Adv. Space Res.Vol. 11, No. 1, pp. (1)57—(1)60,1991 0273—1177/91$0.00+ .50Printedin GreatBritain. All rights reserved. Copyright © 1991 COSPAR

INTERPLANETARY MAGNETIC FLUXROPESOBSERVED BY THE PIONEERVENUS ORBITER

K. Marubashi

CommunicationsResearchLaboratory, Koganei, Tokyo 184, Japan

ABSTRACT

Interplanetary magnetic field data from the Pioneer Venus orbiter (PVO) were surveyed forfive and a half years from December 1978 through May l98~I in search of interplanetary magnet—Ic flux ropes near the Venus orbit. As a result, twenty—six well—defined flux ropes werefound which have characteristics similar to those of flux ropes observed near Earth. In onecase where the Sun, Venus and Earth were closely aligned, an almost Identical structure wasobserved by the PVO and the Earth—orbiting spacecraft with a time delay of about 36 hours.This observation provides evidence that the structure of interplanetary magnetic flux ropesIs maintained during propagation at least from 0.72 AU to 1 AU. A list of the 26 flux ropesidentified is presented for future studies such as comparisons with observations of coronalmass ejections.

INTRODUCTION

A magnetic flux rope is a cylindrical body of magnetized plasma with magnetic topologycharacterized by relatively strong axial fields near the center and weaker, more azimuthalfields towards the outer edge. The concept of interplanetary magnetic clouds /1/ was pro-posed through the recognition of the magnetic field changes in the solar wind which are nowknown to be characteristic of magnetic flux ropes. The topology of interplanetary magneticclouds was later shown to be consistent with that of magnetic flux ropes /2/. Close rela-tionships were also found between the production of interplanetary magnetic flux ropes andthe eruption of solar filaments /2/. The conclusion drawn through subsequent studies /3,4/was that the magnetic fields surrounding an eruptive prominence already have a flux ropestructure at the time of eruption, and that the structure extends or propagates throughinterplanetary space with its orientation maintained.

For further understanding of interplanetary magnetic flux ropes, it is highly desirable toinvestigate flux rope structures at places other than 1 AU. An excellent data base for suchstudies is provided by the magnetic field measurements made by the Pioneer Venus orbiter(PVO) which have been continued since December 1978 /5/. Because of its highly eccentricorbit, the PVO spent the largest part of its orbiting period in Interplanetary space exceptfor some limited intervals when its apoapeis was in the Venus shadow region. Thus the PVOmeasurements provide almost continuous data of solar wind magnetic fields at 0.72 AU. It isexpected that further evidence for the shape and propagation of interplanetary magnetic fluxropes can be obtained by comparing the PVO and near—Earth observations. In addition, the PVOdata may allow us to investigate direct effects on the solar wind of coronal mass ejectionslaunched above the east and west limbs of the Sun provided that the PVO was in the rightplace. This paper reports the results of a search of interplanetary magnetic flux ropes inthe data base provided by the PVO magnetic field measurements.

IDENTIFICATION OF INTERPLANETARY MAGNETIC FLUX ROPES

The top panel of Figure 1 shows variations of 2—minute averages of magnetic field componentsand the total field obtained from the PVO measurements. Here, the Venus Solar Orbital (VSO)coordinate system is used. The middle panel shows directional changes of magnetic field interms of latitudinal (THETA) and longitudinal (PHI) angles of magnetic field vectors. Thebottom panel presents a plot of standard deviations of components and total field. The timeinterval of strong total field centered around 1300 UT on each day indicate that the PVO was

UN3 K. Marubashi

-’ UT -00 i”* 00 06 12 18 00 06 12 18 xvso YVSO

-,z -,,:,

$6 ::0 0.6 -0.2 -0.7 0.9 0.3 -0.5 -0.9 0.7

-4.6 -6.5 -11.3 -11.1 -4.8 -6.4 -11.3 -11.2 zvso -3.6 -0.6 2.4 -4.8 -3.6 -0.6 2.4 -4.B -3.6 -0.6

-;.; .

I:.; .

THE16

UT -00 06 12 18 00 06 12 18 00 06 12 16

UT -00 06 12 16 00 06 12 16 00 06 12 18

Fig. 1. Typical example of the interplanetary magnetic flux rope observed by the PVO. Graphs present variations in magnetic field components and total field (top), direc- tional changes of magnetic field (middle), and variarions of standard deviations of components and total field. Start and end times of the magnetic flux rope are indi- cated by arrows. , .*

The left diagram of Figure 2 is a plot of l-hour averaged magnetic field vectors projected on the X-Y, X-Z and Y-Z planes. Those data obtained behind the bow shock are excluded. This diagram clearly shows that magnetic field vectors rotate very smoothly and nearly parallel to the Y-Z plane from 2300 UT on 4 July through 0000 UT on 6 July 1980. The right diagram presents the result of minimum variance analysis together with directions (4 and 8) of three eigen vectors and the corresponding eigen values (A) of the variance matrix. In general, these forms of diagrams are useful in determining the start and end times of interplanetary magnetic flux ropes and in estimating the degree of orderedness of magnetic field rotation.

For this particular example, model fitting was made with the same method as was applied in a previous work /2/. The result is presented in Figure 3. It is seen that the observed field change can be reproduced well with a rather simple flux rope model.

PVO/OHRG PLOT

1980

ORTE I/ 4 I/ 5 I/ 6 TIME (UT)

5

10 NT ‘t,

t

%

-36

-36 -36 0 36

A, = 449

I$, = 97”

8, = 18'

x2 = 157

$2 = 227"

e2 = 63“

x3= 10

$3= I0

83 = 19"

Fig. 2. Plot of l-hour averaged magnetic field vectors projected on the X-Y, X-X, and Y-Z planes (left), and a hodograph presentation in the principal axis coordinates (right).

IMF Flux Ropes (1)59

MODEL B/Ba ~ =

PARALLEL ~ ~—.l_—..—,= 90°0 i 0 —

DIRECTION 90 1 —

OF ROPE —90 —1 X2 — 0.11

360 1 = 228°

CLOSEST ~ ‘:; .. T.i ~J,d)0.1 R 0.0 0.6 1.0 81/80 03 = 16°

NORIIALIZED DISTANCE

Fig. 3. Results of model calculation for interplanetary magnetic flux rope presented

in Figure 1.

RESULTS OF A SEARCH

A search of’ interplanetary magnetic flux ropes was made with the PVO magnetic field data fromDecember 1978 through May 1984. In this search, first—step candidates were selected by using

plots such as those in Figure 1. Then, each candidate was examined with the use of’ a vectorplot as shown in Figure 2, and finally the minimum variance analysis was carried out to checkthe degree of orderedness of magnetic field rotation using a hodograph in the principal axiscoordinates.

The final selection is presented in Table 1. In the table, E—S—Vindicates the angle betweenthe Sun—Venus line and the Sun—Earth line at the time of magnetic flux rope detection. Inthe TYPE column, P and A denote parallel and anti—parallel types, respectively /2/. Theseparameters are particularly important when these magnetic flux ropes are compared with solarphenomena. The degree of orderedness of field rotation is presented in the last column. Itis subjectively ranked in four classes: excellent, good, fair, and possible.

TABLE 1 List of PVO—observed interplanetary magnetic flux ropes

NO. YR MON START & END DAY/TIME DURATION E-S—V TYPE QUALITY

1. 1979 JAN. 08/1100 — 09/0200 15 37 P FAIR2. OCT. 16/1100 — 17/1400 27 —147~ P GOOD3. OCT. 19/1200 — 19/2000 8 —145~ A GOOD4. OCT. 28/0500 — 29/0900 28 —14o~ P EXCE5. NOV. 25/1600 — 26/1900 27 —123~ P EXCE6. 1980 MAY 26/1200 — 27/1500 27 —12~ P GOOD7. MAY 27/2200 — 28/1300 15 —12’ P PSBL8. JUN. 23/1300 — 24/1100 22 —5’ A FAIR9. JUL. 04/2300 — 06/0000 25 12~ P EXCE

10. OCT. 11/0700 — 11/1500 8 74~ P GOOD11. OCT. 11/2100 — 12/0700 10 75’ A GOOD12. 1981 MAY 05/2300 — 06/1900 20 —162’ A GOOD13. JUL. 06/0800 — 07/1000 26 —121’ P GOOD14. JUL. 20/1600 — 20/2300 7 —1l2~ P GOOD15. OCT. 10/0500 — 11/0300 22 —6l~ P FAIR16. 1982 FEB. 05/0600 — 06/0100 19 9~ A PSBL17. APR. 14/2300 — 15/0800 9 51’ A GOOD18. AUG. 24/1600 — 26/0000 32 l35~ A EXCE19. SEP. 23/1300 — 24/0600 17 l53~ A GOOD20. 1983 MAR. 17/0400 — 17/2300 9 —103’ P GOOD21. APR. 19/1500 — 20/0600 15 —82’ A GOOD22. 1984 JAN. 05/0600 — 05/2300 17 8l~ P GOOD

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pvo~oIqRo PLOT

t 980

ORTE 6/22 6/23 6/24 6/25 6/26

~ __ __

DOTE 8/24 6/25 6/26 6/27 6/28~1: j’ st~’~‘~±ftIFig. 4. Magnetic field vector plots for the PVO and near—Earth observations of thesame interplanetary magnetic flux rope. Time axis for near—Earth data is shifted36 hours ahead.

COMPARISONWITH NEAR-EARTH OBSERVATIONS

It is worthwhile to examine whether the same interplanetary magnetic flux rope was observedby near—Earth spacecraft for each of NOS. 6, 7, 8, 9, and 16, because they were detected atVenus when the E—S—Vangle was small. Such examination reveals an interplanetary magneticflux rope in near—Earth space with almost the same structure as the flux rope of NO. 8. Thetwo observations are compared in Figure 4. The time delay between the two is about 36 hours,being consistent with the solar wind speed of 350 km/s measured on 25 June 1980 by Earth—orbiting spacecraft. ThUS, it is reasonable to conclude that the identical magnetic fluxrope was observed both at 0.72 AU and at 1 AU. This observation provides evidence that thestructure of interplanetary magnetic flux ropes is maintained during propagation at leastfrom 0.72 AU to 1 AU.

Another conclusion drawn from the present examination is that the poesiblity is not high forthe same magnetic flux ropes to be observed at both Venus and Earth. This result is relatedto the typical size of interplanetary magnetic flux ropes. The radius of flux rope istypically 0.2 AU, corresponding to the angular extent viewed from the Sun of only ll~ (0.2radian) at 1 AU. In this context, the interplanetary magnetic flux rope of NO. 8 may be theonly case out of the 26 examples to be observed at Earth.

CONCLUDINGREMARKS

A search of interplanetary magnetic flux ropes at the Venue orbit has been made with the useof the PVO magnetic field data. This allowed identification of many examples of’ magneticflux ropes with characteristics similar to those observed in near—Earth space. A case wasfound which suggests that the identical structure was observed both at Venus and at Earth.It can be emphasized that more detailed comparison between data from the PVO and data fromnear—Earth spacecraft can provide additional information on the shape and propagation of theinterplanetary magnetic flux ropes, and other transient phenomena as well.

ACKNOWLEDGEMENTS

The solar wind data of both the PVO and near—Earth satellites used in this work were suppliedfrom the US National Space Science Data Center, NASA, Goddard Space Flight Center, throughthe Space Observation Data Center, ISAS. The author wishes to thank those people supportingactivities at both Centers.

REFERENCES

1. L.W. Klein and L. F. Burlaga, Interplanetary magnetic clouds at 1 AU, J. Geophys. Res.,87, 613—624, 1982.

2. K. Marubashi. Structure of the interolanetarv mamnetic clouds and their solar orimins.