CHAPTER IV STRUCTURE OF THE HELIOSPHERIC CURRENT SHEET IMF SECTOR STRUCTURE

CHAPTER IV STRUCTURE OF THE HELIOSPHERIC CURRENT SHEET AND IMF SECTOR STRUCTURE page 4.1. Introduction 77 4.2. Conditions for observing four sector IMF structure near 1 AU 80 4.3. Long term changes in (R /R ) 2 1 and IMF data 87 4.4. Mean IMF sector width and asymmetry in HCS 96 4.5. Comparison with asymmetry in sunspot activity and green corona intensity 100 4.6. Discussion 104 4.7. Summary of results 109

77 CHAPTER IV STRUCTURE OF THE HELIOSPHERIC CURRENT SHEET AND I~~ SECTOR STRUCTURE 4.1. Introduction The warped heliospheric current sheet separating heliohemispheres of opposite dominant magnetic polarity exhibits a vlide variety of structural complexity during its evolution during the course of the solar cycle. Near earth's orbit the heliospheric current sheet is observed as the interplanetary magnetic field (IMF) sector boundary separating IMF sectors of opposite polarity. At 1 AU, e~ther two or four IMF sectors are observed per solar rotation period depending on the warpings present in the heliospheric current sheet. Schultz (1973) explained the observed two sector INF structure in terms of the sinusoidal Hes due to a tilted magnetic dipole ln the Sun and the four sector IMF structure in terms of an additional solar nagnetic quadrupole contribution. Subsequent models explaining the IMF sector structure is basically using the idea of Schultz (1973) (Svalgaard et al., 1974, 1976b; Saito, 1975). A good correlation is found between the ItlF polarity predicted by K-corona observations or potential field modelling of

78 photospheric magnetic field and IMFpolarity actually observed near earth during solar cycle 20 and 21 (Saito and Swinson, 1986; Korzhov, 1982; Hoeksema, 1984; Newkirk and Fisk, 1985). Moussas and Tritakis (1982) studied the variation of IMF sector widths of opposite magnetic polarity with the phase of the solar cycle and heliohemisphere of observation. Tritakis (1984a) explained the observed mean IMF sector width changes in the northern and southern heliolatitudes in terms of an asyrmnetric HCS model during solar minima and symmetric HCS model during sunspot maxima. Sawyer (1974) found a semi-annual (heliolatitudinal) and sunspot cycle variation in the observation of four sector IMF structure near earth using inferred and observed IMF polarity data for the past few solar cycles. Svalgaard et al. (1975) using inferred IMF polarity data for nearly five solar cycles found that four sector IMF structure is observed more frequently in low sunspot activity periods. Behannon et al. (1989) studied the variation of IMF sector structure properties over a wide range of heliographic latitudes and radial distance from the Sun using Hes data inferred by Hoeksema and Scherrer (1986) and I~1F data observed by different space probes in and out of ecliptic during solar cycle 21. Tritakis (1979b) also made a study on the change in IMF configuration \vith sunspot cycle. Gonzalez and

79 Gonzalez (1987) found different periodicities in the IMF polarity data observed for the past few solar cycles and showed that there is a continuous change in the predominant IMF configuration from a well defined two sector IMF structure to a well defined four sector IMF structure with periodicities near 1.5 and 3.7 years. The variations in the complexity and configuration of IMF sector structure is now understood in terms of structural changes in the RCS induced by the variations in the nature of large scale solar magnetic field. Kaburaki (1983) proposed a phenomenological model of the RCS and suggested that the transition from a well defined two sector IMF configuration to a four sector IMF configuration is observed due to a change in the ratio of he dipolar and quadrupolar moments in the heliomagnetic field and the tilt of the dipolar axis with respect to solar rotational axis. Other studies also suggested the importance of the quadrupolar contribution to the RCS geometry (R ) ln 2 introducing a four sector IMF structure near earth (Saito and Swinson, 1986; Rakamada and Akasofu, 1981). In this work, the conditions for observing a four sector IMF structure near earth is investigated in terms of the solar magnetic dipolar and quadrupolar contributions to the geometry of the RCS. Fourier analysing the RCS data for the period 1972-1985 the change in the ratio of dipolar and

80 quadrupolar contributions (R 1 /R 2 ) and its role in the observation of four sector HIF structure in the interplanetary medium is discussed. We have also discussed how one can guess the nature of large scale solar magnetic field and HCS from the observations of IMF sector structure, in the past few solar cycles. The influence of solar magnetic quadrupole on IMF mean sector width variations is studied. The use of IMF data to determine asymmetry parameters of RCS like 8 m and 6 is also discussed. Yearly average sign of.l is determined from inferred HlF polarity data for the period 1947-1971 and is compared with the asy~qetry in sunspot activity and green corona intensity about solar equator. e T 4.2. Conditions for observing four sector I~W structure near 1 AU The geometry of the RCS in the interplanetary medium can be approximated by the relation 8 T = Rlsin R 2 (sin2-6) (4-1) The change in geometry of RCS with R l, R 2 and 6 is studied in chapter II. In chapter II the predominant polarity variation of H'lF,lith heliolatitude is investigated to determine the transition latitude 8 m for a given RCS 1 geometry. Apart from the dominant polarity of INF observed during a solar rotation period near 1 AU, the complexity of

81 IMF structure i.e., whether two sectors or four sectors are observed per solar rotation, is another important property of IMF sector structure. The complexity of IHF sector structure can change with heliolatitude of observation which in turn depend on the HCS geometry. In this section the relationship between the change in complexity of IMF sector structure with heliolatitude of observation with change in geometry of HCS determined by the relative dipolar and quadrupolar contributions in the large scale solar magnetic field has been investigated. A condition for observing a four sector structure near 1 AU in terms of the HCS parameters R l, R 2 and &is also obtained. The change in complexity of the IMF structure with heliolatitude will depend on change in the HCS parameters R and 2 O. As a first approximation let us keep The complexity of IMF structure (i.e. whether two sector or four sector INF structure) is then determined for each heliographic latitude where INF sector structure is observed. Since the ratio (R /R ) and the phase factor 2 l 6 are crucial in determining the HCS geometry, it is required to investigate on the HCS geometry for a range of (R /R ) and 2 l 6. The results of this investigation are depicted in figures 4.1a,b,c and d. In figure 4.1a the heliolatitudes of observation of four sector structure near 1 AU is depicted for change in (R /R ) from 0.1 to 1 with steps of 0.1 for a 2 l

82 8 0 (IJ ttl ::s -l-l.r-! -l-l III 4 0 r-l U.r-!..r:: 0.. III H 0 lj1 0 -r-! r-l (IJ ::c: -4 0-16 0-20 0 o 0.4 0.5 0.6 0.8 0.9 1.0 Fig.4.1a. variation in heliolatitudinal distribution of four sector liif structure with R 2 /R 1 for cf=oo

83 I1J '0 4 ::l.j.l.0-1.j.l III r-l 0 0.0-1.G o. III H til 0.0-1 r-l I1J til -4-8 -20 o 0.4 0.5 0.6 0.8 0.9 1.0 Fig.4.1b. Variation in heliolatitudinal distribution of four sector nlf structure \lith TI 2 /R 1 for J=300

84 8 OJ 4 OJ H tn OJ '0 OJ 0 '"d ::l J o.-j J III -4 H u o.-j.c: 0.. III H -8 tjl 0 o.-j H OJ ::t: -12-16 -20 o 0.4 0.5 0.6 0.9 Fig.4.1c. variation in helio1atitudinal distribution of four sector HIP structure with R 2 /R 1 for cf =60 0

85 QJ '0 0 ::l 0.j.J..-f.j.J m r-l u..-f -4 0.c:: P..o m I-l tyl 0..-f -8 0 r-l QJ ::r: -12 0-20 0 o 0.4 0.5 0.6 0.7 R /R 2 1 0.8 0.9 1.0 4.1d. Variation in he1io1atitudina1 distribution of four sector HIP structure viith R 2 /R 1 for cf = 90 0

86 constant value of J (0=0). Here the different values of the maximum heliolatitude of observation of four sector IMP structure corresponding to different values of (R 2 /R l ) are joined together as the V-curve in figure 4.la. Similarly the different values of the minimum heliolatitude of the sahle is joined together as the L-curve. Four sector structure of IMF is observed between the latitudes, indicated by U and L curves for each value of R 2 /R l. Similarly in figure 4.lb, c and 0. the above analysis is repeated for a constant value of coo 0=60 and 6=90 respectively and changing R 2 /R l. can find that four sector structure is observed in interplanetary medium only when (R2/Rl)~0.6 for 6=0 0 For 6=90 0, the ratio (R2/Rl)~0.4 is required to observe a four sector IMF structure in the interplanetary medium. Similarly for vje other constant 6 values the critical ratio of (R 2 /R l ) where a transition from two sector structure to four sector structure occurs in the interplanetary medium varies between 0.4 to 0.6. It is also found from figure 4.1 that the distribution of four sectors with heliolatitude is asymmetric about the heliographic equator. The nature of this asymmetry change with (R /R ) and 2 l 6. From this study we find that the condition for observing four sector IMF structure near heliographic equator for a moderate range of

87 phase values (20 0 <0>90 0 ) can be approximated by (R /R ) ~ 1 2 l or R2~ R. Note that the critical ratio of (R /R ) for the l 2 l transition from 'two sector' to 'four sector' from the IMF configuration near 1 AU is 0.8 for 0=20 0 and it is =1.05 for 0=90 0 near the heliographic equator. So the condition for observing a \Jell defined four sector IMF structure within ecliptic (7.25 0 ) can be approximated by (R /R ) ~ 1. 2 l 4.3. Long term changes in (R 2 /R 1 ) and I~W data One can understand from previous section (4.1) that for periods where four sector structure is observed near earth, (R 2 /R 1 ) of HCS is expected to be greater than or equal to 1. This concept can be used to get an idea of the long term changes in (R 2 /R l ) from IMF sector structure data in the past few solar cycles where direct HCS inferences from solar observations is not available. Before proceeding in that line one need to know about the temporal change in (R /R ) from HCS inferences. Using the HCS data set 2 l described in chapter III (R /R ) value is determined for 2 l each solar rotation between 1971-1985. Digitizing the latitude (8) of HCS at equal intervals of solar longitude ( ) for a given solar rotation and fourier analysing of the form given in equation 4-1 \ve can determine parameters R l, R and R /R. Such a computation of (R 2 2 l 2 /R l ) is made from HCS

2 II I' \ I II" I I I I I I r I I j J I 1.5 ~r-i 1 "- N ~ 0.5- I I II \1 I I r\ II I I I II I \I I I ~ I i II j I \ I II I I, Ii I II i /11, I f I I; II' j 'I, I i' I t I Ii I \1 I, ~i ' Illrl : /\ I ~Il, I 1'1 ilr 1,1, Ii, I,I II JI\ I 1,I 1 II "I '1 I IVilli 1'1 II I III I', I II II,\ f ii, I \I i ~ \,.' I'!i ii 'II I I "',", -I.~ jl It \ I I,,1/ I I 'l I Ii I) l - I~ I,! i i' I ',~! I /'.. 1, I I I,,\ I 'III lv,l I \II'i / I.' I I \'1 i, " I ~ o IL- '. I lit I I!, -:..' 160J 1620 1640 1660 1680 1700 1720 1740 CARRnrGTON ROTATIONS i 00 00 Fi~,4.2. R 2 /R 1 of Hes for the carrington rotations 1580-1765 during 1971-1985

89 data for the carrington rotations 1580-1765 as depicted in figure 4.2. Data gaps are present during solar maximum (1979 1980), where (R /R ) has been equated to zero. It is 2 l interesting to observe from figure 4.2 that the magnitude of (R /R ) fluctuate from one solar rotation to another 2 1 suggesting periodicities in (R /R ) evolution. This period 2 l ranges from few solar rotation to few years. We determine the power-spectra of the time series "of R /R during solar 2 l rotations 1580-1765. The results are shown in figure 4.3. We can find significant power correspond to the following periods:- 6.8 years, 1.85 years and 0.9 year. From IMF daily polarity data (Svalgaard, 1976; Solar geophysical "data bulletins; Couzens and King, 1986; Hoeksema, 1984) the Bartel's rotations where four sector IMF structure is observed near earth are identified for the period 1947-1985. In figure 4.4a the plot of number of Bartel's rotations per year where four sector IMF structure are observed during 1947-1985 is given. We also depict the annual average of (R /R ) values determined from HCS data 2 l for the period 1971-1985 in figure 4.4b. From figures 4.4a and 4.4b we observe that the variation in number of rotations per year where four sector structure is observed is related to the change in average (R /R ). 2 l

28 6.8 yrs 24 20 l-l (J)... '> 0 ~ 16 1 I \ r-i I \0 0 :> -.-1 J co.-i (J) 0::: 12, i I I 8" I a 4 8 12 16 20 HARMONIC Fig.4.3. Power spectra of the R 2 /R 1 for the period 1971-1985

Q) 14 ~.j.j u 10 ~ 14.j.J til \i.t :8 H 14 0.j.J u Q) til 14 ~ 0 ll-l lj-l 0 til Q) U c: Q) 14 14 ~ U U 0 8 ~ Gr I \.I \ J \ J \ " I \ I i 4f I I 2' '!) f-j 47 55 G3 YEAR Fig.4.4a. Annual occurrences of the four sector IMF structure near earth during 1947-1985 71 79 87

'.6 1 4 ' 2 r-i P:: l I \ / \ I "- N P:: (I) 1 0 Ol III 1-1 (I) :> t::r: 0 8 ~ N O G 0 4 70 72 74 76 78 80 82 84 YEAR Fig.4.4b. R /R 2 1 average of HCS for the years 1971-1985

93 Another important result in this line is the calculation of log ((PO/PI) from IMF polarity data for each quarter during an year for the period 1926-1982 (Gonzalez and Gonzalez, 1987). In that study Po is the power (spectral) in the daily IMF polarity having periodicity near 27.5 days denoting the contribution of two sector structure and PI is the spectral power corresponding to a period near 13.5 day denoting the contribution of four sector IMF structure near earth. The parameter log (PO/PI) gives an idea about the relative predominance of four sector structure over two sector structure near earth. The plot of lot (PO/PI) made by Gonzalez and Gonzalez (1987) is reproduced in figures 4.5a and 4.5b. For periods where log (PO/P1»1 (R 2 /R l )av<1 and for log (PO/Pl)~l, we expect (R2/Rl)av~1. Another interesting result to observe from figures 4.5b and 4.4b is that log (PO/PI) and (R /R 2 l )av does not vary in unison with sunspot activity. Further the sunspot cycle variation in (R /R )av for the cycle 20 and 21 and log 2 l (PO/PI) over cycles 17-20 are different from one cycle to another.

2<)t) ZUf-{ I CH NLHv18ER sur JSPOT 150-100 5'0 :) 1~30 19<10 1950 1960 1970 1980 YEAR 2 ~ I I Q('{nr-"PI) _ "I r vi \0 ~ \" \ A ~ ~ h J. ~ tj r, f f;1' ~ I \ \. t. I \ 1\ ",", rl I' I,, I. P, r~. 1 ' i. " I t~. f,~o>.,i '\. I ~ I f\l...,\~ I I I I, 1 ~I. :,1 I ~ \~. r1 I fl.!,., I, I I,, i r: I-..._1_.\_-'~_~_J-'\_t \ \ ~,. I I I \ ': t \: -i.,\\ '! \ rj fl, O..!_L~, \!", -'\i I I V' 1 I \'1 I '( ~. i' r ",,It \' 193() V',945 1 1960 1979~ ~,.,.~ YEAR -l. l Fig.4.5a. Evolution of log (PO/Pl) during 1926-1982 (Gonzalez and Gonzalez, 1987)

\. ~o LOG (PO/PI) '... - CYCLE IF-1 1 -- CYCLE #: 18 CYCLE#: 19 _.- CYCLE #- 20, 00 O'!SO 0 00 -o ~o. Ị I.I 3 :r : i \. \.../ 8 'r I \ ' \,f I 1\ j \ I'~,. " '.10 II YEARS FROM SOLAR MiNIMUM ~ v -I 00 Fig.4.5b. Sunspot cycle variations in log (PO/PI) for the solar cycles 17-20 (Gonzalez ana Gonzalez, 1987)

96 4.4. Mean IMF sector width and asymmetry in HCS Tritakis (1984a,b) and Moussas and Tritakis (1982) studied the mean sector width changes of IMF sectors when earth is in northern or southern heliolatitudes separately. Tritakis (1984a,b) assumed an asymmetric current sheet model to explain the mean sector width variations during sunspot minimum in which a sinusoidal HCS is simply displaced parallel to solar equator. During solar maximum Tritakis assumed a syrametric HCS model to explain the corresponding IMF sector variations near earth. Let us evaluate the mean sector width changes of IMF observed near earth when a quadrupole component in the solar magnetic field 18 also taken into consideration to describe the HCS as in equation 4-1. For simplicity let us assume that the 'mean IMF sector width' as the width of the IMF sector at a mean heliographic latitude o ~7.25(2/71 ) in the northern and southern heliohemispheres. One can evaluate separately the 'mean sector width' for positive IMF sector (=X ) and negative IMF A sector (=X T ) at the mean heliographic latitudes ~7.25 (2/ 7r ) 0 in the northern and southern heliosphcre. We have evaluated these parameters of HCS corresponding to different values of R l, R 2 and J and obtained the valid inequalities between 'mean IMF sector width' differences in the northern and southern heliohemispheres as observed near earth following

97 Tritakis (1984a,b). The inequalities resulted from such a model of asymmetric HCS (due to the presence of the magnetic quadrupole component) for type (A) and type (B) HCS structures is given in table 4.1 where" Ibll = \XA-xTI north, Ib2\ = \XA-xTI south For any current sheet model cyclic every 360 0 X A north X T north = X A south X T south and lall = IX A north - X A south\ = \X south - X southl T T = la 2 \ so the inequalities mentioned in Tritakis (1984a) between lall and la21 are not valid for a cyclic current sheet "and we have considered only the inequalities between I"bll and lb 2 \. The valid inequalities are given for 6 > 0 and 6 < O. It is Horth noting that when the HCS configuration is that of type (B) (e and T 6 are of same sign) the valid inequalities are similar to Tritakis (1984a) model. Similarly a second group of inequalities have been derived corresponding to an HCS configuration type (A). The above inequalities constructed from IMF data during an year cannot be effectively employed to determine the sign of 6. This is because the inequalities are different for type (A) and type (B) RCS configurations and it is difficult to guess the HCS type from IMF data alone. For the asymmetric HCS geometries (A), (B) and (C) the sign of the e~ is the basic parameter which will L

98 Table 4.1 Comparison of valid inequalities of mean IMF sector widths for two different types of RCS Asymmetry in latitudinal Type (B) Type (A) extension of current sheet aysmmetric RCS asymmetric RCS ------------------------------~----------------------- ------ Northward depressed ( 6. > 0) Southward depressed ( ~ < 0)

determill.1e the domill.1~ull.t jpolariity 01 U~F Qb:S~lr\7~d lfll~i&lr ~i&lrlth." $; orbit. Suppose at is in the norith~)fn h.~lio:sjph~)f~ ~Q,\:?>((l) ~ " then the IMP with magnetic polarity id~lflltic~l tq th~ ~Ql@lr south pole will be found to be in e~c~ss n~~lr h.~lioglri&~h.ic equator. The interesting case is that of Res typ~ (D). llfll this case 9 =O and uhen T b is greater than/le:ss th@.lfll ~~lfo" the magnetic polarity of IMP that could be foulflld in e~c~$;~ at earth's orbit will be identical to that of $;QIGr south/north pole. Let Ln and Ls denote the yearly @.v~ra9~ II'IF sector \;1idths observed near earth \~ith magnetic pqlarity identical with solar north pole and south pole respe~tively. The yearly average sign of at could be inferred a~ (as~uming type (D) RCS types are rarely observed). et>o for Ln < Ls and at < 0 for Ln> Ls Such an inference of at is done for the period 1972-1985, using IMF observations near earth's orbit (Svalgaard. 1916; Solar Geophysical Data; Hoeksema, 1984). In table 4.2 we give a schematic representation of the sign of the yearly average values of at evaluated from inferred MCS data described in chapter III. We also give the at signs inferr~d from It1F observations and the sign of the average 6 obtained using inferred HCS observations. The symbol '0' m~nn~ no asymmetry or the sign is indeterminablc'. EXCQpt <1Ud.. fhj 197')= 1977 and 1983-1985 the average configuration ()f Ile:;; i tj\l\t. of system (A). During 1977 the average picturo ib ~mb\guo\l~

100 due to the fact that for some solar rotations during that period RCS configuration resembled that of type (D). During 1983-1985 the asymmetry in RCS resembles that of type (B). It is seen from table 4.2 that the sign of 8 T inferred IMF observations agree well with the yearly average sign from of 8 T obtained from RCS data except for the year 1975. This implies that one could infer the sign of e T using IMP observations for the past few solar cycles. Such an attempt is made as shown in table 4.3 for the period 1947-1971 using catalogue of inferred IMP polarities of Svalgaard (1976). For the years between solar maxima and solar polar reversal period (1947-1949; 1957-1958 and 1969-1970) 8 T has been assigned '0' sign in table 4.3. 4.5. Comparison with asymmetry in sunspot activity and green corona intensity To compare the yearly average sign of 8 T with the sign of asymmetry in sunspot activity for the period 1947-1984 the parameter 6. s = Rn/RnRs has been used, where 6 s denotes the asymmetry ln sunspot activity about the heliographic equator, Rn and Rs are the annual mean sunspot number in the northern and southern he1iohemispheres (Koyama, 1985; swinson et al., 1986). The parameter A is w s assigned sign if 6 s<0.5 and it is assigned sign if

G1 :2J 5 O:2J9 101 Table 4.2 Schematic representation of yearly average sign of ~ & 9 T inferred from RCS observations (1972-1985), asymmetry in sunspot activity ~ (1972-1984), s and asymmetry in green corona intensity (1972-1984) (~g ) year 9 T (HCS) 6. As (HCS) (Sunspot) 6. g (green corona) ------------------------------------------------------------ 1972 1973 1974 1975 1976 o 1977 o o 1978 1979 o o o 1980 o o o 1981 1982 o 1983 1984 o 1985

102 Table 4.3 Schematic representation of yearly average sign of em.l inferred from Ir1F data, asymmetry in sunspot activity ~, and asymmetry in green corona s intensity 6 for the period 1947-1971 g year year 6 ' s ---------------------------------------~-------------- ------ 1947 0 1960 1948 0 1961 1949 0 1962 1950 1963 1951 1964 1952 1965, 1953 1967 1954 1968 1955 1969 0 1956 1970 0 1957 0 1971 ;... 1958 0 1959

103 ~ s > 0.5. The sign of 6 s for the period 1972-1984 is shown in table 4.2 and for the period 1947-1971 is shown in table 4 3 Similarly to compare the average annual 8 sign T with the asymnetry in green corona intensity about the heliographic equator we use the data published by Tritakis et al. (1988) using green corona observations from Pic-du- Iiidi observatory for the period 1947-1974 and from Rusin et al. (1988) and r'loussas et al. (1983) for the period 1975-1984. The sign of asyddetry in green corona intensity about the heliographic equator (positive if the average intensity of the green corona in the northern solar hemisphere exceeds the same in the southern solar hemisphere) be called as A U g and the same is depicted in table 4.2 and 4.3 for the periods 1972-1984 and 1947-1971 respectively. It is seen from the above tables that 6 and 6 signs looks very s g similar for the period 1947-1984. The important feature to notice that on an average the sign of 6 and {:;. remained s g as positive for a long duration between 1949-197l. But 8 T has several sign reversals during the above period. Hence from the above results one cannot find any reliable relationship betvleen asyr,unetry in HCS and asymmetry in sunspot or green corona activities about solar equator.

104 4.6. Discussion In this work it is shown how IMF data is used to determine certain properties and features concerning the large scale solar magnetic field and associated structure of the heliospheric current sheet. IMF sector data (two sector or four sector structure occurrences near earth) is shown to give an idea of the relative dipolar and quadrupolar contributions to the RCS geometry in the past where direct solar observation of RCS structure is not available. It is worth noting from studies in chapter II and this chapter that the presence of the quadrupolar component ln the heliomagnetic field inferred by a non-zero R 2 contributing to the RCS geometry can cause an asy~netry about heliographic equator not only in the heliolatitudinal variations in predominant polarity of IMF but also in the heliolatitudinal variations in IMF sector complexity. In fact the derived heliolatitudinal variations in lle1iospheric sector structure complexity obtained from RCS inferences for the period 1977-198', as reproduced in figure 4.6, shows an observable asymmetry about the heliographic equator (Behannon et al., 1989). Analysis of the number of occurrences of four sector. structure per year (Fig. 4.4a) or the par.ameter log (PO/P l ) of Gonzalez and Gonzalez (1987) (Fig. 4.5a,b) for the past

- 24-20 -16-12 LU 0-8 ::J -4 i- 0 i- «4...J -8 12 16-20 24-28... -32 _ 37 a> <0 <0 J I a> CO <0 ~ i t ~ r-..... -r;'1:iii I~ ff i ~i '!1 0) N r-..... a> ~ a> <0 r-.. f-' o ljl -42-47 -53-60 1 I I a: r-.. CO >- r-.. r-.. 0) r-.. I o CO!It co N CO M CO 0; 10 CO <0 CO EO Fig.4.6. ije1io1atitudina1 cha~ge in the two and four sector heliospheric structure during 1977-1986. Shaded regions correspond to four sector structure. (Behannon et al., 1989)

106 several solar cycles indicate that the relative quadrupolar and dipolar contribution to the current sheet varies with sunspot activity differently in different sunspot cycles. Temporal change in (R /R ) derived from RCS data 2 l for the period 1971-1985 shows several periodicities less than solar cycle period. This result gives support for the periodicities derived from the analysis of IMP polarity by Gonzalez and Gonzalez (1987). The influence of the 'solar magnetic quadrupole' on the mean sector width variations of IMP near 1 AU is studied. It 1S also shown how one could determine the asymmetry parameters of RCS such as 8 by comparing RCS and T IMP data during 1971-1985. The yearly average sign of 8 T is evaluated from IMP data for the period 1947-1971 which suggests periodicities less than the solar cycle period in the evolution of asymmetry in RCS. Short term periodicities less than the solar cycle period are also observed in the case of evolution of north-south asymmetry in solar phenomena viz., green corona (Ozguc and Ucer, 1987), solar prominences (Vizoso and Ballester, 1989) and solar flares (Vizoso and Ballester, 1987). The asymmetry in RCS given by the yearly average sign of 8 is cor,1pared Hith annual sign of asymmetry 'r in sunspot activity ~ s and green corona intensity 6 g about the solar equator. The sign of asymmetry in sunspot activity

107 and green corona line intensity on an average remained positive for the period 1949-1971 while e T has several sign reversals during the same period. Thus the above results and the results in chapter II comparing e and monthly values of T sunspot asymmetry parameter 6 s for the period 1972-85 suggests that one should be careful in relating the asymmetry in sunspot activity and related parameters with asymmetry in HCS geometry about the solar equator (Saito et al., 1977; Tritakis, 1984a,b). One can also expect that the relationship of the north-south asymnetry in other parameters like annual sums of magnetic field associated with the sunspots (Tritakis, 1984b) and sunspot area (Swinson et al., 1986) with the asymmetry in heliospheric current sheet will be no better than the asymmetry in sunspot number or green corona as investigated in this section. Various heliomagnetic nultipoles can exhibit different periodicities, e.g., the 22-year periodicity is exhibited by the north-south dipole harmonic. The amplitude or the intensity of the solar magnetic multipoles is a timevarying quantity. So the superposition of different magnetic modes existing in the Sun with different phases and amplitudes can determine the nature and evolution of asyoonetric phenomena about the solar equator such as sunspot activity and HCS. Long term changes in the different

108 magnetic modes in the Sun is studied by Gokhale and Javaraiah (1990) using sunspot observations. The investigations of Stenflo and Vogel (1986) and 'Stenflo and Gudel (1988) will be helpful for a detailed study in the similar direction.

109 4.7. Summary of results i) The role of solar magnetic quadrupole contribution to the Res on the heliolatitudinal dependence of the observation of four sector structure of IHF near 1 AU is studied. ii) Using Res and IMF polarity data the possibility inferring the relative solar magnetic dipolar and iv) Using solar and IMF observations (1947-1984) it is sho\jn that the asymmetry in green corona intensity or sunspot activity about the solar equator is not related consistently \lith the north-south quadrupolar contributions to RCS (R /R ) from IMF 2 l polarity data in the past few solar cycles is investigated. iii) The influence of the north-south asymmetry in,rcs on HIF sector \'lidth observations near 1 AU is studied. It is shown that one could infer the yearly average sign of 8 from IOF polarity data T comparing RCS and IMF observations during 1972 1985. The yearly mean sign of 8 is inferred from T IMF polarity data (deduced from geomagnetic observations) during 1947-1971. asymmetry in heliospheric current sheet about the heliographic equator.