GEOPHYSICAL RESEARCH LETTERS, VOL. 18, NO. 9, PAGES 1663-1666, SEPTEMBER 1991

GIOTTO'S MISSION TO PLANET EARTH

K.-H. Glass•neier 1, F. M. Neubauer 1, G. Brach 1, H. Marschall I M. H. Acufia 2. L. F. Burlaga 2, F. Mariani 3, G. Musmann 4 N. F. Ness 5, M. K. Wallis 6, E. Ungstmp 7, H. U. Schmidt 8

Abstract. After its successful encounter with comet P/Halley and a four-years hibernation period ESA's Giotto spacecraft has been reactivated in February 1990 and performed the first-ever Earth gravity-assisted maneuver on July 2, 1990 to be retar- geted for comet P/Grigg-Skjellerup. This swing-by is of unique scientific interest due to Giotto's hyperbolic, high-inclination orbit. Here, we shall report on scientific results of the Giotto magnetic field experirnent. Due to the high fly-by velocity and the relative quietness of the magnetosphere during the swing- by period these measurements present a snapshot view of the Earth magnetosphere with clearly identified inbound and out- bound bow shock and magnetopause crossings. The outbound crossings are of particular interest as surface waves at the po- !ar magnetopause at a distance of 28 R E as well as a strong quasi-perpendicular bow shock at a distance of about 64 R E are observed

Introduction

Giotto - the spacecraft, which has been sent into space to unravel the secrets of comet P/Halley has been reactivated in February 1990 after a four-year hibernation period. Cur- rently, ongoing debate exists on the possibility to extend the Giotto mission toward another encounter with comet P/Grigg- Skjellerup.

To enable such a continuation a close swing-by with the Earth has been necessary and was performed on July 2, 1990, 10:01:18 LIT perigee time. Coming from deep space Giotto encountered the Earth on a nearly hyperbolic orbit with perigee at a distance of about 22731 km from the Earth's surface. As the check-out

period after the spacecraft reactivation in February 1990 yealed that the Giotto magnetometer experiment [Neubaner et al., 1986] was fully operational, the flux gate rnagnetometer (as well as the energetic particle experiment EPONA [Mc Kenna- Lawlor, 1086]) was switched on during the swing-by. Thus, Giotto's mission to planet Earth is the first encounter of an observing spacecraft, coming from deep space, with our home planet.

We shall present first scientific results of the magnetic field observations made during this Earth swing-by. Due to tech- nical constraints real time telemetry was prohibited during the encounter. Thus the magnetometer experiment was operated in

1institut f'dr Geophysik und Meteorologie, UniversitKt zu K51n 2NASA-GSFC, Greenbelt 3Dipartimento di Fisica, Universira di Roma, Roma 4Institut fdr Geophysik und Meteorologie, TU Braunschweig 5Bartol Research Foundation, University of Delaware, Newark 6University College, Cardiff ?Danish Space Research Institute, Lyngby 8Max-Planck-Institute for Astrophysics, Garching

Copyright 1991 by the American Geophysical Union.

Paper number 91GL00550 0094-8534/9 !/91GL-00550503. O0

its memory mode, and was switched-on and off on July 1, 23:48 UT, and July 4, 10:48 UT, respectively. With a sampling rate of 8 vectors per spin period of about 4 s and averaging over 64 spins the 15 kbyte onboard memory allowed the measurement of one full average magnetic field vector about every 256 s. Due to the close fly-by distance, maximum magnetic field magnitudes of 500-600 nT had been estimated, and the measuring range was set at q- 1024 nT, which yields a quantization uncertainty of q- 0.25 nT.

As Giotto is not a magnetically clean spacecraft in-flight de- termination of contaminating spacecraft fields was necessary. Regular snapshot intervals, during which 8 vectors per space- craft spinperiod were measured, allowed the removal of the com- ponents of contaminating fields perpendicular to the spin axis. Determination of the spacecraft field component along the spin axis is not readily amenable to techniques that have been suc- cessfully used on interplanetary spacecraft due to the low time resolution, and the short tirne interval Giotto measured in the interplanetary medium. Along the spin axis a spacecraft field correction was applied, which is based on the ground systern magnetic field test.

Observational Results

Observations as well as the spacecraft orbit will be presented in a geocentric solar magnetospheric (GSM) coordinate systern. It should be noted that while the dipole always lies in the x-z plane, the angle with the z-axis undergoes a diurnal variation. At perigee passage the dipole tilt angle was 20.5 ø towards the Sun.

Giotto entered the magnetosphere-solar wind interaction re- gion coming from south of the nc-y plane at a local time of about 11:00 MLT (Figure 1). After its perigee passage at (-1.06, 2.89, -3.38) RE, i.e. close to the dusk side, the spacecraft left the magnetosphere in the midnight sector on an orbit inclined to the z-axis by about 30 ø. The oscillatory motion in the x-y plane (Figure 1) is due to the dipole axis rocking around the z-axis. Nominal bow shock and magnetopause positions [Formisano et al., 1979] for a magnetic tilt angle 0 ø are also given in Figure 1.

An overview of the magnetic field variations is given in Figure 2. The dominating feature is the rapid increase of the magnetic field magnitude when approaching the Earth, i.e. the influence of the geornagnetic main field. The maximum magnitude, 567 nT, was observed at the time of closest approach. Using a mag- netospheric magnetic field model, modified by D.P. Stern after Tsyganenko and Usmanov [Tsyganenko and Usmanov, 1982], at closest approach a mean deviation of 26 nT between modelled and actually measured magnetic field components was found. This difference of about 4.6% is partly due to ill defined space- craft fields, and in a larger part due to ring current fields which are different from the nominal ones used in the model. Figure 3 exhibits a more detailed comparision between the actual ob- servations and the Tsyganenko-Stern model. In particular, the tail magnetic field observed very well agrees with the modelled tail. Differences found are generally less than 4 nT.

The inbound bowshock and magnetopause crossings occurred at about 04:49 UT and 06:44 UT at distances of 13.4 R E and

!663

1664 Glassmeier et al: Mission to Planet Earth

lOO

80

40

o

-/,o

lOO

o

-2o

Giotto's Earth Swing-by

....... GS• X-Y Plane

........,./Bow Shock

! ! ,

•, ...... GSH lone

12:00 UT Tramarks / /' every /. h

-•00-80-•0-•0-2o 0 2o •0

Fig. 1: The trajectory of Giotto through the Earth magneto- sphere and adjacent regions, projected onto the x-y and planes of a geocentric solar magnetospheric coordinate sys- tem. The trajectory shown starts at July 2, 00:00 UT and ends at about July 4, 08:00 UT. Nominal bow shock and magnetopause boundaries are shown skematically, too.

9.8 R•, respectively (Figure 4). The bow shock is most clear!y visible as an increase of the field magnitude by about 11 nT within a distance of nearly 5000 km, i.e. many times the solar wind proton gyroradius. Due to the low-time resolution of the magnetic field experiment during the swing-by period further details of the shock have not been detected.

In the upstream region the interplanetary magnetic field only exhibited directional and minor magnitude variations. The av- erage magnetic field vector was inclined with respect to the ecliptic plane by about 45 ø . The angle between the Sun-Earth line and the average field direction is calculated as-15 ø in the ecliptic plane, as compared to 45 ø for an Archimedean spiral orginating from the Sun. The field direction thus defined is inclined to the normal of a model bow shock by about 39 ø, which points toward Giotto having traversed a quasi-parallel bow shock.

Giotto Magnetic Field Experiment Univ. Cologne lOOO

• 100 B.$ M ca 10

360 •_.. ! .• i i I i...z_...,_ , , , , , , , , , , ,

, , ..

T: 0 z, 8 12 16 20 0 L 8 12 16 H

R: 25 16 7 7 16 25 34 43 51 60 68 R• July 2.1990 Universal Time (SCET) July3.1990

Fig. 2: The intensity of the magnetic field (B) and the azimuth angle • as well as the elevation angle O based on 256 s averages during the Earth swing-by of Giotto. • is the angle between the GSM x-axis and the vector projection into the GSM x-y pIane, and O the angle between the x-y plane and the magnetic field vector

5o ................ Difference-] X 0 ........ - ----- --' .•_• •:

-40

i i i ! ! i i ! I i i i,, i/ i

"" /.o [ GSM CoordinotesJ

• •o .._½.....CIo•est Approach '-' I

N 0

-/,0

-60 I I ,I I I ! I I, I I I I, I 07 09 11 13 15 17 19 21 h

July 2, 1990 Universel Time (SCET)

Fig. 3: lq, esiduat magnetic field components of the observed and modelled magnetospheric field along the Giotto trajectory.

in the magnetosheath the magnitude stayed almost constant while the x and y components exhibit a gradual decrease from positive to negative values (Figure 4), i.e. the field vector in the x-y plane was slowly deflected, indicating complex draping of the interplanetary magnetic field around the obstacle of the Earth's magnetic field as, for example, analysed in more detail by Behannon and Fairfield [1969].

The inbound magnetopause crossing is most visible by the large magnetic field shear at 06:44 UT, when, for example, the component changed from -8.4 nT to 25.11 nT. Determination of the magnetopause normal was again prohibited due to the low- time resolution. The magnetopause thickness, however, may be estimated from the spacecraft velocity component normal to the nominal magnetopause, va • 4 kin/s, and the crossing time, •' •_ 256 s, to be •_ 1024 km, a value quite in order with previous estimates [Russell and Elphic, 1978; Lepping and Burlaga, 1979].

Immediately after the inbound crossing the influence of the geomagnetic main field becomes dominant as indicated by the further rapid increase of the field magnitude. While crossing the inner part of the nightside magnetosphere, no major tem- poral variations, such as substorm activity, have been observed. Giotto's encounter with the Earth's magnetosphere was with a moderately quiet one, as also witnessed by low values of the

22 0 -22

22 0 -22

22 0

-22

, , ! , i I s , i i , i

, .... i , , i , , i , , i ....

•c 22

, , l, , , • , , . ! , ,

3 /. 5 6 7

July 2, 1990 UniversQI Time (SCET)

Fig. 4: Magnetic field observations around the inbound bow shock and magnetopause crossings at about 04:49 UT and 06:40 UT, respectively.

Glassmeier et al: Mission to Planet Earth 1665

•1o N

5

0

-5

Giotto's Earth Magnetotail Encounter

-•_--%___.Midnight Oipol• 7 .... Cusp Region

• • Tr• o -s -•o -•s -•o -3 -3o

XGSM [ RE]

Fig. 5: Projections of the measured magnetic field vectors onto the x-z plane during part of Giotto's magnetotail traversal. The vector projections have their origin on the trajectory. The first vector was plotted for 10:55 UT and every 25 rain onwards. Also shown is the polar magnetopause boundary as well as the Earth dipole at 13:00 UT, i.e. the time of the crossing of the tailward extension of the midnight cusp region.

planetary geomagnetic activity index Kp •_ 3 during the en- counter period.

The entry into the tail is marked by the crossing of the tail- ward extension of the midnight cusp region, i.e. the transi- tion region between fieldlines still closed in the magnetotail and those connected to the far down stream solar wind magnetic field. This region is characterized by a sign change of the z- component from positive to negative values at about 14:30 UT and at distance of 23 R.• (Figure 5). The magnetic field mag- nitude in the tail decays from about 42 nT at 14:30 UT down to about 25 nT at 18:00 UT (22 P•j•), when the gradual decrease of the magnitude suddenly ceased and the magnitude stayed constant until about the outbound magnetopause crossing. As no plasma observations are available, we cannot unambigously identify the nature of this transition region. However, compar- ison of the Giotto data with Heos 2 observations [Rosenbauer et al., 1975] suggests that Giotto entered the plasma mantle at about 18:00 UT at position (-14.1, 0.8, 16.5) R.•.

The lobe field structure (Figure 5) exhibits a clear tail flaring with a flaring angie of about 12 ø. Coroniti and Kennel's [1972] tail flaring model can be used to check our present observations against theoretical predictions. Their model essentially assumes pressure balance between the solar wind dynamic pressure and the tail magnetic field, and a constant tail flux, i.e. it neglects the magnetic field component normal to the magnetopause. It allows to determine parameters such as the tail flaring angle, i.e. the angle of attack of the solar wind at the tail magnetopause or the distance at which flairing ceases. ISEE 3 observations in the distant tail [Slavin et al., 1985] strongly confirm this model. Using nominal values [Slavin et al., 1985] for the initial tail ra- dius Ro = 20 Rj• at a downstream distance Xo = 20 R.•, a terminal tail radius Rt = $0 R•, and for the square root of the ratio of ram pressure to the solar wind thermal and magnetic pressure M = 7, a flaring angle a = 21.2 ø results. Using the same values as above, but It.t = 23 R•, a flaring angle a = 12 ø results, much in agreement with our observations. As the terminal tail radius depends on magnetospheric activity, i.e. in- creases during the growth phase of a magnetospheric substorm,

32

• 0

x _ 16 ; 32

>' - 16

• 0 N _ 16

7.32

0

, I ,i I • I • I i t ,I , I

, I • t i,,, I [ I I 1 • I

•__L I , ! , i i !

17 18 19 20 21 22 23 2/. July 2,1990 Universal Time [SCET)

Fig. 6' Magnetic field observations around the outbound polar magnetopause crossing at 21:20 UT.

we take this smaller terminal tail radius as a further hint of

having traversed a moderately quiet time magnetosphere. The outbound magnetopause was traversed at 9.1:20 UT at

(-17.4, -3.3, 22.1) R•. The magnetopause is characterized by a clear change in magnetic field as well as a magnitude jump by about 18 nT (Figure 6). However, as compared with the inbound magnetopause the outbound one exhibits significant temporal variations. A possible explanation of the rapid mag- nitude variations is in terms of surface waves. Surface waves

at the magnetopause have extensively been studied by, for ex- ample, Aubry et al. [1971]. They are a probable consequence of a Kelvin-Helmholtz instability at the magnetopause [Pu and Kivelson, 1983; Lepping and Burlaga, 1979]. This instability sets up an oscillatory motion of the boundary which eventually moves several times past the moving satellite.

Following Lepping and Burlaga [1979] we suggest to interpret the field magnitude changes after 21:20 UT (Figure 6) as such multiple magnetopause crossings. The magnitude increase at about 20:20 UT and the decrease at about 20:50 UT could be a

signature of the spatially decaying magnetopause surface wave as well. A KHI generated surface w•ve is of the fast mode type [Pu and Kivelson, 1983]. F¾om the magnetic field observations (Figure 6) the decay length normal to the magnetopause is es- timated to be about 17/2 •r Ri• , where a spacecraft velocity of 2.6 (3.0) km/s normal (tangential) to the magnetopause was used. As for a magnetopause surface wave 2 •r times the hori- zontal wave length ,• is comparable to the decay length, we have ,X m 1TRi•. This value is comparable to the value of Lepping and Burlaga and similar to those determined by e.g. Hones et al. [1981] for plasma vortices at the inner edge of the low-latitude boundary layer. With a magnetic field strength of 22 nT in the plasma mantle (l*igure 6) and a typical proton density of order I cm -3 an Alfv•n velocity of about 480 km/s results, which allows us to estimate a wave period of 225 s , a value familiar from ground-magnetic pc5 pulsations. Lepping and Burlaga's [1979] method to determine the horizontal wavelength is not applicable in the present case due to the low time resolution of our data. X rough estimate, however, yields ), • 602EE, i.e. indicates significant undersampling of the magnetopause structure.

We are not aware of any other reported observation of sur- face waves at the distant polar magnetopause, such as those presented here. Detection of these waves, however, is of par-

1666 Glassmeier et al: Mission to Planet Earth

ticular interest for the ongoing debate on possible causes of geomagnetic substorms. We note that in particular the ther- mal catastrophe model proposed by Goertz and Smith [1989] requires surface waves at the distant polar magnetopause as an energy and momentum transfer mechanism from the solar wind to the magnetosphere.

Having passed the outbound magnetopause, Giotto entered a quite extended magnetosheath region. We identify a sharp decrease of the magnetic field magnitude at July 3, 13:49 UT (see Figure 2) and spacecraft position (-34.8, 4.1, 53.1) R E as the final entry of Giotto back into the solar wind. Thus Giotto spent about 16.5 h in the polar magnetosheath, when it travelled over a distance of nearly 36

The magnetosheath field is found to be rather turbulent be- hind the magnetopause until about July 3, 05:30 UT at a dis- tance of 46 RE, when the field magnitude starts to increase by about 4 nT up to about 19 nT. Within this highly turbulent sheath region there occurs at least one tangential discontinu- ity as can be judged by the magnetic hole (field value 1.2 nT) at July 3, 03:29 UT and its associated directional variations (Figure 2).

The high-field region extends over about 8 h or 16 K E. Its most remarkable feature is a magnitude depression at about 12:50 UT on July 3, 1990 (Figure 2). As magnitude and field direction within this region are comparable with that of the adjacent solar wind plasma it is justified to assume an inward motion of the bow shock as an explanation.

The final bow shock itself may be classified as a quasi-perpen- dicular shock, both on grounds of the magnetic field structure as well as due to the direction of the interplanetary magnetic field. The mean field vector upstream of the bow shock is in- clined with respect to the ecliptic plane by about 35 ø , but, as compared with the upstream direction, the field vector is now pointing anti-sunward. The angle between the Sun-Earth line and the mean field direction is about 63 ø , i.e. comparable to the Archimedean spiral direction at 1 AU. During the Earth en- counter an interplanetary sector boundary crossing must have been encountered.

Summary

The Earth's swing-by of ESA's Giotto, the first encounter of an observing spacecraft coming from deep space with our home planet, was a successful undertaking as far as magnetic field ob- servations are concerned. Giotto traversed the quiet time mag- netosphere proper within about 15 h and thus provides us with a snapshot view of some major regions of the magnetoshere. In- bound a quasi-parallel and outbound a quasi-perpendicular bow shock has been observed. The outbound crossing is of interest as no other observations of the distant polar magnetosheath and shock have been made to date.

Temporal variations at the outbound magnetopause reveal the existence of surface waves at the polar magnetopause, which are an important mechanism for energy and momentum cou- pling from the solar wind into the magnetosphere in the ther- mal catastrophe model for magnetospheric substorms [Goertz and Smith, 1989],

Finally, magnetic field variations observed in the inner mag- netosphere fit very well the model fields derived using a modified Tsyganenko-Usmanov field model [Tsyganenko and Usmanov, 1982].

Acknowledgement,:We thank the many colleagues in the Giotto Extended Mission project teams at ESTEC and ESOC, who made

these Earth swing-by observations a reality. Particular thanks are to D.P. Stern and P•. v. Stein for making available magnetospheric model magnetic field data. We gratefully acknowledge financial support by the various national funding agencies.

•eferences

Behannon, K.W., D. H. Fairfield, Spatial variations of the mag- netosheath magnetic field, Planet. Space Sci., I7, 1803-1816, 1969.

Coroniti, F.V., C.F. Kennel, Changes in magnetospheric configu- ration during the substorm growth phase, g. Geoph•s. Res., 77, 3361-3375, 1972.

Formisano, V., V. Domingo, K.-P. Wenzel, The three-dimensional

shape of the magnetopause, PlaneL Space Sci., •7, 1137-1150, 1979.

Goertz, C.K., I{.A. Smith, The thermal catastrophe model of sub- storms, Y. Geophys. Res, õ4, 6581-6596, 1989.

Hones, E.W., et fl., Further determination of the characteristics of

magnetospheric plasma vortices with ISEE 1 and 2, g. Geophy•. /res., $6, 814-820, 1981.

Aubry, M.P., M.G. Kivelson, C.T. Russell, Motion and structure of the magnetopause, J. Geoph•ls. Res., 76, 1673-1696, 1971.

Lepping, P•.P., L.F. Burlaga, Geomagnetopause surface fluctua- tions observed by Voyager 1, if. Geophys. Res., 84, 7099-7!06, 1979.

McKenna-Lawlor, S., et fl., Energetic ions in the environment of comet Halley, Nature, $œ1, 347-349, 1986.

Neubauer, F.M., et al.• The Giotto magnetic-field investigation, Euvop. Space Ag. SP-I077, p. 1-14, 1986.

Pu, Z.Y., M.G. Kivelson, Kelvin-Helmholtz instability at the mag- netopause: solution for compressible plasmas, 88, 841-852, 1983.

Rosenbauer, H., et al., HEOS 2 Plasma observations in the distant polar magnetosphere: the plasma mantle, Y. GeoI•h•s. Res., 80, 2723-2737, 1975.

P•ussell, C.T., R..C. Elphic, Initial ISEE magnetometer results: magnetopause observations, Space Sci. Rev., œœ, 681-690, 1978.

Slavin, J.A., et al., An ISEE $ study of average and substorm conditions in distant magnetotail, Y. Geoph•s. Res., •0, 10875- 10895, 1985.

Tsyganenko, N.A., A.V. Usmanov, Determination of the magne- tospheric current system parameters and development of exper- imental geomagnetic field models based on data from IMP and HEOS Satellites, Planet. Space Sci., $0, 985-995, 1982.

K.H. Glassmeier, F.M. Neubauer, O. Brach, H. Marschall, In- stitut fdr Oeophysik, Universitit K$1n, D-5 K5!n 41, FRO.

M.H. Acuna, L.F. Burlaga, Lab. f. Extraterr. Phys., NASA OSFC, Greenbelt, MD 20771, USA.

17. Mariani, Dipartimento di Fisica, II Universita di Roma, 1-00173 Roma, Italy.

O. Musmann, Institut ffir Oeophysik und Meteorologie, TU Braunschweig, D-33 Braunschweig, FRO.

N.F. Ness• Bartol Research Foundation, University of Delaware, Newark, DE. 19716, USA

M.K. Wallis, University College, Cardiff CF1 1XL, UK. E. Ungstrup, Danish Space P•esearch Institute, DK-2800 Lyn-

gby, Denmark. H.U. Schmidt, MPI f. Astrophysik, D-8046 Oarching, FRO.

(l•eceived November 8, 1990; (January 2, 1991)

(accepted February 20, 1991)