Originally published in: Critical Problems of Magnetospheric Physics, edited by E. R. Dyer, 1-16, IUCSTP Secretariat, Washington, D.C., 1972.
The earth's magnetic field presents an obstacle to the solar wind flow about 50 earth radii (Re) across (transverse to the flow) and about 1000 Re long (in the direction of flow). As illustrated in Figure 1, the supersonic solar wind is decelerated and deflected by a collisionless shock wave, called the bow shock, which stands in front of the magnetic cavity. The region of shocked solar wind plasma surrounding the cavity is called the magnetosheath. Although there is yet much to be learned about both the earth's bow shock and magnetosheath, these regions will not be discussed in depth during this symposium.
|Figure 1. The noon-midnight cross section of the magnetosphere. The size of the polar cusp has been exaggerated; the neutral point has been drawn at 30 Re although it must commonly be beyond the orbit of the moon; and the plasma sheet boundary is not necessarily field aligned.|
The magnetic cavity, generally called the magnetosphere, effectively shields the earth and its atmosphere from the direct influx of the solar wind, so that there is very little flow of mass or momentum across the magnetopause, the boundary of the magnetosphere. However, the departure from perfect shielding, although small, is extremely important. It is generally believed that auroral particles and Van Allen belt particles originate in the solar wind, and the magnetotail would not exist without the flow of momentum from the solar wind into the magnetosphere. Unfortunately, we know very little about the internal structure of the magnetopause, nor of its cousin, the polar cusp, a region to which the magnetosheath plasma has direct access.
As I will discuss in more detail below, the long extension of the magnetosphere in the antisolar direction, the magnetotail, acts as a storage reservoir for the energy released during substorms. This energy is stored principally in the magnetic field. In the center of the tail is the plasma sheet in which the energy density of the particles is comparable to that of the field. Located within the plasma sheet is the current sheet, called the neutral sheet, responsible for the reversal of the tail field from the north to south lobe. This current sheet is seldom neutral, i.e., has zero magnetic field, and is not always a well defined subregion within the plasma sheet. The plasma sheet is shown to be field aligned in Figure 1 but this has not been proven experimentally. A neutral point is postulated to exist in some theories but if it is ever present within the orbit of the moon it must be a transient phenomenon. The magnetic flux in the tail changes with time. Magnetic flux is added through flux transfer from the dayside of the magnetosphere and is removed by convection from the tail into the night time magnetosphere. This transport of magnetic flux is manifested in magnetospheric and ionospheric electric fields. These electric fields are critically important for low energy and thermal plasma, especially for the formation and dynamics of the plasmasphere.
The plasmasphere is an important subregion within the magnetosphere in which the mass density is dominated by cold particles (T~2000 K). It is thought to control wave-particle instabilities and, hence, affect the loss and transport of radiation belt particles. The radiation belt particles in turn are energized by various processes occurring during substorms when the magnetic field in the tail is changing rapidly and the convection electric fields are enhanced. As we can see from this brief introduction and as will become clearer during the following papers, the various regions of the magnetosphere and the physical processes occurring therein are not independent. It is difficult or even impossible to study effectively any magnetospheric phenomenon in isolation. Thus, any successful investigation of the magnetosphere must be comprehensive. The proposed International Magnetospheric Study is directed towards this goal but every effort must be made to ensure that it is comprehensive.
In the following sections we will discuss the consequences of an open magnetosphere, in particular flux transport and the growth phase of substorms, then examine a model for the expansion phase of substorms, and finally review what still needs to be done in these areas.
Strictly speaking, magnetic field lines have no ends. However, it is convenient and generally unambiguous to speak of field lines which are closed and open. A closed field is one which if followed through space from the surface of the earth, returns to the surface of the earth within a finite path length. Otherwise a field line is open.
As evidenced by the existence of the radiation belts and conjugate wave phenomena, the majority of magnetospheric field lines are closed. The field lines from the polar cap which extend into the tail do not contain radiation belt particles nor conjugate wave phenomena. However, this alone does not prove these lines are open, since other properties of the tail may be responsible for their absence.
In situ measurements of the tail magnetic field do not show whether the tail is open or closed, either. This is because the ratio of the length to the width of the tail (of the order of 20) implies that, if the tail is open the magnetic field strength perpendicular to the magnetopause in the tail, is of the order of one half gamma. This is unresolvable by present techniques. In situ, measurements of the dayside magnetopause have been interpreted to indicate that at times, some magnetospheric field lines are open (Sonnerup and Cahill, 1967; 1968; Sonnerup, 1971), but the evidence is far from clear (Aubry et al., 1971).
Figure 2 shows the Dungey reconnection model of the magnetosphere (Dungey, 1963) for southward and northward oriented interplanetary fields. As sketched here, there is quite a difference in topology when the interplanetary field switches from southward to northward. When the interplanetary is southward there are field lines with two, one and no feet on the earth, but when the interplanetary field is northward, field lines either intersect the earth twice or not at all. Thus, in the Dungey model the magnetosphere is open for southward inter- planetary fields and closed for northward interplanetary fields. Another difference between the north and south models, is the role played by the merging of the magnetospheric and inter- planetary fields. For southward interplanetary fields flux is added to the tail by this merging; for northward interplanetary fields flux is removed from the tail. Either process is a steady state one, however, since flux is removed from the tail for southward fields by merging at a neutral point in the tail, and added to the tail for northward fields by convection from the dayside.
|Figure 2. The Dungey model of the magnetosphere. The letter N denotes a neutral point. Arrows indicate the direction of plasma flow. The model is highly qualitative and in particular no attempt has been made to draw these diagrams to scale (after Dungey, 1963).|
Observations of solar particles provide the best evidence to date that the tail field lines are open to the solar wind. However, both solar electron data (West and Vampola, 1971) and solar proton data (Evans and Stone, 1971; Morfill and Scholer, 1972) indicate that the tail is open at all times, regardless of the sign of the north-south component of the interplanetary magnetic field. Thus, the Dungey model for northward fields sketched in Figure 2 is not consistent with these results. The necessary modification to it is simple, however. We just take note of the fact that it is highly improbable that the same field line will connect to both the north and south tail neutral points. This is particularly true if the interplanetary magnetic field is not exactly north.
Figure 3 illustrates the time history of a field line merging at the north tail neutral point. The field line Bb in the interplanetary medium is convected to the magnetopause where it, B' b', merges at the north tail neutral point N with the north lobe tail line ' E forming two lines b' E and B' '. The field line B' does not intersect the earth and is therefore removed from the tail. The line b E is attached to the earth and to the solar wind. It is dragged down the tail, b' E, and replaces the lost line B' '. Thus, the merging of the magnetospheric and interplanetary fields involves no transfer of flux from the dayside magnetosphere to the tail or vice versa. The neutral point in the center of the tail can only exist if there is another neutral point at infinity (Dungey, 1963). However, it must exist to separate the closed and open lines. If reconnection took place at this neutral point, the tail would be depleted in the steady state, thus this neutral point must evolve to a Y-type neutral point when the interplanetary field turns northward if the tail is not to disappear.
|Figure 3. A modified Dungey model of the magnetosphere illustrating merging when the interplanetary field is not exactly northward. In this situation field lines are expected to merge with only one of the two neutral points on the surface of the tail. Only merging at the north neutral point is illustrated. Other field lines merge at a south neutral point (not shown) in an analogous manner. The sequence of field lines Bb, B'b, etc. indicates the time history of one field line or a set of field lines at one particular time. The dashed portion of b''E indicates that this field line is out of the magnetic meridian. It is carried around the magnetosphere by the solar wind.|
For every line Bb there is another line Aa which merges with a south lobe field line. For simplicity this line has not been drawn in Figure 3; its merging history is strictly analogous to that of Bb. We note the following properties of bE and AE, the merged field lines. First, the field line connected to the north lobe is connected to the interplanetary medium south of the magnetosphere and the field line connected to the south lobe is connected to the interplanetary medium north of the magnetosphere. Secondly, the lines bE and AE will probably convect around the magnetosphere on opposite sides. If we let be the angle about the earth sun line in solar magnetospheric coordinates where = 0 along the Y-direction, and /2 along the Z- direction (Russell, 1971), then AE lines will tend to slip to the dawn side and bE lines will tend to slip to the dusk side for /2 and vice versa for 0 /2.
Figure 4 shows the history of field line motion external to the magnetosphere for merging in the presence of a northward interplanetary field. In a realistic situation, i.e., / 2, the majority of the A and the majority of the B lines would be on opposite sides of the magnetosphere. We note that the models illustrated in Figures 2, 3 and 4 do not depend on the merging mechanism, but rather only on geometrical considerations given that merging takes place. The strength and importance of these effects, however, does depend on the mechanism.
|Figure 4. The connection of interplanetary field lines to the magnetopause for northward interplanetary fields. "A" field lines have merged with the south lobe of the magnetotail and "B" field lines have merged with the north lobe. Lines A and a, B and b, etc. were connected before merging with the tail field. If merging took place on the dayside for northward fields, the field lines a, B, and B' would not be present and field lines A and b would be the products of the breaking of a single interplanetary field line. This figure has been drawn to emphasize its consistency with Frank's model of the magnetosphere (see figure 7, Frank and Gurnett, 1971).|
The major practical importance of the magnetotail lies in its role as the reservoir of energy for substorms. During the growth phase of substorms, the field normal to the neutral sheet decreases (Fairfield and Ness, 1970; Russell et al., 1971) and the plasma sheet thins (Hones et al., 1971). These are both indications that the number of closed field lines in the tail is decreasing before the sudden energy release. Thus, the tail energy state must be determined by the number of open lines in the tail, and to know the potential of the tail for energy release we must make an accounting of the transfer of flux from open to closed field lines and vice versa. Conceptually, we can do this by observing the flow in the ionosphere across the last closed field lines, or measuring the area of the ionosphere enclosed by the last closed field lines. If the interplanetary field merges with the dayside magnetosphere, then flux is opened up and flows into the polar cap, loosely defined here as the area in the ionosphere enclosed by the last closed field lines. Reconnection within the tail, i.e., at the neutral sheet, causes flux to leave the polar cap. If the interplanetary field merges with the tail field and not with the dayside closed field lines as illustrated in Figure 3 , then no new flux is opened up. In this case, the size of the polar cap remains constant and flow does not cross the last closed field line.
In order to study the effect of the solar wind magnetospheric interaction on the polar cap, or to use polar cap observations to study the magnetospheric interaction with the solar wind, it is desirable to map the polar cap into the tail. Such a map is complicated by the existence or possible existence of electric fields parallel to magnetic fields and by nonpotential electric fields. However, there can be little doubt that the magnetopause maps into the last closed field line in the polar cap and that field lines in the plasma sheet in the center of the tail reach the earth near midnight. Similarly motions along the tail, i.e., parallel to the magnetic field should have no effect in the polar cap, but motions around the tail axis should map into local time motions in the polar cap. Finally, vertical motions through the tail, e.g., from the magnetopause to the plasma sheet should map into motions across the polar cap. We must remember, however, that such a map is qualitative and thus can serve only to describe very general features of the flow patterns.
Figure 5 schematically illustrates the possible flows over the polar cap. If there is dayside merging, closed field lines open up and are convected into the polar cap near noon. The polar cap increases in area. If flux is reconnected at the neutral sheet, open flux closes and is convected out of the polar cap near midnight. The polar cap then decreases in area. Although neither process can proceed alone in steady state, since the polar cap cannot expand or shrink indefinitely, these two processes can act simultaneously in steady state. If the interplanetary field is southward, the expected polar cap flow is essentially directly over the polar cap. This is simply because the north polar cap is connected to the solar wind north of the magnetosphere and the south polar cap to the south solar wind. In this situation newly reconnected field lines are piled on top of old field lines, resulting in a sinking of field lines into the lobes of the tail.
Connection of the interplanetary field to tail field lines which are already open, does not cause flux to enter or leave the polar cap. However, it does cause a convective flow. This flow should be concentrated at the edge of the polar cap for a northward interplanetary field since there is no piling on of tail flux but rather a redistribution of flux on the surface of the tail. The flow should go from noon to midnight since the south polar cap is connected to the solar wind north of the magnetosphere and vice versa for the north polar cap. As illustrated in Figure 5, this flow may have a dawndusk asymmetry, if the azimuthal angle, , about the earth-sun line is not /2, i.e., if the interplanetary field is not exactly northward. The asymmetry is, of course, reversed in the opposite hemisphere. We note that there should be a distributed return flow in the antisolar direction in this case, and that this process is a steady state process because the polar cap area is left unchanged. We note in passing, that the connection of the north lobe to the south solar wind and the south lobe to the north solar wind, together with the dawn-dusk flow asymmetry in the two hemispheres will cause a twisting of the tail for northward directed interplanetary fields so that the neutral sheet will not lie parallel to the solar magnetospheric equator far from the earth. The direction of this twist is a function of the azimuthal angle about the earth-sun line.
|Figure 5. Qualitative polar cap convection patterns due to merging of the interplanetary field with the dayside magnetospheric field lines, and with the tail field lines and due to merging of open tail field lines across the neutral sheet. Plus signs indicate the area enclosed by the last closed field lines (LCF) is increasing; minus signs indicate it is decreasing. Dawn-dusk asymmetries in the flow may arise when there is a non-zero east-west component of the interplanetary magnetic field. The middle panel indicates the situation expected when the interplanetary field has a negative solar magnetospheric Y component.|
Finally, the bottom two panels show the case for dayside merging with northward inter-planetary fields. If this can occur, the north tail field lines will be connected to the solar wind south of the magnetosphere as in the previous case and thus the flow will tend to occur along the edge of the polar cap. As in the previous case, also, there may be a dawn-dusk asymmetry in the flow. However, as in the case of a southward directed interplanetary field, neither dayside merging nor neutral sheet reconnection can form a steady state process alone because the former increases the area of the polar cap and the latter decreases it.
Experimentally we cannot measure the size of the polar cap or the total flux in the tail with a satellite because the boundaries of these regions can change shape as well as scale. Thus, a single point measurement of the last closed field line or the magnetopause in the tail is essentially useless. Another method would be to measure the flow velocity through the last closed field line boundary. Such a measurement would require a knowledge of the orientation of this boundary as well as one position on it. Ideally, we need the integrated flow across the boundary over a wide segment of the boundary. Typically polar satellite orbits cross this boundary at right angles to it, giving a flow velocity over a rather limited region of space.
Another method would be to measure the flow velocity across a closed path normal to the flow (e.g., a dawn-dusk satellite orbit) and find the net flow in the polar cap and the net flow exterior to the polar cap. Ideally these should be equal and opposite and in turn equal to the merging rate if the polar cap was not increasing in size. Unfortunately, such a measurement would require magnetospheric processes to remain steady for the order of an hour, which is an unusual situation during interesting periods. Further, such a measurement requires at least a two component measurement of the electric field with both high accuracy and precision.
Another method for determining the merging rate is to measure the magnetic field normal to the magnetopause. This has been done for the dayside magnetopause, but with much ambiguity. The problem with this method is that the magnetopause moves while the normal component is being measured. A dual satellite measurement would be useful in this regard by enabling spatial and temporal variations to be removed, as well as providing a measure of the velocity and thickness of the boundary. However, the total flux merged per unit time depends on the dimensions of the merging region, as well as the local merging rate, and this can only be obtained statistically by this approach. We note that this technique is not useful for studying the component normal to the magnetopause in the tail because the expected size of this component (about l/2nT) is at about the limit of accuracy of present day magnetometers.
The purpose of measuring the dayside merging rate is to determine what controls this rate and the functional form of this control. It is expected theoretically (cf., Petschek, 1964) and observed experimentally that the merging rate is controlled to a major degree by the magnitude of the north-south component of the interplanetary magnetic field. It is, therefore, important that this quantity be measured in the course of any comprehensive magnetospheric investigation.
At present, we have only hints about the form of this relationship. On the one hand, Arnoldy (1971) found a high correlation between the hourly integrated southward component and geomagnetic activity, if he assumed no interaction of the magnetosphere with the solar wind when the interplanetary field was northward. Russell and McPherron (1972) in a study of the semiannual variation of geomagnetic activity, found that if the southward component were responsible for the semiannual variation, then the interaction assumed by Arnoldy was consistent with the observed variation, but, the interaction proposed theoretically by Petschek in which the merging rate varies slowly and continuously as the interplanetary field direction goes from north to south, was inconsistent with the observations. On the other hand, the convection patterns measured on Injun 5 (Cauffman and Gurnett, 1971) and OGO6 (Heppner, 1972a) often resemble the sum of the bottom two convection patterns of Figure 5, which were drawn for dayside reconnection with northward fields. The same convection pattern could, of course, be constructed by switching between the upper and middle convection patterns at the proper times as the satellite passed over the polar cap. This frequent occurrence of these patterns, however, argues that this is a real pattern and thus, low altitude electric field measurements are consistent with dayside merging for northward fields as Petschek's mechanism would predict. We note that Heppner (1972b) has shown that the dawn-dusk asymmetry in the polar cap convection pattern is controlled by the east-west component of the interplanetary magnetic field. However, as stated in the previous section, this is a geometrical effect essentially independent of the merging mechanism.
Despite the large gaps in our understanding of the solar wind magnetosphere interaction, attempts have been made to predict the orientation of the interplanetary field using polar cap magnetograms. Svalgaard (1968) and Mansurov (1969) noted that there was a broad minimum for a few hours near local noon in the vertical component of a station near the northern pole such as Thule (magnetic latitude 86.8o) and an increase in the horizontal component of a lower latitude station such as Godhavn (magnetic latitude 77.5o) during an "away" sector in the interplanetary magnetic field, and the opposite in a toward sector. Friis-Christensen et al. (1971) used this effect to predict rather successfully the sector structure for 1969. Further studies, however, (Friis-Christensen et al., 1972) have shown that the eastwest component of the interplanetary magnetic field rather than the radial component controls the polar cap current system. This, as we have mentioned above, is a geometrical consequence of the solar wind flow around the magnetosphere in the presence of merging. Jorgensen et al. (1972) have pointed out that this pattern is consistent with Dungey's model and Petschek's merging mechanism.
A substorm is a sequence of auroral zone phenomena which occur repeatedly with roughly the same behavior. There are auroral substorms, magnetic substorms, xray substorms, etc. These have been described by Akasofu (1964). We are concerned, however, with magnetospheric substorms (Parks et al., 1968; McPherron et al., 1968; Coroniti et al., 1968), of which auroral substorms, magnetic substorms, etc., are simply manifestations. The magnetospheric substorm can be divided into three stages: the growth phase when the energy reservoir in the tail is being filled; the expansion phase when the energy is being most rapidly released; and the recovery phase when the currents set up during the expansion phase are decaying. Although there is some controversy as to whether the growth phase can be unambiguously recognized in ground based data, data taken in the magnetosphere show the growth phase quite clearly.
Starting on the dayside, the growth phase is seen as an inward motion, or erosion, of the magnetopause, caused by a southward turning of the interplanetary field. A case study of such an event was first carried out by Aubry et al. (1970). During this event, the solar wind velocity and number density remained roughly constant, while the magnetopause moved inwards over 2 Re. This is equivalent to the transport of 10 Maxwells to the tail which would increase the flux content of the tail by about 15%.
This sequence of events studied on a single pass of OGO5 is supported by statistical studies of Meng (1970) and Fairfield (1971). Meng examined the position of the magnetopause as a function of the AE index. He found that distant magnetopause encounters invariably correspond to quiet times, whereas, abnormally earthward encounters could occur at either quiet or disturbed times. Fairfield examined the magnetopause position as a function of the sign of the north-south component of the magnetosheath field. He found that when the magnetosheath field was southward, the magnetopause was on the average 1 Re closer to the earth than when it was northward.
This transport of flux has some important consequences in the tail. In the region where the tail boundary is parallel to the undisturbed solar wind, the magnetic flux density is determined by the normal pressure of the solar wind which is usually found to be constant in this process. Thus, the area of the tail must increase to accommodate the additional flux. However, the stress of the solar wind on the tail equals the energy density of the magnetic field times the area of the tail cross section. Siscoe and Cummings (1969) have proposed that the tail current system must move closer to the earth in order that the force between the tail current system and the earth's dipole be strong enough to balance the additional stress.
It is very difficult to verify experimentally this proposed inward motion of the tail current system, because the field measured locally at a satellite is also determined by distant currents. However, satellite observations do show the existence of tail-like fields quite close to the earth in the night hemisphere preceding substorms, with moderate distortions from a dipolar field at 6.6 Re (Coleman and Cummings, 1971), and almost complete tail-like character at 8 Re (Russell et al., 1971; McPherron et al., 1972). On the other hand, since the dayside magnetosphere shrinks and the distant tail cross section expands, the flaring angle of the tail or the angle of the tail boundary to the undisturbed solar wind, will increase in the near earth region. This, in turn, compresses the tail flux requiring larger currents in the near tail region, which would also increase the tail-like character of the midnight magnetospheric field. This increase in the tail field in the region from 10 to 30 Re behind the earth during the growth phase is readily observable in data obtained far from the plasma sheet (cf. Aubry and McPherron, 1971; Meng et al., 1971). Finally, we note that the two dimensional magnetospheric model of Unti and Atkinson (1968) predicts that the tail currents will move inwards as the flux in the tail increases. Thus, we conclude that the increasing tail-like character of the magnetic field in the midnight magnetosphere is related to the flux transport by both inward motion and increased strength of the tail current system.
Another phenomenon occurring during the growth phase is the thinning of the plasma sheet (Hones et al., 1971). The plasma sheet is defined by and best studied with measurements of the low energy plasma in the tail (E~1 kev). However, the plasma sheet has only been extensively studied this way at 18 Re. Fortunately, the motion of the plasma sheet may be studied by observing its signature in the magnetic field (Fairfield and Ness, 1970; Meng et al., 1971; Russell et al., 1971); the energetic electrons (Meng et al., 1971), or energetic protons (Buck et al., 1972). It is observed to drift with apparent velocities from 4 to 20 km/sec during the growth phase. We note that this thinning and the subsequent expansion of the plasma sheet during the expansion phase need not represent convective motions, but in fact may represent the loss and acceleration of plasma sheet particles.
In summary, the essential features of the growth phase in the magnetosphere are: a transport of flux from the dayside magnetosphere which causes the dayside magnetosphere to shrink; an increase in the strength of the near tail current systems due to the increased flaring angle of the tail, and possibly an inward motion of the current system; and a thinning of the plasma sheet.
A priori, the expansion phase of substorms could be initiated by a trigger in the solar wind, an event in the distant tail, in the near tail or in the ionosphere since all these regions are observed to participate to some degree in the substorm process. However, there is evidence to indicate that the "disturbance" causing the expansion phase is initiated in the near tail and propagates tailward. This evidence stems primarily from a comprehensive study of observations made in the near tail region during a single pass of the OGO5 satellite. The first piece of evidence is the simultaneity of events in the near tail with the onset of the ground signature of substorms (McPherron et al., 1972). This, however, essentially determines only the field line on which the disturbance is initiated and the substorm could be triggered anywhere along that field line from the ionosphere to the equator. The evidence for the equator rather than the ionosphere, is that during this pass the thinning of the plasma sheet proceeded until the plasma sheet reached 0.5 Re thickness at the 8 Re behind the earth and was converging with radial distance (Buck et al., 1972), when it suddenly began to expand at the time of a substorm onset as determined from ground-based magnetograms. This one observation is consistent with the expansion onset being determined by the thickness of the plasma sheet in the near tail region, possibly occurring when the plasma sheet reached zero thickness. There was no evidence of the plasma sheet motion being stopped or checked prior to the expansion.
|Figure 6. A model for the onset of the expansion phase of a magnetospheric substorm. The x marks the position of the OGO-5 satellite during a well documented substorm on August 15, 1968. The asterisk marks a typical location of a VELA satellite during a substorm. At time A, the plasma sheet is thinning but thins faster near the earth. At time B, an x-type neutral point has formed near OGO, and OGO-5 leaves the plasma sheet. The neutral point moves away from the earth and OGO is enveloped in the expanding plasma sheet. Finally, at D, the neutral point has moved far down the tail enveloping VELA in the plasma sheet also.|
The model that best explains these observations is illustrated in Figure 6. During the growth phase, the plasma sheet thins. Satellites such as OGO, denoted by the X and VELA by the star, see evidence of this thinning. At the onset of the expansion phase, a X-type neutral point forms near the earth. This may form on closed field lines in the plasma sheet or the plasma sheet may disappear over a limited interval of local time so that merging is initiated on the first open field lines. OGO might leave the plasma sheet temporarily at this time only to rapidly reenter the plasma sheet. VELA need not leave the plasma sheet until much later as the merging region passes by VELA. This explains the variable timing of the thinning events relative to expansion phase onsets seen at the VELA satellite. Eventually, the expanding plasma sheet on the earthward side of the neutral point overtakes VELA.
There are several good reasons for invoking a moving neutral point model such as this one. First, there should exist a pressure imbalance across the merging region because the flow is impeded by the earth on one side, but unimpeded on the other. Second, since the average field is northward in this region, any neutral point formed close to the earth must have transient behavior and third, southward magnetic fields are observed prior to plasma sheet expansions (cf. Figures 6, 7, 8 and 9 of Fairfield and Ness, 1970). However, a conclusive test of this model awaits the observation of this southward component with the appropriate time delay by two suitably spaced satellites in the tail, such as on a mother-daughter mission.
The models of the interaction of the solar wind with the magnetosphere which we have presented here are only qualitative models. If we are to make them quantitative as we must, if we ever hope to predict or control magnetospheric phenomena, we need to continue our observational program. This includes new satellite missions as well as continued groundbased observations. The most important requirement is a solar wind monitor as close to the earth as possible to minimize uncertainties in arrival times of solar wind events. This argues for a earth orbiting probe. If this is done, then two spacecraft would be needed for 100% solar wind coverage. The extra cost of two launches would be repaid in the security of a redundant spacecraft, and the added reliability in measuring the orientation of discontinuities when both spacecraft are in the solar wind.
The second most important new mission is an eccentric orbiting spacecraft with an apogee of about 15 Re to make measurements of the magnetopause and near tail region. The height of apogee should be kept to a minimum to maximize the duration of magnetospheric measurements. Apogee and perigee should occur as close as possible to the earth's equator, although the inclination of the orbit may be arbitrary. This will maximize observation time near the neutral sheet and at low latitudes on the magnetopause. This mission should be a mother-daughter type.
Thirdly, a similar mission with apogee well above the earth's equator to probe the polar cusp should be flown. Since the polar cusp moves, and its dimensions are quite important parameters, this should also be a mother-daughter mission.
Low altitude polar orbiters can also make a fundamental contribution to magnetospheric studies and should not be overlooked. They can be used to map the convective motions over the pole and probe the length of the tail via studies of solar particle events. A dual launch with spacecraft 180 degrees apart would be quite useful in this regard.
Finally, although we have stressed observations in the distant magnetosphere, and the growth phase and the onset of the expansion phase of substorms, we must remember that we need also to understand the consequences of the solar wind interaction and substorm processes deep within the magnetosphere. Synchronous orbiters are good, convenient platforms for such studies, and may be ideal locations for future magnetospheric monitors. However, at the present we know little about the magnetosphere just inside and just outside of the synchronous orbit. Thus, serious consideration should be given to a plasmaspheric explorer, perhaps with a 24 hour orbit, but with an apogee of about 10 Re and a perigee around 3 Re to probe the inner magnetosphere.
In conclusion, we have just finished a decade of in situ magnetospheric exploration. This exploratory phase has provided us with qualitative models for most magnetospheric phenomena. The task ahead of us, in particular during the International Magnetospheric Study, is to make these models quantitative. If we are to achieve this goal, we must have a comprehensive set of synoptic measurements on the ground, in the magnetosphere, and, most importantly, in the near earth solar wind.
Discussions with my colleagues, especially M.G. Kivelson and R.L. McPherron, have been very helpful in preparing this review. During the preparation of this report the author received support from the National Aeronautics and Space Administration under contract NAS 5-9098.
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Ness: In your first figure and in your discussion you referred to the possible existence of a single neutral line which, if it exists, probably occurs beyond the distance to the Moon. Recently Schindler and Ness (1971) have shown that magnetic field data from Explorer 34 are consistent with multiple neutral lines inside the orbit of the Moon. What is the basis for your belief that only one neutral line beyond the Moon can exist?