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Multiple substorm injections and the new substorm paradigm: Interpretation of the CDAW 7 substorm
Mv. Space Res. Vol. 13, No.4, ~. Printed in GreatBritain.
(4)213—(4)216, 1993
MULTIPLE SUBSTORM INJECTIONS AND THE NEW SUBSTORM PARADIGM: INTERPRETATION OF THE CDAW 7 SUBSTORM G. D. Reeves Space Science and Technology Group, Los Alamos NationalLaboratory, Los Alamos, NM 87545, U.SA
ABSTRACT
In the near-Earth neutral line model, the substorm is triggered by reconnection in the magnetotail and its effects radiate from there. In recent years new observations have suggested a new paradigm for substorms in which there are two coupled acceleration regions. In the current sheet disruption model the substorm is thought to begin in an acceleration region atthe inner edge of the plasma sheet (5-10 Re) and to propagate down tall where it may trigger reconnection in the mid-tail (typically >25 Re). We examine the CDAW 7 substorm in the context of this new substorm paradigm, with emphasis on the role of the substorm injection. BACKGROUND For about 10 years the near-Earth neutral line model (e.g., /1/) has provided the most complete framework for the interpretation of substorm observations. It is based on the idea that the substorm starts in the magnetotail with magnetic reconnection. Particles are accelerated in the reconnection region and precipitate, brightening the aurorae. As more magnetic flux is reconnected, the auroral brightening expands poleward. At the same time the cross-tail current is diverted into the ionosphere forming the substorm current wedge. Along with the collapse (or “dipolarization”) of the field inside the current wedge comes the injection of energetic particles into the vicinity of geosynchronous orbit. This is referred to as the convection surge /2/. Recently, observations have been obtained which are irreconcilable with the existing paradigm. New field line models have been used with auroral observations to suggest that the region of aurora! substorm brightening maps very close to the Earth to the vicinity of 10 Re (e.g., /3,4,5/). Studies of plasma sheet flows imply that the neutral line rarely forms inside X=-19 Re /6/. In addition there is evidence that the substorm injection is produced by the energization of particles locally, near where they are observed rather than by energization further down tail and then convection Earthward. The substorm injection has been observed within minutes of substorm onset and there is evidence that the region of dipolarization of the field lines expands outward /7,8/ which is inconsistent with a convection surge scenario. These problems have led some researchers to propose two substorm acceleration regions /9,10,11/ one atthe inner edge of the plasma sheet (=10 Re) and one in the magnetotail (>=25 Re). -
Thus a new substorm paradigm is emerging which synthesizes the near-Earth neutral line model with the new observations. In one such synthesis model, proposed by Lui /9/, substorm onset begins with the initiation of an instability near the inner edge of the plasma sheet, at about 10 Re. Several candidates for such an instability exist but all involve the disruption and diversion of strong cross-tail currents into the ionosphere. The instability energizes and pitch angle scatters particles into the loss cone, the aurora brightens, and particles are injected into geosynchronous orbit. This process is similar to that envisioned in tbe near-Earth neutral line model but the field line mapping is quite different and no reconnection of field lines takes place because of the relatively strong Bz componentcloser to the Earth. Injected particles can be (4)213
(4)214
G. D. Reeves
observed locally or could be convected a short distance Earthward on dipolarizing field lines. As the substorm progresses the instability region grows in radius and longitude, propagating as a rarefaction wave. It is accompanied by the expansion of the active auroral region poleward and in longitude. Also expected is the expansion of the substorm current wedge, dipolarization of more of the tail field lines, and the expansion of the substorm injection region. If the instability propagates tailward to a region of sufficiently weak Bz reconnection of magnetic field lines is expected to occur. Reconnection could then produce a plasmoid in the usual waybut plasmoid fonnation would occur in the late expansion phase or in the recovery phase /9/. In this scenario, reconnection is a consequence rather than a cause ofthe substorm. 1114
THE CDAW 7 SUBSTORM OBSERVATIONS A substorm that occurred on April 24, 1979 was the subject of investigation for the 7th Coordinated Data Analysis Workshop (CDAW 7). It is a very well characterized substorm with observations from ground magnetometers, from geosynchronous orbit, and from ISEE in the tail. The first signatures of onset were observed in ground magnetometers at College, Alaska at 1110 UT ±1miii (Figure 1). Virtually all the expected signatures of reconnection were observed on the ISEE spacecraft minutes later. A tailward streaming beam of energetic particles appeared in the central plasma sheet and expanded across the ISEE 1 and 2 spacecraft beginning at 1112 UT. This streaming layer reached the lobe boundary at =1117 UT /12/. Since no energy dispersion was observed in the beams the acceleration region was no more than 4.5 Re Earthward of ISEE’s position at =21 Re down tail /12/. The beam observations were interpreted as the expansion of a reconnection separatrix layer /13/. Shortly after the separatrix layer crossed ISEE 2 a prolonged period of southward Bz was observed /13/ and after the reconnection of the last closed field line at 1117 UT the ISEE spacecraft were left in the northern lobe /13/.
(~,
__________
I)
‘“‘.,.~
Z
H
~oll2I~~ ____________
______________
io 11 12 13 14 UT Fig. 1. The magnetogram from College Alaska. Substorm onset begins with the negative bay at approximately 1110 UT. The three current untensifications at 1114, 1118, and 1135 UT correspond to the three substorm injections at geosynchronous orbit. 09
At geosynchronous orbit, spacecraft 1977-007 was located at 35°east of local midnight where it was in a good position to observe drifting, injected energetic particles. There were three distinct injections in the course of the substorm (Figure 2). Reeves et al. /11/ modeled the drift motion of the injected particles, subtracting the propagation delay time from the observation time. They found that the actual times of injection (rounded to the nearest minute) were 1114 UT, 1118 UT, and 1135 UT. The three injections overlappedin local time, not only with each other, but also with the local time of the ISEE spacecraft /11/. Accompanying each injection was an intensification of ionospheric currents that deepened the magnetic bays (Figure 1) and increased the AE index. The peak AE and AL and the deepest magnetic bay were observed within a few minutes of the 1135 UT injection which was the largest both in flux and in local time extent /11/. The recovery of the plasma sheet at ISEE was observed around 1141 UT /13/. INTERPRETATION IN THE NEW SUBSTORM PARADIGM
The initial sequence of events in the CDAW 7 substorm is readily interpreted within the Context of the nearEarth neutral line model but that model must be substantially modified to account for the later evolution of the substorm. An interpretation based on a modified near-Earth neutral line model has been presented elsewhere/il/. Here we consider whether the CDAW 7 observations can be interpreted equally well within the context of the new paradigm in which reconnection is a consequence of near-Earth current disruption. In this model, the onset of the magnetic bay at 1110 UT was the signature of a current disruption
Multiple Substorm Injections
1977.007 -35 E
30.45 k~V 45.65 koV
~: 1030
I
I
1130
instability. Then, presumably, a rarefaction wave was launched down the tail and triggered reconnection there. In the mean time, the disruption region near the Earth produc:d the
~
1100
(4)215
140-200 IceV 1200
1230
‘l’he timing ofevents for the CDAW 7 substorm •
puLs some impo an cons am s on is Fig. 2. Three substorm injections seen at geosynchronous interpretation and hence on the new substorm orbit. Differential energy fluxes are shown on an offset paradigm in general. In contrast to the model scale so the three dispersed peaks can be clearly seen. The proposed by Lui /9/ reconnection in this location of spacecraft 1977-007 is shown in the inset. substorm did not occur in the late expansion or recovery phases. The first reconnection signatures atISEE were observed =2 miii after onset so reconnection must have occurred within a few minutes of the onset of current disruption. ISEE was left in the lobes at 1117 UT, 7 mm after onset. The expansion of substorm activity continued well beyond that time. The recovery phase of the substorm began 13 to 18 mm later. The peaks in the AE and AL indices were seen several minutes before the third geosynchronous injection at 1135 UT and the deepest excursion of the College H-component occurred a few minutes after 1135 UT (Figure 1). ISEE observed the recovery of the plasma sheet at 1141 UT. Therefore 1130-1140 UT is the most likely interval for the beginning of the recovery phase. (Even if one interprets the signatures atISEE as evidence of an instability other than reconnection this timing argument remains valid.) UnIvufs.Illm.(hoijrs)
From observations alone we cannot determinethe location of the initiation of current disruption but the observations allow the possibility that onset occurred in the near-Earth region prior to the start of reconnection. In that case, it is most likely that onset occurred taitward of 6.6 Re. The first injection at geosynchronous orbit occurred at 1114 UT, 4 mm after onset and 2 mm after reconnection signatures were observed in the tail. If the onset of current disruption had occurred Earthward of 6.6 Re the rarefaction wave could not have triggered reconnection before producing an injection at geosynchronous orbit. Therefore we place the probable location of substorm onset between 6.6 Re and the ISEE location (X=-21 Re) and suggest that the disturbance expanded both Earthward and tailward. If the onset of current disruption occurred tailward of geosynchronous orbit we must interpret the Earthward propagation of the substorm disturbance as either a slow convection of energized particles toward geosynchronous orbit or an Earthward expansion of the current disruption region itself. Convection lasting 4 ruin is difficult to reconcile with known injection observations. It is also possible that the instability requires several minutes to energize the particles before convecting them. A third possibility is that the injection at 1114 UT represents an Earthward expansion of the disruption region. In this view, the injection at 1114 UT is an event separate from onset rather than a propagated signature. This view is consistent with the observation of four intensifications of the College H-bay only three of which were associated with geosynchronous injections. The third important constraint on the interpretationof the CDAW 7 substorm in the new paradigm is that it must account for those three injections and the observation that they overlap in local time /11/. The fact that the three injection times agree so closely with the three times of maximum activity in the College magnetogram clearly suggests that the three injections are producedby disruptions that divert current into the ionosphere. However, the current disruption region in which the particles are energized must expand in three discrete steps rather than expanding as a continuous wave front. Therefore, it is possible that the coupling of the ionosphere to the magnetosphere plays an important role in controlling the current instability that produces the injections.
(4)216
0. D. Reeves
CONCLUSIONS & UNANSWERED QUESTIONS We have re-examined the CDAW 7 substorm in the context of a new paradigm for substorm evolution in which the onset of the substorm is located in the vicinity of the inner edge of the plasma sheet We found that the CDAW 7 observations can be interpreted consistently within this framework providing certain revisions are made and constraints applied. • Reconnection signatures are observed within 2 mm of substorm onset and 13-18 nun before the beginning of the recovery phase. Therefore, in contrast to the suggestion of Lw /9/, current disruption onset and reconnection onset in this substorm are nearly simultaneous processes. • Ifthe substonn process begins with current disruption then the location of onset is most likely between geosynchronous orbit (6.6 Re) and the ISEE location (21 Re). The initial disruption may launch a rarefaction wave whichpropagated quickly down tail but the later geosynchronous injections are more consistent with an Earthward motion ofthe disruption region. • Each ofthe major intensifications of the westward electrojet is accompanied by a burst of injected energetic particles at geosynchronous orbit. This suggests that the later evolution of the disruption occurs indiscrete steps rather than as a continuous wave. The reason the instability does not proceed to completion continuously after onset may be a result of magnetosphere-ionosphere coupling. Comparing this study and the conclusions of Reeves et al. /11/ we find that the onset of the CDAW 7 substorm can be placed at either the reconnection region (=20 Re) or closer to geosynchronous orbit (perhaps =10 Re) without contradiction of the observations. Both interpretations require that there be two distinct regions of acceleration. Both leave important questions unanswered questions that are, perhaps, more important than the exact location of onset of the substorm. • Why does the near-Earth current disrupt in multiple steps? It has been reported /14/that over 75% of all substorms are multiple-injectionsubstonns so the CDAW 7 substonni is by no means unique in this respect. • What is the instability that is responsible for the disruption of the current and the energization of particles? Several competing mechanisms have been proposed and we need testable predictions to discriminate betweenthem. • What role does magnetosphere-ionosphere coupling play in activating or moderating this instability? Is it responsible for pseudo-breakups and multiple onset substorms? • How are the two magnetospheric acceleration regions coupled? Is the order of triggering these regions dictated by the means oftransmitting information? These uncertainties remain in any substorm paradigm, new or old. -
REFERENCES 1. Hones, E. W., Jr., Space Sci. Rev., 23, 393, 1979. 2. Moore, T. E., R. L. Arnoldy, J. Feynman, and D. A. Hardy, J. Geophys. Res., 86, 6713, 1981. 3. Klumpar, D. M., J. M. Quinn, and E. G. Shelly, Geophys. Res. Lett., 15, 1295, 1988. 4. Zelenyi, L. M., R. A. Kovrazkhin, and J. M. Bosqued, .1. Geophys. Res., 95, 12,119, 1990. 5. Elphinstone, R. D., D. Hearn, J. S. Murphree, and L. L. Cogger, J. Geophys. Res., 96, 1467, 1991. 6. Baumjohann, W., G. Paschmann, and C. A. Cattell, J. Geophys. Res., 94, 6597, 1989. 7. Lopez, R. E., R. W. McEntire, T. A. Potemra, D. N. Baker, and R. D. Belian, Planet. Space Sci., 38, 771, 1990. 8. Ohtam, S., K. Takahashi, L. J. Zanetti, T. A. Potemra, R. W. McEntire, and T. lijima, in Magnetospheric Substorms, AGU Monograph 64, 131, 1991. 9. Lui, A. T. Y., J. Geophys. Res., 96, 1849, 1991. 10. Cowley, S. W. H., Adv. Space Res., 11, 99, 1991. 11. Reeves, G. D., G. Kettmann, T. A. Fritz, R. D. Belian, J. Geophys. Res., 97, 6417, 1992. 12. Kettmann, G., T. A. Fritz, and E. W. Hones, Jr., J. Geophys. Res., 95, 12,045, 1990. 13. Hones, E. W., T. A. Fritz, J. Birn, J. Cooney, and S. J. Bame, J. Geophys. Res., 91, 6845, 1986. 14. Belian, R. D., D. N. Baker, E. W. Hones, Jr., and P. R. Higbie, J. Geophys. Res., 89, 9101, 1984.
(4)213—(4)216, 1993
MULTIPLE SUBSTORM INJECTIONS AND THE NEW SUBSTORM PARADIGM: INTERPRETATION OF THE CDAW 7 SUBSTORM G. D. Reeves Space Science and Technology Group, Los Alamos NationalLaboratory, Los Alamos, NM 87545, U.SA
ABSTRACT
In the near-Earth neutral line model, the substorm is triggered by reconnection in the magnetotail and its effects radiate from there. In recent years new observations have suggested a new paradigm for substorms in which there are two coupled acceleration regions. In the current sheet disruption model the substorm is thought to begin in an acceleration region atthe inner edge of the plasma sheet (5-10 Re) and to propagate down tall where it may trigger reconnection in the mid-tail (typically >25 Re). We examine the CDAW 7 substorm in the context of this new substorm paradigm, with emphasis on the role of the substorm injection. BACKGROUND For about 10 years the near-Earth neutral line model (e.g., /1/) has provided the most complete framework for the interpretation of substorm observations. It is based on the idea that the substorm starts in the magnetotail with magnetic reconnection. Particles are accelerated in the reconnection region and precipitate, brightening the aurorae. As more magnetic flux is reconnected, the auroral brightening expands poleward. At the same time the cross-tail current is diverted into the ionosphere forming the substorm current wedge. Along with the collapse (or “dipolarization”) of the field inside the current wedge comes the injection of energetic particles into the vicinity of geosynchronous orbit. This is referred to as the convection surge /2/. Recently, observations have been obtained which are irreconcilable with the existing paradigm. New field line models have been used with auroral observations to suggest that the region of aurora! substorm brightening maps very close to the Earth to the vicinity of 10 Re (e.g., /3,4,5/). Studies of plasma sheet flows imply that the neutral line rarely forms inside X=-19 Re /6/. In addition there is evidence that the substorm injection is produced by the energization of particles locally, near where they are observed rather than by energization further down tail and then convection Earthward. The substorm injection has been observed within minutes of substorm onset and there is evidence that the region of dipolarization of the field lines expands outward /7,8/ which is inconsistent with a convection surge scenario. These problems have led some researchers to propose two substorm acceleration regions /9,10,11/ one atthe inner edge of the plasma sheet (=10 Re) and one in the magnetotail (>=25 Re). -
Thus a new substorm paradigm is emerging which synthesizes the near-Earth neutral line model with the new observations. In one such synthesis model, proposed by Lui /9/, substorm onset begins with the initiation of an instability near the inner edge of the plasma sheet, at about 10 Re. Several candidates for such an instability exist but all involve the disruption and diversion of strong cross-tail currents into the ionosphere. The instability energizes and pitch angle scatters particles into the loss cone, the aurora brightens, and particles are injected into geosynchronous orbit. This process is similar to that envisioned in tbe near-Earth neutral line model but the field line mapping is quite different and no reconnection of field lines takes place because of the relatively strong Bz componentcloser to the Earth. Injected particles can be (4)213
(4)214
G. D. Reeves
observed locally or could be convected a short distance Earthward on dipolarizing field lines. As the substorm progresses the instability region grows in radius and longitude, propagating as a rarefaction wave. It is accompanied by the expansion of the active auroral region poleward and in longitude. Also expected is the expansion of the substorm current wedge, dipolarization of more of the tail field lines, and the expansion of the substorm injection region. If the instability propagates tailward to a region of sufficiently weak Bz reconnection of magnetic field lines is expected to occur. Reconnection could then produce a plasmoid in the usual waybut plasmoid fonnation would occur in the late expansion phase or in the recovery phase /9/. In this scenario, reconnection is a consequence rather than a cause ofthe substorm. 1114
THE CDAW 7 SUBSTORM OBSERVATIONS A substorm that occurred on April 24, 1979 was the subject of investigation for the 7th Coordinated Data Analysis Workshop (CDAW 7). It is a very well characterized substorm with observations from ground magnetometers, from geosynchronous orbit, and from ISEE in the tail. The first signatures of onset were observed in ground magnetometers at College, Alaska at 1110 UT ±1miii (Figure 1). Virtually all the expected signatures of reconnection were observed on the ISEE spacecraft minutes later. A tailward streaming beam of energetic particles appeared in the central plasma sheet and expanded across the ISEE 1 and 2 spacecraft beginning at 1112 UT. This streaming layer reached the lobe boundary at =1117 UT /12/. Since no energy dispersion was observed in the beams the acceleration region was no more than 4.5 Re Earthward of ISEE’s position at =21 Re down tail /12/. The beam observations were interpreted as the expansion of a reconnection separatrix layer /13/. Shortly after the separatrix layer crossed ISEE 2 a prolonged period of southward Bz was observed /13/ and after the reconnection of the last closed field line at 1117 UT the ISEE spacecraft were left in the northern lobe /13/.
(~,
__________
I)
‘“‘.,.~
Z
H
~oll2I~~ ____________
______________
io 11 12 13 14 UT Fig. 1. The magnetogram from College Alaska. Substorm onset begins with the negative bay at approximately 1110 UT. The three current untensifications at 1114, 1118, and 1135 UT correspond to the three substorm injections at geosynchronous orbit. 09
At geosynchronous orbit, spacecraft 1977-007 was located at 35°east of local midnight where it was in a good position to observe drifting, injected energetic particles. There were three distinct injections in the course of the substorm (Figure 2). Reeves et al. /11/ modeled the drift motion of the injected particles, subtracting the propagation delay time from the observation time. They found that the actual times of injection (rounded to the nearest minute) were 1114 UT, 1118 UT, and 1135 UT. The three injections overlappedin local time, not only with each other, but also with the local time of the ISEE spacecraft /11/. Accompanying each injection was an intensification of ionospheric currents that deepened the magnetic bays (Figure 1) and increased the AE index. The peak AE and AL and the deepest magnetic bay were observed within a few minutes of the 1135 UT injection which was the largest both in flux and in local time extent /11/. The recovery of the plasma sheet at ISEE was observed around 1141 UT /13/. INTERPRETATION IN THE NEW SUBSTORM PARADIGM
The initial sequence of events in the CDAW 7 substorm is readily interpreted within the Context of the nearEarth neutral line model but that model must be substantially modified to account for the later evolution of the substorm. An interpretation based on a modified near-Earth neutral line model has been presented elsewhere/il/. Here we consider whether the CDAW 7 observations can be interpreted equally well within the context of the new paradigm in which reconnection is a consequence of near-Earth current disruption. In this model, the onset of the magnetic bay at 1110 UT was the signature of a current disruption
Multiple Substorm Injections
1977.007 -35 E
30.45 k~V 45.65 koV
~: 1030
I
I
1130
instability. Then, presumably, a rarefaction wave was launched down the tail and triggered reconnection there. In the mean time, the disruption region near the Earth produc:d the
~
1100
(4)215
140-200 IceV 1200
1230
‘l’he timing ofevents for the CDAW 7 substorm •
puLs some impo an cons am s on is Fig. 2. Three substorm injections seen at geosynchronous interpretation and hence on the new substorm orbit. Differential energy fluxes are shown on an offset paradigm in general. In contrast to the model scale so the three dispersed peaks can be clearly seen. The proposed by Lui /9/ reconnection in this location of spacecraft 1977-007 is shown in the inset. substorm did not occur in the late expansion or recovery phases. The first reconnection signatures atISEE were observed =2 miii after onset so reconnection must have occurred within a few minutes of the onset of current disruption. ISEE was left in the lobes at 1117 UT, 7 mm after onset. The expansion of substorm activity continued well beyond that time. The recovery phase of the substorm began 13 to 18 mm later. The peaks in the AE and AL indices were seen several minutes before the third geosynchronous injection at 1135 UT and the deepest excursion of the College H-component occurred a few minutes after 1135 UT (Figure 1). ISEE observed the recovery of the plasma sheet at 1141 UT. Therefore 1130-1140 UT is the most likely interval for the beginning of the recovery phase. (Even if one interprets the signatures atISEE as evidence of an instability other than reconnection this timing argument remains valid.) UnIvufs.Illm.(hoijrs)
From observations alone we cannot determinethe location of the initiation of current disruption but the observations allow the possibility that onset occurred in the near-Earth region prior to the start of reconnection. In that case, it is most likely that onset occurred taitward of 6.6 Re. The first injection at geosynchronous orbit occurred at 1114 UT, 4 mm after onset and 2 mm after reconnection signatures were observed in the tail. If the onset of current disruption had occurred Earthward of 6.6 Re the rarefaction wave could not have triggered reconnection before producing an injection at geosynchronous orbit. Therefore we place the probable location of substorm onset between 6.6 Re and the ISEE location (X=-21 Re) and suggest that the disturbance expanded both Earthward and tailward. If the onset of current disruption occurred tailward of geosynchronous orbit we must interpret the Earthward propagation of the substorm disturbance as either a slow convection of energized particles toward geosynchronous orbit or an Earthward expansion of the current disruption region itself. Convection lasting 4 ruin is difficult to reconcile with known injection observations. It is also possible that the instability requires several minutes to energize the particles before convecting them. A third possibility is that the injection at 1114 UT represents an Earthward expansion of the disruption region. In this view, the injection at 1114 UT is an event separate from onset rather than a propagated signature. This view is consistent with the observation of four intensifications of the College H-bay only three of which were associated with geosynchronous injections. The third important constraint on the interpretationof the CDAW 7 substorm in the new paradigm is that it must account for those three injections and the observation that they overlap in local time /11/. The fact that the three injection times agree so closely with the three times of maximum activity in the College magnetogram clearly suggests that the three injections are producedby disruptions that divert current into the ionosphere. However, the current disruption region in which the particles are energized must expand in three discrete steps rather than expanding as a continuous wave front. Therefore, it is possible that the coupling of the ionosphere to the magnetosphere plays an important role in controlling the current instability that produces the injections.
(4)216
0. D. Reeves
CONCLUSIONS & UNANSWERED QUESTIONS We have re-examined the CDAW 7 substorm in the context of a new paradigm for substorm evolution in which the onset of the substorm is located in the vicinity of the inner edge of the plasma sheet We found that the CDAW 7 observations can be interpreted consistently within this framework providing certain revisions are made and constraints applied. • Reconnection signatures are observed within 2 mm of substorm onset and 13-18 nun before the beginning of the recovery phase. Therefore, in contrast to the suggestion of Lw /9/, current disruption onset and reconnection onset in this substorm are nearly simultaneous processes. • Ifthe substonn process begins with current disruption then the location of onset is most likely between geosynchronous orbit (6.6 Re) and the ISEE location (21 Re). The initial disruption may launch a rarefaction wave whichpropagated quickly down tail but the later geosynchronous injections are more consistent with an Earthward motion ofthe disruption region. • Each ofthe major intensifications of the westward electrojet is accompanied by a burst of injected energetic particles at geosynchronous orbit. This suggests that the later evolution of the disruption occurs indiscrete steps rather than as a continuous wave. The reason the instability does not proceed to completion continuously after onset may be a result of magnetosphere-ionosphere coupling. Comparing this study and the conclusions of Reeves et al. /11/ we find that the onset of the CDAW 7 substorm can be placed at either the reconnection region (=20 Re) or closer to geosynchronous orbit (perhaps =10 Re) without contradiction of the observations. Both interpretations require that there be two distinct regions of acceleration. Both leave important questions unanswered questions that are, perhaps, more important than the exact location of onset of the substorm. • Why does the near-Earth current disrupt in multiple steps? It has been reported /14/that over 75% of all substorms are multiple-injectionsubstonns so the CDAW 7 substonni is by no means unique in this respect. • What is the instability that is responsible for the disruption of the current and the energization of particles? Several competing mechanisms have been proposed and we need testable predictions to discriminate betweenthem. • What role does magnetosphere-ionosphere coupling play in activating or moderating this instability? Is it responsible for pseudo-breakups and multiple onset substorms? • How are the two magnetospheric acceleration regions coupled? Is the order of triggering these regions dictated by the means oftransmitting information? These uncertainties remain in any substorm paradigm, new or old. -
REFERENCES 1. Hones, E. W., Jr., Space Sci. Rev., 23, 393, 1979. 2. Moore, T. E., R. L. Arnoldy, J. Feynman, and D. A. Hardy, J. Geophys. Res., 86, 6713, 1981. 3. Klumpar, D. M., J. M. Quinn, and E. G. Shelly, Geophys. Res. Lett., 15, 1295, 1988. 4. Zelenyi, L. M., R. A. Kovrazkhin, and J. M. Bosqued, .1. Geophys. Res., 95, 12,119, 1990. 5. Elphinstone, R. D., D. Hearn, J. S. Murphree, and L. L. Cogger, J. Geophys. Res., 96, 1467, 1991. 6. Baumjohann, W., G. Paschmann, and C. A. Cattell, J. Geophys. Res., 94, 6597, 1989. 7. Lopez, R. E., R. W. McEntire, T. A. Potemra, D. N. Baker, and R. D. Belian, Planet. Space Sci., 38, 771, 1990. 8. Ohtam, S., K. Takahashi, L. J. Zanetti, T. A. Potemra, R. W. McEntire, and T. lijima, in Magnetospheric Substorms, AGU Monograph 64, 131, 1991. 9. Lui, A. T. Y., J. Geophys. Res., 96, 1849, 1991. 10. Cowley, S. W. H., Adv. Space Res., 11, 99, 1991. 11. Reeves, G. D., G. Kettmann, T. A. Fritz, R. D. Belian, J. Geophys. Res., 97, 6417, 1992. 12. Kettmann, G., T. A. Fritz, and E. W. Hones, Jr., J. Geophys. Res., 95, 12,045, 1990. 13. Hones, E. W., T. A. Fritz, J. Birn, J. Cooney, and S. J. Bame, J. Geophys. Res., 91, 6845, 1986. 14. Belian, R. D., D. N. Baker, E. W. Hones, Jr., and P. R. Higbie, J. Geophys. Res., 89, 9101, 1984.