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Spatial Variation and Evolution of Heliospheric Sector Structure

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Spatial Variation and Evolution of Heliospheric Sector Structure JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. A2, PAGES 1245-1260, FEBRUARY 1, 1989 Spatial Variation and Evolution of Heliospheric Sector Structure K. W. BEHANNON AND L. F. BURLAGA Laboratory jbr Extraterrestrial Physics,NASA Goddard Space Fli•tht Center, Greenbelt, Maryland J. T. H OEKSEMA Centerfor Space Scienceand Astrophysics,Stanford University, Stanford, California L. W. KLEIN Applied Research Corporation, Landover, Maryland The magneticsector polarity at the Sun and in interplanetaryspace has been surveyedduring approxi- mately three quarters of sunspot cycle 21 using photosphericmagnetic field observationsfrom the Wilcox Solar Observatory at Stanford University and spacecraftobservations by Voyagers 1 and 2, Pioneer Venus Orbiter, and, to a lesser extent, Helios A, IMP 8, and ISEE 3. During the more than 8-year period of the study, late 1977-1985, the Voyagers measured the interplanetary magnetic field (IMF) over a heliocentricdistance range of 1-25 AU and a heliolatitude range of -5ø-27 ø. The photo- spheric data were used together with a potential field (PF) model to calculate the location of the heliosphericcurrent sheet near the Sun throughout the same period, which included sunspotmaximum and much of the declining phase.The large-scalemagnetic polarity structure was found to be in almost continuousevolution throughout the period of study. This included an alternation betweentwo-sector and four-sector patterns, with a mean duration in each case of approximately seven solar rotations. Although the azimuth of the IMF was found by the Voyager spacecraftto be oriented more than 60ø from the theoretical Parker spiral angle 20% of the time, application of smoothingtechniques produced IMF polarity patterns which could be comparedwith the pattern derived at the Sun. Such comparisons revealedvery good agreement(82%) betweensolar and IMF sectorpatterns observed by Pioneer Venus in the inner heliosphereand significantly poorer agreement (61-64ø/,,) between solar and outer heli- ospherepatterns observedby Voyagers 1 and 2. This may be the result of dynamical processesin the solar wind which alter the position and shape of the current sheet beyond a few astronomical units. Sector pattern stability was found to decreasewith increasingheliocentric distance, also. Evidence was found for the more frequent occurrenceof four-sectorstructure near the Sun than in the outer heli- osphere.In 1985, when Voyager 1 reached a heliolatitude of 25ø, it observedthe disappearanceof the sector structure• consistent with the tilt of the current sheet deduced from photospheric magnetic field measurements. 1. INTRODUCTION has been demonstrated that the observedsectoring of the IMF It has now been more than 20 years since the discoveryof is the result of a large-scale heliospheric current sheet rooted the sector structure of the interplanetary magnetic field (IMF) at the Sun. Altschuler and Newkirk [1969], Schatten et al. and the first attempts to find a correspondencebetween the [1969], and Schatten [1972] developed use of the potential IMF and observedsolar magnetic fields. In that initial work, field (PF) model to calculate the coronal magnetic field and using measurementsmade by the IMP 1 satellite,Ness and current sheet as sourcesof IMF structure. Schulz [1973] pro- Wilcox [i 964] establishedthe photosphericsource of the IMF posed a theoretical model in which the heliomagnetic equator and demonstrated that near the sunspot cycle minimum in is extended by the solar wind into a warped annular neutral 1964 the IMF was sectoredinto a quasi-stationary pattern of surface.An inclination of the solar magnetic axis to the Sun's alternatingregions of dominantmagnetic field polarity, either rotation axis causes the surface to wobble, creating a two- toward or away from the Sun along the Parker spiral direc- sector pattern, with a quadrupole contribution necessary to account for four sectors.Sval.qaard et al. [1974] developed a tion [Wilcox and Ness, 1965]. Although the pattern remained similar, phenomenological model combining the large-scale relatively stable during that epoch, observationstaken at polar field of the Sun and solar sector structure. It has been other times showed lessstability, with changesin the number concluded that the observed solar cycle variations in IMF of sectorsand in their sizes[e.g., Colemanet al., 1966; Fairfield sector structure result from changes in the inclination, shape, and Ness, 1967; Coleman et al., 1967; Ness and Wilcox, 1967; and topology of that sheet. Wilcox and Colburn, 1969, 1970, 1972; Fairfield and Ness, Rosenbertdand Coleman [1969] found that the dominant 1974]. polarity of the IMF either north or south of the current sheet With the collective studiesover nearly two complete 11-year is simply that of the dipolar component of the Sun's field in sunspotcycles since the earliest spacecraftinvestigations, it the same hemisphere. Thomas and Smith [1981] found the results of their analysis of Pioneer 10 and 11 measurements during 1972-1975 (approachingsolar minimum) in the region Copyright 1989 by the American GeophysicalUnion. 1-8.5 AU largely consistentwith a simple tilted sheet model, Paper number 88JA03516. with the tilt decreasing with time from 30ø to 15ø . When Pio- 0148-0227,'89/88JA-03516 $05.00 neer 11 reached 16ø heliographic latitude in 1976, the IMF 1245 1246 BEHANNON ET AL.: HELIOSPHERICSECTOR VARIATION AND EVOLUTION was found to be almost totally of one polarity [Smith et al., overall purpose of the work, however, is to demonstrate the 1978], suggestinga sheet confined to the equatorial region extent to which the polarity structure inferred near the Sun, with at most small-amplitude warps. Svalgaard and Wilcox and assumedto be frozen into the solar wind at that point, is [1974] had shown that the latitudinal extent of current sheet in fact observedby spacecraftin the IMF over a wide range of warp is consistentlylower at sunspotminimum than at other distancesfrom the Sun, over a modest range of heliographic times in the sunspotcycle. Using Helios 1 and 2 observations latitudes,and over a large fraction of a solar cycle. in the inner solar system,[/illante et al. [1979] obtained simi- In the next section we discussthe current sheet configura- lar resultsduring 1973-1976. tion at the Sun calculatedfrom the PF model, in particular its Wilcox et al. [1980] and Wilcox and Scherrer [1981] latitudinal and temporal variationsduring the period of study. argued that large-scale warping of the sheet results from We then describethe sectorpatterns observedin the IMF by photospheric magnetic field structure, with only small-scale the Voyager spacecraftthroughout the period. In section4 the "ripples" from the ballerina skirt effect proposed by Alfv•n polarity patterns observedat PVO, V1, and V2 are compared •1977]. Burlaga et al. [1981] found that the current sheet with patterns inferred on the Sun, both at the equator and at extended northward significantly less near sunspot minimum higher latitudes. Supportingobservations by Helios A, IMP 8, than Wilcox et al. and Wilcox and Scherrer estimated, and and ISEE 3 are also discussed.Section 5 presentsthe resultsof they suggestedthat proper allowance must be made for the stability analyses of the sector structure at the Sun and in the polar magnetic field of the Sun in calculating current sheet IMF. In the final section we interpret and discussthe results, structure using the PF approach. Bruno et al. [1982] con- comparing where applicable with the results of earlier studies. cluded that the four-sector pattern observed near solar mini- mum (1976-1977) was produced by a warped current sheet 2. CURRENT SHEET STRUCTURE AT THE SUN that was nearly symmetric about the solar equatorial plane The structure of the heliosphericcurrent sheet at the Sun and that the amplitude of the warps was small. The results during the period of this study has been calculated from were attributed to a magnetic dipole nearly aligned with the photospheric magnetic field observations made at the Stan- Sun's rotation axis and a quadrupole component 17% as large ford Solar Observatory during 1976-1986. By use of a PF as the axial dipole. model the structure is determined on a source surface, the Hoeksema et al. [1983] calculated the shape of the current spherical surface at which solar magnetic field lines become sheet at the Sun using their P F model that included the polar radial [Schatten eta!., 1969; Altschulerand Newkirk, 1969]. magnetic fields computed from polar observations [Hoeksema The radius of the sourcesurface (2.5 Rs) and the strengthof et al., 1982]. Near solar minimum they inferred a sheet with the polar field were optimized by correlation with IMF obser- predominantly two northern and two southern excursions(i.e., vations [Hoeksemaeta!., 1983; Hoeksemaand Scherrer,1986]. four sectors) of small amplitude, approximately symmetric The polar field strength changesduring the solar cycle and about the helioequator. They found a good correlation be- therefore requires a variable correction [Hoeksema et al., tween the PF model predictions and the IMF polarity ob- 1983]. servedat Earth. Using this form of the PF model, Wilcox and The resulting calculatedlarge-scale structure has been illus- Hundhausen[1983] and Bruno et al. [1984] demonstrated that trated for each solar rotation and discussedin detail by Hoek- the quasi-stationary, four-sector structure of the heliospheric sema et al. [1983] for Carrington rotations (CR) 1641-1729. current sheet inferred early in cycle 21 (1976-1977)
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