, the diurnal anisotropy vectors have shiftedto early hours with respect to the average diurnalanisotropy vector for the entire period ofinvestigation, showing an oscillatory nature of it.This is further supported by earlier findings duringthe period of positive PSMF, i.e., 1953-54, withthe shift in the diurnal phase towards earlierhours 10. Thus, it may be concluded from thesefindings that the phase of the diurnal anisotropyhas a periodic nature with respect to PSMF.

4 ConclusionsFollowing conclusions may be made from the

present investigation:

(i) From the analysis performed on all days,120 quiet days and 60 quietest days, it isconcluded that QestDsare more suitable forthe anisotropic studies on long/short termbasis.

(ii) The phase of the diurnal anisotropy onQestDswhich is shifted maximum (::0:: 3 h )during 1976 has again shown a shift toearly hours during .1995.

(iii) With the present findings it has beenfurther established that the diurnalanisotropy, in particular, the diurnal phaseof CR intensity has a periodic nature withrespect to the polarity of solar magneticfield which is applicable for all the levelsof solar activity.

•

Finally, it is quite likely that the shift towardsearly hours in the diurnal anisotropy during 1995continues further till the minimum level of solaractivity is reached.

AcknowledgementsThe authors are indebted to vanous

experimental groups, in particular, to Profs. MBercovitch, K Nagashima and Miss Aoi Inoue, forproviding the neutron monitor data.

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