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Produced by the NASA Center for Aerospace Information (CASI) THE EXTEM)ED CORONAL MAGNETIC FIELD

_N 1 - 3 2 3 5_0 (ACCE5SION NUMBER) (THRU)

O (PAGES) (CODE,

(NASA CR OR TMX OR AD tf'UN DER) (CATE6;V)

Series 11, Issue 74 October 16, 1970 UNIVERSITY Of CALIFORNIA, BERKELEY .'0

THE EXTENDED CORONAL MAGNETIC FIFID

John M. Wilcox

Technical Report

ONR Contract N00014-69-A-0200-1016, Project NR 021 101 NASA Grant NGL 05-003-230 and NSF Grant GA-1319

Distribution of this document is unlimited.

Reproduction in whole or in part is permitted for any purpose of the United States Government.

To be published in the proceedings, NATO Advanced Study Institute on Physics of the Solar Corona, Athens, September 1970. r n

sad x_

THE EXTENDED CORONAL MAGNETIC FIELD

John M. Wilcox

Space Sciences Laboratory

University of California

I Berkeley, California 94720, USA

Abstract. The coronal magnetic field should contain many field lines

connecting the photosphere to interplanetary space. A sharp boundary

separates two adjacent sectors of opposite polarity. The large-s,1,%le

structure of the corona is related to the photospheric sector pattern.

The corona may frequently contain transient magnetic loops reaching out

to five to ten solar radii.

As has already been mentioned in the introductory talk there is a

good correspondence between the large-scale photospheric magnetic field

and the large-scale interplanetary magnetic field observed with space-

craft near the earth. This situation means that the coronal magnetic

field problem is bounded, although the outer bound is perhaps at a rather

large distance Fran the region of the corona most often discussed. In

the first portion of this paper we shall indicate the nature of the large-

scale patterns in the photospheric field and then indicate some implications

and related observations for the extended coronal field.

The classical Babcock (1961) model of solar magnetism utilizes the

stretching and field amplication effects of differential rotation to explain

a number of the observed solar magnetic phenomena, including the observation 2.

that the polarity of a bipolar magnetic region in the northern solar hemisphere is opposite from the polarity of a bipolar region in the southern hemisphere. An additional large-scale pw,tern in the photospheric magnetic field having rather different prop,-rt ns has recently been discovered (Wilcox and Howard, 1968). Figure 1 40., a schematic showing some of the main properties of this solar sector pattern. A boundary exists approximately in the north-south direction. On one side of the boundary the large-scale weak photospheric field is predominantly directed out of the sun, and on the other side of the boundary this field is predominantly directed into the sun. This pattern exists over a wide range of latitudes on both sides of the equator. The boundary rotates in an approximately rigidly rotating coordinate system, stnce it is very little influenced by the shearing effects to be expected from differential rotation. The solar sector patt y: - ,i thus differs from the Babcock model in two fwidamental respects: 1) The solar sector pattern rotates in an a most rigidly rotatirK coordinate system while the Babcock model depends on differential rotation to produce the observed effects, and 2) the solar sector pattern has the same polarity on both sides of the equator while the Babcock model (and observations) show that bipolar magnetic regions have opposite polarities on either side of the equator. Yet the two patterns coexist on the sun.

The solar sector pattern is the source (Wilcox and Ness, 1965) of a corresponding interplanetary sector pattern, an example of which is shown in Figure 2. Some of the solar magnetic field lines are carried outward by the radially flowing solar wind plasma. The combination of the radial plasma flow and the solar rotation leads to an Archimedes spiral shape for an average interplanetary field line. Thus a solar sector boundary is 3• transported into interplanetary space in the form of an Archimedes spiral.

The interplanetary sector pattern rotates with the sun so that a complete pattern sweeps past the earth every 27 days (the solar rotation period).

The pattern shown in Figure 2 was approximately stationary in time for one year (1964) near the minimum of the 11-year sunspot cycle. The evolution with time of the sector pattern (Wilcox and Colburn,

1970) is shown in Figure 3, which is basically a 27-day calendar. The top raw represents the first 27-day rotation, the second row is the next

27-day rotation and so on. The shaded regions indicate the polarity of the interplanetary magnetic sector pattern. The stationary pattern with four sectors per solar rotation can be seen during the year 1964. With the rise of solar activity in 1965 the sector pattern begins to change.

Usually one solar rotation is quite similar to the preceding rotation, but in the course of several rotations an appreciable change in the sector pattern may occur. From the diFc:us siun so far we see that the coronal magnetic field should contain many field lines connecting the photosphere to interplanetar!r space. A sharp boundary separates two adjacent sectors of opposite polarity.

We will next establish that the region to the west of a sector boundary

(before the boundary in the nense of solar rotation) is a quiet region while the region just east of the boundary (after the boundary) is an active region.

First we may examine the location of flares. Do they occur at random with respect to sector boundaries? We see in Figure 4 that the region close to a sector boundary is the most likely site for a flare. Figure 4 is a histo- gram of flare occurrence as a function of distance from a sector boundary, where distance is measured in terms of days of rotation (one day equals 130 longitude). The results by Bumba and Obridko (1969) have been extended by 4.

Vladimirsky (private communication) using a larger body of observations.

Vladimirsky confirms these results and shows that the most likely position

for a flare is just eastward of (after) a sector boundary. Is this a parti-

cular property of flares or does it extend to other solar activity? Figure 5

shows the average position of plages in the sectors observed near solar

minimum (Wilcox and Ness, 1967). in Figure 5 the, preceding boundary of a

sector is at about 50°w and the fallowing sector boundary is at about 500E. We can see that the plages are more numerous in the areas just after the

sector boundary.

Does this same property exist in the extended coronal field near the

earth? To answer this question we may use the earth's magnetic field as

a probe, since this field is influenced by interplanetary conditions.

Figure 6 shows the average response of geomagnetic activity as a sector

boundary sweeps past the earth (Wilcox and Colbaarn, 1970). The abscissa

labeled zero represents the time at which a sector is observed near the

earth and the graph shows the situation four days before and four days

after this time. We see that in the days before the sector boundary geo-

magnetic activity is monotonically decreasing, with an almost discontinuous

increase near the boundary and a petals shortly Vaereafter. Thus we see again

that the region just before the boundary is quiet and the region after the

boundary is active.

Having established this boundary situation in the photosphere and at

the distance of the earth we may inquire if the same effect exists in the ^y corona. The vertical hatched regions in Figure 7 (Couturier and Leblanc,

1970) represent coronal enhancements observed with the Nancay radio inter-

ferometer at 169 MRz (1.77m). These are regions of enhanced electron density

and temperature at altitudes between 0.2 and 0.5 solar radii above the p.ioto- 5•

sphere. At the bottom of Figure 7 the interplanetary sector polarity is

indicated. It can be seen that just after each sector boundary a coronal

enhancement occurs. Thus we see that the large-scale structure of the corona is related to than photospheric and interplanetary sectorap tterns. We examine now some unique observations of the coronal magnetic field

M. the ragions 5 to 10 solar radii that were obtained when the Pioneer 6

It spacecraft was occulted by the sun. The Faraday rotation of the microwave

telemetry signal from the spacecraft was observed (Stelzried et al., 1970)

during the time in which the line-of-sight from earth to Pioneer had its

closest approach to the sun in this region. The positions of Pioneer, sun

and earth are shown in Figure 8. The Faraday rotation observation gives

the product of the line-of-sight magnetic field multiplied by the density

along the column from earth to Pioneer. Most of the rotation occurs near

the shortest distance to the sun where the densities and field magnitudes

are largest. Occasionally the observed Paraday rotation showed a change .y, of 300 within a time interval of 2 hours, as shown in Figure 9. This observation has been interpreted (Schatten, 1970) in terms of loops of magnetic

field transported outward from the sun by plasma ejected from flares. The

three sketches in Figure 10 show the situation before, during and a lter the

event shown in Figure 9. The Faraday observations in Figure 9 agree in

direction and in magnitude with observations of an active region in the . photosphere assumed to be the source of the flare. The flares producing

events of this kind are not particularly large, and we may therefore assume

that often the corona contains such magnetic loop structures. It is rare to find evidence of such loops in the observations of the interplanetary

field near the earth. It therefore seems that most of the loops return

to the sun after having reached perhaps a distance of 10 solar radii. I

Thus the corona mad frequently contain transient magnetic loops reaching out five to ten solar radii.

Acknowledgements

This work was supporter) in pert by the Office of Naval. Research under contract N00014-69-A-0200-1016 1 by the National Aeronautics and

Space Administration under grant NGL 05-003-230 1 and by the National Science Foundation under grant GA-1319. ..

7•

References

Babcock, H. W.: 1961, Astrophys. J. 1339 572•

Bumba, V. and Obridko, V. N.: 1969, Solar Physics 6, 104.

Couturier, P. and Leblenc, Y.: 1970, Astron. & Astrophys. 7 9 254.

Schatten, K. H.: 1970, Solar Physics 12, 484.

Stelzried, C. T., Levy, G. S., Sato, T., Rusch, W. V. T., Ohlson, J. E.,

Schatten, K. and Wilcox, J. M.: Solar Physics, to be published.

Wilcox, J. M. and Colburn, D. S.: J. Geophys. Res., to be published.

Wilcox, J. M. and Hovmrd, R.: 196a, Solar Physics 5, 564.

4;',' I c (YA, J. M. and Ness N. F.: 1965, J. Geophys. Res. 70, 5793•

Wilcox, J. M. and Ness, N. F.: 1967, Solar Physics 1, 437.

Wilcox, J. M., Severny, A. and Colburn, D. S.: 1959, Nature 224, 353•

Y 8.

Figure Captions

Figure 1. A schematic of the average position of a solar sector

boundary during 1965. On each side of the boundary the weak background

photospheric magnetic field is predominantly of a single polarity in

equatorial latitudes on both sides of the equator (after Wilcox et al.,

10 1969)•

Figure 2. The plus signs (away fram the sun) and minus signs (toward

the sun) at the circumference of the figure indicate the direction of

the measured interplanetary magnetic field during successive 3-hour

intervals. The inner portion of the figure is a schematic representation

of a sector structure of the interplanetary magnetic field that is suggested

by these observations. The deviations about the average streaming angle

that are actually present are not'shown (after Wilcox and Ness, 1965).

Figure 3. Observed sector structure of the interplanetary magnetic field,

overlayed on the daily geomagnetic character index C9, as prepared by the

Geophysikalisches Institut in G8ttingen. Light shading indicates sectors

with field predominantly away from the sun, and dark shading indicates

sectors with field predominantly toward the sun. Diagonal bars indicate t an interpolated quasi-stationary structure during 1964 (after Wilcox and

I Colburn, 1970).

+I 9.

Figure 4. Histograms of frequency distribution of the time difference between the central meridian passage of spot groups and the position of solar sector boundaries for the groups: (a) with flares of impo'.stance

1 + or greater; (b) with a number of flares equal or greater than 10 (after Sumba and Qbridko, 1969).

Figure 5. Superposed-epoch analysis of calcium plage structure obtained frcm the daily Fraunhofer Institute maps of the sun. The sectors are approximately centered at central meridian, so that the leading edge of the sector is at about 50 OW and the trailing edge of the sector about 50 0E longitude (after Wilcox and Ness, 1967).

Figure 6. Superposed epoch analysis of the magnitude of the plane- tart' magnetic 3-hour-range indices Kp as a function of position with respect to a sector boundary. The abscissa reFresent position with respect to the sector boundary, measured in days, as the sector pattern sweeps past the earth. The solid line represents similar results obtained near solar minimum, the dots represent results ill 1967 and the Xs represent results during 1968 (after Wilcox and Colburn, 1970).

Figure 7. Solar wind activity during solar rotation 1768. The vertical hatched regions represent CIP of coronal enhancements. a, solar wind velo- city, b, proton density, c, temperature (upper and lower limits), d, interplanetary magnetic field magnitude, and e, sector polarity pattern (after

Couturier and Leblanc, 1970). 10.

Figure 8. positions of the Pioneer 6 spacecraft, the sun and the earth at the time of observations of the Faraday rotation of the spacecraft tele- matey signal.

Figure 9. Observed polarization of the telemetry radio signal from Pioneer 6 as a function of time (after Stelzried et al., 1970).

Figure 10. View from the north of the magnetic field model proposed to explain the transient events observed with Pioneer 6. The line-of- sight between Pioneer 6 and the earth is shown to the right of each panel (after Schatten, 1970)(, ad V 0 l V Na W I 0 +- N s 0 c OL a Q oC mi Q c C 0 0 0 6. a V$ Z v^ W M uoW C^ A m^ Q m 3 N

Z ^ W ti ^O C11 cV V O ^ ON ^++++ +++ + + V _ W I I !I j 1 I ' 1 1 , I i i I I++++ 1 1, II ' I I ++ ++ I ±+ Z + III CD +++ ++ c^ + ++ ++

+++++++

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Figure 5

3.5

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-4 -3 -2 -1 0 1 2 3 4 POSITION WITH RESPECT TO SECTOR SOUNDARY, DAYS

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Figure 8

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TO PIONEER VI

MAGNET I C FIELD LINES

i DURING QUIET PERIOD SUN - M SECIUR BOUNJARY

SECTOR

112 DA Y AFTER FLARE SUN ^ ^ BOUNDARI I

-- 2 DAYS AFTER FLARESUN SECTOR BOUNDARY

TO EARTH Figure 10 UNCLASSIFIED ^Securlty Cla .-,.oration DOCUMENT CONTROL DATA R^.D ♦ (i.tur/fy tla^.11f1.111u►I of fill• b..Jr of obefte, l enJ b o,Ifwrw I wh.n raw ..roll r.r-,FI of 4 1e.0I1,w,1i l ORIGINATING AC TIVI • Y Wnrp,-far. @-rthar) 20 R(POtwT aaCURI T ♦ C LA 1 11FICA TION Space ^ciences Laboratory Unclassified Urd veraity of California a h Gmau: Berkeley, California 94720 REPORT TITLE —^- —

THE E)CI'FMKD COI?OrU+L MAGNETIC FIEP,

A DESCRI P TIVE NO' E`- ;Type of re1wrt on., Inclusive date@) Technical Report ~ S AUTHOR(S) (Loot rune, ttrer name. Initial)

W t 1 c ox T .John M.

' 6 RE PO PT VATE...__^^.-- 70 TOTAL NO OF PA TES 70 NO O P R[P/ Ch: tober 16, 1970 19 10 a ON T R A C - O A • (. R A N T N - ^a 0NI0INATCR'1 0%tPO l1T NU•A8F P(S) UNR Corr;ract N00014-69-A-0 00-101.6

b P 4C:9C1 h^ NR 021 101 T'A.,A Grant FCL 02-003-230 C. •b O T +ER wFPOA T NO(S) (Any other numb Pro tl.at may b@ asvlan@d this report) d NSF Grant GA-- 1319 10 AVAILABILITY 'LIMITATION NOTICES

Qualified requesters m1y obtain copies of this report from DDC.

I I SUPPLEMENTAnY NOTES 12 SPONSO IUNG MILITARY ACTIVITY Nuclear Physics Branch Office of Naval Research Washington, D. C. 20;60

13 ABSTRACT

The coronal magnetic field should contain marry field lines con-

netting the photosphere to interplanetary space. A sharp boundary

separates two adjacent sectors of opposite polarity. The large-scale

Etructure of the corona is related to the photospheric sector pattern.

The corona may frequently ^.obtain transient magnetic loops reaching

out to five to ten solar radii. (U)

r F DD I AA , 6e 1473 UNCLASS IFIED -- C^!`^l rf tL l l .Eln:J— -- -- _LTNCT ASS IF_IED_ Securov Cla —Aica! ion

FORM DD , a. 1473 ( BACK) vNcLASS IF= Security Classification