RADIO M A P P I N G OF THE H E L I O S P H E R I C C U R R E N T S H E E T

R. T. S T E W A R T

Division of Radiophysics, CSIRO, P.O. Box 76, Epping, N S W 2121, Australia

(Received 4 November, 1986; in revised form 18 February, 1987)

Abstract. Synoptic plots of solar radio noise storms in the interval 1973 to 1984 are described. The dividing line between opposite noise storm polarities appears to be a good representation of the heliospheric current sheet out to displacements in latitude of ~ + 50 ~ from the solar equator. This result is surprising, because noise storms are closely associated with closed magnetic field regions near sunspots. The possibility that noise storm polarity is determined by mode coupling high in the corona, where field lines are open, ean be ruled out by the available evidence. This leads us to conclude that it is the clustering in longitude of active region complexes which determines the sector structure of the interplanetary magnetic field.

I. Introduction

The existence of magnetic sectors of opposite polarity co-rotating with the Sun was discovered by the Imp 1 spacecraft (Ness and Wilcox, 1964). Since then a number of spacecraft observations of the interplanetary magnetic field structure have been made near the ecliptic plane, at distances of 0.4 to 1 AU from the Sun (see Bruno et al., 1982, and references therein). Originally it was thought that the magnetic sectors were meridional structures with boundaries running north and south but later observations showed this to be incorrect (Rosenberg and Coleman, 1969). The latest interpretation (Schulz, 1973) is that the magnetic sectors are due to multiple crossings of the solar equatorial plane by a large-scale, warped heliospheric current sheet. Schulz suggested that the two-sector pattern sometimes observed arises from a tilted solar magnetic dipole component and the more commonly observed four-sector pattern from a quadrupole component.

Strong support for these ideas has come from potential-field modelling of low- resolution measurements of the line-of-sight components of the photospheric magnetic fields (Schatten et al., 1969; Altschuler and Newkirk, 1969; Wilcox et al., 1980). A spherical source surface is assumed to exist at some height, usually ~ 2.5 Ro, below which the coronal magnetic field is potential (current-free) and above which the field lines are stretched radially through the action of the solar wind. The neutral line on this surface is taken to be the footpoints of the heliospheric current sheet. Despite the inherent simplifications in this model the agreement with interplanetary magnetic field measurements in the ecliptic plane is quite good. (Hoeksema etal . , 1982, 1983). Furthermore, Hoeksema and Scherrer (1986, Figure 8) have plotted the relative r.m.s. contributions of the dipole, quadrupole, and octopole components of the source surface field strength throughout the solar cycle. Near minimum the dipole component dominates but during much of the cycle the quadrupole component is comparable and even the octopole component is not negligible.

Solar Physics 109 (1987) 139-147. �9 1987 by D. Reidel Publishing Company

140 R.T. STEWART

Another method of approximating the current sheet is to plot the band of enhanced electron density or K-coronal brightness encircling the Sun; the base of the bright structures being associated with closed magnetic field lines below the source surface (Pneuman et al., 1978). Comparisons with interplanetary magnetic field measurements at distances from 0.3 to 1.0AU and within +7.~ heliographic latitude during 1976-1977 confirm that the heliospheric current sheet was within 7 ~ of the K-coronal simulation, the latter predicting a four-sector warped boundary with an average latitudinal displacement of 8 ~ and a maximum deviation of 15 ~ (Bruno et aL, 1982). During 1974 the current sheet had a two-sector structure, according to K-coronal evidence, with displacements extending to ~ 30 ~ latitude (Hundhausen, 1977). Such large displacements highlight the importance of having a method, other than that of taking spacecraft measurements in the ecliptic plane, of determining the structure of the interplanetary magnetic field.

Apart from potential field solutions and K-coronal brightness simulations there is another method which uses solar noise storm polarities to plot the magnetic field direction in the high corona. Stewart (1985) has shown that yearly plots of 160 MHz noise storm polarization during 1976-1977 and 1980-1983 formed large-scale cellular structures with polarities and dividing neutral lines similar to those derived for the interplanetary magnetic field. The purpose of this paper is to extend this work over the entire period of Culgoora radioheliograph observations at 160 MHz, from 1973 to 1984, and to follow in detail the evolution of large-scale magnetic field structures in the solar corona.

2. Comparison of Noise Storm Polarities with Heliospherie Current Sheet Simulations

Figures 1 to 4 show data in synoptic plot format, with each point representing a daily measurement of the 160 MHz noise storm position and its polarity (see Stewart et al.,

1985, for details). The heavy line for comparison was selected from published simula- tions of the heliospheric current sheet and chosen to represent a typical example during the interval of each plot: for Figures l(a) and l(b) see Levine (1978); Figure l(c), Wilcox and Hundhausen (1983); Figures l(d)-3(d), Hoeksema et al. (1982); and Figure 4(a), Hoeksema and Scherrer (1986).

At the commencement of this solar cycle in 1976 the current sheet shows four warps extending to latitudes + 15 ~ (see Figure l(c)). The noise storm polarities straddle the current sheet and are associated with active regions where the dominant sunspot polarity is positive north of the current sheet and negative to the south (Stewart, 1985, Figure 5). In each heliomagnetic hemisphere the coronal holes and solar polar regions have the same polarity as that of the noise storms (Stewart, 1985, Figure 3(a)).

As the solar cycle progresses the warps in the current sheet become deeper and vary more rapidly, reaching solar latitudes ~ 50 ~ by early 1978 (Figures l(d) and l(e)) and ,-, 60 ~ by early 1979 (Figures 2(a) and 2(b)). By April 1979 the current sheet has extended > 70 ~ indicating a drastic weakening of the polar component owing to the

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arrival of trailing field flux from remnant active regions of the new cycle (see Bumba and Howard, 1965, for details). This process continues until polarity reversal occurs around March 1980 for the north pole and around September 1980 for the south pole (Webb et aL, 1984). During this period of almost zero polar fields the global coronal field resembles almost an equatorial dipole (Figures 2(c) to 2(0).

The current sheet continues to undergo rapid convolutions during 1980-1981 (Figures 3(a) and 3(b)) owing to the dominating influence of the active region component and the appearance of mid-latitude coronal holes with new cycle trailing flux (Webb et al.,

1984). By 1982 (Figure 3(e)) the global field has stabilized into a two-sector structure, similar to that observed during the latter part of the Skylab mission of early 1974 (Figure l(b)). (Note, however, that the sectors have opposite polarities in 1974 and 1982.) This structure is associated with the development of large-scale coronal holes extending from the poles towards the solar equator (Stewart, 1985, Figure 3; also Hundhausen, 1977). According to Bruno et aL (1982) the coronal field in 1974 can be interpreted as a tilted dipole with a 30 ~ tilt to the solar axis, implying a dipole component

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and minus) of (b). (b) Coronal hole polarities (plus and minus) taken from published Ha synoptic plots (Solar-Geophysical

Data, 1984).

RADIO MAPPING OF THE HELIOSPHERIC CURRENT SHEET 145

in the equatorial plane about half that of the axial dipole component. During 1982 the noise storm polarity also reflects this tilted dipole form (Figure 3(e)). During 1983 the global structure resembles that of a quadrupole (Figure 3(f)) but reverts to that of a tilted dipole again in 1984 (Figure 4(a)).

The influence of large-scale coronal holes on the global structure during the declining phase of the solar cycle is again evident in 1984 (Figure 4(b)). In Figure 4(a) the full line is the derived neutral line from potential field solutions and the dashed line is our estimate from combining the noise storm and coronal hole data. It is evident from this comparison and also from Figure 3 of Stewart (1985) that the latter method is quite sufficient for determining the heliospheric current sheet over most of the solar cycle.

3. Discussion

The generally good association shown in Figures 1 to 4 between 160 MHz solar noise storm polarities and global magnetic fields may at first sight seem rather surprising when one considers the close association between noise storm activity and large sunspots (Payne-Scott et al., 1950). However, as Stewart (1975) has pointed out, there are two possible explanations for this agreement. Firstly, there is the possibility that the noise storm polarity is imposed by propagation conditions between the source and the observer. If the field direction, relative to the ray path, reverses between the source region and the high corona, then the ray must pass through a region of quasi-transverse propagation. If weak coupling applies in this region (Cohen, 1960) the observed polarity will be determined by the polarity of the high magnetic fields. However, if strong coupling occurs the polarization is maintained in its original sense and reflects the magnetic field direction of the source region.

To test for mode coupling effects we have applied the following argument. If weak coupling were to occur between the site of emission (at say a height of 1.3 Ro) and the observer, then one would expect the noise storm polarities to show a better correlation with the derived source surface fields at 2.5 R@. Taking noise storm data for the interval May 1976 to December 1977 and comparing with magnetic field data in Hoeksema and Scherrer (1986), we derive the correlations given in Table I, where it can be seen that

TABLE I

Correlation between noise storm and solar field polarity during May 1976 to December 1977

Agreeing with No, Percentage

Photospheric field 209 87

Source surface field 173 72

146 R.T. STEWART

noise storms are in better agreement with photospheric fields (87 ~ correlation) than with source surface fields (72~)*.

Hence, the coupling is strong. The result is in apparent contradiction with theory (Melrose, 1974), which predicts that strong mode coupling will only occur if we have

f > (5L s f ~ f I 4 3 ) 1/4 , (1)

where f is observing frequency and fp and fz-i are the plasma and gyro-frequencies in megahertz and LB is the characteristic distances in kilometres over which the direction of the magnetic field varies. It is highly unlikely that we would have L B < 105 km, so Equation (1) requires a magnetic field strength B of < 0.1 G. From a comprison of noise storm data and derived magnetic field strengths (Hoeksema and Scherrer, 1986) it is apparent that at heights less than the source surface the fields in the vicinity of the noise storm must be > 0.1 G. Hence, contrary to observation, the mode coupling should be weak. We conclude that inequality (1) is invalid for solar coronal conditions.

It would appear, therefore, that noise storm polarity is determined by active-region magnetic flux; more specifically, it is closely associated with dominant sunspot flux. For the years 1976 and 1982, Stewart (1985) found an 86~o correlation between the polarity of the dominant sunspot in noise-storm-associated active regions and the polarity of the sector structure. This result is comparable with the correlation found above for noise storms and the photospheric polarity and suggests strongly that the magnetic sectors result mainly from the clustering in longitude of large sunspots (Gaizauskas et aL, 1983). Why clustering should occur and why the dominant sunspot polarity should agree with the sector polarity is not understood.

References

Altschuler, M. D. and Newkirk, G.: 1969, Solar Phys. 9, 131. Bruno, R., Burlaga, L. F., and Hundhausen, A. J.: 1982, J. Geophys. Res. 87, 10339. Bumba, V. and Howard, R.: 1965, Astrophys. J. 141, 1502. Cohen, M. R.: 1960, Astrophys. J. 131, 664. Gaizauskas, V., Harvey, K. L., Harvey, J. W., and Zwaan, C.: 1983, Astrophys. J. 265, 1056. Hoeksema, J. T. and Scherrer, P. H.: 1986, Report UAG-94, World Data Center A for Solar Terrestrial

Physics, NOAA, Boulder, Colorado. Hoeksema, J. T., Wilcox, J. M., and Scherrer, P. H.: 1982, J. Geophys. Res. 87, 10331. Hoeksema, J. T., Wilcox, J. M., and Seherrer, P. H.: 1983, J. Geophys. Res. 88, 9910. Hundhausen, A. J.: 1977, in J. B. Zirker (ed.), Coronal Holes and High Speed Streams, Colorado Associated

University Press, Boulder, p. 225. Levine, R. H.: 1978, J. Geophys. Res. 83, 4193. Melrose, D. B.: 1974, Aust. J. Phys. 27, 31. Ness, N. F., and Wilcox, J. M.: 1964, Phys. Rev. Letters 13, 461. Payne-Scott, Ruby and Little; A. G.: 1950, Aust. a r. Sci. Res. Ser. A4, 508. Pneuman, G. W., Hansen, S. F., and Hansen, R. T.: 1978, Solar Phys. 59, 313. Rosenberg, R. L. and Coleman, P. J.: 1969, J. Geophys. Res. 74, 5611. Schatten, K. H., Wilcox, J. M., and Ness, N. F.: 1969, Solar Phys. 6, 442.

* Note that we have adopted the Wilcox and Hundhausen (1983) correction to current sheet 1664; otherwise the correlation with source surface fields is worse (68%).

RADIO MAPPING OF THE HELIOSPHERIC CURRENT SHEET 147

Schulz, M.: 1973, Astrophys. Space. Sci. 24, 371. Solar Geophysical Data: 1984, NOAA, Boulder, Colorado. Stewart, R. T.: 1985, Solar Phys. 96, 381. Stewart, R. T., Eason, H., and Heisler, L. H.: 1985, Proc. Astron. Astron. Soc. Aust. 6, 231. Webb, D. F., Davis, J. M., and Mclntosh, P. S.: 1984, Solar Phys. 92, 109. Wilcox, J. M. and Hundhausen, A. J.: 1983, J. Geophys. Res. 88, 8095. Wilcox, J. M., Scherrer, P. H., and Hoeksema, J. T.: 1980, Science 209, 603.