Adv. Space Res. Vol. 4, No. 8, pp. 125—128, 1984 0273—1177/84 $0.00 + .50Printed in Groat Britain. All rights reserved. Copyright © COSPAR

NORTH-SOUTH ASYMMETRIES OFTHE SOLAR MAGNETIC FIELD INCONJUNCTIONIWITH THE LOCATIONOF THE HELIOSPHERIC CURRENTSHEET ON BOTH SIDES OF THESOLAR EQUATOR

Vasilis P. Tritakis

ResearchCenterfor Astronomyand AppliedMathematics,Academyof Athens, Anagnostopoulou14, str., GR-106 73Athens, Greece

ABSTRACT

Reported heliospheric current—sheet displacements from the equatorial planehave been found to be in acireement with north—south asymmetries of the solarmagnetic field. Mean heliospheric sector width estimations in the period1947—1977 have shown that the heliospheric current sheet demonstrates anasymmetric placement with respect to the solar equator. This asymmetry isvery prominent in the epochs of the solar cycle minima while it almostdisappears in the epochs of maxima. At the same time, the sums of the maximavalues of the sunspot magnetic field intensity showed in the epochs of mi-nimum a characteristic asymmetry which implies an essential conjul-ictionamong the heliospheric current sheet, the solar cycle and the solar macineticfield. The main conclusion which could be derived of these observations isthat the heliospheric current sheet has its orinin on the solar surfacewhile its location with respect to the solar equator appears to be affectedby the variability of the lower layers of the solar interior.

INTRODUCTION

North—south asymmetries of the solar activity have been mentioned a loncitime ago. Since the early times of the satellite era J.M. Wilcox had assumedthat asymmetric activity on the two solar hemispheres could cause the ave-rage solar wind flow to bulge out /5,’. Later on, a detailed study ofsolar wind speed data for various heliolatitude zones 10°wide had pointedout that solar wind is faster in the higher latitudes than the lower. Inaddition, significant differences Of activity between the solar hemisphereshad been also detected. These differences were particularly c’reat near theepoch of minimum (1973-1976) while they almost disappeared in the epoch ofmaximum /1/. Similar indications of a varying activity between the solarhemispheres in the epoch of minimum of the previous solar cycle No.20 have beenalso reported by Hewish and Symonds /2/. It is very important that anoverall view of the observations reported about a N—S asymmetry of thesolar activity seem to confirm the aspect that this asymmetry tends tofollow a quasi 22-year magnetic solar cycle, namely every 22years thisasymmetric solar activity changes direction.

In the following, we present evidence that the N—S asymmetry of solaractivity affects the heliospheric current sheet displacements with respectto the solar equator.

DATA

A possible North—South asymmetry of the solar activity could be easilydetected in a solar magnetic field time—series extended at least to 2—3solar cycles, but such a long and detailed macinetic field time series isnot available. However, an appropriate, long and uniform time series ofsolar magnetic field data has been found to be the sunis of the maximumvalues of the sunspot magnetic field intensity observed for each ciroup ofsunspots during its passage over the visible solar hemisphere. This datarecord has been compiled in the Mount Wilson Observatory and extends from1917 to 1958 /6/.

On the other hand a possible North-South asymmetric solar activity

125

126 V. P. Tritakis

manifestation in the interplanetary medium could be easily ~detected by the heliosphericcurrent sheet location withr~spect to the solar equator. ,~

the estimated average lengthsThe displacement of the currentsheet from the solar equatorcan be derived from the mean isector widths (MSW), which areof the interplanetary magnetic 4000

field (IMF) sector structure 8during Northward and Southward ~excursions of the Earth in re-lation to~the solar equator.Consequently, for a six—monthperiod when Earth has North orSouth heliolatitude we canestimate the MSIJ for both ‘away’ ~22 ~ ~ 28193030 ~ ~ 3819.90.924446481950525456 ~

or (+) and ‘toward” or (-) IMFyears—ssector structures. It is clear

Fiq. 1. Annual sums of the maximum inten—that “away” or “toward” IMF sity of the sunspot magnetic field.sector patterns correspond to Asterisks show macinetic fields beloncoinci topositive or negative solar ma- new solar cycles. -

gnetic field polarity, respe-ctively.

EVIDENCE OF NORTH-SOUTHASYMMETRIC SOLAR ACTI-VITY. ~ N.6 N=7 N.5 N.5 N=4

~ S~1 S~3 S.5 S.5 S=1

Figu~eLshows the yearly sumsof the maximum values of the moo

0

sunspot magnetic field inten— 5sity observed for each group X eoo

of sunspots during its passage •o0

over the visible solar hemi— �oo

~ *~sphere, for the north and south I -~ ~\I ‘\~hemispheres separately. -400 Ufeatures could be determined: ~In this figure the following 600 V j *a.— There is no coincidence z ‘°~

between the extrema of the ~ 1200

ma~gnetic cycles formed in ~400 ~N N-~6OC’ ~ ( )

the two solar hemispheres. 1918 ~ 192022 24 26 28183032 34 36 36184042 44 46 48195052 54~8 501980

It is evident that in mostyea rs—~of the cases the maxima and

the minima in the two herni— FiçJ. 2. Differences between the values ofthe upper and the lower panel of the ficiurespheres are shifted 1—2 3. Asterisks show magnetic fields belongingyears with respect to each to new solar cycles.Vertical lines separateother, a fact which sugges— -successive solar cycles while the number ofted by the unequal distri— years where the mannetic fields predominatebution of the solar activi— in the North (N) or the South (S) solar he—ty in the two solar hemi— misphere are denoted.spheres.

b.— This feature may be relatedto the different start times of the new magnetic cycle in the two solarhemispheres. In the epochs of minima 1924 and 1935, the new magneticcycle starts first in the North hemisphere while in the following minima1944, and 1955 the new cycle starts in the south hemisphere (see theasterisks) in fici. 1. This property of new magnetic cycles to start atdifferent solar hemispheres every two solar cycles implies a 22—yearcycle feature, which possibly plays an important role in the formationof a North—South asymmetric distribution of the solar activity.

c.— The magnetic field values of the North hemisphere look greater, in mostcases, than the corresponding values of the South hemisphere. This lastfeature could be shown descriptively in the figure 2, where the diffe-rences EHN_EHS are rerpesented. Vertical lines separate successivemagnetic solar cycles while the number of the years where the magne-tic field values of the North (N) or the South (5) solar hemispheres

Solar Magnetic Field 127

—Fpredominate, are denoted at the topIn 6

of each cycle. From this denotation ~04 /

it is obvious that the North solar ~ 2 •hemisphere, in most of the cases, ismagnetically more active than the _~/_\__/\

South. From figure 2 it is clear ,~ -~ 1 \ / ——

that in the first two cycles the -~ .North solar hemisphere appears more -.

active than the South (in 6 and 7 NV MN NV MIN INY MN~ ~ 1’ ~ -~ ‘I Myears of the first two cycles res-pectively, the magnetic field ofthe North hemisphere was greater 0

than the corresponding field inten- ~ _~_/__v_:\sities of the South). By contrast 2 vin the next two solar cycles(1934—1954) the North and South solar ‘~

hemispheres seem to be equally active ~ ~ 55 57 ~ 918 187 II 197173 75 77 79

(5 to 5 years) while in the lastcycle (1954—1958) it appears that Fig. 3. Mean sector width differencesthe North hemisphere starts to pre— between “away” (lower panel) anddominate again (4 to 1). “toward” (Upper panel) mean sector

widths.These properties noted in figure 2 ______________________________seem to support the conclusion that e ~ II+4*****

the North solar hemisphere appears e

domination of the North hemispheremore active than the South, while the 402

shows a 22 year cycle variation. ~Figure 3 shows the MSWdifferences of ~2

both ‘away’ (lower panel) and “toward” -~ \j

(upper panel) sectors detected on both ‘~4-6sides of the solar equator. If ~N’~S

are the MSWvalues estimated during 8NV MN INV MN NV MN

the North or South heliolatitude pe—nod of the earth in a certain year 6

respectively, then each point of the 4

figure 3 represents the difference 2 V~( —~ of the away (low panel) ort~e ~toward” (upper panel) sectors. -~

Encircled points denote statistically ~ -2

significant differences at a 0.05 IX -

confidence level. It is clear thatvalues equal or close to zero imply ~ ,~, 53 ~ 53 65 87 69197173 75 77

that the relevant MSWin both solar Ficj. 4. Mean sector width differences,hemi~pheres are almost equal during north (upper panel) and south(X —x )~0 (~N~X5). In addition, (lower panel) heliolatitudes of thevaI~ue~ deviating significantly from Earth.zero show that sectors detected inone hemisphere are larger on the ave-rage than the same kind of sectors —

detected in the other hemisphere; i.e.if X1—X5~0then ~N~S• Positive

values depicted in figure 4 imply X1>X while neciative values meanXN~XS for “away” or “ toward” sectors~ As a matter of fact, it is veryinteresting that in figure 3, significant MSWdifferences for both kindsof sectors deviating in opposite directions with respect to the zero levelare found in the epochs of the minima. In the next figure 4 we illustratethe MSWdifferences of the “away” from the “toward” sectors detected duringNorth (upper panel) and South (lower p~nel)_heliolatitudes of the Earth.Hence the parameters (X — ~ ~NORT~ (XA,,_ ~~T~SoUTH express the dtfferencesbetween the mean sectorAwidt~s or sway (X ) and”toward” (XT) sectorsduring Northward and Southward excursions o~ the Earth, with respect tothe solar equator, respectively. It is evident that ficiure 4 is similar tofigure 3 and reproduces its most prominent features. The MSV differences offigure 4 predominate in the epochs of minima (MIN) in the same way as infigure 3 while in the epochs of the solar magnetic field inverstion (INV)they are insignificant and close to the zero level. It is apparent thatwhen (XAXT)NORTH or (~A~~T)SOUTH~0then (XA~~T)N th or (XA~T)Sth

Similar relations are valid when A~T~Nortl-~ 0 or AXT)South~ 0.

128 V. P. Tritakis

DISCUSSION

There are strong indications that in most of the cases the North solar hemi-sphere is more active than the South. Actually, from figure 2 it is clearthat the annual sum values of the maximum solar maanetic field strength isgreater in the North hemisphere than the South. In addition, great deviationsof the annual sums differences (EHN_EHS) from the zero level close to theepochs of minima can be determined. On the other hand, great deviationsof the MSWdifferences (XN_XS) for the “away” and “toward” sector patternsas well as the MSWdifferences (XA_X ) for North and South excursions of theEarth close to the epochs of minima ~ave been detected in the ficiures 3 and4. The meaning of the above mentioned deviations of the MSWdifferencesfrom the zero level is that sector patterns on both sides of the solarequator are not equal. In general the sector of each solar hemisphere whichis in agreement with the polarity of the nearest solar pole is broader thanthe other polarity sector of the same hemisphere i.e. in 1953—54 and 1975—76when the North solar pole is positive (“away”) the inequalities (X~-X5)AwA~O

and (~N-x5)TOWARD~0are valid, namely (XN)xS)AWAY and (xN~X5)TOWARD

The opposite inequalities are valid for the interval 1964—66 where thesolar polarity was inverted (North pole is negative or “toward”)Nevertheless, from the fiaure 4 we can obtain additional information beyondthat figure 2. It is very interesting that in two out of the three epochsof minima (1953—55, 1964—66) the absolute MST differences of the North hemi-sphere (upper panel)_ar~ signific~tl~ greater than those of the southhemisphere, namely I(XA_XT)NorthI~~XA_xT)sout~, while at the same time MSWdifferences of the minima - - 1953—55, 1964—66 are statistically si-gnificant in the North hemisphere (upper panel) but insiqnificant in theSouth (lower panel). Quite opposite conditions are valid for the minimum(1975—77), namely MSWdifferences of the South hemisphere are statisticallysignificant as well as absolutely greater than those of the North hemisphere.The significant deviations of the MSWdifferences from the zero level dete-cted in the epochs of minima could be interpreted by a Southward displace-ment of the heliospheric current sheet creating inequalities between sectorpatterns of the North and the South hemisphere. It is resonable to assumethat this Southward displacement of the current sheet could he originatedby a N—S asymmetry of the solar activity which is manifested by the asymme-tric distribution of the annual sums of the maximum values of the solarmagnetic fields, in the epochs of minima, detected in figure 2.Finally, it is reasonable to assume that the heliospheric current sheet hasits origin on the solar surface while its placement with respect to thesolar equator is highly affected by the variability of the layers belowthe surface , especially the asymmetric distribution of the solar activitywhich seems to predominate in the epochs of minima.

REFERENCES

Coles, U.A., Rickett, B.J., Rumsey, V.H., Kaufman, J.J., Turley, D.G.,Ananthakrishnan, S., Armstronci, J.W., Harmon , J. K., Scott.,S.L., and D.G. Sine, Solar cycle changes in the polar solarwind. Nature, 286, 239, (1980).

Hewish, A. and M.D. Symonds, Radio investiqation of the solar plasma, Planet

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space Sci. 17, 313, (1969).t:ilcox, J. M., On a possible North—South asymmetry in the solar wind, Irish

Astron. J.,~ 82, (1965).Xanthakis, S., The different indices of solar activity and the time of rise,

Solar Physics” edit. by J. Xanthakis, Interscience Publishers,(196 7).