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Multipoint measurements of magnetotail dynamics
Adv. Space Res. Vol. 8, No. 9—10, pp. (9)97—(9)108, 1988 Printed in Great Britain. All rights reserved.
0273—1177/88 $0.00 + .50 Copyright © 1989 COSPAR
MULTIPOINT MEASUREMENTS OF MAGNETOTAIL DYNAMICS D. H. Fairfield Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.
ABSTRACT Multipoint spacecraft measurements have played an important role in documenting the morphology of magnetospheric substornis. Widely separated measurements have quite clearly delineated many large scale features such as, (1) plasma sheet thinning associated with the onset of substorms, (2) energy storage and release in the magnetotail associated with the growth and expansion phase of substorms, (3) the earthward and tailward field aligned currents associated with the dawn and dusk portions of the substorm current wedge and (4) the deep tail response that follows substorms detected near synchronous orbit. Smaller scale phenomena that must be observed on finer time scales and detected in more limited spatial regions have been studied by ISEE 1 and 2 and a number of spacecraft near geosynchronous orbit. Injection fronts that are sometimes thought to propagate earthward toward synchronous orbit are difficult to detect and observed time delays may be due at least partially to longitudinal expansions of the substorm current wedge. Measurements near the equatorial current sheet where substorm onsets are often thought to originate, frequently reveal rapid time variations yet nearby spacecraft are seldom available to help determine whether these variations are due to local turbulence or more coherent behavior on a somewhat larger scale. High time resolution measurements from the Cluster and ISTP programs should help rectify these ambiguities. INTRODUCTION To understand the dynamics of a complex system such as the earth’s magnetotail it is necessary to obtain knowledge of time changes in different locations throughout a vast volume of space. To obtain such knowledge one can either (1) assume the existence of a repeatable pattern of changes and combine a large number of measurements at different locations at different times to give a statistical pattern that may approximate an individual event, or (2) make measurements at different locations during individual events. Both of these approaches have been used to contribute to our current understanding of the magnetotail. This review emphasizes the important contributions of the second technique. CONTRIBUTING SPACECRAFT 1963-1988 Figure 1 indicates intervals during which 51 different spacecraft made measurements between 1963 and 1988. Solid lines in the top portion of the figure indicate 14 fields and particles spacecraft arranged vertically according to their apogee and plotted during their lifetime of useful operation. Off scale at the top of the figure and designated by dotted lines are Explorer 35 in lunar orbit (~6ORE) and Explorer 33 with an apogee beyond 76 R Off scale at the bottom and again designated by dotted lines are 12 YELA spacecraft (VEL launches included 6 pairs of spacecraft) which carried plasma/particle payloads into approximately circular orbits near 18 RE. In the bottom portion of the figure are 22 spacecraft in geosynchronous and near geosynchronous (GEOS 1, SCATIIA, DE 1,and CCE) orbits which are included because of their unique ability to define substorm variations in the inner magnetotail. Several trends are apparent in Figure 1. The mission of early spacecraft was to explore the magnetospheric environment and hence early spacecraft carried fields and particles experiments into highly eccentric orbits where they could explore a wide variety of phenomena. The combined high frequency of launches in the late 1960’s along with increasing spacecraft lifetimes produced an optimum opportunity for multipoint measurements in the period of roughly 1969—73. As this exploratory phase of magnetospheric research tapered off, applications spacecraft carrying auxiliary scientific experiments began to proliferate in synchronous orbit which led to unprecedented opportunities for multipoint measurements in this more limited region of space after ~l975. Primary among these geosynchronous applications spacecraft are the U. S. SMS—GOES series of weather satellites making magnetic field and very energetic (>~0.5MeV) particle measurements and the U. S. Los Alamosinstrumented DOE/DOD series of spacecraft (designated by numbers in Figure 1) which make (9)97
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D. H. Fairfield
MAGNETOTAIL SPACECRAFT 1963—1988 EXP 35
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Fig. 1. Intervals indicating the useful lifetimes of 51 spacecraft that have contributed to magnetotail research. In the top portion of the figure solid lines indicate the spacecraft apogee according to the scale at the left whereas dashed lines indicate spacecraft off the scale. In the lower portion of the figure dotted lines indicate spacecraft in near geosynchronous orbits and solid lines indicate spacecraft in geosynchronous orbit, all spaced on an arbitrary vertical scale. Tic marks indicate the begin time of the associated year. intermediate energy (30-2000 keV) particle measurements. Another trend discernible in Figure 1 is the increasing lifetime of the more recently launched spacecraft. In particular, the IMP 8 and SCATHA spacecraft are still operating after 14 and 9 years respectively and other spacecraft such as the IMP 5, 6 and ISEE 1/2 spacecraft continued to operate until they reentered the atmosphere. Finally we note that the launch of the well instrumented AMPTE IRM and CCE spacecraft in 1984 along with the ongoing IMP 8, ISEE 1/2 and geosynchronous spacecraft have provided another unprecedented opportunity for near earth and distant tail multipoint measurements in 1985—87 which we should be hearing more about in the future. The ongoing series of GOES and Los Alamos spacecraft along with CCE should provide important support for the ISTP and Cluster spacecraft in the 1990’s.
MULTIPOINT MEASUREMENTS Plasma Sheet Dynamics Associated with Substorms The thinning of the plasma sheet associated with substorm onset and its subsequent thickening during recovery are among the most prominent dynamical changes in the magnetotail. Thinning at the time of onset was first suggested by the disappearance of plasma at ‘48 RE but it was not immediately clear that this disappearance was caused by actual thinning rather than simple motion of the plasma sheet. Actual thinning was verified by examples where 2 VELA spacecraft located near 18 RE in the northern and southern portions of the plasma sheet both detected the disappearance of the plasma sheet at the same time /1/. This thinning of the distant plasma sheet at the time of substorm onset has been most recently demonstrated by Sauvaud et al. /2/ and Huang et al. /3/. Sauvaud et al. found that the disappearance of plasma at ISEE 1 in 100 events (primarily beyond ‘45 RE) occurred within a few minutes of plasma injections (substorm onsets) detected at the GEOS 2 spacecraft in geosynchronous orbit. Probably the best illustration of a thin current sheet within a thinned plasma sheet after substorm onset is the ISEE 1/2 example shown in Figure 2 /4/. ISEE 1 was 15.4 RE from the earth near the 0200 LT meridian plane and ISEE 2 was 1.5 RE closer to the earth near the
Magnetotail Dynamics
same meridian.
In an appropriately tilted coordinate system the north— south separation of these spacecraft was 0.6 RE. At 1057 30 s ISEE 2 (the lighter trace in Figure 2) crossed the current sheet from the northern to the southern hemisphere and at 1106 45 s ISEE 1 did the same. For 9 minutes in between these times the two spacecraft were observing oppositely—directed lobe-like magnetic fields of the northern and southern hemispheres. The current sheet and high fi plasma sheet must have been located between these spacecraft and must have been considerably thinner than the 3700—km north-south spacecraft separation. Measurements at different radial distances have shown that plasma sheet thinning within “15 RE of the earth occurs before substorm onset /5,6/. This near earth thinning is generally detected by the disappearance of energetic trapped particles at the synchronous orbit and beyond. Multispacecraft measurements have demonstrated that this pre-onset thinning is a growth phase phenomena produced by a southward IMP /7,8/. This phenomena can even be used as a short term predictor of an imminent substorm /9/.
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Fig. 2. ISEE 1 and 2 measurements where the spacecraft were separated by 1.5 RE in ~ and 0.6 RE in an appropriate north-south coordinate system. For nine minutes a thin current sheet must have lay between the two spacecraft which were measuring strong lobe— like fields of the northern and southern hemispheres respectively.
Although the morphology of plasma sheet thinning phenomena is rather well established, the velocities and the detailed structure of the outer boundary of the plasma sheet at these times are more difficult to determine, In principle one would like to see the boundary cross two spacecraft in succession so that the velocity of the boundary could be determined by dividing the separation by the time delay. Unfortunately this procedure requires knowledge of the orientation of the boundary and assumptions about its uniformity, which is information that is not readily obtained by two spacecraft. In spite of such difficulties individual two— spacecraft examples have been presented using such spacecraft as VELA and IMP 3 /10,11/, and IMP 7 and 8 /12/ all of which give velocities in the range of a few to a few tens of km/s. The ISEE 1-2 satellite pair has also been used /,13,14,15/ but these spacecraft are so close together (often <1000 km apart) that the time delays are often difficult to measure. Forbes et al. found average thinning velocities near 20 km/s both from interspacecraft timing and from direct measurements of north-south plasma flows, but only about half the time could time delays even be discerned. Their plasma sheet expansion velocities were considerably higher but this may have been due to waves on the plasma sheet boundary during substorm recovery which were perhaps due to the Kelvin—Helmholz instability. Substorm Current Wedge Many years of research have made it abundantly clear that the well known dipolarization of the magnetic field in the inner magnetotail at substorm onset is particularly clear in the region near geosynchronous orbit. These dipolarizations along with their associated Pi 2 pulsations and ground perturbations [e. g. /16,17/ provide the best indicator of the time of substorm onsets and hence these data are of considerable importance for comparison with measurements in the more distant tail. The concept of the substorm current wedge has become very well established as a result of measurements at geosynchronous orbit /e.g. 18,19 and references therein/. At the time of substorm onset equatorial cross-tail currents are disrupted with the diverted currents flowing earthward along field lines in the post—midnight sector and through the ionosphere where they create the westward electrojet. Finally the currents complete their circuit by flowing back out to the magnetotail along field lines of the pre—midnight sector. In recent years many geosynchronous spacecraft have contributed to refinements of this simple current wedge picture. Multipoint measurements have shown that the current wedge develops in a longitudinal sector perhaps 2 to 3 hours wide in local time /20/ which then JASR 8:9/10—C
D. H. Fairfield
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expands in longitude over the ~iext 10 to 20 minutes /21,20/, sometimes in association with additional intensifications in adjacent and overlapping longitudinal sectors /22,23/. The east—west D perturbation at geosynchronous orbit generally begins at the time of ground onset whereas the accompanying increase in the north-south or H component is generally delayed until the expanding current wedge reaches the longitude of the observing spacecraft /21,24/. This delay means that the depressed 11 component marking the substorm growth phase can persist outside the current wedge sector even after substorm onset has begun within the wedge. This local time dependence of substorm phenomena introduces additional complications to substorm research and points up the need for multipoint measurements. Figure 3a illustrates magnetic field measurements from the AMPTE/CCE and GOES 5 and 6 spacecraft in the inner magnetotail /19/. Data are in a left hand coordinate system where B is parallel to the dipole axis and positive northward, B~is aligned in the meridian p’ane with positive in an earthward direction, and B~is transverse to the meridian plane with positive in an eastward direction as it is at ground observatories. The onset times and central meridians of two substorms have been determined from ground magnetograms and are indicated by vertical lines in the bottom portion of the figure and at appropriate longitudes in the inserts which graphically indicate the spacecraft location. It is important to note that all of these spacecraft are north of the magnetic equatorial plane. Prominent features in Figure 3a are the well known increase in B and decrease in B~or 9 that is characteristic of field line stretching during the growth phase preceding substorm onsets /e. g. 25,8/. Other equally prominent features are the B decrease and B 5 or 0 increase that is associated with each substorm onset. The latter effect is, however, delayed by some 20 minutes at GOES 5 for the second substorm due to the fact that this spacecraft has moved well into the morning hours where it is well to the east of the onset
meridian. Figure 3b displays AMPTE/CCE and GOES 5 data for the earlier substorm at 6 and 3 s resolution respectively. This figure shows increases in Bz at the two spacecraft which are simultaneous within a fraction of a minute and which occur without any change, in B. These ONSET
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Fig. 3b. High resolution AMPTE/CCE and 9OES 5 data for the first substorm interval of figure 3a. The three components are characterized by a simultaneous onset in B~, a time delay iii the decrease in B and different signs for the pertur~ationin By.
Magnetotail Dynamics
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facts suggest that the increases are due to the integrated effects of the wedge currents. In contrast, the field magnitude decreases abruptly at GOES 5 at 0359 and at AMPTE/CCE at about 0407 which suggests an expanding plasma sheet boundary that successively crosses these spacecraft /see also 26/. The behavior of the By components is quite different at the two spacecraft and can be interpreted in terms of an earthward field aligned current that is initially equatorward of both spacecraft and then passes over the lower latitude GOES 5 at 0359 and the slightly higher latitude AMPTE/CCE at 0410. This equatorward location for the wedge currents is unusual and is probably due to the large intensity of this substorm. Other multipoint measurements have suggested that the outward pre—midnight leg of the current wedge is confined to a relatively small flux tube which is associated with the westward traveling surge /27/ while the earthward directed dawnside currents assume a more conventional sheet configuration. The inward and outward currents probably even overlap in the near midnight region and produce a region of great complexity /28,19/. Injection Fronts Even prior to much of the current wedge research discussed above there was considerable interest in the precise timing of dipolarizations at different locations in the inner magnetotail. A fortuitous alignment of 000-5 and ATS-1 on 7 August 1968 to provided important information on this question /29/. These two spacecraft were separated in a nearly radial direction by 2.3 RE near the 0100 LT meridian (solar magnetospheric coordinates -8.2, -2.0, 2.1 RE and -5.9, -2.1, 2.2 RE respectively) when they observed characteristic B and B~field increases (see figure 4). These increases were observed 94 seconds earlier at the more distant spacecraft which led to the suggestion that the increases were caused by a field compression wave propagating inward with a velocity of ‘450 km/s. Several similar near-radial alignments of the ATS-5 and SCATRA spacecraft were found, again with corresponding equatorial field and particle increases that occurred first at the spacecraft more distant from the earth /30/. These observations were taken as evidence supporting the idea of an inward propagating “injection front” as it was called by Moore et al. /30/. These authors also suggested that hot plasma from the more distant plasma sheet 8 moved earthward behind front andatproduced plasma “injections frequently observed in the the injection inner magnetotail the time the of hot substorms. Two other examples /31,28/ support the injection front picture but are not entirely definitive. MAGNETOSPHERIC
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D. H. Fairfield
Although the injection front remains a popular idea, a closer look at the supporting data reveals some additional complications. Russell /32/ in a further discussion of the 7 August 1968 event notes that the substorm associated with this event occurred about 10 minutes earlier on the ground than at the spacecraft which is an unusually long delay for a spacecraft near the onset meridian of a substorm. A careful examination of the magnetograms for this day indeed clearly defines the onset at 1120 and at a meridian centered near or slightly east of Guam which places it several hours west of the spacecraft meridian. North American observatories near and east of the spacecraft meridian exhibit negative D and H perturbations demonstrating that this region is initially east of the current wedge. A larger negative D perturbation at western U. S. stations at 1130 suggests an impulsive eastward expansion of the wedge at this time which would move it toward the spacecraft pair where OGO 5 is ~6° west of ATS 1. A close look at the ATS—5 — SCATIIA locations of Moore et al. /30/ also reveals that the second spacecraft to see the disturbance is invariably the spacecraft slightly further from midnight as well as the one closer to the earth. Since the more recent work /21,20/ has demonstrated that the effects associated with the substorm current wedge expand longitudinally from the onset sector near midnight, one wonders whether this longitudinal expansion might be more relevant than the radial propagation that had been invoked to explain the earlier examples. In summary, it may be concluded that the injection boundary idea remains attractive idea, however on close examination the evidence for its existence may not be as strong as is sometimes supposed. Small scale magnetotail phenomena: ISEE1/2 The ISEE 1/2 spacecraft pair with distance separations of typically 100—1000 km provide and unprecedented opportunity to measure rapid boundary motions and fine scale phenomena. In addition to the plasma sheet boundary motion discussed above there have been high resolution studies of field aligned currents showing oscillations /33/ and considerable complexity /34/. One field aligned current event has been interpreted as a magnetic flux rope /35/. Large Scale Behavior of the Magnetotail Measurements at widely spaced locations throughout the magnetotail have been instrumental in demonstrating the coherent behavior of the magnetotail during substorms. An early multipoint study /36/ combined published IMP 4 data /37/ with 000 5 and ATS 1 data and solar wind measurements from Explorer 33 and 35 to provide some of the first indications of how a southward IMP produces a thin plasma sheet and stores energy in the tail prior to substorm onset. This solar wind control was pursued further by Pytte and West /7/ which was one of a comprehensive series of papers /6,38,39,40/ that used OGO 5 and VELA magnetotail data and also made extensive use of ground observations to accurately define substorm onset times. An example from /6/ shown in figure 5 compares VELA particle data measurements in the premidnight region at ‘48 Rg just north of the magnetotail equatorial plane with 000-5 particle and field measurements near the same latitude and longitude but between 16 and 9 R~. Near 13 RE at “2200 LT 000 5 detected particle increases and magnetic field dipolarizations at the time of substorm intensifications as defined by the onset of Pi 2 pulsations on the ground. In contrast, the particles measured by VELA at the greater distance gradually disappeared and did not return until the recovery phase of the substorm shortly before 2300 LT. Similar effects were seen during a second series of weaker intensifications after 0054. Another paper of this series /38/ made the distinction between magnetic activity driven directly by a southward IMP and that related to energy released from the magnetotail during sudden onset substorms marked by Pi 2 pulsations. This paper suggests that both modes of generation are important, which is a conclusion generally accepted today, but only after many years of controversy. Other papers of this series demonstrated the association between the poleward boundary of the aurora and the high latitude boundary of the plasma sheet /40/ and the movement of the two during the recovery phase of substorms /39/. Another multipoint study of large scale magnetotail dynamics using IMP 6, 7 and 8 measurements was the first to combine plasma and field measurements and allow a study of the total tail pressure variation at different locations /41/. An example from this paper is shown in Figure 6 which displays IMP 8 and IMP 8 magnetic field data sampled at a similar radial distance and local time but in opposite hemispheres. The corresponding plot of total pressure (figure la in /41/ but not shown here) reveals only a smooth slow variation in the total pressure on going from the lobe to the plasma sheet which indicates that the total pressure tends to be the same in both regions. Another primary result of this multipoint study is illustrated in Figure 6 by the gradual decrease in the total pressure at both spacecraft during the periods of high AE activity. This behavior indicates that tail energy loss during substorms is a large scale process involving the whole tail. Figure 6 and other figures in this paper also illustrate the tendency for the B~component to be small before substorm onset, not only at a single point, but over a substantial portion of the magnetotail and well beyond the synchronous orbit. The B~increase indicating the field dipolarization is typically abrupt at distances within roughly 15 RE but is more gradual as it is in Figure 8 at larger distances.
Magnetotail Dynamics
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(9)104
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to the tail. Some 25 minutes after synchronous orbit onsets, ISEE 3 at “200 RE frequently detected the characteristic high velocity flows and north—south B~variations characteristic of a passing plasmoid. If ISEE 3 happened to be located in the magnetosheath near 200 RE after substomm onsets it often moved into the tail lobe as if a tail bulge were produced by a plasmoid moving down the tail. ISEE 3 frequently ended this sequence in the magnetosheath as if the tail radius had decreased. Additional supporting evidence for the above conclusions was provided by a superposed epoch analysis keyed to the arrival of high velocity tailward flowing plasma at ISEE 3 some 220 RE down the tail /49/. These statistical results shown in successive panels of Figure 7 indicate that (1) a low velocity characteristic of the tail lobe precedes the t=0 velocity increase, (2) a high field strength characteristic of the lobe also precedes the t=0 velocity increase, (3) a north—south B~variation characteristic of a of a plasmoid follows the arrival of the high velocity flow, (4) a substorm associated increase in energetic particle flux at synchronous orbit occurs some 25 minutes before the high velocity flows, and (5) a decrease in the lower envelope of the AE index AL also precede the flows, again supporting the idea of a causal substorm onset. Other ISEE 3 deep tail observations have been presented behavior and are supported by ISEE 1 and geosynchronous These papers include further evidence for plasmoids and associated earthward and tailward particle streaming in layers respectively.
which emphasize energetic particle data nearer earth /50,51,52,53/. evidence for simultaneous substormthe near earth and distant boundary
Magnetotail Dynamics
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Comprehensive Case Studies
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In addition to the numerous multipoint studies of of event studies /54,55 and preceding papers, 56/ particular phenomena cited above, there are a number that are frequently associated with organized workshops /57,58 and following papers/. These papers have been important in demonstrating the existence of the substorm growth phase and the importance of the substorm current wedge /55/ and the role of both directly driven and tail unloading components of substorms /57,58 and following papers/. Most recently Sauvaud et al. /56/ for a major substorm onset, direct access of solar concluded that although tail energy is the source wind energy is the cause of smaller events that have many substorm features that precede and follow the main onset.
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The magnetotail Workshops (CDAW oriented 6 and 8) Coordinated have been particularly Data Analysis effective in gathering together multipoint tail measurements and supporting data for intensive analysis. CDAW 6 /58/ focused on a substorm where ISEE 1/2 spacecraft were very near the equatorial current sheet at the time of substorm onset (see
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CDAW 8 focused on an interval when ISEE 3 was in the deep tail and supporting data were available from a number of spacecraft nearer earth. These results which are only now approaching the publication stage, are generally supportive of the reconnection model of substorms and the existence of plasmoids, but again they reveal more complexity than is implied by a simple 2 dimensional model of plasmoids /59,60,61/. Magnetotail reconnection is strongly supported by an event where a substorm—associated tail reconfiguration with energy loss observed near 20 and 30 R~is associated with both the observation of a plasmoid at ISEE 3 at ‘410 Ru’ and a decrease in the area of the dark polar cap po~ewardof the auroral zone as seen in DE 1 images /59/. DISCUSSION AND CONCLUSIONS
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Fig. 7. Results of a superposed epoch analysis of deep tail and geosynchronous data [Baker et al., 1987). The analysis is keyed to a velocity increase at ISEE 3 and shows both a following north-south B~ signature of a plasmoid and a preceding decrease in geosynchronous particle flux and AL index characteristic of a substorm.
It has been emphasized how a large collection of scientific spacecraft in the early 1970’s were instrumental in demonstrating the global nature of magnetospheric substorms - the storage of energy in the tail during the growth phase, the thinning of the plasma sheet relative to onset, and the reconfiguration and energy loss after onset and how all these phenomena are controlled by the IMP. As the launch rate of scientific spacecraft decreased in the early 1970’s an increasing number of experiments on applications spacecraft became available in synchronous orbit. These measurements along with ground observations have been used extensively to study the time development of the preeminent substorm current system, the substorm current wedge. As these geosynchronous measurements become better understood they are increasingly able to serve as routine monitors of substorms fo7 comparison with data further back in the tail. Such geosynchronous measurements have significantly enhanced the value of ISEE 3 measurements in the distant magnetotail by identifying substorms associated with plasmoids observed at the deep tail location. It is suggested that similar geosynchronous measurements can be similarly used for comparison with ISEE 1/2 and AIPTE/IRM measurements in the mid 1980’s and especially with ISTP and CLUSTER missions in the 1990’s when these planed multipoint missions with state—of—the—art instrumentation revisit locations explored in a cursory manner by earlier missions.
REFERENCES 1.
H. W. Hones Jr., J. R. Asbridge, and S. J. Bame, Time variations of the magnetotail
.
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plasma sheet at 18 RE determined from concurrent observations by a pair of Vela satellites, J. Geophys. Res., 76, 4402, (1971) 2.
J. A. Sauvaud, A. Saint-Marc, J. Dandouras, H. Reme, A. Korth, G. Kremser, and 0. K. Parks, A multisatellite study of the plasma sheet dynamics at substorm onset, Geophys Res. Lett, 5, 500, (1984)
3.
C. Y. Huang, L. A. Frank, and T. E. Eastman, Observations of plasma distributions during the Coordinated data analysis workshop substorms of March 31 to April 1, 1979, J. Geophys. Res., 93, 2377, (1987)
4.
D. H. Fairfield, Magnetotail energy storage and the variability of the magnetotail current sheet, in Magnetic Reconnection in Space and Laboratory Plasmas, ed. E. W. Hones Jr., American Geophysical Union, Washington D. C., 1984, p 168.
5.
A. Nishida and K. Fujii, Thinning of the near-earth (l0’45 RE) plasma sheet preceding the substorm expansion phase, Planet. Space Sci., 24, 849, (1976)
6.
7. Pytte, R. L. McPherron, M. 0. Kivelson, H. I. West, Jr., and H. W. Hones, Jr., Multiple-satellite studies of magnetospheric substorms: radial dynamics of the plasma sheet, J. Geophys. Res., 81, 5921, (1976)
7.
T. Pytte and H. I. West Jr., Ground—satellite correlations during presubstorm magnetic field configuration changes and plasma sheet thinning in the near-earth magnetotail, J. Geophys. Res., 83, 3791, (1978)
8.
D. N. Baker, E. W. Hones Jr., R. D. Belian, P. R. Higbie, R. P. Lepping, and P. Stauning, Multiple—spacecraft and correlated riometer study of magnetospheric substorm phenomena, J. Geophys. Res., 87, 6121, (1982)
9.
D. N. Baker, P. R. Higbie, E. W. Hones, Jr., and R. D. Belian, High-resolution energetic particle measurements at 6.6 RE 3. Low—energy electron anisotropies and short—term substorm predictions, J. Geophys. Res., 83, 4863, (1978)
10. 0.—I. Meng, E. W. Hones and S.—I. Akasofu, Simultaneous observations of an energetic electron event in the magnetotail by the Vela 3A and Imp 3 satellites, 1, J. Geophys Res., 75, 7294, (1970) 11. S.-I. Akasofu, E. W. Hones Jr., and C.-I. Meng, Simultaneous Observations of an energetic electron event in the magnetotail by the Vela 3A and Imp 3 satellites, 2, J. Geophys Res., 75, 7296, (1970) 12. R. J. DeCoster and L. A. Frank, Observations pertaining to the dynamics of the plasma sheet, J. Geophys. Res., 84, 5099, (1979) 13. 0. K. Parks, C. S. Lin, K. A. Anderson, R. P. Lin, and H. Reme, ISEE 1 and 2 particle observations of outer plasma sheet boundary, J. Geophys. Res., 84, 6471, (1979) 14. T. C. Forbes, E. W. Hones Jr., S. J. Bame, J. R. Asbridge, G. Paschmann, N. Sckopke, and C. T. Russell, Substorm—related plasma sheet motions as determined from differential timing of plasma changes at the Isee satellites, J. Geophys. Res., 86, 3459, (1981) 15. K. Takahashi and H. W. Hones Jr., ISEE-l and -2 observations of ion distributions at the plasma sheet — tail lobe boundary, J. Geophys. Res., 93, 8558, (1988) 16. H. J. Singer, W. J. Hughes, P. F. Fougere and D. J. Knecht, The localization of P1 2 pulsations: ground—satellite observations, J. Geophys. Res., 88, 7029, (1983) 17. C. Gelpi, W. J. Hughes, H. J. Singer, and M. Lester, Mid—latitude Pi 2 polarization pattern and synchronous orbit magnetic activity, J. Geophys. Res., 90, 6451, (1985) 18. J. N. Barfield, N. A. Saflekos, R. H. Sheehan, R. L. Carovillano, T. A. Potemra and D. Knecht, Three—dimensional observations of Birkeland currents, J. Ceophys. Res., 91, 4393 (1986) 19. D. H. Fairfield and L. J. Zanetti, Three point magnetic field observations of substorms in the inner magnetotail, to be submitted 1988. 20. R. L. Arnoldy and T. E. Moore, Longitudinal structure of substorm injections at synchronous orbit, J. Geophys. Res., 88, 6213, (1983) 21. T. Nagai, Observed magnetic substorm signatures at synchronous altitude, J. Geophys Res., 87, 4405, (1982) 22. T. Nagai, D. N. Baker, and P. R. Higbie, Development of substorm activity in multiple—
:1
.
Magnetotail Dynamics
onset substorms at synchronous orbit, J. Geophys. Res, 88, 6994,
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(1983)
23. H. J. Singer, W. J. Hughes, C. Gelpi, and B. G. Ledley, Magnetic disturbances in the vicinity of synchronous orbit and the substorm current wedge: A case study, J. Geophys Res., 90, 9583, (1985) 24. T. Nagai, Field-aligned currents associated with substorms in the vicinity of synchronous orbit 2. GOES 2 and GOES 3 observations, J. Geophys. Res. , 92, 2432, (1987) 25. D. N. Baker, E. W. Hones, Jr., P. K. Higbie, R. D. Belian, and P. Stauning, Global properties of the magnetosphere during a substorm growth phase: A case study, J. Geophys Res., 86, 8941, (1981) 26. D. P. Smits, W. J. Hughes, C. A. Cattell, and C. T. Russell, Observations of fieldaligned currents, waves, and electric fields at substorm onset, J. Geophys. Res., 91, 121, (1986) 27. C. Gelpi, H. J. Singer, and W. J. Hughes, A comparison of magnetic signatures and DMSP auroral images at substorm onset: three case studies, J. Geophys. Res., 92, 2447, (1987) 28. T. Nagai, H. J. Singer, B. G. Ledley, and K. C. Olsen, Field-aligned currents associated with substorms in the vicinity of synchronous orbit, 1. The July 5, 1979, substorm observed by SCATHA, GOES 3, and GOES 2, J. Geophys. Res., 92, 2425, (1987) 29. C. 7. Russell and R. L. McPherron, The magnetotail and substorms, Space Science Rev., 11, 111, (1973) 30. T. E. Moore, R. L. Arnoldy, J. Feynman, and D. A. Hardy, Propagating substorm injection fronts, J. Geophys. Res., 86, 6713, (1981) 31. J. N. Barfield, S. H. DeForest and D. J. Williams, Simultaneous observations of substorms electrons: Explorer 45 and ATS 5, J. Geophys. Res., 82, 531, (1977) 32. C. T. Russell, The solar wind and magnetospheric dynamics, in Correlated Interplanetary and Magnetospheric Observations, ed. D. H. Page, D. Reidel Pub. Co., Dordrecht, Holland, 3. 1974, p 33. T. J. Kelly, C. 7. Russell, and K. J. Walker, ISEE 1 and 2 observations of an oscillating outward moving current sheet near midnight, J. Geophys. Res., 89, 2745, (1984) 34. T. J. Kelly, C. T. Russell, R. J. Walker, G. K. Parks, and J. T. Gosling, ISEE 1 and 2 observations of Birkeland currents in the earth’s inner magnetosphere, J. Geophys. Res., 91, 6945, (1986) 35. R. C. Elphic, C. A. Cattell, K. Takahashi, S. J Bame, and C. T. Russell, ISEE-1 and 2 observations of magnetic flux ropes in the magnetotail: FTE’s in the plasma sheet?, Geophys. Res. Lett. , 13, 648, (1986) 36. M. P. Aubry and R. L. McPherron, Magnetotail changes in relation to the solar wind magnetic field and magnetospheric substorms, J. Geophys. Res., 76, 4381, (1971) 37. D. H. Fairfield and N. F. Ness, Configuration of the geomagnetic tail during substorms, J. Geophys. Res. , 75, 7032, (1970) 38. T. Pytte, K. L. McPherron, H. W. Hones Jr., and H. I. West, Multiple-satellite studies of magnetospheric substorms: Distinction between polar magnetic substorms and convection driven magnetic bays, J. Geophys. Res., 83, 663, (1978) 39. 7. Pytte, R. L. McPherron, M. C. Kivelson, H. I. West Jr., and H. W. Hones Jr., Multiplesatellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral zone activity, J. Geophys. Res.. 83, 5256, (1978) 40. T. Pytte, J. A. Lundlbad, F. Soraas, H. W. Hones Jr., and H. I. West, Three-satellite measurements and field-line mapping of the outer plasma sheet boundary from rl.2 to 18 RE during substorms, Geophys. Res. Lett., 5, 585, (1978) 41. D. H. Fairfield, K. P. Lepping, E. W. Hones Jr., S. J. Bame, and J. R. Asbridge, Simultaneous measurements of magnetotail dynamics by IMP spacecraft, J. Geophys. Rem., 86, 1396, (1981) 42. B. W. Hones Jr., J. Birn, J. R. Asbridge, C. Paschmann, N. Sckopke, and C. Haerendel, Further determination of the characteristics of magnetospheric plasma vortices with Isee 1 and 2, J. Geophys. Res., 86, 814, (1981) 43. IL. A. Saunders, D. J. Southwood, E. W. Hones Jr., and C. 7. Russell, A bydromagnetic
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vortex seen by ISEE-1 and 2, J. Atmos. Terr. Phys., 43, 927, (1983) 44. U. A. Saunders, D. J. Southwood, T. A. Fritz, and B. W. Hones Jr., Hydromagnetic vortices — I. The 11 December 1977 event, Planet. Space Sci., 31, 1099, (1983) 45. IL. A. Saunders, D. J. Southwood, and H. W. Hones Jr., Hydromagnetic vortices - II. Further dawnside events, Planet. Space Sci., 31, 1117, (1983) 46. H. W. Hones Jr., J. Birn, S. J. Bane, and C. T. Russell, New observations of plasma vortices and insights into their interpretation, Ceophys. Res. Lett., 10, 674, (1983) 47. J. Him, H. W. Hones, Jr., S. J. Bame, and C. T. Russell, Analysis of 16 plasma vortex events in the geomagnetic tail, J. Geophys. Res., 90, 7449, (1985) 48. D. N. Baker, S. J. Bane, R. D. Belian, W. C. Feldman, J. T. Gosling, P. R. Higbie, B. W. Hones, Jr., D. J. McComas, and R. D. Zwickl, Correlated dynamical changes in the nearearth and distant magnetotail regions: ISEE 3, J. Geophys. Res., 89, 3855, (1984) 49. D. N. Baker, K. C. Anderson, R. D. Zwickl and J. A. Slavin, Average plasma and magnetic field variations in the distant magnetotail associated with near-earth substorm effects, J. Geophys. Res., 92, 71, (1987) 50. IL. Scholer, G. Gloeckler, D. Hovestadt, F. IL. Ipavich, B. Klecker, D. N. Baker, W. Baumjohann, B. 7. Tsurutani, and R. D. Zwickl, Simultaneous observation of the plasma sheet in the near earth and distant magnetotail: ISEE—l and ISEE-3, Geophys Res. Lett., 11, 1034, (1984) 51. IL. Scholer, IL., D. N. Baker, S. J. Bane, W. Baumjohann, C. Gloeckler, F. U. Ipavich, E. J. Smith, and B. T. Tsurutani, Correlated observations of substorm effects in the nearearth region and the deep magnetotail, J. Geophys. Res., 90, 4021, (1985) 52. IL. Scholer, D. N. Baker, C. Gloeckler, B. Klecker, F. IL. Ipavich, T. Terasawa, B. T. Tsurutani, and A. B. Calvin, Energetic particle beams in the plasma sheet boundary layer following substorm expansion: Simultaneous near—earth and distant tail observations, J. Geophys. Res. , 91, 4277, (1986) 53. I. C. Richardson, S. W. H. Cowley, H. W. Hones, Jr., and S. J. Bane, Plasmoid-associated energetic ion bursts in the deep geomagnetic tail: properties of plasmoids and the postp1a~smoidplasma sheet, J. Geophys. Res., 92, 9997, (1987) 54. H. W. Hones Jr., S.-I. Akasofu, J. H. Wolcott, S. J. Bane, D. H. Fairfield, and C.-I. Meng, Correlated observations of several auroral substorms on February 17, 1971, J. Geophys. Res., 81, 1725, (1976) 55. R. L. McPherron, C. T Russell, and M. P Aubry, Satellite studies of magnetospheric substorms on August 15, 1968 9. Phenomenological model for substorms, J. Geophys. Res., 78, 3131, (1973) 56. J. A. Sauvaud, J. P. Treihou, A. Saint-Marc, J. Dandouras, H. Reme, A. Korth, C. Kremser, G. K. Parks, A. N. Zaitzev, V. Petrov, L. Lazutine, and K. Pellinen, Large scale response of the magnetosphere to a southward turning of the interplanetary magnetic field, J. Geophys. Res., 93, 2365, (1987) 57. A. Nishida and Y. Kamide, Magnetospheric processes preceding the onset of an isolated substorm: a case study of the March 31, 1978 substorm, J. Geophys. Res., 88, 7005, (1983) 58. R. L. McPherron and R. H. Manka, Dynamics of the 1054 UT March 22 1979, substorm event: CDAW 6, J. Geophys. Res., 90, 1175, (1985) 59. D. H. Fairfield, D. N. Baker, J. D. Craven, R. C. Elphic, J. F. Fennell, L. A. Frank, I. G. Richardson, H. J. Singer, J. A. Slavin, B. T. Tsurutani, and K. D. Zwickl, Substorms, plasmoids, flux ropes, and magnetotail flux loss on March 25 1983: CDAW 8, J. Geophys. ~ submitted (1988) 60. J. A. Slavin, D. N. Baker, J. D. Craven, B. Calvin, W. J. Hughes, R. H. Manka, D. D. J. Sibeck, H. J. Singer, H. J. Smith, plasmoid structure and dynamics: CDAW-8,
K. C. Elphic, D. H. Fairfield, L. A. Frank, A. C. Mitchel, I. C. Richardson, T. K. Sanderson, and R. D. Zwickl, ISEE 3 observations of submitted to J. Geophys. Res., (1988)
61. D. N. Baker, J. D. Craven, R. C. Elphic, D. H. Fairfield, L. A. Frank, H. J. Singer, J. A. Slavin, I. C. Richardson, C. J. Owen, and R. D. Zwickl, The CDAW-8 substorm event of 28 January 1983: A detailed global study, submitted to J. Geophys. Res., (1988)
0273—1177/88 $0.00 + .50 Copyright © 1989 COSPAR
MULTIPOINT MEASUREMENTS OF MAGNETOTAIL DYNAMICS D. H. Fairfield Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.
ABSTRACT Multipoint spacecraft measurements have played an important role in documenting the morphology of magnetospheric substornis. Widely separated measurements have quite clearly delineated many large scale features such as, (1) plasma sheet thinning associated with the onset of substorms, (2) energy storage and release in the magnetotail associated with the growth and expansion phase of substorms, (3) the earthward and tailward field aligned currents associated with the dawn and dusk portions of the substorm current wedge and (4) the deep tail response that follows substorms detected near synchronous orbit. Smaller scale phenomena that must be observed on finer time scales and detected in more limited spatial regions have been studied by ISEE 1 and 2 and a number of spacecraft near geosynchronous orbit. Injection fronts that are sometimes thought to propagate earthward toward synchronous orbit are difficult to detect and observed time delays may be due at least partially to longitudinal expansions of the substorm current wedge. Measurements near the equatorial current sheet where substorm onsets are often thought to originate, frequently reveal rapid time variations yet nearby spacecraft are seldom available to help determine whether these variations are due to local turbulence or more coherent behavior on a somewhat larger scale. High time resolution measurements from the Cluster and ISTP programs should help rectify these ambiguities. INTRODUCTION To understand the dynamics of a complex system such as the earth’s magnetotail it is necessary to obtain knowledge of time changes in different locations throughout a vast volume of space. To obtain such knowledge one can either (1) assume the existence of a repeatable pattern of changes and combine a large number of measurements at different locations at different times to give a statistical pattern that may approximate an individual event, or (2) make measurements at different locations during individual events. Both of these approaches have been used to contribute to our current understanding of the magnetotail. This review emphasizes the important contributions of the second technique. CONTRIBUTING SPACECRAFT 1963-1988 Figure 1 indicates intervals during which 51 different spacecraft made measurements between 1963 and 1988. Solid lines in the top portion of the figure indicate 14 fields and particles spacecraft arranged vertically according to their apogee and plotted during their lifetime of useful operation. Off scale at the top of the figure and designated by dotted lines are Explorer 35 in lunar orbit (~6ORE) and Explorer 33 with an apogee beyond 76 R Off scale at the bottom and again designated by dotted lines are 12 YELA spacecraft (VEL launches included 6 pairs of spacecraft) which carried plasma/particle payloads into approximately circular orbits near 18 RE. In the bottom portion of the figure are 22 spacecraft in geosynchronous and near geosynchronous (GEOS 1, SCATIIA, DE 1,and CCE) orbits which are included because of their unique ability to define substorm variations in the inner magnetotail. Several trends are apparent in Figure 1. The mission of early spacecraft was to explore the magnetospheric environment and hence early spacecraft carried fields and particles experiments into highly eccentric orbits where they could explore a wide variety of phenomena. The combined high frequency of launches in the late 1960’s along with increasing spacecraft lifetimes produced an optimum opportunity for multipoint measurements in the period of roughly 1969—73. As this exploratory phase of magnetospheric research tapered off, applications spacecraft carrying auxiliary scientific experiments began to proliferate in synchronous orbit which led to unprecedented opportunities for multipoint measurements in this more limited region of space after ~l975. Primary among these geosynchronous applications spacecraft are the U. S. SMS—GOES series of weather satellites making magnetic field and very energetic (>~0.5MeV) particle measurements and the U. S. Los Alamosinstrumented DOE/DOD series of spacecraft (designated by numbers in Figure 1) which make (9)97
(9)98
D. H. Fairfield
MAGNETOTAIL SPACECRAFT 1963—1988 EXP 35
ISEE 3
EXP33 IMP8 40
IMP7
1Mp3 IMP1 IMPS OGO 1
20
OGO
ISEE 1/2
~‘
0G03 IMP 2 VELA3
.......
VELA2
VELA 1 DE
VELA 4 1 GEOS 2 129 GEOS1 025 053 037 059 019 007 CC GOES 1 SCATHA ATS6 ATSS GOES6 SMS2GOES3 GOES 5
o Z o
I
L__1~i
65
VELA6.. VELA5
ATS1 70
S~~1 75 YEARS
G~ES2 80
I
85
Fig. 1. Intervals indicating the useful lifetimes of 51 spacecraft that have contributed to magnetotail research. In the top portion of the figure solid lines indicate the spacecraft apogee according to the scale at the left whereas dashed lines indicate spacecraft off the scale. In the lower portion of the figure dotted lines indicate spacecraft in near geosynchronous orbits and solid lines indicate spacecraft in geosynchronous orbit, all spaced on an arbitrary vertical scale. Tic marks indicate the begin time of the associated year. intermediate energy (30-2000 keV) particle measurements. Another trend discernible in Figure 1 is the increasing lifetime of the more recently launched spacecraft. In particular, the IMP 8 and SCATHA spacecraft are still operating after 14 and 9 years respectively and other spacecraft such as the IMP 5, 6 and ISEE 1/2 spacecraft continued to operate until they reentered the atmosphere. Finally we note that the launch of the well instrumented AMPTE IRM and CCE spacecraft in 1984 along with the ongoing IMP 8, ISEE 1/2 and geosynchronous spacecraft have provided another unprecedented opportunity for near earth and distant tail multipoint measurements in 1985—87 which we should be hearing more about in the future. The ongoing series of GOES and Los Alamos spacecraft along with CCE should provide important support for the ISTP and Cluster spacecraft in the 1990’s.
MULTIPOINT MEASUREMENTS Plasma Sheet Dynamics Associated with Substorms The thinning of the plasma sheet associated with substorm onset and its subsequent thickening during recovery are among the most prominent dynamical changes in the magnetotail. Thinning at the time of onset was first suggested by the disappearance of plasma at ‘48 RE but it was not immediately clear that this disappearance was caused by actual thinning rather than simple motion of the plasma sheet. Actual thinning was verified by examples where 2 VELA spacecraft located near 18 RE in the northern and southern portions of the plasma sheet both detected the disappearance of the plasma sheet at the same time /1/. This thinning of the distant plasma sheet at the time of substorm onset has been most recently demonstrated by Sauvaud et al. /2/ and Huang et al. /3/. Sauvaud et al. found that the disappearance of plasma at ISEE 1 in 100 events (primarily beyond ‘45 RE) occurred within a few minutes of plasma injections (substorm onsets) detected at the GEOS 2 spacecraft in geosynchronous orbit. Probably the best illustration of a thin current sheet within a thinned plasma sheet after substorm onset is the ISEE 1/2 example shown in Figure 2 /4/. ISEE 1 was 15.4 RE from the earth near the 0200 LT meridian plane and ISEE 2 was 1.5 RE closer to the earth near the
Magnetotail Dynamics
same meridian.
In an appropriately tilted coordinate system the north— south separation of these spacecraft was 0.6 RE. At 1057 30 s ISEE 2 (the lighter trace in Figure 2) crossed the current sheet from the northern to the southern hemisphere and at 1106 45 s ISEE 1 did the same. For 9 minutes in between these times the two spacecraft were observing oppositely—directed lobe-like magnetic fields of the northern and southern hemispheres. The current sheet and high fi plasma sheet must have been located between these spacecraft and must have been considerably thinner than the 3700—km north-south spacecraft separation. Measurements at different radial distances have shown that plasma sheet thinning within “15 RE of the earth occurs before substorm onset /5,6/. This near earth thinning is generally detected by the disappearance of energetic trapped particles at the synchronous orbit and beyond. Multispacecraft measurements have demonstrated that this pre-onset thinning is a growth phase phenomena produced by a southward IMP /7,8/. This phenomena can even be used as a short term predictor of an imminent substorm /9/.
(9)99
20 —
ISEEI
B(nT) 60 0
___________________
_________
60 B 0 -60 60 B 0
~.
- -
-60 _____________________________ I 60 .~-...
Bx(flT)
- - -
_________
~ -
-60 0:56
1:00
11:04 MARCH 22,1979
11:08
11:12
Fig. 2. ISEE 1 and 2 measurements where the spacecraft were separated by 1.5 RE in ~ and 0.6 RE in an appropriate north-south coordinate system. For nine minutes a thin current sheet must have lay between the two spacecraft which were measuring strong lobe— like fields of the northern and southern hemispheres respectively.
Although the morphology of plasma sheet thinning phenomena is rather well established, the velocities and the detailed structure of the outer boundary of the plasma sheet at these times are more difficult to determine, In principle one would like to see the boundary cross two spacecraft in succession so that the velocity of the boundary could be determined by dividing the separation by the time delay. Unfortunately this procedure requires knowledge of the orientation of the boundary and assumptions about its uniformity, which is information that is not readily obtained by two spacecraft. In spite of such difficulties individual two— spacecraft examples have been presented using such spacecraft as VELA and IMP 3 /10,11/, and IMP 7 and 8 /12/ all of which give velocities in the range of a few to a few tens of km/s. The ISEE 1-2 satellite pair has also been used /,13,14,15/ but these spacecraft are so close together (often <1000 km apart) that the time delays are often difficult to measure. Forbes et al. found average thinning velocities near 20 km/s both from interspacecraft timing and from direct measurements of north-south plasma flows, but only about half the time could time delays even be discerned. Their plasma sheet expansion velocities were considerably higher but this may have been due to waves on the plasma sheet boundary during substorm recovery which were perhaps due to the Kelvin—Helmholz instability. Substorm Current Wedge Many years of research have made it abundantly clear that the well known dipolarization of the magnetic field in the inner magnetotail at substorm onset is particularly clear in the region near geosynchronous orbit. These dipolarizations along with their associated Pi 2 pulsations and ground perturbations [e. g. /16,17/ provide the best indicator of the time of substorm onsets and hence these data are of considerable importance for comparison with measurements in the more distant tail. The concept of the substorm current wedge has become very well established as a result of measurements at geosynchronous orbit /e.g. 18,19 and references therein/. At the time of substorm onset equatorial cross-tail currents are disrupted with the diverted currents flowing earthward along field lines in the post—midnight sector and through the ionosphere where they create the westward electrojet. Finally the currents complete their circuit by flowing back out to the magnetotail along field lines of the pre—midnight sector. In recent years many geosynchronous spacecraft have contributed to refinements of this simple current wedge picture. Multipoint measurements have shown that the current wedge develops in a longitudinal sector perhaps 2 to 3 hours wide in local time /20/ which then JASR 8:9/10—C
D. H. Fairfield
(9)100
expands in longitude over the ~iext 10 to 20 minutes /21,20/, sometimes in association with additional intensifications in adjacent and overlapping longitudinal sectors /22,23/. The east—west D perturbation at geosynchronous orbit generally begins at the time of ground onset whereas the accompanying increase in the north-south or H component is generally delayed until the expanding current wedge reaches the longitude of the observing spacecraft /21,24/. This delay means that the depressed 11 component marking the substorm growth phase can persist outside the current wedge sector even after substorm onset has begun within the wedge. This local time dependence of substorm phenomena introduces additional complications to substorm research and points up the need for multipoint measurements. Figure 3a illustrates magnetic field measurements from the AMPTE/CCE and GOES 5 and 6 spacecraft in the inner magnetotail /19/. Data are in a left hand coordinate system where B is parallel to the dipole axis and positive northward, B~is aligned in the meridian p’ane with positive in an earthward direction, and B~is transverse to the meridian plane with positive in an eastward direction as it is at ground observatories. The onset times and central meridians of two substorms have been determined from ground magnetograms and are indicated by vertical lines in the bottom portion of the figure and at appropriate longitudes in the inserts which graphically indicate the spacecraft location. It is important to note that all of these spacecraft are north of the magnetic equatorial plane. Prominent features in Figure 3a are the well known increase in B and decrease in B~or 9 that is characteristic of field line stretching during the growth phase preceding substorm onsets /e. g. 25,8/. Other equally prominent features are the B decrease and B 5 or 0 increase that is associated with each substorm onset. The latter effect is, however, delayed by some 20 minutes at GOES 5 for the second substorm due to the fact that this spacecraft has moved well into the morning hours where it is well to the east of the onset
meridian. Figure 3b displays AMPTE/CCE and GOES 5 data for the earlier substorm at 6 and 3 s resolution respectively. This figure shows increases in Bz at the two spacecraft which are simultaneous within a fraction of a minute and which occur without any change, in B. These ONSET
~ 2
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______
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____ I
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6 JIJNE7 1985
8
Fig. 3a. AMPTE/CCE and GOES 5 and 6 magnetic field measurements during a period when two substorm onsets occurred as indicated by vertical lines. The insets at the top indicate the XV and xz plane locations of the spacecraft at the two onset times,
I I
3~45 10
I
I I I
I I
4:0045 JUNE71985
Fig. 3b. High resolution AMPTE/CCE and 9OES 5 data for the first substorm interval of figure 3a. The three components are characterized by a simultaneous onset in B~, a time delay iii the decrease in B and different signs for the pertur~ationin By.
Magnetotail Dynamics
(9)101
facts suggest that the increases are due to the integrated effects of the wedge currents. In contrast, the field magnitude decreases abruptly at GOES 5 at 0359 and at AMPTE/CCE at about 0407 which suggests an expanding plasma sheet boundary that successively crosses these spacecraft /see also 26/. The behavior of the By components is quite different at the two spacecraft and can be interpreted in terms of an earthward field aligned current that is initially equatorward of both spacecraft and then passes over the lower latitude GOES 5 at 0359 and the slightly higher latitude AMPTE/CCE at 0410. This equatorward location for the wedge currents is unusual and is probably due to the large intensity of this substorm. Other multipoint measurements have suggested that the outward pre—midnight leg of the current wedge is confined to a relatively small flux tube which is associated with the westward traveling surge /27/ while the earthward directed dawnside currents assume a more conventional sheet configuration. The inward and outward currents probably even overlap in the near midnight region and produce a region of great complexity /28,19/. Injection Fronts Even prior to much of the current wedge research discussed above there was considerable interest in the precise timing of dipolarizations at different locations in the inner magnetotail. A fortuitous alignment of 000-5 and ATS-1 on 7 August 1968 to provided important information on this question /29/. These two spacecraft were separated in a nearly radial direction by 2.3 RE near the 0100 LT meridian (solar magnetospheric coordinates -8.2, -2.0, 2.1 RE and -5.9, -2.1, 2.2 RE respectively) when they observed characteristic B and B~field increases (see figure 4). These increases were observed 94 seconds earlier at the more distant spacecraft which led to the suggestion that the increases were caused by a field compression wave propagating inward with a velocity of ‘450 km/s. Several similar near-radial alignments of the ATS-5 and SCATRA spacecraft were found, again with corresponding equatorial field and particle increases that occurred first at the spacecraft more distant from the earth /30/. These observations were taken as evidence supporting the idea of an inward propagating “injection front” as it was called by Moore et al. /30/. These authors also suggested that hot plasma from the more distant plasma sheet 8 moved earthward behind front andatproduced plasma “injections frequently observed in the the injection inner magnetotail the time the of hot substorms. Two other examples /31,28/ support the injection front picture but are not entirely definitive. MAGNETOSPHERIC
SU8STORM
1120
UT AUGUST 7, 29:03
1968
30:37
v
~
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~
~
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1110
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1130
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~
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UNIVERSAL TIME Fig. 4. 000 1 and ATS 1 magnetic field data which was the original and remains the best example of a substorm injection front. The increase in B and B~at the more earthward ATS 1 spacecraft occurred 94 seconds after that at the 000 1 spacecraft 2 3 RE further from the earth.
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D. H. Fairfield
Although the injection front remains a popular idea, a closer look at the supporting data reveals some additional complications. Russell /32/ in a further discussion of the 7 August 1968 event notes that the substorm associated with this event occurred about 10 minutes earlier on the ground than at the spacecraft which is an unusually long delay for a spacecraft near the onset meridian of a substorm. A careful examination of the magnetograms for this day indeed clearly defines the onset at 1120 and at a meridian centered near or slightly east of Guam which places it several hours west of the spacecraft meridian. North American observatories near and east of the spacecraft meridian exhibit negative D and H perturbations demonstrating that this region is initially east of the current wedge. A larger negative D perturbation at western U. S. stations at 1130 suggests an impulsive eastward expansion of the wedge at this time which would move it toward the spacecraft pair where OGO 5 is ~6° west of ATS 1. A close look at the ATS—5 — SCATIIA locations of Moore et al. /30/ also reveals that the second spacecraft to see the disturbance is invariably the spacecraft slightly further from midnight as well as the one closer to the earth. Since the more recent work /21,20/ has demonstrated that the effects associated with the substorm current wedge expand longitudinally from the onset sector near midnight, one wonders whether this longitudinal expansion might be more relevant than the radial propagation that had been invoked to explain the earlier examples. In summary, it may be concluded that the injection boundary idea remains attractive idea, however on close examination the evidence for its existence may not be as strong as is sometimes supposed. Small scale magnetotail phenomena: ISEE1/2 The ISEE 1/2 spacecraft pair with distance separations of typically 100—1000 km provide and unprecedented opportunity to measure rapid boundary motions and fine scale phenomena. In addition to the plasma sheet boundary motion discussed above there have been high resolution studies of field aligned currents showing oscillations /33/ and considerable complexity /34/. One field aligned current event has been interpreted as a magnetic flux rope /35/. Large Scale Behavior of the Magnetotail Measurements at widely spaced locations throughout the magnetotail have been instrumental in demonstrating the coherent behavior of the magnetotail during substorms. An early multipoint study /36/ combined published IMP 4 data /37/ with 000 5 and ATS 1 data and solar wind measurements from Explorer 33 and 35 to provide some of the first indications of how a southward IMP produces a thin plasma sheet and stores energy in the tail prior to substorm onset. This solar wind control was pursued further by Pytte and West /7/ which was one of a comprehensive series of papers /6,38,39,40/ that used OGO 5 and VELA magnetotail data and also made extensive use of ground observations to accurately define substorm onset times. An example from /6/ shown in figure 5 compares VELA particle data measurements in the premidnight region at ‘48 Rg just north of the magnetotail equatorial plane with 000-5 particle and field measurements near the same latitude and longitude but between 16 and 9 R~. Near 13 RE at “2200 LT 000 5 detected particle increases and magnetic field dipolarizations at the time of substorm intensifications as defined by the onset of Pi 2 pulsations on the ground. In contrast, the particles measured by VELA at the greater distance gradually disappeared and did not return until the recovery phase of the substorm shortly before 2300 LT. Similar effects were seen during a second series of weaker intensifications after 0054. Another paper of this series /38/ made the distinction between magnetic activity driven directly by a southward IMP and that related to energy released from the magnetotail during sudden onset substorms marked by Pi 2 pulsations. This paper suggests that both modes of generation are important, which is a conclusion generally accepted today, but only after many years of controversy. Other papers of this series demonstrated the association between the poleward boundary of the aurora and the high latitude boundary of the plasma sheet /40/ and the movement of the two during the recovery phase of substorms /39/. Another multipoint study of large scale magnetotail dynamics using IMP 6, 7 and 8 measurements was the first to combine plasma and field measurements and allow a study of the total tail pressure variation at different locations /41/. An example from this paper is shown in Figure 6 which displays IMP 8 and IMP 8 magnetic field data sampled at a similar radial distance and local time but in opposite hemispheres. The corresponding plot of total pressure (figure la in /41/ but not shown here) reveals only a smooth slow variation in the total pressure on going from the lobe to the plasma sheet which indicates that the total pressure tends to be the same in both regions. Another primary result of this multipoint study is illustrated in Figure 6 by the gradual decrease in the total pressure at both spacecraft during the periods of high AE activity. This behavior indicates that tail energy loss during substorms is a large scale process involving the whole tail. Figure 6 and other figures in this paper also illustrate the tendency for the B~component to be small before substorm onset, not only at a single point, but over a substantial portion of the magnetotail and well beyond the synchronous orbit. The B~increase indicating the field dipolarization is typically abrupt at distances within roughly 15 RE but is more gradual as it is in Figure 8 at larger distances.
Magnetotail Dynamics
(9)103
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(9)104
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to the tail. Some 25 minutes after synchronous orbit onsets, ISEE 3 at “200 RE frequently detected the characteristic high velocity flows and north—south B~variations characteristic of a passing plasmoid. If ISEE 3 happened to be located in the magnetosheath near 200 RE after substomm onsets it often moved into the tail lobe as if a tail bulge were produced by a plasmoid moving down the tail. ISEE 3 frequently ended this sequence in the magnetosheath as if the tail radius had decreased. Additional supporting evidence for the above conclusions was provided by a superposed epoch analysis keyed to the arrival of high velocity tailward flowing plasma at ISEE 3 some 220 RE down the tail /49/. These statistical results shown in successive panels of Figure 7 indicate that (1) a low velocity characteristic of the tail lobe precedes the t=0 velocity increase, (2) a high field strength characteristic of the lobe also precedes the t=0 velocity increase, (3) a north—south B~variation characteristic of a of a plasmoid follows the arrival of the high velocity flow, (4) a substorm associated increase in energetic particle flux at synchronous orbit occurs some 25 minutes before the high velocity flows, and (5) a decrease in the lower envelope of the AE index AL also precede the flows, again supporting the idea of a causal substorm onset. Other ISEE 3 deep tail observations have been presented behavior and are supported by ISEE 1 and geosynchronous These papers include further evidence for plasmoids and associated earthward and tailward particle streaming in layers respectively.
which emphasize energetic particle data nearer earth /50,51,52,53/. evidence for simultaneous substormthe near earth and distant boundary
Magnetotail Dynamics
(9)105
Comprehensive Case Studies
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In addition to the numerous multipoint studies of of event studies /54,55 and preceding papers, 56/ particular phenomena cited above, there are a number that are frequently associated with organized workshops /57,58 and following papers/. These papers have been important in demonstrating the existence of the substorm growth phase and the importance of the substorm current wedge /55/ and the role of both directly driven and tail unloading components of substorms /57,58 and following papers/. Most recently Sauvaud et al. /56/ for a major substorm onset, direct access of solar concluded that although tail energy is the source wind energy is the cause of smaller events that have many substorm features that precede and follow the main onset.
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The magnetotail Workshops (CDAW oriented 6 and 8) Coordinated have been particularly Data Analysis effective in gathering together multipoint tail measurements and supporting data for intensive analysis. CDAW 6 /58/ focused on a substorm where ISEE 1/2 spacecraft were very near the equatorial current sheet at the time of substorm onset (see
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CDAW 8 focused on an interval when ISEE 3 was in the deep tail and supporting data were available from a number of spacecraft nearer earth. These results which are only now approaching the publication stage, are generally supportive of the reconnection model of substorms and the existence of plasmoids, but again they reveal more complexity than is implied by a simple 2 dimensional model of plasmoids /59,60,61/. Magnetotail reconnection is strongly supported by an event where a substorm—associated tail reconfiguration with energy loss observed near 20 and 30 R~is associated with both the observation of a plasmoid at ISEE 3 at ‘410 Ru’ and a decrease in the area of the dark polar cap po~ewardof the auroral zone as seen in DE 1 images /59/. DISCUSSION AND CONCLUSIONS
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Fig. 7. Results of a superposed epoch analysis of deep tail and geosynchronous data [Baker et al., 1987). The analysis is keyed to a velocity increase at ISEE 3 and shows both a following north-south B~ signature of a plasmoid and a preceding decrease in geosynchronous particle flux and AL index characteristic of a substorm.
It has been emphasized how a large collection of scientific spacecraft in the early 1970’s were instrumental in demonstrating the global nature of magnetospheric substorms - the storage of energy in the tail during the growth phase, the thinning of the plasma sheet relative to onset, and the reconfiguration and energy loss after onset and how all these phenomena are controlled by the IMP. As the launch rate of scientific spacecraft decreased in the early 1970’s an increasing number of experiments on applications spacecraft became available in synchronous orbit. These measurements along with ground observations have been used extensively to study the time development of the preeminent substorm current system, the substorm current wedge. As these geosynchronous measurements become better understood they are increasingly able to serve as routine monitors of substorms fo7 comparison with data further back in the tail. Such geosynchronous measurements have significantly enhanced the value of ISEE 3 measurements in the distant magnetotail by identifying substorms associated with plasmoids observed at the deep tail location. It is suggested that similar geosynchronous measurements can be similarly used for comparison with ISEE 1/2 and AIPTE/IRM measurements in the mid 1980’s and especially with ISTP and CLUSTER missions in the 1990’s when these planed multipoint missions with state—of—the—art instrumentation revisit locations explored in a cursory manner by earlier missions.
REFERENCES 1.
H. W. Hones Jr., J. R. Asbridge, and S. J. Bame, Time variations of the magnetotail
.
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plasma sheet at 18 RE determined from concurrent observations by a pair of Vela satellites, J. Geophys. Res., 76, 4402, (1971) 2.
J. A. Sauvaud, A. Saint-Marc, J. Dandouras, H. Reme, A. Korth, G. Kremser, and 0. K. Parks, A multisatellite study of the plasma sheet dynamics at substorm onset, Geophys Res. Lett, 5, 500, (1984)
3.
C. Y. Huang, L. A. Frank, and T. E. Eastman, Observations of plasma distributions during the Coordinated data analysis workshop substorms of March 31 to April 1, 1979, J. Geophys. Res., 93, 2377, (1987)
4.
D. H. Fairfield, Magnetotail energy storage and the variability of the magnetotail current sheet, in Magnetic Reconnection in Space and Laboratory Plasmas, ed. E. W. Hones Jr., American Geophysical Union, Washington D. C., 1984, p 168.
5.
A. Nishida and K. Fujii, Thinning of the near-earth (l0’45 RE) plasma sheet preceding the substorm expansion phase, Planet. Space Sci., 24, 849, (1976)
6.
7. Pytte, R. L. McPherron, M. 0. Kivelson, H. I. West, Jr., and H. W. Hones, Jr., Multiple-satellite studies of magnetospheric substorms: radial dynamics of the plasma sheet, J. Geophys. Res., 81, 5921, (1976)
7.
T. Pytte and H. I. West Jr., Ground—satellite correlations during presubstorm magnetic field configuration changes and plasma sheet thinning in the near-earth magnetotail, J. Geophys. Res., 83, 3791, (1978)
8.
D. N. Baker, E. W. Hones Jr., R. D. Belian, P. R. Higbie, R. P. Lepping, and P. Stauning, Multiple—spacecraft and correlated riometer study of magnetospheric substorm phenomena, J. Geophys. Res., 87, 6121, (1982)
9.
D. N. Baker, P. R. Higbie, E. W. Hones, Jr., and R. D. Belian, High-resolution energetic particle measurements at 6.6 RE 3. Low—energy electron anisotropies and short—term substorm predictions, J. Geophys. Res., 83, 4863, (1978)
10. 0.—I. Meng, E. W. Hones and S.—I. Akasofu, Simultaneous observations of an energetic electron event in the magnetotail by the Vela 3A and Imp 3 satellites, 1, J. Geophys Res., 75, 7294, (1970) 11. S.-I. Akasofu, E. W. Hones Jr., and C.-I. Meng, Simultaneous Observations of an energetic electron event in the magnetotail by the Vela 3A and Imp 3 satellites, 2, J. Geophys Res., 75, 7296, (1970) 12. R. J. DeCoster and L. A. Frank, Observations pertaining to the dynamics of the plasma sheet, J. Geophys. Res., 84, 5099, (1979) 13. 0. K. Parks, C. S. Lin, K. A. Anderson, R. P. Lin, and H. Reme, ISEE 1 and 2 particle observations of outer plasma sheet boundary, J. Geophys. Res., 84, 6471, (1979) 14. T. C. Forbes, E. W. Hones Jr., S. J. Bame, J. R. Asbridge, G. Paschmann, N. Sckopke, and C. T. Russell, Substorm—related plasma sheet motions as determined from differential timing of plasma changes at the Isee satellites, J. Geophys. Res., 86, 3459, (1981) 15. K. Takahashi and H. W. Hones Jr., ISEE-l and -2 observations of ion distributions at the plasma sheet — tail lobe boundary, J. Geophys. Res., 93, 8558, (1988) 16. H. J. Singer, W. J. Hughes, P. F. Fougere and D. J. Knecht, The localization of P1 2 pulsations: ground—satellite observations, J. Geophys. Res., 88, 7029, (1983) 17. C. Gelpi, W. J. Hughes, H. J. Singer, and M. Lester, Mid—latitude Pi 2 polarization pattern and synchronous orbit magnetic activity, J. Geophys. Res., 90, 6451, (1985) 18. J. N. Barfield, N. A. Saflekos, R. H. Sheehan, R. L. Carovillano, T. A. Potemra and D. Knecht, Three—dimensional observations of Birkeland currents, J. Ceophys. Res., 91, 4393 (1986) 19. D. H. Fairfield and L. J. Zanetti, Three point magnetic field observations of substorms in the inner magnetotail, to be submitted 1988. 20. R. L. Arnoldy and T. E. Moore, Longitudinal structure of substorm injections at synchronous orbit, J. Geophys. Res., 88, 6213, (1983) 21. T. Nagai, Observed magnetic substorm signatures at synchronous altitude, J. Geophys Res., 87, 4405, (1982) 22. T. Nagai, D. N. Baker, and P. R. Higbie, Development of substorm activity in multiple—
:1
.
Magnetotail Dynamics
onset substorms at synchronous orbit, J. Geophys. Res, 88, 6994,
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23. H. J. Singer, W. J. Hughes, C. Gelpi, and B. G. Ledley, Magnetic disturbances in the vicinity of synchronous orbit and the substorm current wedge: A case study, J. Geophys Res., 90, 9583, (1985) 24. T. Nagai, Field-aligned currents associated with substorms in the vicinity of synchronous orbit 2. GOES 2 and GOES 3 observations, J. Geophys. Res. , 92, 2432, (1987) 25. D. N. Baker, E. W. Hones, Jr., P. K. Higbie, R. D. Belian, and P. Stauning, Global properties of the magnetosphere during a substorm growth phase: A case study, J. Geophys Res., 86, 8941, (1981) 26. D. P. Smits, W. J. Hughes, C. A. Cattell, and C. T. Russell, Observations of fieldaligned currents, waves, and electric fields at substorm onset, J. Geophys. Res., 91, 121, (1986) 27. C. Gelpi, H. J. Singer, and W. J. Hughes, A comparison of magnetic signatures and DMSP auroral images at substorm onset: three case studies, J. Geophys. Res., 92, 2447, (1987) 28. T. Nagai, H. J. Singer, B. G. Ledley, and K. C. Olsen, Field-aligned currents associated with substorms in the vicinity of synchronous orbit, 1. The July 5, 1979, substorm observed by SCATHA, GOES 3, and GOES 2, J. Geophys. Res., 92, 2425, (1987) 29. C. 7. Russell and R. L. McPherron, The magnetotail and substorms, Space Science Rev., 11, 111, (1973) 30. T. E. Moore, R. L. Arnoldy, J. Feynman, and D. A. Hardy, Propagating substorm injection fronts, J. Geophys. Res., 86, 6713, (1981) 31. J. N. Barfield, S. H. DeForest and D. J. Williams, Simultaneous observations of substorms electrons: Explorer 45 and ATS 5, J. Geophys. Res., 82, 531, (1977) 32. C. T. Russell, The solar wind and magnetospheric dynamics, in Correlated Interplanetary and Magnetospheric Observations, ed. D. H. Page, D. Reidel Pub. Co., Dordrecht, Holland, 3. 1974, p 33. T. J. Kelly, C. 7. Russell, and K. J. Walker, ISEE 1 and 2 observations of an oscillating outward moving current sheet near midnight, J. Geophys. Res., 89, 2745, (1984) 34. T. J. Kelly, C. T. Russell, R. J. Walker, G. K. Parks, and J. T. Gosling, ISEE 1 and 2 observations of Birkeland currents in the earth’s inner magnetosphere, J. Geophys. Res., 91, 6945, (1986) 35. R. C. Elphic, C. A. Cattell, K. Takahashi, S. J Bame, and C. T. Russell, ISEE-1 and 2 observations of magnetic flux ropes in the magnetotail: FTE’s in the plasma sheet?, Geophys. Res. Lett. , 13, 648, (1986) 36. M. P. Aubry and R. L. McPherron, Magnetotail changes in relation to the solar wind magnetic field and magnetospheric substorms, J. Geophys. Res., 76, 4381, (1971) 37. D. H. Fairfield and N. F. Ness, Configuration of the geomagnetic tail during substorms, J. Geophys. Res. , 75, 7032, (1970) 38. T. Pytte, K. L. McPherron, H. W. Hones Jr., and H. I. West, Multiple-satellite studies of magnetospheric substorms: Distinction between polar magnetic substorms and convection driven magnetic bays, J. Geophys. Res., 83, 663, (1978) 39. 7. Pytte, R. L. McPherron, M. C. Kivelson, H. I. West Jr., and H. W. Hones Jr., Multiplesatellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral zone activity, J. Geophys. Res.. 83, 5256, (1978) 40. T. Pytte, J. A. Lundlbad, F. Soraas, H. W. Hones Jr., and H. I. West, Three-satellite measurements and field-line mapping of the outer plasma sheet boundary from rl.2 to 18 RE during substorms, Geophys. Res. Lett., 5, 585, (1978) 41. D. H. Fairfield, K. P. Lepping, E. W. Hones Jr., S. J. Bame, and J. R. Asbridge, Simultaneous measurements of magnetotail dynamics by IMP spacecraft, J. Geophys. Rem., 86, 1396, (1981) 42. B. W. Hones Jr., J. Birn, J. R. Asbridge, C. Paschmann, N. Sckopke, and C. Haerendel, Further determination of the characteristics of magnetospheric plasma vortices with Isee 1 and 2, J. Geophys. Res., 86, 814, (1981) 43. IL. A. Saunders, D. J. Southwood, E. W. Hones Jr., and C. 7. Russell, A bydromagnetic
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vortex seen by ISEE-1 and 2, J. Atmos. Terr. Phys., 43, 927, (1983) 44. U. A. Saunders, D. J. Southwood, T. A. Fritz, and B. W. Hones Jr., Hydromagnetic vortices — I. The 11 December 1977 event, Planet. Space Sci., 31, 1099, (1983) 45. IL. A. Saunders, D. J. Southwood, and H. W. Hones Jr., Hydromagnetic vortices - II. Further dawnside events, Planet. Space Sci., 31, 1117, (1983) 46. H. W. Hones Jr., J. Birn, S. J. Bane, and C. T. Russell, New observations of plasma vortices and insights into their interpretation, Ceophys. Res. Lett., 10, 674, (1983) 47. J. Him, H. W. Hones, Jr., S. J. Bame, and C. T. Russell, Analysis of 16 plasma vortex events in the geomagnetic tail, J. Geophys. Res., 90, 7449, (1985) 48. D. N. Baker, S. J. Bane, R. D. Belian, W. C. Feldman, J. T. Gosling, P. R. Higbie, B. W. Hones, Jr., D. J. McComas, and R. D. Zwickl, Correlated dynamical changes in the nearearth and distant magnetotail regions: ISEE 3, J. Geophys. Res., 89, 3855, (1984) 49. D. N. Baker, K. C. Anderson, R. D. Zwickl and J. A. Slavin, Average plasma and magnetic field variations in the distant magnetotail associated with near-earth substorm effects, J. Geophys. Res., 92, 71, (1987) 50. IL. Scholer, G. Gloeckler, D. Hovestadt, F. IL. Ipavich, B. Klecker, D. N. Baker, W. Baumjohann, B. 7. Tsurutani, and R. D. Zwickl, Simultaneous observation of the plasma sheet in the near earth and distant magnetotail: ISEE—l and ISEE-3, Geophys Res. Lett., 11, 1034, (1984) 51. IL. Scholer, IL., D. N. Baker, S. J. Bane, W. Baumjohann, C. Gloeckler, F. U. Ipavich, E. J. Smith, and B. T. Tsurutani, Correlated observations of substorm effects in the nearearth region and the deep magnetotail, J. Geophys. Res., 90, 4021, (1985) 52. IL. Scholer, D. N. Baker, C. Gloeckler, B. Klecker, F. IL. Ipavich, T. Terasawa, B. T. Tsurutani, and A. B. Calvin, Energetic particle beams in the plasma sheet boundary layer following substorm expansion: Simultaneous near—earth and distant tail observations, J. Geophys. Res. , 91, 4277, (1986) 53. I. C. Richardson, S. W. H. Cowley, H. W. Hones, Jr., and S. J. Bane, Plasmoid-associated energetic ion bursts in the deep geomagnetic tail: properties of plasmoids and the postp1a~smoidplasma sheet, J. Geophys. Res., 92, 9997, (1987) 54. H. W. Hones Jr., S.-I. Akasofu, J. H. Wolcott, S. J. Bane, D. H. Fairfield, and C.-I. Meng, Correlated observations of several auroral substorms on February 17, 1971, J. Geophys. Res., 81, 1725, (1976) 55. R. L. McPherron, C. T Russell, and M. P Aubry, Satellite studies of magnetospheric substorms on August 15, 1968 9. Phenomenological model for substorms, J. Geophys. Res., 78, 3131, (1973) 56. J. A. Sauvaud, J. P. Treihou, A. Saint-Marc, J. Dandouras, H. Reme, A. Korth, C. Kremser, G. K. Parks, A. N. Zaitzev, V. Petrov, L. Lazutine, and K. Pellinen, Large scale response of the magnetosphere to a southward turning of the interplanetary magnetic field, J. Geophys. Res., 93, 2365, (1987) 57. A. Nishida and Y. Kamide, Magnetospheric processes preceding the onset of an isolated substorm: a case study of the March 31, 1978 substorm, J. Geophys. Res., 88, 7005, (1983) 58. R. L. McPherron and R. H. Manka, Dynamics of the 1054 UT March 22 1979, substorm event: CDAW 6, J. Geophys. Res., 90, 1175, (1985) 59. D. H. Fairfield, D. N. Baker, J. D. Craven, R. C. Elphic, J. F. Fennell, L. A. Frank, I. G. Richardson, H. J. Singer, J. A. Slavin, B. T. Tsurutani, and K. D. Zwickl, Substorms, plasmoids, flux ropes, and magnetotail flux loss on March 25 1983: CDAW 8, J. Geophys. ~ submitted (1988) 60. J. A. Slavin, D. N. Baker, J. D. Craven, B. Calvin, W. J. Hughes, R. H. Manka, D. D. J. Sibeck, H. J. Singer, H. J. Smith, plasmoid structure and dynamics: CDAW-8,
K. C. Elphic, D. H. Fairfield, L. A. Frank, A. C. Mitchel, I. C. Richardson, T. K. Sanderson, and R. D. Zwickl, ISEE 3 observations of submitted to J. Geophys. Res., (1988)
61. D. N. Baker, J. D. Craven, R. C. Elphic, D. H. Fairfield, L. A. Frank, H. J. Singer, J. A. Slavin, I. C. Richardson, C. J. Owen, and R. D. Zwickl, The CDAW-8 substorm event of 28 January 1983: A detailed global study, submitted to J. Geophys. Res., (1988)