Magnetic flux rope type structures in the geomagnetic tail

OO?l-Yl69,‘Yl $3.00+ .oO Pergamon Press plc

Magnetic flux rope type structures in the geomagnetic tail A. E. ANTONOVA and A. P. KROPOXIN Institute of Nuclear Physics, Moscow State University. Moscow, 119899. U.S.S.R.

Abstract-We identify some structures in the geomagnetic tail observed by the Prognoz 9 and ISEE spacecraft as ‘magnetic flux ropes’. The following new features are emphasized : (1) The structures are associated with considerable fluxes of energetic ions and electrons. (2) Particles are effectively energized at magnetic field discontinuities, resulting in the generation of spectra extending up to MeV energies. (3) An external field source (i.e. the interplanetary magnetic field) may be ofessential importance for the generation of the flux ropes whose axes lie in the cross-tail direction.

1. INTRODUCTION A variety of related magnetic field features and associated cold and hot plasma populations are observed in the geomagnetic tail. ANTONOVA et al. (1989a,b) analysed magnetic flux rope (MFR) structures in the magnetotail using simultaneous magnetic field and energetic electron and ion (assumed to be protons) measurements made by the Prognoz 9 satellite. SIBECK cut al. (1984), SCHOLER et al. (1985), KENNEL et al. (1986) and ELPHIC et al. (1986), have discussed flux ropes in the Earth’s magnetotail. SCHOLERet al. (1985) presented energetic ion and electron observations which indicated sharp increases in the flux of energetic ions at the time of the events, but only minor increases in the flux of energetic electrons at those times. In contrast to those results, we will present an MFR which is accompanied by a strong increase in the flux of energetic electrons. Here we summarize the results obtained by ANTONOVA et al. (1989a.b) and provide additional information concerning the proton spectra associated with Prognoz 9 MFR observations. Moreover, we have compared an MFR observed on 23 March 1979 with the magnetic field observations in the solar wind, in the magnetosheath and near the terrestrial bow shock.

2. RESULTS Table 1 compares characteristics of the particle fluxes observed in MFRs by ISEE 3 on 25 March 1983 with those observed by Prognoz 9 on 21 August 1983. The MFR locations and maximum to background flux ratios (J/Jb) are also presented. Figure 1 shows the 164-s averaged vector magnetic field measurements from Prognoz 9 on 21 August

1983, from 0000 to 0500 UT, when the inbound spacecraft crossed the southern boundary of the plasma sheet (PS) and entered the magnetotail lobe. Note that the entrance into the PS (20 August 1983, at 0800 UT) occurred at comparatively low aurora1 activity (AE g 150 nT) and the magnetic field strength (B) in the northern tail lobe was 15 nT. In the following hours. the geomagnetic disturbance intensified and the hourly averaged AE increased to z 670 nT during the period 0100-0300 UT on 21 August 1983. A 2-h long gradual increase in the magnetotail magnetic field strength began at 2300 UT on 20 August 1983, during which the field increased from E 10 to ~20nT (at OlOOUT). This increase indicated a change of conditions in interplanetary space-probably a solar wind pressure increase. Figure 2 presents more detailed 10.24-s magnetic field vector data and the 164-s averaged energetic proton and electron fluxes at 0108-0148 UT, 21 August 1983, when Prognoz 9 was in the pre-midnight sector. south of the neutral sheet, and near the PS boundary, its coordinates being given in Table 1. Characteristic ‘bipolar’ variations of the By- and &-components accompanied by increasing energetic particle intensity were observed along with strong geomagnetic disturbances detected by ground-based stations (e.g. Molodegnaya, Heiss, Narssarssuaq). From 0126:37 to 0135:19 UT, Prognoz 9 observed magnetic field vector rotations accompanying three discontinuities at 0128:19, 0129:51 and 0132:15 UT. Magnetic field strength variations (ABIB) across these discontinuities were about unity and the variations occurred abruptly in less than l&20 s. The data show that bipolar magnetic field variations characteristic of MFRs were observed in the 01144137UT interval on 21 August 1983. generally similar to those observed in the ISEE 3 and ISEE

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A. E. ANTONOVA and A. P. KROP~TKIN

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Table 1. Magnetic

flux rope characteristics Spacecraft Prognoz

ISEE 3 Date Time (UT) Coordinates

21 August 1983 0105-0149 - 30.65 7.88 -3.12

25 March 1983 07054735 -109 0.0 3.6

GSM (Re) A’ Y z

Flux ratio

J/Jb

J/Jb E

E (keV) Protons

30-I 10

Electrons

75-l 10

l/2 (ELPHIC et al., 1986) data. However, there are important distinctions. The characteristic rotation of the magnetic induction vector in this event, which occurs in the YZ plane at the background of a sharp decrease of the Bxcomponent, is presented in Fig. 3. It is seen here that the event observed on Prognoz 9 indeed differs from the flux rope identified. For example, in the ISEE 1 data on 23 March 1979 (ELPHIC et al., 1986). the bipolar variation is observed mainly in the By-com-

(keV)

3.10’ 80-120 120-200 60&1250 loo&1250

30

112-157

5

ponent rather than in the &-component. Thus the interpretation involving a flux rope oriented from dawn to dusk appears to be inadequate, with the core field oriented just in this way, as it was in the case for the ISEE 1 event. More adequate interpretation involves a flux rope with an earthward-directed magnetic field-aligned current along its axis, being presumaLly due to the substorm current wedge. Then the positive-then-negative By variation corresponds to the inward (outward) motion of the inward (outward)

-1oh 20 m 10PP 0

’ Kp=4+ 5 ”

Fig.

lo3 lo2 30 2 3.10’

80-120

Prognoz 9 21.08.83

01

9

;

3

X Y -29.81 7.49 Z -3.78

I. Magnetic field intensity near the magnetotail

i

UT

-28.08 8.5 -5.14 plasma

sheet.

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Magnetic flux rope type structures

600-l 250 keV 2 w

164s

(Olh21m30s)

3 (Olh32m25s)

UT

Fig. 2. Prognoz 9 crossing of a magnetic flux rope structure. Panels (a)-(c) show the magnetic field B-_.BJ and Bx components, respectively; panels (d))(g) show proton fluxes with energies 80-120, 12S200, 20s 600 and 600-1250 keV. The vertical lines marked 2 and 3 show the two times (0121:30 and 0132:25. respectively) for which the proton spectra are presented in Fig. 6.

region of that current along the geomagnetic tail past the satellite position. Here the Bz sign reverses, that is the appearance of the negative Bz variation is related to the plasma sheet current decrease due to the Birkeland current, associated with the possible generation of a new, near-Earth neutral line. In this case. the core magnetic field is the geomagnetic tail main field oriented in the X-direction, and its positive variation also means the decrease of that field in the southern tail lobe below the Birkeland current. The magnetic field variations under consideration were accompanied by significant increases of energetic proton and electron intensities. Note that only one of the three ISEE 3 events presented by SCHOLER et al. (1985) (that on 25 March 1983) was accompanied by a noticeable change of the 75-l 10 keV electron flux. (To our knowledge the background intensity, Jb, of g 100 keV electrons for Prognoz 9 was considerably less than that for ISEE 3.)

Figures 4 and 5 show Prognoz 9 observations of 164-s averaged fluxes of 80-120 keV electrons and of protons with Ep = 80-120 and 12G200 keV, respectively. The angle between the particle detector axis and the Sun-Earth line was 45”. Energetic particle intensities in the MFR-type structure exceed those detected by Prognoz 9 elsewhere during this pass through the tail and, in particular, those observed when the spacecraft crossed the northern PS boundary (an uncertainty exists for the neutral sheet crossing when the detectors were, unfortunately, in a calibration mode). The MFR-like structure was observed for 4min. The simultaneous observation of proton and electron flux increases at different energies, together with sharp magnetic field changes, as shown in Fig. 2, may indicate in-situ particle acceleration. The 100 keV electron intensity increased by approximately three orders of magnitude and the flux of protons with Ep = 120-

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A. E. ANTONOVAand A. P. KROPOTKIN

the normal component of the magnetic field becomes very small in the near-Earth neutral line vicinity and the convection electric field penetrates the thinned plasma sheet. Acceleration of energetic particles by that electric field occurs during their cross-tail nonadiabatic ‘drift’ motion. The mechanism has been subsequently used many times for the interpretation of experimental data on the energetic particle accel1 I eration at magnetic field discontinuities (e.g. WIL10nT Olh 32m15S 28m19S -.J LIAMS et al., 1990). I .1 I The proton integral energy spectra based on 164-s UT averaged flux measurements are shown in Fig. 6 for three typical time intervals : Prognoz 9 in the tail lobe near the PS boundary during quiet (1) and disturbed (2) ambient magnetic field conditions, and in the MFR structure (3) observed on 21 August 1983. The spectra tend to become harder during disturbances. ELPHIC et al. (1986) analysed ISEE I and 2 MFR observations on 23 March 1979, from 1840 to 1930 UT. in the near-Earth magnetotail close to the Fig. 3. Rotation of By. Bz components and depression of Bs neutral sheet (GSM X = -21.3. Y = -4.2. Z = 3.3 component in the MFR event. Earth radii) and identified a MFR structure at 19211923 UT. The MFR axis was oriented along the Yaxis, with the Y-component of the disturbance field being predominant. Simultaneous interplanetary 200 keV increased by two orders at the discontinuities. magnetic field observations. obtained from IMP 8 and These energetic particle intensity enhancements at the ISEE 3. reveal that the IMF magnitude and oriendiscontinuities may provide evidence for an effective tation differed greatly from their average values; particle acceleration mechanism, perhaps that first proposed by ALEXEEV et al. (1970) and SHABANSKY namely the &-component was predominant and its (1972). As the plasma sheet gets very thin at the beginhourly averaged value reached IOnT. A disturbance similar to that observed in the magnetotail was also ning of a substorm expansion phase, simultaneously

Bz

B’;i”=-12 (4 nT) By’“=-12 (7 nT)

Prognoz 9 21.08.83

8

E,=80-120

102

10°

0

1

32’26’

Fig. 4. Electron

:6’10’

3

keV

4

fluxes at energies of 80-120 keV.

5

UT

Magnetic flux rope type structures

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Prognoz 9 21.O8.a3

E,=80-120keV

E,=120-200keV

UT

Fig. 5. Proton fluxes. at energies of 80-120 and 120-200 keV

seen in simultaneous magnetic data from Prognoz 7 located at that time in the magnetosheath

10-l loo

200

300

600

Ep tkeV1 Fig. 6. Magnetotail proton integral energy spectra: (1) and Q-near the plasma sheet; (3)~-in the MFR.

Zse = 31.17 Earth (Xse = -0.29, Yse = -1.87, radii). Characteristic variations observed by Prognoz 7 are shown in Fig. 7a and b. In both cases as the Bxcomponent decreases, a wave-like variation occurs in the 8~ and B,- traces, similar to MFR variations in the magnetotail. In particular at the Prognoz 7 location an abrupt variation of the R~-component was recorded. corresponding well to the bipolar structure in 8: existing in the magnetotail at 1922 UT. However, we cannot at present identify a particular coupling mechanism for variations observed in the magnetosheath (and presumably due to IMF variations) and MFR effects in the magnetotail. (Note, however, that the By-component in the magnetosheath. whose 7-min averaged value was as large as 24nT. had an opposite sign as compared with the IMF.) Presumably, these three observations considered together provide evidence of a significant IMF effect on the MFR structure formation. There was a strong increase in the AE index (from

120 nT at 18 UT to 328 nT at 1900 UT) on 23 March 1979. The dynamical pressure (nMv’) of the solar wind remained quite constant for almost the entire day from 2 1000 UT on 23 March 1979 to 2 1OOOUT

A. E. ANTONOVA and A. P. KROPOTKIN

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23.03.79

Prognoz 7

Event B

40

F 5 m

20

+30 F +20 : N. Z +lO m 0

I

Fig. 7. Magnetic variations in the magnetosheath observed in the period when MFR events were recorded in the magnetotail. on 24 March 1979. However, just during the period of interest (on 23 March from 1700 to 1900 UT), the gas kinetic pressure (nkT) diminished by a factor of 2. Correspondingly, Prognoz 7 data indicate multiple bow shock crossings, which caused the spacecraft briefly to emerge into the solar wind. Moreover, the enhanced geomagnetic activity during this well-documented CDAW-6 event is accompanied by increases in the intensity of protons with Ep > 100 keV observed by Prognoz 7 in the magnetosheath and near the bow shock front (ANTONOVAet al., 1988). Presumably this was due to particle leakage out of the disturbed magnetosphere.

3. SUMMARYAND

DISCUSSION

Examination of magnetic field and energetic particle data from Prognoz 9, when it was in the geomagnetic tail, reveals a quite rare event which we identify as a magnetic flux rope. The following new features are emphasised. 1. The structure is associated with considerable fluxes of energetic ions and electrons. 2. Particles are effectively energized at magnetic field discontinuities, resulting in the generation of spectra extending up to MeV energies. 3. An external field source (i.e., the interplanetary magnetic field) may be of essential importance for the generation of flux ropes whose axes lie in the crosstail direction.

The appearance of magnetic flux ropes in the geomagnetic tail seems to be in general associated with processes of induced reconnection taking place in that region during substorm activity. In the plasma sheet there appear structures localized in a relatively narrow local time sector. They are bordered by one or several pairs of field-aligned currents, flowing into and out of the ionosphere (the substorm current wedge). In an idealized two-dimensional geometry, with By = 0, the appearance of new X-lines bordering such structures would produce isolated ‘magnetic islands’ with closed field lines. However, in a real situation, with the dawndusk irregularity pointed out above, and By # 0, a topologically different configuration results. That is. a limited (in the Y direction) flux rope, in which field lines of three different types are wound up together : (a) open field lines of the south and north lobes, (b) closed field lines crossing the current sheet many times, and (c) fully untied lines; that is, IMF lines distorted by the presence of the geomagnetic tail (BIRN et al., 1989). Lines of force of these three types are tightly intermingled in the flux rope, and this has great importance from the point of view of the magnetic reconnection mechanism operating in the geomagnetic tail during a substorm. The flux rope orientation in such a situation may be greatly distorted when compared with the initial ‘magnetic island’, dawn-dusk orientation, due to the presence of intense field-aligned currents bordering the flux rope configuration in that direction and forming the substorm current wedge. This may explain

Magnetic flux rope type structures

the polarization of magnetic variations which was observed by Prognoz 9 in the reported event. It is well known that the substorm effects in the geomagnetic tail, which are interpreted as the appearance of new neutral lines and magnetic reconnection. follow an intense plasma sheet thinning and a decrease of the field Bn component perpendicular to the sheet. As a result, in the region with Bn _N0 adjacent to the flux rope, a plasma sheet with the least possible thickness (being of the order of a thermal ion gyroradius) is formed, with convection electric field penetrating the sheet and producing the induced magnetic reconnection. This provides conditions for energetic ion and electron ‘drift’ acceleration in that field, as they move on the sheet plane by a distance nearly equal to a doubled gyroradius in the Bn field, quickly

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oscillating at the same time near the sheet plane. Such a mechanism of particle acceleration in the geomagnetic tail has been proposed by ALEXEEV et 01. (1970) and subsequently has been many times used for observational data interpretation (e.g. WILLIAMS et al., 1990). The mechanism may be responsible for energetic particle spikes being often observed in the geomagnetic tail, including the variations presented here and associated with flux rope observations.

Acknowledgements-The authors wish to thank their colleagues 0. R. Grigorian and V. G. Stolpovsky (INP, Moscow State University). Also E. G. Eroshenko and V. A. Styazkin (IZMIRAN), who provided the Prognoz 7 and 9 observations ; we appreciate their helpful comments. They are also grateful for the suggestions of the referee.

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On the topology and dynamics of the magnetotail. Paper presented at the 7th IAGA Assembly, Exeter, 1989. Magnetic flux ropes in the magnetotail. In Mathematical Models in Space Physics. Proceedings of the Conference held in the memory of V. P. Shabansky, Moscow, 1988, KROPOTKIN,A. P. (ed.), p. 84. Moscow University Press, Moscow. J. geophys. Res. 94,241. Geophys. Res. Lett. 13,648.

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