JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. A12, PAGES 14,465-14,477, DECEMBER 1, 1988

A MULTI-INSTRUMENT STUDY OF FLUX TRANSFER EVENT STRUCTURE

] C. J. Farrugia, R. P. Rijnbeek, ]'2 M. A. Saunders, ] D. J. Southwood D. J. Rodgers, 3 M. F. Smith,3, 4 C. P. Chaloner, 5 D. S. Hall, 5

P. J. Christiansen, 6 and L. J. C. Woolliscroft 7

Abstract. We present simultaneous measure- ments at high temporal resolution of flux transfer events (FTEs) earthward of the magnetopause using four instruments on the Active Magnetospheric Particle Tracer Explorers (AMPTE) UKS spacecraft. The seven events in our study are distributed in local time from early dawn to midafternoon over a GSM latitude range of ~40 ø , from 24øN to ]7øS. A multilayered structure is revealed with systematic behavior in fields and charged particle populations. The plasma pressure varies in anticorrelation with the field pressure, the total pressure in general exceeding that of the surrounding medium. While it is not the prime concern of this paper to discuss the validity of models proposed for FTEs, the new, detailed experimental results it contains make it clear that the Russell-Elphic []978] model needs, at the very least, more detailed development.

7. Introduction

Magnetic reconnection is a process important in astrophysics, solar physics, and laboratory plasma devices as well as in the Earth's magnetosphere. However, the magnetosphere affords the only opportunity to make detailed measurements of plasma and fields in situ.

At the dayside magnetopause, reconnection appears to occur in both quasi-steady and time- dependent forms. Here we report on measurements made in flux transfer events (FTEs), which are thought to be the result of sporadic and localized reconnection [Russell and Elphic, ]978; Haerendel et al., ]978]. Over the last decade, ISEE 7 and 2 and Heos 2 spacecraft data have

, , ,,

7 The Blackett Laboratory, Imperial College,

London SW7 2BZ, England. 2Now at Institut f•r Weltraumforschung,

Inffeldgasse ]2, A-8070 Graz, Austria. 3Mullard Space Science Laboratory, University

College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, England.

4Now at Instrumentation and Space Research Division, Southwest Research Institute, San Antonio, TX 78284.

5 Rutherford Appleton Laboratory, Chilton,

Didcot, Oxon, OX77 OQX, England. 6School of Mathematical and Physical Sciences,

University of Sussex, Brighton, Sussex, BN7 9QH, England.

7physics Department, University of Sheffield, Sheffield S3 7RH, England.

Copyright 7988 by the American Geophysical Union.

Paper number 7A9272. 0748-0227/88/007A-9272505.00

been the source of many studies. New data on flux transfer events are now available from the

spacecraft of the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission, launched in August 7984. The new studies emerging [e.g., Southwood et al., ]986; LaBelle et al., ]987; Rijnbeek et al., 7987] suggest there remains much to be learnt.

This report presents a multi-instrument analysis of FTEs detected when the AMPTE UKS spacecraft was situated earthward of the magnetopause. Seven events were examined, from three orbits (listed in Table ]). Our study extends most previous work on FTEs through the use of a multi-instrument data set and also

through the high time resolution used. A brief account of instrumentation is given

in the next section. In section 3, we commence our analysis with a detailed description of one event seen on October 28, ]984. The remainder of the events are discussed and compared with the October 28 event in section 4. Our

concluding section summarizes our findings and discusses the implications for our understanding of the structure of magnetospheric FTEs.

There is already a body of literature on FTEs. Theoretical works and interpretations are often based on the simple original "connected flux tube" model of Russell and Elphic [7978], but other proposals have been made, e.g., the multiple X line reconnection model of Lee and Fu [7985]. Conceptually, it is useful to have a framework to organize our observations, and we shall use the Russell-Elphic model thus, but the final conclusion from our data is that a much

more elaborate model is needed.

2. Instrumentation

The three-axis orthogonal flux gate magneto- meter on board the AMPTE UKS spacecraft [Southwood et al., ]985] transmits in science mode the components of the magnetic field at a rate of ]6 or 8 samples/s. We shall use 0.5-s averages in the magnetic field displays. At present, calibration yields the absolute magnetic field to an accuracy of 2 nT (W. A. C. Mier-Jedrzejowicz, private communication, 7986). The AMPTE UKS ion spectrometer [Coates et al., ]985] uses a spin- synchronized energy sweep to yield a complete three-dimensional energy distribution of positive ions (assumed to be protons) in the range ]0 eV/q to 20 keV/q once every spin period (-5 s). The three-dimensional electron spectro- meter [Shah et al.• 7985] measures in the energy range 72 eV to ]8 keV, an energy sweep being made once or twice per second. Here we use 2-s resolution. Neither charged particle instrument samples the full energy range of the ambient plasma, and at times discrepancies may appear in the derived ion and electron density signifying

14,465

14,466 Farrugia et al.' Multi-instrument Study of Flux Transfer Events

TABLE ]. List of the Magnetospheric FTEs Studied

LOCATION

DAY, ]984 UT POLARITY LATITUDE, deg LT EVENT NUMBERS

Sept. 28 ]05] reverse -]7 ]340 a

Sept. 28 ]054 reverse -]7 ]340 a 2 Sept. 28 ]057 reverse -]7 ]340 a 3 Oct. 28 ]046 standard 26 0855 a 4 Oct. 28 ]]27 standard 24 0904 a 5 Dec. 6 0722 standard ]5 0638 a 6 Dec. 6 0727 standard ]5 0638 a 7

The terms "standard" and "reverse" polarity refer to up/down and down/up oscillations in the magnetic field component normal to the magnetopause. The latitudes and local times are given in the GSM coordinate system. The

letters a] to a 7 designate the flux transfer events in this study.

the presence of undetected charged particles outside the energy range of the instruments and/ or a significant number of ions other than protons. Plasma wave electric field data come from the wave instrument, described by Darbyshire et al. []985]. In the mode used here, an on-board signal processor calculates an autocorrelation function over ]024 data samples. The frequency spectrum subsequently derived on the ground yields a resolution of 74 Hz over the 0.]] to 3.9-kHz band. The spectra used have been averaged over half a spin period, thus removing the effects of any spin modulation in the electric dipole antenna signal. It must be mentioned that the electric dipole length (7 m) is less than ideal to ascertain that coupling with effects local to the spacecraft is eliminated altogether. However, we believe that the wave results reflect real changes in the ambient wave field.

3. The Observations

3.]. The October 28, ]984, Event' Overview

At ]046 UT on October 28,]984, UKS encountered a flux transfer event while outbound in the mid-

morning at 26 ø N GSM (geocentric solar magnetospheric coordinates).

Figure ] displays magnetic field data for a 6-min interval. Field components are given in the L, M, N boundary normal coordinate system introduced by Russell and Elphic []978]. For want of a better estimate of the local normal

direction at the time of the event, the N coordinate direction is along the outward pointing magnetopause minimum variance normal [Sonnerup, ]976] determined from the subsequent crossing at ]2]0 UT. L is parallel to the component of the Earth's dipole in the magnetopause plane, and M completes the set. The field strength is shown in the fourth panel. The last panel displays the angle, •LM, between the field component in the LM plane and the L axis (measured positive toward the M axis). The fact that B N does not show near-zero mean is likely to be due to our uncertainty as to the

actual local normal direction at this time and

is not of physical significance. The bipolar oscillation in B N evident in the

third panel is the basic signature of an FTE. Its peak-to-peak amplitude is 26 nT, well above the amplitude criterion set for survey purposes [Rijnbeek et al., ]984; Southwood et al., ]986].

The outer pair of vertical guidelines on the figure delimit the period where BN is perturbed and are separated by 5.4 min. The lowest panel shows that the field undergoes a series of deflections in the magnetopause plane also. It points initially northward and deflects first westward and then eastward. In the center of

the event the field rotates westward again to adopt a north-west orientation for about 35 s. The sequence of rotations on entry is then reversed as the spacecraft exits the structure.

Short-duration (•3-s period), large-amplitude (•]0 nT) magnetic field oscillations appear in all field components in the central segment.

The field strength (fourth panel) is enhanced over the values recorded before and after the

event, but clear peaks occur on each edge of the central segment.

Plasma bulk parameters are shown in Figure 2. Subscripts refer to the ions (which we assume to be protons) and electrons, respectively. The first panel displays the total density in cm -3. Energetic ( >]0 keV for ions; >2keV for the electrons) particle densities are shown in the second panel. The third panel gives the ion and electron temperatures in electron volts. The fourth panel shows the plasma bulk flow velocity in kilometers per second. Note that due to the not fully calibrated effect of penetrating radiation on the ion instrument, ion density and temperature may be slightly overestimated and underestimated, respectively, when fluxes are low. The qualitative behavior described here is at no point in doubt.

Magnetospheric particle populations are seen at the edges of the event. In the central region, the plasma parameters attain values typical of the magnetosheath. Previously, Paschmann et al. []982] and Daly and Keppler []982] have pointed out that values intermediate

Farrugia et al.' Multi-instrument Study of Flux Transfer Events 14,467

28 Oct 1984 (LMN) UKS ' ' ' I .... I .... I .... I ' ' ' I I ' ' i I .... I ' ' I "," ' ' , .... i .... , '"

_ .... ".' . B L 40 I I IR i R2' 20 - R1 I R2 ,R3 I R4 I 3', El' I I

o ........ I I i ', I I

. I I . _ ) 40- 20- i • • i -

- ..... •LM 0 1 ................... I ' , ..... •..' .... I ...............

Time 10:43 44 45 46 47 48 10:49 (UT) R 9.79 9.85

LTGs M 0855 0855 LATGs M 25.8 25.7

Fig. ]. Eight minutes of magnetic field data for event a 4 in Table ]. The resolution of the data is ] sample/0.5 s. The panels show, from top to bottom, the boundary normal components of the field (BN, BM, BL), the field strength (B), and a measure of the orientation of the field in the (LM) plane, •LM; •LM = 0ø and •LM = 90ø correspond to a field pointing along +L and +M, respectively.

between the magnetosheath and magnetosphere are attained in the center of magnetospheric FTEs.

Figure 3 shows the pressure variation. We have ignored the electron contribution to the plasma pressure in this paper. The contribution of the electrons, particularly in the central regions (see Figure 2), is small (at most 25%). The plasma, magnetic, and total (magnetic plus plasma) pressures are shown. The outer (large) dashed lines mark the region of enhanced density (cf. top panel of Figure 2). An increase in the total pressure by about 20-25% is clear, in agreement with earlier studies [Paschmann et al., ]982]. The better time resolution of the ion instrument reveals an interior structure

where the plasma and field pressures oscillate in strict antiphase, while maintaining the total pressure.

structure in the event is given in the color spectrograms from the ion, electron, and wave instruments in Plate ]. (Plate ] is shown here in black and white. The color version can be

found in the separate color section in this issue). The color-coded ion count rates (top panel)and electron intensities (middle panel) are displayed against time (horizontal) and energy (vertical). The white dotted trace in the electron spectrogram indicates the average energy. The electron energy scale is linear at low energies and quasi-logarithmic at the higher energies. Thus, tick marks shown in the electron sPectrogram correspond to ]00, 200, 500, and ]00D eV. The wave spectrogram (bottom

panel) shows intensity against frequency (vertical) and time (horizontal). Waves at or near noise level are suppressed by a threshold. There is an interference band between ].0 and

].5 kHz, and some small contamination may still exist in this frequency range. The black traces give calculated values of fce, the electron gyrofrequency, and fce/2.

Before ]045'00 UT, the plasma populations are those of the ambient magnetosphere. A distribution intermediate between that of the

magnetosheath and magnetosphere distributions then appears which•seems to be a mix of plasma from the two regions. This regime matches the steepest rise in the density plots of Figure 2 up to ]045'36 UT. In the Russell-Elphic model this region would be identified as the entry on to open field lines. Deeper still inside

energetic particles is observed (see also Figure 2), and no mixing is evident. The ion population resembles completely that in the sheath between 1045'54 •T and 1046'30 •T.

Immediately before and after this interval, the ion population is very similar, but it is more isotropic, and the count rates are lower than in the sheath proper. The electrons show a similar picture. Note also how, as in the previous figures, the characteristic types of population encountered are repeated in reverse order in the latter half of the event.

Where the mixture is present, both particle spectrograms show evidence of energization. If one follows the trace corresponding to peak

14,468 Farrugia et al.' Multi-instrument Study of Flux Transfer Events

Np,e 10

Np, e 10-1

Tp, e 103

Vp 100

28 October 1984 UKS These emissions have a power spectral density above 10-8.4 V2 m-2 Hz-], which is more than an order of magnitude higher than the background magnetospheric emissions. Total power in the 0.]]- to 3.9-kHz band is shown in Figure 5. The emissions are in two bands, above and below fce/2. Both are present between 1043:]5 UT and ]043:35 UT. The lower band is more pronounced between ]043:50 UT and ]044:]5 UT and between

]047:]5 UT and ]047:50 UT. After ]048:00, the upper band is hardly detectable. The wave bursts appear to be strongest at times of local minima in fce and hence in the magnetic field. The search coil on the companion AMPTE IRM spacecraft detected the waves and showed that they are electromagnetic (R. A. Treumann, private communication, ]986), and indeed some IRM wave data from the same event are shown in

the recent paper by LaBelle et al. []987]. Note that Figure 5 shows an asymmetry in the

strength of the emissions before and after the center of the event; an asymmetry is also seen in the electron intensities in the corresponding

10:43 10:49 regions (Plate ] ). Time (UT) In the central region, from about ]045' 30 UT

Fig. 2. Plasma bulk parameters for event a 4. The more jagged trace refers to the electrons. Top panel: proton and electron densities (cm-3); second panel: energetic proton (> ]0 keV) and electron (> 2 keV) densities (cm-3); third panel: temperatures (electron volts); bottom panel: bulk plasma velocity (kilometers per second). The vertical guidelines correspond to those-in Figure ].

to ]047:00 UT, there are smaller-amplitude, short-lived, broadband spiky emissions. It is difficult to estimate the amplitude confidently as the measurements may be limited by the correlator response time.

There is a further form of wave phenomenon present in the event, namely the fluctuations evident in the core in the magnetic field data. These were investigated by high-pass filtering the data and rotating it to a coordinate system aligned along the average field in the central region. The magnetic signal is polarized in

count rates in the ion spectrograms in Plate ], one finds that the maximum ion count rate

occurs at about 5 keV, an energy intermediate between those where the maxima occur in the

surrounding magnetosphere and central FTE region. These count rates exceed the sum of ions of the same energy observed outside and in the core. Similarly, in the electron spectrograms, the intensity of electrons near 0.5 keV is enchanced with respect to both neighboring regions.

There is another important feature of the enhanced fluxes of 0.5-keV electrons in the

intermediate region. In the large panel of Figure 4, we plot the electron intensity against energy for three different directions, parallel, perpendicular, and antiparallel to the field. It is clear that the increase in

intensity is associated with the field-aligned electrons. The inset illustrates this

explicitly with the full pitch angle distribution for 270-eV electrons. The

computed net heat flux along the field of these counterstreaming electrons is negligible (< ]0-4 J m-2 s-I).

The prominent feature in the bottom panel of Plate ] is the observation of strong bursts of wave power at and below about ].5 kHz on either side of what we have identified as the

open field line region; i.e., they occur where the densities and temperatures have not yet changed from ambient values. In the Russell and Elphic model, this corresponds to the region of draped, unreconnected field lines.

28 October 1984 U KS .... I .... i ........ i ........ 30 ................... I .... i";' • • I

- I i •/•\ i I -

I PTOT - I :

¾\ ' I \•.1 ' ,.., B -

I L/ I !L/! ! po I . , . I

................. I .... ':..! ..... I..': .... t ...............

25

20

PTOT

15

PP,B 10

5

0 10:43 10:49

Time(UT)

Fig. 3. Pressures during flux transfer event a 4 in units of ]0-]0 N m-2. The dashed trace is the ion pressure (the dominant contributor to plasma pressure), the solid curve in the middle is the magnetic field pressure, and the upper solid curve (labeled total pressure) is the sum thereof. Again, the vertical lines are similar to those in Figure ]. The two outermost solid vertical lines bracket the event, and the next innermost, dashed guidelines indicate the region within which the density departs from ambient values.

Farrugia et al.' Multi-instrument Study of Flux Transfer Events 14,469

10 0 .

1.0 '1: .. :.

1.0KEY

::. c:, 1-Ke V

10:43 10:46

CTS

•....,... 10:5

.,

i' •..' 10o

1015

lO 9

li):49

fce (kHz)

- - - fce/2 (kHz)

Time (UT)

Plate ]. Three color spectrograms for event a . The color version and a complete description of this figure can be found in the4separate color section in this issue.

the plane perpendicular to the field. No systematic sense of rotation was found except where the oscillations were largest in amplitude and most coherent (1046'00 - ]046:]2 UT). There they were approximately left-hand elliptically polarized with respect to the field direction as illustrated in the

hodogram shown in Figure 6. The frequency is

below the local ion cyclotron frequency (near ] Hz in a 64-nT field). Assuming the waves travel at near the Alfv•n speed (190 km/s) in the moving plasma (100 km/s, from Figure 2), one concludes that the Doppler shift is not big enough to change the frequency substantially from the emitted frequency nor to change the polarization sense in the plasma frame. Hence,

14,470

1015

1014

1015

Norrnolized Intensity (270eV)

Farrugia et al.' Multi-instrument Study of Flux Transfer Events

F signature into nested regions on the basis of • the magnetic field measurements. We shall follow this approach and define four distinct

•! • • nested regions, R],•R2, R3, and R4. These are I • labeled in Figure ], and all can be 45 90 135 10 characterized by distinct behavior in the magnetic field component in the plane of the

Pitch Angle magnetopause although all have distinct

84/302 10:45:20

tO 10:45:40

Porollel

....... Perpendiculor Antiporollel

',- 1011

0 1010 0 ß

• lO 9 101 10 2 10 • 10 4 10 5

Energy/eV Fig. 4. An electron energy spectrum over a 20-s integration period within region R2 in Figure ]. The parallel and antiparallel contribution are practically equal, but the perpendicular fluxes are a factor of 4-5 lower in intensity. In the inset the intensities of 270-eV electrons are

shown as a function of pitch angle. One notes the pronounced bidirectional streaming of electrons of this energy in this region.

we conclude that the waves are left-hand

polarized ion cyclotron mode waves propagating along the magnetic field.

3.2. Discussion of the Observations

Rijnbeek et al. []987], in a study of the same event, proposed subdividing the FTE

particle properties as well. A parameter that distinguishes the regions

very effectively is the ratio of plasma to field pressure, 8, plotter in Figure 7. The anti- phase relationship between plasma and field pressure noted earlier enhances the variation in 8. The regions are marked by the vertical lines as in the earlier figures, and we have introduced a labeling R]', R2', and R3', to distinguish the encounters on the exit leg.

Let us examine the variation in field

orientation represented by the angle •LM and the normal field component B N. In R], B N and •LM differ from ambient, but the particle populations do not. Theories based on the Russell and Elphic model [Southwood, ]985; Farrugia et al., ]987] identify this as Where the field is draped over the connected tube. The •LM panel in Figure ] shows the field reversing its sense of rotation at the R]-R2 boundary. In R3 the field slews in the L, M plane in the opposite sense and then returns. In R3, the low-pressure region, the field change is less dramatic. In the central region, R4, the systematic rotation ceases, and the direction fluctuates about this mean

position (cf. the fluctuations in Figure ]). On exiting, the spacecraft encounters each region again, in reverse order.

With reference to the regions marked on Figures ]-6 and Plate ], let us summarize the physical properties of the various regions. In R], the field line draping region, the field and flow are perturbed in accord with previous studies which have shown that magneto- hydrodynamic flow can well model what is seen

-7.6 , ' , , ,

-8.2

m. -8.6

•.. -8.8 • -9.0

-9.4 ....

10:49

Time (UT) Fig. 5. A power spectrum for the wave emissions in the previous figure. The regions corresponding to the times before 45'06 UT and after 47']8 UT are the fore and after draping regions respectively, the rest is the open field line region. The vertical axis shows the logarithm of the square of the wave power, and the horizontal axis is the time.

Farrugia et al.' Multi-instrument Study of Flux Transfer Events 14,471

-2

-4

28 October 1984 10:46:15 to 10:46:26 UT

' i I i

t

._

• I .' : : • :

' x••,• ' X--•

-- 89.6 + 20.0 Deg

-6 -6 -4 -2 0 2 4 6

Fig. 6. A hodogram showing the field

perturbations in the central part of event a 4 in a plane perpendicular to the direction of the average field there. In actuality, the perturbations have an inclination of 89.8 ø + 2].9 ø to this field.

[Southwood, ]985; Farrugia et al., ]987]. The particle population is magnetospherelike. The plasma pressure increases as we approach R2. In the wave instrument, R] is characterized by intensified choruslike emissions not present in the rest of the FTE. Their detection indicates

the presence of a population of electrons with a velocity distribution anisotropy to drive the waves via the familiar Doppler-shifted electron cyclotron interaction.

Rijnbeek et al. []987] identified R2 in magnetometer data and noted the presence of enhanced fluxes of intermediate energy electrons. We have seen that these electrons

have a marked pitch angle anisotropy and are bidirectionally streaming along the field. There is no electron heat flux detectable in

the layer in contrast to similar layers reported by Scudder et al. []984] in sheath events. The ion population is a mixture with evidence of heating. The ion flow characteristics in R2 are very intriguing; the highest flow speed seen in the event occurs in R2'. Inspection of the vector flow (not shown) shows that the flow at the peak is at an angle of ]20 ø to the field in contrast to an angle of 25 ø in the interior regions R3 and R4.

The short-lived, broadband, spiky wave emissions seen in the interior are first

observed in R2 and appear to correspond to the medium frequency waves reported in magneto- spheric FTEs by LaBelle et al. []987]. As noted by Anderson et al. []982], they are similar to waves in the magnetosheath but have an enhanced occurrence frequency. (One may note also that other electron waves are present

in this region but not detected by our instrument; LaBelle et al. []987, Figure ]0] show high-frequency noise bursts near the electron plasma frequency for this same event. Note that in our data we do detect electron

beams in this region which might account for •he presence of these emissions). In the example under discussion the field strength maximizes in R3. The field lines here are

certainly open, and the particle population is sheathlike except that it has lower intensities and is more isotropic. The broadband, spiky emissions also persist in this region.

The particle population of R4 is indistinguishable from the sheath. The field strength changes with respect to R3. In this case it decreases. The spiky emissions are evident here too, and region R4 also contains ultralow-frequency magnetic oscillations identified as ion cyclotron waves. The presence of the waves could indicate velocity anisotropy in some part of the ion distribution in R4.

4. Other Magnetospheric Events

In this section we summarize the properties of the six other events listed in Table ] and, in particular, compare them with the detailed report in section 3.

In Figures 8a, 8b and 8c, we show a selection of 6-min segments of magnetic field data at 0.5-s resolution for all the events studied.

Figures 9a, 9b, and 9c show (from top to bottom) the ion density (solid trace) and the density of energetic ions (dotted trace); the plasma bulk velocity (second panel); the ion (solid) and electron (dotted) temperature (third panel); and the pressures (bottom panel). The upper solid trace in the latter refers to the total

pressure, the dashed trace to the plasma pressure, and the lower solid trace to the magnetic pressure. The events are labeled

a i, a2, ....,a 7 in Figure 9, event a 4 being that

October 1984 10 ......... . .........

R1 R2

5

0 10:43

R3'I R2'

Time (UT) 10:49

Fig. 7. The variation of the plasma beta through event a.4. The values of the beta, shown on a linear scale along the vertical axis, are multiplied by a factor of ]0.

14,472 Farrugia et al.: Multi-instrument Study of Flux Transfer Events

60

B L 40 20

20

B M 0 -20

28 Sept 1984 (LMN) UKS .... i ,. • ß i .... ! .... i .... ! .... i .... i .... i .... i .... i .... i ....

aN

B (NT)

20

0

-20

6O

4O

2O . . . i .... i .... i ...... I i i i i I ! • , , I , •

Time 12:49 ...... 5'0 ......... 5'1' 52 '•3 54 i :•:55 (UT) R 10:10 10:04 9:97

LTGsM 1338 1339 1339

LATGs M -16.7 -16.8 -16,9

Fig. 8a

Fig. 8. Three 6-min magnetic field plots at 0.5-s resolution showing the components in boundary normal coordinates and the field strength. Two reverse polarity FTEs are seen on the inleg, September 28, ]984, at southerly GSM latitudes (Figure 8a, events a. and a in Table 1) Figures 8b and 8c show two normal polarity events on the ! 2 ' outbound passes on October 28, ]984, and December 6, ]984, respectively, this time at

northern GSM latitudes (events a5, a• in Table ]). There is practically an annihilation of the field in the ear ier part of the December 6 event.

studied in section 3. Position information in

GSM coordinates is given in Figure 8. Figures 8a and 9a are from an inbound pass in

the early afternoon sector (]340 LT) at southerly GSM latitudes (]7øS) September 28, ]984. The FTEs marked are of reverse type [Rijnbeek et al., ]982] as are the vast majority seen at southern latitudes [Southwood et al., ]986]. The ]8 min of data shown in Figure 9a include a magnetopause crossing where there is clear evidence of reconnection, namely the accelerated flow at ]247 UT, accompanied by velocity bursts and variations in the density. The perturbationsin field at this time indicate the presence of one or possibly two FTEs, but the signatures are confused by the magnetopause signatures, and the events were not included in our study. FTEs occur at ]25] UT (a]), ]254 UT (a), and ]257 UT (a3). Note the anticorrelation of the densities of bulk and energetic particles in each event, the structured high-speed flow associated with each, and the increases in total pressure. The total pressure shows less structure than the particle and field pressures individually. The anti- correlation between field and particle pressure in the central regions, as in our paradigm, is clear.

Figures 8b and 9b are from October 28, ]984, and show a second event, 40 min after the event

discussed in detail. As noted earlier, the spacecraft is above the equator in the morning local time sector (0840 LT). Figures 8c and 9c show data from an outbound pass at ]5 ø N GSM near dawn (0638 UT) on December 6, ]984.

All events show structure in the field

strength. The magnetic field profile of events

a], a4, and a 5 shows a pair of shoulders surrounding a plateau of depressed field

strength. Others (a2, a3, a6, and, to a lesser extent, a 7) exhibit two dips in the field strength surrounding a central peak. The lowest field strength values within the events are usually below levels outside. In the most

extreme instance, a•, the field strength drops to 8 nT from an ambient value of 28 nT. Every event except one (a 2) has the large-amplitude ULF oscillations. All events exhibit multiple field deflections in the angle •LM in the plane of the magnetopause. A few of these, notably

in a6, are more complicated than those of a4, the event chosen as a paradigm.

The bulk parameter plots (Figures 9a-9c) show behavior common to all events. In each case the

density increases over local magnetospheric values to approach sheath levels at the center (see, for example, Figure 9a) where in addition the energetic ions drop out.

Evidence of an energized population at the edges is seen in some of the events (a], a4, a6,

Farrugia et al.: Multi-instrument Study of Flux Transfer Events 14,473

70 60 50

B L 4O 30 20 10

30 20

B a lO o

-lO

lO

BN .100 70 60

B 50 (NT) 40

30 20

28 October 1984 (LMN) UKS ,' ß ' ß i .... i .... i .... i .... i .... I .... i .... i .... i .... I .... i .... j

- I

lO : i ...... i ...... i ...... i ...... i . , , i i i

Time 11 ;24" '2'5 '2'6 ':•7; 2'8' 20 i i :30 (UT) R 10:63 10:69 10:75

LTGs M 0903 0904 0905

LATGs M 24.4 24.3 24.2

Fig. 8b

a7). Energy spectrograms confirm the presence of bidirectionally streaming electrons in a certain energy range in all events in this region.

Although the structure of field and plasma pressure varies in detail from event to event, the anticorrelated behavior of the plasma and field pressures is a repeatable feature seen in all events. The total pressure, in all

events except perhaps event a7, is above that in the surrounding medium. In some events this excess of pressure is maintained

over an extended period (a•, a4, as, a6); in others, it is sharp and lo alized (events a2, a3). The total pressure excess inside the events is maintained even in extreme circum-

stances such as event a 6 where the field pressure drops to almost zero.

It is hard to generalize about the flow characteristics in the events. Substantial

enhancements of flow speed can be seen in all. The peak flow in the interior of the events is usually also above that expected in the nearby magnetosheath (cf. events a], a2, a3). However, as we described in our detailed description, flow bursts at either the leading or trailing edges of the events, or both, are not uncommon.

In fact the flows detected in FTEs are more

complicated than indicated so far. We have not given vector information. Vector flow data show shears are common. To illustrate, we display in Figure ]0 the field and flow components in L,M,N coordinates at 5-s resolution for the events a], a2, and a 3. In event a2, whose flow profile shows a single central enhancement (Figure 9a), the plasma is flowing consistently in the -L and -M direction. In events a! and a3, however, the leading flow burst is toward

+L while the second is toward -L. It follows

that in the first burst in a], the angle between the flow direction and the field changes from being about 20 ø to the field at the leading edge to ]40 ø at the trailing edge.

The wave emissions seen in the events are

in general similar to those discussed in our detailed study of event a 4 with spikelike (sheath) noise within the center "open" field line region and an intensification of emissions near the electron cyclotron frequency outside. The power is generally weaker than in event a 4.

In event a 6 the emissions have very low power and, where the pronounced dip in magnetic field strength occurs, the wave intensity has the same level as in the undisturbed magneto- sphere. As the field starts to recover, spiky emissions are seen.

5. Discussion and Conclusions

We have described magnetic field, ion, electron, and wave data from a number of flux transfer events encountered by UKS inside the Earth's magnetosphere in late ]984.

Originally, FTEs were identified by their magnetic signature alone. Subsequently, a large number of papers have shown that a whole complex of signatures in particle, field, and wave data are associated with them. No papers (except for LaBelle et al.'s []987] recent paper on waves in FTEs) have taken data from as many types of instrument as this study, and only rarely has comparable time resolution been used.

We chose one event as a paradigm. The event was fairly symmetrical, matching our

14,474 Farrugia et al.: Multi-instrument Study of Flux Transfer Events

50 40 30

BE 20 lO

o -lO

5o 40 30

Bu 20 lO

o -lO

BN 1• , -10

5O

B 40 30

(NT) 20 10

Time 07:19

6 Dec 1984 (LMN) UKS ,_ .... i .... i .... I ', ß ß ß i .... ! .... i .... i .... i .... i .... i .... • .... ,

::

; .... i .... i .... i .... i .... i .... i .... i .... i .... I.llll,l,,llll I

20 21 22 23 24 07:25(UT) R 12.89

LTGs M 0638

LATGs M 15.2

12.93 12.98

0638 0638

15.3 15.4

Fig. 8c

preconceived idea that FTEs have a basically cylindrical symmetry centered on a tube of open flux. In an earlier report of the same event, Rijnbeek et al. []987] proposed that there were several layers of different plasma and field regimes about the central core which the simple flux tube model [Russell and Elphic, ]978] or developments of it [Cowley, ]982; Paschmann et al., ]982] do not explain. The new data used here have borne out the layering hypothesis (cf. the plot of plasma 8, in Figure 6).

Further analysis of the other events revealed not complete repeatability, but strong similarities. Bearing in mind that even were each structure simply layered and cylindrically symmetric, there would be no guarantee that the combination of spacecraft orbit and possibly irregular motion of the structure as a whole would produce the same sampling in every event, there is no real concern that all features are

not reproduced. We have too few events to attempt a

statistical study but it still is worth outlining what effects are necessary features of FTEs and which are contingent. Evidently, almost by definition, the bending of the field associated with the bipolar B N signature heads the list. There are two potential sources, draping in the outer regions as the FTE obstacle moves the surrounding plasma out of its way and, secondly, twisting of the field to contain a pressure excess in the core. The B N signature is found well outside the region where pressure is enhanced, and yet it extends right into the interior where something must balance the total pressure excess [Paschmann et al., ]982]. Hence, our results are consistent with both effects being present in different regions.

The skewing of the field in the L,M plane, represented by the angle •LM, is also likely to have its basis in the same two effects.

The field strength may increase or decrease in FTEs as Rijnbeek et al. []984] first pointed out. L•hr and Kl•cker []987] and LaBelle et al. []987] have furthermore identified and elaborated upon what they call the "crater"-like signature which is also seen in our events. One event here (a 6) showed an extreme minimum field value and, in this respect, resembled the "holes" near the magnetopause reported by L•hr and Kl•cker [1987]. Indeed, L•hr and Kl•cker [1987] compare the hole signature to that of FTEs. Although the field pressure may vary above and below exterior values, the total pressure is consistently larger in the central regions than that outside. A new feature brought out in our work is that the excess may be maintained by field or plasma (ion) pressure and either may be counterbalanced by the other within a given event. A corollary is that the variation in plasma 8 in a given event may be large.

The simplest layered model is one in which no flow crosses the interfaces where field and

plasma pressure change but the presence of some wavelike structures in the interior cannot be

precluded. In past analyses, field and plasma parameter changes like those associated with the maintenance of stress balance in an Alfv•n

wave have been proposed [cf. Saunders et al., 1984]. The antiphase pressure variations that we detect are more like those associated with

an obliquely propagating slow mode MHD wave. Streaming electrons are a distinct feature

of the boundary regions of events and have been associated with the occurrence of slow shocks

Farrugia et al.' Multi-instrument Study of Flux Transfer Events 1•,•75

Np

200

Vp 100

28 Sept 1984 AMPTE - UKS

1

0.1

0.01

' ' t ,,

i ' ,,

I I I I I

i- .. I I Ill I I I I [ /A A I,,• I Ill I I II• I ;.,.;v;: •, 'd"- . v ,! .... v ---

PP,B '" ' ' ,, ' V • .... , v I /vwv :I•,:'"."• ....... '• ' ' ..... ';i ........ ?' ' ', ......... -" .... I .... I, ---I .... iI .... i,l...iL ....... i ...i....i .... i ....

Time 12.44 47 50 53 56 59 13.02(UT)

R 10.21 i0.02 9.88 LT 1356 1358 1400 LAT -1.5 -1.6 -1.6

10

1

Np 0.1

0.01

vp 100

28 October 1984 AMPTE - UKS ß l'','l'''l .... i .... i .... i .... i .... ,---'-,-•-'1 .... I-I''l''l'

ILII•I I /• IJ

I I I

i I

', I

• ! ! I I I I I I

fl i . I It

Tp 103

I

I

I I

'"" r I/'1• ..... • • ,,,_ •

PP,B [',.[i: :• ......................................... .., ,-, ,:•," I ,,

Time 10.43 51 59 11.07 15 23 11.31 (UT)

R 9.79 10.28 10.75

Fig. 9b

Fig. 9a

Fig. 9. Ion bulk parameters for all the flux transfer events in this study, for (a) September 28, (b) October 28, and (c) December 6. The top panel shows the total ion densities (assumed protons) in cm-3 (solid curve) and the density of energetic (> ]0 keV) protons (dotted curve). The second panel shows the ion flow velocity, and the third panel the temperatures. The bottom panel shows the total pressure (upper solid curve), the plasma pressure (dashed curve), and the field pressure (lower solid curve). The open field line region for each event is indicated between vertical guide- lines.

in the magnetotail [Feldman et al., ]•84]. Similar electron distributions have been

reported in FTEs by Scudder et al. []984]. However, unlike this earlier report, we do not detect significant heat flux because of the bidirectional streaming. If the electrons originate in the sheath, they have been energized on their way to the spacecraft, perhaps at a neutral line. A sheath origin,explains the counterstreaming well. The electrons move rapidly, and those flowing away from the Earth along the field have already been reflected by the mirror force closer to Earth. Their high speed means there is a need to postulate a heating mechanism that is continuously present. Continued reconnection is a clear candidate

[see also Scudder et al., ]984]. Models like that of Lee and Fu []985]

10

1

Np 0.1

0.01

3OO

Vp 200

100

Tp 103

PTOT 10 PP,B

Time 7.18 20

6 Dec 1984 AMPTE - UKS

. a6 _,

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

i

i i i i i i i i i

i ' I

............. I ................. ! .......... ! ..... ! ........... I i I I

I I I 1 i I I

, I [; ,.,•. /., ,...., ',•; ;- .... -,,-,.'.-.,

""" 22 24 26 28 7.30(UT)

R 12.87 12.96 13.06

Fig. 9c

14,476 Farrugia et al.' Multi-instrument Study of Flux Transfer Events

VL

VM

VN

V 200 100

28 Sept 1984 AMPTE - UKS • .... ' .... I .... ''''1' .... '•1''''1'''' .... ' .... I .... ''1'''' .... '

.,• I ̂ I i I I I ,uur I// I I I I , I

..•1 I I I_ •_ ^ _ I

i

I I I I i

i

•0 • •• • i

I i i I

•0 i •0• I •

0 ........... ---7 L • I • i •

•0 • I AI I kl I

-20 ........ • ,

i i

issue. Work to be published elsewhere is proceeding on detailed comparison of field and ion flow velocity patterns in FTEs. Although shears in flow are a natural feature of flux

tube models, the dramatic instances of the reversal of the flow component along the field direction recorded here were not expected. We have no simple explanation of the effect.

There have been few studies of the electro-

magnetic spectrum of FTEs in the past [Anderson et al., 1982; LaBelle et al., 1987]. The signals recorded here near the ion and electron gyrofrequencies may simply be a parasitic effect driven by the interstreaming plasmas resulting from reconnection. No one has proposed that electromagnetic turbulence plays a primary role in FTE formation, but this turbulence may still be important in the overall physics. In a collision-free plasma, waves can be an agent in catalytically facilitating the substantial redistribution of energy and momentum. The waves seen here can have significantly influenced the detailed evolution of the plasma in FTEs.

Acknowledgments. The authors gratefully acknowledge helpful discussions with D. A. Bryant, S. W. H. Cowley, A.D. Johnstone and M.P. Freeman. We would also like to thank

the referees for many helpful suggestions. We Time 12.48 50 52 54 56 58 13.00(UT) are very grateful to Sue Hewes for carefully

typing the manuscript. During part of this research C. J. Farrugia was supported by a European Space Agency felloyship and R. P. Rijnbeek was sponsored by a Royal Society European Exchange program.

The Editor thanks G. Paschmann, B. U. O. Sonnerup, and a third referee for their assistance in evaluating this paper.

R 10.13 10.00 9.88

Fig. 10. Flow and field data for events a I to a 3 in boundary normal coordinates. Note the flow enhancements during the three events shown.

There are flow shears in events a 1 and a 3 but not in event a 2.

postulate the creation of FTE-like structures during tearing mode reconnection at the magneto- pause. In simulations [Ding et al., 1986], magnetic bubbles form and move away while reconnection continues. The hot electron

boundary layer fits well with this aspect of the tearing mode model. However, this is not the only possibility. Biernat et al. [1987] present a two-dimensional theory of time- dependent reconnection in the Petschek limit where a bubblelike structure naturally forms. The two dimensionality of the model precludes application directly to FTEs, but the authors do point out the geometrical similarity to FTEs.

Sonnerup [1987] has attempted to extend the basic Russell and Elphic connected tube model and account for many subsequent observations. He too, however, introduces reconnection on the outer layers of the tube to explain the hot electrons.

Flows and shears within FTEs deserve further

study. Early reports by Saunders et al. [1984] proposed that field and flow perturbations were proportional and related much as an AlfvEn wave. Farrugia et al.'s [1987] study showed that far from the center even simple two-dimensional incompressible flow, in which field and flow perturbations also are proportional, gave good qualitative agreement. In fact, quantitative tests using carefully calibrated particle instrumentation are required to resolve the

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(Received July ]3, ]987; revised February 8, ]988;

accepted February ]], ]988.)