In the eight preceding papers, two magnetospheric substorms on August 15, 1968, were studied with data derived from many sources. In this, the concluding paper, we attempt a synthesis of these observations, presenting a phenomenological model of the magnetospheric substorm. On the basis of our results for August 15, together with previous reports, we believe that the substorm sequence can be divided into three main phases: the growth phase, the expansion phase, and the recovery phase. Observations for each of the first three substorms on this day are organized according to this scheme. We present these observations as three distinct chronologies, which we then summarize as a phenomenological model. This model is consistent with most of our observations on August 15, as well as with most previous reports. In our interpretation we expand our phenomenological model, briefly described in several preceding papers. This model follows closely the theoretical ideas presented more quantitatively in recent papers by Coroniti and Kennel (1972a, b; 1973). A southward turning of the interplanetary magnetic field is accompanied by erosion of the dayside magnetosphere, flux transport to the geomagnetic tail, and thinning and inward motion of the plasma sheet. Our observations indicate, furthermore, that the expansion phase of substorms can originate near the inner edge of the plasma sheet as a consequence of rapid plasma sheet thinning. At this time a portion of the inner edge of the tail current is ‘short circuited’ through the ionosphere. This process is consistent with the formation of a neutral point in the near‐tail region and its subsequent propagation tailward. However, the onset of the expansion phase of substorms is found to be far from a simple process. Expansion phases can be centered at local times far from midnight, can apparently be localized to one meridian, and can have multiple onsets centered at different local times. Such behavior indicates that, in comparing observations occurring in different substorms, careful note should be made of the localization and central meridian of each substorm.

On March 27, 1968, the UCLA magnetometer on board the inbound Ogo 5 satellite recorded an inward motion of the magnetopause by about 2 RE in two hours. It is shown that this inward motion was associated with a reversal of the vertical component of the interplanetary field from northward to southward, the solar wind momentum flux remaining constant. The inward shift did not produce any compression of the magnetospheric cavity, which implies a transfer of magnetic flux from the dayside magnetosphere to the tail: the Imp 4 satellite saw the magnetic tail field increasing at the end of this interval and later the substorm‐associated collapse of this field. The substorm was also recorded on the ground. It is emphasized that the position of the magnetopause after the inward shift cannot be explained in terms of the available numerical models.

Data obtained during a 2-hour sequence of multiple'crossings of the magnetopause in the equatorial plane near 0900 LT with the Ogo 5 UCLA triaxial fluxgate magnetometer and electron spectrometer show that the magnetopause motion was composed of two different oscillations: large-amplitude oscillations with periods from 3.5 to 6 min, and smaller amplitude oscillations with periods as short as 10 sec. The amplitude of the short-period oscillation increased abruptly when the magnetosheath field turned 90 ø southward, producing an extremely variable boundary. The particle boundary showed the same oscillations as the magnetic field boundary, but the two were not coincident and their relative position was quite variable. The direction of the normal to the magnetopause during successive crossings shows that these oscillations do not represent pulsation of the whole boundary but are ripples moving tailward with a velocity of the same order as the plasma flow velocity. The observed structure of the boundary was not consistent with a rotational discontinuity. Since the component of the magnetic field normal to the boundary was often nonzero, however, the structure was not consistent with a steady state tangential discontinuity either. We will conclude that our observations do not 1673

Substorm activity is known to be associated with changes in the solar wind parameters and the magnetotail configuration. In this paper we investigate whether the magnetotail changes occur only as a consequence of substorms or also as a direct consequence of changes in the solar wind parameters. Using data from several satellites (Ogo 5, ATS 1, Imp 4, Explorer 33 and 35) and 17 ground magnetic observatories, we conclude that the tail responds to both changes in the north‐south orientation of the interplanetary field and substorm activity. Specifically, we show the following. (1) A change from a northward to a southward interplanetary field causes a slow increase of the field to be recorded by a satellite within the lobe of the tail, and a thinning of the plasma sheet. (2) A change from a southward to a northward interplanetary field causes the plasma sheet to expand. In contrast, it seems that in the inner magnetosphere the distortion of the magnetic field due to a period of southward interplanetary field is not relieved by an interval of northward field but only through the occurrence of a substorm expansion. (3) A substorm expansion causes a slow decrease of the field within the lobe of the tail and an expansion of the plasma sheet.

Ogo 5 magnetic-field and energetic-electron (E > 50 kev) data are used to study both the quiet-time, steady-state configuration of the outer magnetosphere or near tail region near midnight and the disturbed time changes of this configuration. The nighttime cusp is found to be a distinct feature within the plasma sheet at quiet times but indistinguishable from the plasma sheet at disturbed times. The sequence of thinning and expansion of the plasma sheet in this region in association with the substorms is studied. The response of the plasma, sheet in the near tail at ~10 R• is found to be similar to that in the more distant tail at >20 R•. Finally, the nature of field-aligned currents flowing on the plasma-sheet boundary is investigated. Assuming infinite current sheets, the sheet current density at Ogo 5 is found to be approximately 10 -3 amp m -•. At ionospheric altitudes in the auro,ral zone, these currents scale to. 10 -• amp rn% in good agreement wlth low-altitude measurements. These currents in space often appear in double or multiple sheets. The sequence of events that occqrs during a magnetospheric substorm is gradually becoming clear as a result of recent studies. However, attempts to define this sequence are hampered by the apparent variability of the signature in the tail [Hones et al., 1971a], difficulties in assigning the exact onset time for the various phases of a substorm from ground-based data, and the different velocities of propagation of the effects parallel and perpendicular to the neutral sheet [Akasoiu et al., 1970; Meng et al., 1970]. Most studies of data obtained in the tail beyond distances of 15 R• show that the plasma sheet thins during the growth phase or an early phase of the substorm and expands with a variable delay after the ground onset of the expansion phase [Fairfield and Ness, 1970; Hones et al., 1971b; Me•g et al., 1971]. In addition, Aubry and McPherron [1971] have shown that at 30 Rr a plasma-sheet expansion can be triggered by a reversal of the interplanetary field from southward to northward,