Cometary boundaries: VEGA observations at Halley

Adv. Space Rca. V

01. 6. No. 1. pp. 217—~8.1966 Printed in Great Bntain. All rights resersed.

027~1t7766 9)00 + .50 Copvrieht © COSPAR

COMETARY BOUNDARIES: VEGA OBSERVATIONS AT HALLEY K. Schwingenschuh,* W. Riedler,* G. Schelch,* Ye. G. Yeroshenko,** V. A. Styashkin,** J. G. Luhmann,*** C. T. Russell*** andJ. A. Fedder~ Space Research Institute, Austrian Academy of Sciences, Inffeldgasse 12, A-8010 Graz, Austria **Jz~f~~~y p/a Academgorodok, Podolskij District, Moscow Region, U.S.S.R. * * * Institute of Geophysics and Planetary Physics, University of California, Los Angeles, U.S.A. fNaval Research Laboratory. Washington, DC 20375, U.S.A. *

ABSTRACT This paper describes the boundaries which have been observed by the magnetic field experiment aboard the VEGA—i and VEGA—2 spacecraft during the Halley encounters on March 6 and 9, 1986. The outer boundaries are found to be closely related to the predicted cometary shock front. The inner boundaries near the closest approach are compared with plasma observations and with the results of MHD computer simulations in three dimensions. INTRODUCTION

The interplanetary magnetic field plays an important role in the interaction of the solar wind with the atmosphere of unmagnetised bodies (e.g. Venus and comets) in the solar system /1/. In order to interact with the interplanetary field in the solar wind the neutral atmospheres have to be ionised. At present we know four ionisation processes: photoionisation by the solar EIJV radiation, impact ionisation through energetic electrons, charge exchange between solar wind protons and planetary or cometary neutrals and critical velocity ionisation. The freshly ionised particles are picked up by the ambient solar wind. These picked up ions add mass to or mass—load the solar wind. Due to the mass—loading the solar wind is decelerated thus causing a draping of the frozen—in—magnetic field lines. The result is a tail—like configuration with two oppositely polarised magnetic lobes. The slowing of the flow can also produce a shock front. However, the interaction of the solar wind with the atmosphere of comets can be different from the interaction with unmagnetised planetary objects possessing an atmosphere. The solar wind flows through the comet and not around it. There is slowing down and deflection of the solar wind but this process is not as abrupt as it is at a planet. Thus it is not obvious that a bow shock forms. In most theoretical models of the cometary - solar wind interaction the comet is treated as a source of gas within a flow of magnetised plasma /10/. These models predict an outer and inner shock as well as a contact discontinuity separating the solar wind from the cometary plasma. Modern computers can simulate this interaction in three dimensions calculating the total mass density, velocity, temperature and magnetic field /5,9/. The flyby of the two VEGA spacecraft at comet Halley now provides a means to observe in—situ these boundaries of the cometary — solar wind interaction. OUTER BOUNDARIES: EVIDENCE FOR A BOW SHOCK? When the two VEGA spacecraft approached Halley the magnetic field magnitude and turbulence in the range 0.04 — 0.7 Hz increased gradually at distances of 2 — 1.2 X km (Fig. 2). Intense fluctuations with periods from 5 to 6 minutes have also been detected. The high— frequency plasma—wave analyser (APV—V) on board the VEGA spacecraft observed also an increase of electric field strength at about the same distances /3/. The IMF in this region showed an away polarity when VEGA—l passed and a distinct ‘toward’ polarity during the VEGA-2 flyby (Fig. 1 and Fig. 3). During the VEGA—l encounter at a distance of 1.18 X 10~ km (3:10 UT) the magnetic field turbulence in the frequency range 0.04 — 2 Hz increased rapidly (label Sl in Fig. 2) but no abrupt changes in the total magnetic field were observed (Fig. 4.3.) . Table 1 shows 10 mm averages and standard deviations of the magnetic field components arid of the magnitude during this interval. The low—frequency plasma—wave analyser (APV—N) on board VEGA—l detected at a distance of 9.9 ~ lO~ km (3:46 UT; label S1’ in Fig. 2) a sharp rise in the 1.5 Hz range of the electric field channel /11/ and the plasma analyser observed in the same region a broadening of the proton distribution /7/. Fig. 4.2 shows that this boundary was not so well observed by VEGA-2.

217

jAsR

~,:—o

K. Schwingenschuh et a!.

218

TABLE 1 VEGA—i magnetic field averages and standard deviations of the S1 boundary, which can be interpreted as an inbound bow shock. The shock occurred most probably at 3:46 UT (distance 9.9 X i0~ kin) /2/ when the APV—N instrument showed a sharp increase of the electric field in the 1.5 Hz frequency range /11/. Using these magnetic field values and the coplanarity theorem a eB of 2O~±l5~ (see also Fig. 3.3) was calculated. This is typical for a quasi—parallel shock. distance

1.14

—

1.09 (106 kin)

1.02

—

9.75

UT

3:20

—

3:30

3:45

—

3:54

B

—

(nT) x SD_BX (nT)

6.2

—

2.4

2.4

3.7

6.2

3.3

SB~

1.8

2.3

Bz

8.2

13.5

SB

1.2

3.0

BT

12.4

14.8

1.3

3.0

SBT

VEGA—I

—

MISCNA MAGNETIC FIELD DATA

SE COORDINATES 19—FEB—86

~

E0

~

19 21 19—FEB—86

(106 J<;~)

23

25

27

1

3

5

:°

~

~7 ~

9~ 11 ~

13

15

17

19 21 UT 21—MAR—86

Fig. 1. The IMF measured by VEGA—i 15 days before and 15 days after the VEGA—i encounter. The magnetic field during the VEGA—i and VEGA—2 encounters is indicated by the labels Vi and V 2. After VEGA—i had passed the inner regions of the coma with the typical inverse—V shaped magnetic field magnitude signature the spacecraft entered a region where the total field decreased slowly to the undisturbed IMP. Contrary to the inbound part of the encounter the B5 component had a positive (‘toward’) polarity. During the outbound phase of the encounter the VEGA—i magnetometer observed in this outer regions two boundaries. The first one was crossed at a distance of about 4.5 X i05 km and is indicated by an increase of magnetic field fluctuations (label S2 in Fig. 2) . The solar wind velocity increased also (Fig. 4.4) . The second boundary is marked by a sudden decrease of magnetic field turbulence down to undisturbed solar wind values (label s3 in Fig. 2) Several years before the first in—situ measurements near a comet the interaction between the solar wind and the cometary atmosphere was modelled using advanced computer simulation techniques /9,5/. The position of the bow shock from the nucleus during the VEGA Halley flyby was simulated by numerical MMD models /12,8/ and by particle simulations /13/ and was found to be approx. 106 km assuming an outgassing rate of io~° molecules/sec. If we compare these

Cometary Boundaries at Halley

219

2 10~

6

0 UT

I

I

y~s~~

I

2 106

106

10~

~

2 106

Xcss(km) to Sun

16 Ui

VEGA-i MAGNETIC FIELD DATA Bx AND BT

0TALCSE.COORD.

~

~/\

~50 ~

0

I

_________________

0

2

4

6

~2

I

8

10

S3 I

I

12

14

I

16UT

VEGA-i MISCHA BX-SPECTRAL CHANNEL 0.039-0.664 Hz

S1~1

0.4

~

2

8 10 CA 2092 1521 951 381 190 761 distance to the nucleus (1O~km) 0

4

6

~L2 12

14

1331

1902

16 UT

2472

MARCH 6, 1986

Fig.

2. The lower panels show the total magnetic field, the B component and the spectral density of this component during the VEGA—i encounter. The dipper sketch show the VEGA—i trajectory in the Xc E — Zc55 plane and a model bow shock from /13/. S1 and S2 are boundaries closely related ~o the cometary bow shock. The boundaries ~i and S3 are most probably foreshocks. results with the magnetic field /14/ and plasma measurements /7,11,13/ the detected boundary Si’ is a candidate for an inbound shock and the boundary S2 is a potential outbound bow shock crossing. Fig. 3.1 and Fig. 3.2 show the magnetic field along the VEGA—i and VEGA—2 trajectories projected onto the ~ — plane together with the model bow shock from ref. /13/. The subsolar stand—off distance used is 2.7 X ~ km and the semi—latus rectum of the fitted hyperbola is 5.6 x iO~ km. The angle S between the model shock normal and the measured magnetic field is shown in Figs. 3.1 an~3.2. Due to these calculations the VEGA—i inbound shock should be quasi-perpendicular (8 = 85 +1- lOs) and the VEGA-i outbound shock (6~ = 50 +/- 10) more quasi-parallel. Using t~ same method the VEGA—2 model inbound shock sho~id have a S of 60 +1— 20*. Due to the large standard deviation the shock cannot be 8B4.1 = 620) quasi— classified. A si~ation similar to the VEGA—i encounter is shown in Fig. whenand the a Pioneer parallel outbound = 26°). Comparing the magnetic Venus Orbiter (PVO)shock passed(6~ a quasi—perpendicular inbound Venusfield shock data ~ of the VEGA—i flyby and of the PVO shock cross~ngs both VEGA—i crossings are more like the PVO quasi—parallel outbound shock. This is also consistent with the coplanarity calculations where a ~Bn from 2° to 30°was calculated (see Table 1) The boundaries

S

1 and S3 outbound bow shocks.

in

Fig. 2

can

be

interpreted

as

foreshock

of the

inbound

and

220

K. Schwingenschuh

VEGA—i

—

MISCHA MAGNETIC FIELD VECTORS 23:00

er a!.

CSE COORDS.

-

Fig.

UT

85°+/-lo°

=

-

projected X — ~CSE plane.onto The the hybe~~la is a model bow shock from

33~05 Lu in

—

5o~+i-io.

‘~‘

vs - BY

0 o

15:00

7’

0 Cs 0

05—MAR-65 —4d00

‘

during the encounter

/13/. inbound Parameters (5 and MMS) of the and outbound (~j 3n ) bow shock crossings are Bn shown.

,

0

nT20 BX qL~._J

magnetic

0

°

MHS

The

field vectors VEGA—i

o Cs

,8 BN

3.1.

0 .0

I

—3doo

I ‘

—2doO

‘

—1~00 X — CSE

I

0 1000 ~IQ00 kin]

VEGA—2 — MISCHA MAGNETIC FIELD 23:00 VECTORS liT

2000

3000

CSE COORDS.

—

2 = Y2 + Z2) of 3.2. Vector plot CX and U; U the VEGA-2 encounter. Only inbound data are Fig.

0 0

shown.

0 ~0 0 0

-

-o 0

the magnetic field during

Si in

mu

0I ~0

nT 0 20 L_1 BXvs

U

0 0 ~0

06—MAR—86 —4000

‘

: I

—

3d00

06—MAR—86 DAY:

—2600

—idoo X — CSE

0 1000 ~4000 kin]

I

I

2000

3000

65

VEGA 1

90

NO~4ALVECTOR. 0.525 —0.527 —0.668

80 70

9AI$

—

29.210

IBM

—

tie

60

40 30

cJLJ~rJLJLL

to 20

30

Fig.

25

magnetic the between magnetic

~

~T

15 to 5

field angle the field

The

total

(B I

and

I (TBN) upstream and the

shock normal during the VEGA—i inbound shock •

00 03:

3.3.

10: 00

I

•

I

-

20:00

IDE IN

•

10:00 MI/SE

I

•

•

40:00 80:00 CSE COa~S

0:00

10:00

04:

10: 00

crossing. The normaiwas determined using the coplanarity theorem.

Cometary Boundaries at Halley

401 O’b~t~5 ~

~j

3~9~9

Fig. 4.1. Outbound (lower panels) and inbound (upper panels)

1

0~

bow shock crossing of the Pioneer Venus Orbiter.

4O~ ~

r

40-

BM (,T(

~

s~ ~ 40 L 181

2.8

~

______________________

01:

40

t~’T)

r[

]

________________________________________________

~

834

1835

~

I.

~“~‘

-

~

0 63330 4C

8L

221

835

Orbit 95 Out

. 40L 40r

.

j

83630

3/9/TO

26”

M,.,3.” 2.8

9M (CT)

40 40 InTl

L

r

0

181

L

~,

0

____________________________________

193730

1936

939

940

VEGA -2 BOW SHOCK CROSSING C ~ 600

94030

Umn.csal Tim.

MAR. 9, 1986 Fig. 4.2. velocity,

PLASM~-;

I I

I

I

Solar wind temperature,

ion flux (1.5 Hz band), electric field (1.5 Hz range) and total magnetic field during the inbound bow shock crossing of the I

PtA St~4G-;

VEGA—2 /2/).

1— ~

I

,

ION F1LD( /5Hr

I A~1N

120 I

I

I

~ 2~E~EC~R.tC FIELD l5~

A~N

0

2t7

I

I

I

~

0

I

1.8

1.6

,~

I

.

1.4

I ~.

,.

1.2

R, 10’km

spacecraft

(from

K. Schwingenschuh et at.

222

VEGA-i BOW SHIXK CFd7SSING MAR. 6, ~9R~

Fig.

plasma

________________________________________________________

~ ~.

4.3. The same parameters as in

Fig. 4.2 during the VEGA-i inbound bow shock crossing. The shock at

—-—----.—--.--.——-----..-——--———

4:45 UT by

.~o —f

.

is

characterized

increase

of

the

electric field in the 1.5 Hz range (from /2/)

I

P45M46 I

an

10

I

I

ION flux L5.’fr

~

I

4PV-N

6.’

___________________________________________________

I

~

•

I

£LECTR!C FIEi.D 15Hz v-~

~

..,—

Is

ci

10

‘liSt HA

5-

,—,

90

I

1.5

~1

1.3

VEGA-i BOW SHOCK CROSSING ~

I

•

3.~

Z30

UT R, 1O’km

MAR. 6, 1986

Fig. 4.4. Solar wind velocity, temperature and

I

the magnetic field magnitude during the VEGA-i outbound bow shock

600 I

~4OO~

crossing

~200

I

I

PASMAO -1 ~10

1

I

I

I

20

L,

I

MISCHA

10

0

____ 1i~

___________________

0.4

0.5

0,6

0.7

1.1

•

11.~

Z2

UT 1.~ R

(from /2/).

Cometary Boundaries at Halley

223

INNER BOUNDARIES

The main features of the magnetic field in the inner part of the coma are the increasing total field up to a peak value of 75 nT (VEGA—i) and 80 nT (VEGA—2) and the oscillatory structure of the B and B components near the closest approach (Fig. 6). A first boundary or layer (see label Mj in Fig. 6) was detected at a distance of 350 x ~ km during the VEGA—i encounter and at 390 X io~ kin during the VEGA—2 encounter and is characterized by a sudden increase of the slope of the total magnetic field. At about the same distance the solar wind proton population becomes comparable with the cometary implanted ions /7/. The most interesting region is between the boundaries C 1 and C2 (see Fig. 6) observed during the VEGA—i encounter. These boundaries are separating two regions with different draping patterns (see Fig. 5) . The plasma instruments aboard the VEGA spacecraft detected at a distance of about 161 N io~ km (6:45 UT) a boundary (label CP in Fig. 6), which separates the solar wind controlled region in the ‘cometosheath’ from the cometary plasma region dominated by heavy cometary ions /7/. The plasma experimenters called this surface cometopause. The region C1—C2 detected by the magnetometer is inside and the boundary M1 outside the cometopause.

VEGA—I

— MISCHA

MAGNETIC

FIELD VECTORS

—

CSE

COORDS.

\

I 7~00~

-o — —

E

I

-~ LI1 0 0

—--

-o

7~iO

U

‘-4

I.

Ui

En

00

>-

fT O 20 L......JBXvs.BY 06—MAR—86 I

~

I ,I 1

07:00 ‘

—160

40.0

—~o —

CSE

~,

\

minut

‘

X

\

‘

‘

~“//

so

i~o

[1000 km]

Fig. 5. The magnetic field vectors along the VEGA—i trajectory in the inner regions of the coma. The hyperbola intersects the trajectory at the location of the boundaries C1 and C2 from fig. 6 and can be interpreted as an isochrone of the interplanetary magnetic field. The two different draping patterns can be explained by a remnant interplanetary magnetic field in region II. The boundary M1 was observed during both VEGA encounters and is therefore most probably a stationary structure of the inner coma. This boundary can be explained by a sudden change of the solar wind velocity thus causing a different piling up of the interplanetary magnetic field lines. Fig. 7.1 and Fig. 7.2 show a minimum variance analysis of the diffuse C1 and C2 layers. The boundaries are discontinuities with a small normal component. Fig. 9 shows the result of a computer simulation of the Halley encounter using Fedder’s model (see Fig. 8). The deviation of the experimental results from these simulations can be explained by a remnant interplanetary magnetic field, which due to the plasma properties of this region stagnated. Fig. 10 shows a possible configuration of an old field in the center and a newer field further Out. The structure in the center between the boundaries C1 and C2 shows a draping pattern with a different IMP than the structure outside this region. Eight hours before the encounter VEGA—i observed a change of the IMP polarity from +B to -B~ (see Fig. 1) . This is an additional proof for the hypothesis that the magnetometer aboard VEGA-i observed a remnant IMP in the inner coma of comet Halley.

224

K. Schwingenschuh cc at.

VEGA—1 — MISCHA MAGNETIC FIELD OaTs

I 40j

—

Fig.

056 COORDINATES 06MAR28

$

I

~1 -i

$

c 2

-40

VEGA—2 ~-4O

I

I

I

The

field

and

total the

components during the VEGA—i (upper panels) and

F I

6.

magnetic

I

(lower

panels)

encounters. Ml and M2 indicate boundaries detected by the magneto—

E

me~st~F~ I

L40

~I

indicated

by

the

label

CP. ~—4O

~,-,‘, 65/05: 00

05: 30

VEGA—2

06: 00

05: 30

07: 00

07: 30

08: 00

96

1.8

190

523

711. —

MISCHA MAGNETIC FIELD DATA

CSE COORDINATES

08: 30 UT

333

13~km

09—MAR—86

~I

~I

~I1

II’ 68/05: 00

61.5

05: 30 507

06: 00

05: 30

07: 00

36c

230

92

07: 30 .7

08: 00 181.

08: 30 UT3krn 323 lO

CONCLUSIONS Two (Si

and S

2) of the four outer boundaries which have been observed by the magnetic field experiments during the VEGA encounters are probably bow shock crossings. This classification is mainly based on low—frequency plasma—wave measurements. The boundaries S1 and S3 can be interpreted as foreshocks. The inner boundaries Cj and C2 observed during the VEGA—i encounter are separating two regions with different draping patterns. This can be explained by a remnant magnetic field in this region caused by a polarity change of the IMP eight hours before the encounter. The observations near the boundaries C1 and C2 have been simulated with Fedder’s model using two different IMP directions. The mantle boundaries Mj and M2 have been observed during both encounters and are therefore most probably stationary structures of the inner part of the coma.

Cometary Boundaries at Halley

225

VEGA I SI

-

06—MAR—96 JAY:

65

F9014 07:09.45.000 TO 07’: t~35.0OO

II

1S

________________ 9 32 48 64 99

‘p 04.4

19

~

—~

0

‘

1.

—Is

\~ HII4VAR AVEPA9ES 91

//

9.1

—3.10 —36.61

81

791<

5.4.4.4

56.59

1.509

0.886

EIG~I YAOEB

/)

1294.931 106.513 0.70704.6496 EASES VECTORS

t~

//‘

SAS

0.0711

~9.29TA1~ —0. 0.9.431 1103—0.2348—0.9658 0.7240—0.2495

SE

—

_______________________________________________________________

Fig. 7.1.

84< 2.80

1<

4.77

6.7

Hodograrn of the magnetic field components of the boundary C

and B are the maximum, medium and minimum variance axis. start.~ngpoint of this 3—d hodograin.

1. The axis B B The dashed lines indicat~th~

•0

.1

VEGA

I

64

06—MAR—86 DAY:

65

FROM 07: 21: 28.000 TO 07:29.45.000

4.

32~ 16

140_SI

—19

‘

—32

‘

—18

0

15

32

19

5.4

99

—16

,r7 -32 MIES~AYFRASS

I -~

81

9.1

—17.25 —47.62

ECse(

21<

ST

79(1

7.53

56.20

81.32

VALL9.S

9.48

EASER VECTORS —0.3851—0.8346 0.3936

1271.586 156.766 2.687 0.9072—0.2689 0.1443—0.1691—0.8713 IA’(CERTAINTY 5.90 84< 1<0.3088 9.9

Fig. 7.2.

Hodogram of the boundary C2.

0.625

226

K. Schwingenschuh

GIACOBINI

—

Fig. 8. Magnetic field lines of a Giacobini-

ZINNER SIMULATION

_______________________________________________

—

1810~

et a!.

Zinner and Halley simulation using Fedders model. The massloadinc

I

Y

0km

rates used were

4 x 1o2~

(GZ) and 3 X 1O~~ (Halley) proton masses per second.

—1x10~ 1”10~

N

—

_______________________________________________

0

_______

jolO

—lolO°

—2o10°

—3o10~

HALLEY SIMULATION

0

-1x~0’

0

X

(km)

Vega Halley Encounter BIME T (O~3.5,,3.5)niT

Fig. ,

9.

the

Simulation

of field along the

magnetic

measurements

VEGA trajectory using one

20 0

BX

—20 20

BY

I

magnetic IMP direction. field The observations during the VEGA—2 Fig. 5) these

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

—~

—~

BZ

gB~ C

I

encounter (see are similar to predicted

signatures.

o -20 20 0 —20

7\__-_4-_~

~1~—

—~

—~

Hours from Sun—Comet Axis Crossing

Cometary Boundaries at Halley

32

-

16

B~

0 —16

±32

~ I

I

I I

I

N

16

B~ o

I

I

I

I

I

I.

227

Fig. 10. Simulation of the oscillatory structure observed by the magneto— meters during the VEGA—i encounter. This simulation is using Fedder’s model calculations and two different IMP directions (see regions I and II in fig. 5) . The model

t~to:t~fiy:y with a different orientation centered on closest approach.

-16 ±32

16

B~ 0

I

‘I

I

Time From Closest Approach (hours)

ACKNOWLEDGEMENTS

The authors thank F. Pieber, W. Ramsauer, R. Schmidt, H. Stdilner, H. Valavanoglou, G. Berghofer, ö. Aydogar and G. Suppan for the technological implementation of the magnetometer and are grateful to A.V. Dyatchkov, N.J. Havenson and K. Pirsch for the experimental data preparation. Thanks are due to T.K. Breus, A.A. Gaieev, K. Gringauz, M. Heyn, V.H. Orayevski, V.G. Shapiro, J. Phillips and K. Torkar for useful discussions. The financial support of the Austrian Federal Ministry of Science and Research is gratefully acknowledged. REFERENCES 1.

M.A. Saunders, C.T. Russell and J.L. Luhxnann, Interaction with planetary ionospheres and atmospheres: a review, Proc. of conference on Comparative study of magnetosperic systems, La Londe des Maures, France Sept. 1985.

2.

A.A. Gaieev, B. N. Gribov, T. Gombosi, S. I. Klimov, P. Oberc, A. Remizov, K. Gringauz, W. Riedler, R. Z. Sagdeev, S. P. Savin, A. Yu. Sokoiov, V. D. Shapiro, V. I. Shevchenko, K. Szego, M. I. Verigin and Ye. Yeroshenko, The position and structure of comet Halley’s bow shock: VEGA—i and VEGA—2 measurements, Geoph. Res. Lett., in press (1986)

3.

R. Grard, A. Pedersen, J.G. Trotignon, 0. Molchanov and V. Formisano, Observations comet Halley, Nature 321, 290—291 (1986)

4.

C.T. Russell, Planetary bow shocks, in: Coiiisionless Shocks in the Heliospere: Review of Current Research, 109—130 (1985)

C. Beghin, M. Mogiievsky, Y. Mikhaiiov, of waves and plasma in the environment of

228

K. Schwingenschuh er a!.

S.H.

Brecht and

J.G.

Lyon,

MPG

simulation

of

a

comet,

Icarus,

in

press

5.

J.A. Fedder, (1986)

6.

W. Riedler, K. Schwingenschuh, Ye. Yeroshenko and C.T. Russell, The VEGA magnetic field experiment, in: Field—, Particle— and Wave—Experiments on Cometary Missions, K. Schwingenschuh and W. Riedier (ed.), 111—124, 1986, Graz, Austria

7.

K.I. Gringauz, T.I. Gombosi, A.p. Remizov, I. Apathy, I. Szemerey, M.I. Verigin, L.I. Denchikova, A.V. Dyachkov, E. Keppler, I.N. Klimenko, A.K. Richter, A.J. Somogyi, K. SzegO, S. Szendrd, M. Tatrailyay, A. Varga and G.A. Viadimirova, First in situ plasma and neutral gas measurements at comet Halley, Nature 321, 282—285 (1986)

8.

W. Riedler, K. Schwingenschuh, Ye. Yeroshenko, C.T. Russell, J.G. Luhmann and J.A. Fedder: The VEGA Mission Magnetic Field Experiment: Preliminary Results and Plans for Halley Flyby, EOS 66 (46) , 1017—1018 (Abstract) , 1985.

9.

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