Electron and proton data from Isis 1 and 2 have been used to examine the change in the latitudinal morphology of auroral particle fluxes as a function of substorm time in the 2100-0300 dipole magnetic local time period. Clear and repeatable systematics in the latitudinal morphology were observed during the various phases of a substorm, allowing one to identify the substorm phase based on particle data alone. Based on this study, a new phenomenological substorm model is presented that relates particle precipitation from various parts of the magnetotail to auroral oval morphology. which the data were taken frequently (though not necessarily on successive orbits) and then to study how the distribution of electrons varied during those periods as substorms grew and subsided. This method led us to recognize several types of distribution patterns which were repeatedly seen in the satellite records in the 2100-0300 MLT period and which can be Copyright ¸ 1975 by the American Geophysical Union. reasonably well classified in terms of substorm times: the expansion phase, the recovery phase, as well as the quiet time distribution. This method was supplemented by a second method in which a large number of individual passes were examined in terms of substorm time. Both methods provided consistent results.The substorm phases were inferred on the basis of either the AE index or individual magnetic records from the midnight sector. It should be noted that the AE index is far from ideal in determining the phases of a substorm, so that it will be used merely as a guide. When available, the interplanetary magnetic field (IMF) data were provided for a qualitative reference.In section 2 we shbw that one can, in general, distinguish several precipitation regions whose characteristics are distinctly different. In section 3 we show typical examples of the precipitation pattern during the quiet period, the expansive phase, and the recovery phase. Then in section 4 we show several extended periods during which a number of satellite passes are available. It will be seen that most of the features discussed in sections 2 and 3 are clearly recognized in successive passes. In section 5 we propose an improved precipitation pattern morphology for different phases of a substorm. For details of the format of Isis 1 and 2 data, see Heikkila and Winningham [1971], Winningham et al. [1973], and the caption ofFigure la. 2. CLASSIC EXAMPLES OF LATITUDINAL MORPHOLOGY Quiet. Time.Observations of electrons on the night side by the polarorbiting satellites Isis 1 and 2 have revealed characteristic and repeatable morphological regions. Figure 1 (electron and proton energy-time spectrograms) is an example which clearly shows all the regions observed during a magnetically quiet period in the midnight sector. Figure 2a shows the superimposed H component records from 10 standard auroral zone stations; the upper envelope and lower envelope of the H component traces provide A U and AL, respectively. The distance between A U and AL provides the AE index. 3148 WINN...

IGY sudden commencements of magnitude greater than 20 gammas simultaneously (±3 min) triggered magnetospheric substorms with a very high probability (∼90%). The magnitude dependence gives rise to a part of the large sunspot cycle variation of the probability of the occurrence of simultaneous events: 49% during the IGY and only 4% during the IQSY. Also, the degree of presubstorm disturbances (measured in terms of ΣKp), types of interplanetary discontinuities, and the direction of interplanetary magnetic field do not appear to be important parameters for ssc‐triggered substorms. Therefore, these results may indicate that a sudden compression of the magnetosphere alone, if intense enough, can sometimes trigger substorms.

The Alaska meridian all‐sky photographs from 1966 and 1970 are compared with the corresponding interplanetary magnetic field data. More than 50 auroral substorms are identified, and their onset times (T = 0) determined. Some of the important findings in this correlative study are as follows: (1) During quiet periods, auroral substorms are quite common along the contracted oval even when the north‐south component Bz is positive; such substorms occur beyond the field of view (both photographically and magnetically) of an AE station. (2) During moderately disturbed periods, even when allowance is made for the IMF signal transition time, it is difficult to find any consistent IMF signature that can be readily identified as that which can be related to the onset of auroral substorms; specifically, we examined |B|, Bz > 0, Bz < 0, ∂Bz/∂t, and so forth. From (1) and (2) we conclude that Bz < 0 is not always a necessary condition for triggering mechanisms and that Bz < 0 is primarily responsible for shifting the oval equatorward to the latitude of AE stations.

Calculations have been made of the effects of intense poleward‐directed electric fields upon the nighttime ionospheric E‐region. The results show the Pedersen and Hall conductivities are substantially changed, thereby decreasing the ionospheric electrical load seen by magnetospheric sources. It appears that relatively large electric fields can exist in the absence of accompanying large field‐aligned currents, as long as the underlying ionosphere remains in darkness and/or energetic particle precipitation is absent.

Plasma flow during plasma sheet expansions is studied with extensive data from Vela 5 and 6 and IMP 6 satellites in the range X ≃ 0 to −30 RE. By combining statistically 44 flow observations by Vela 5 and 6 and 102 flow observations by IMP 6 it is concluded that the three‐dimensional plasma flow direction within the expanding plasma sheet at −10 RE > X > −30 RE is, in general, sunward and approximately parallel to the equatorial plane. The flow speed is usually high (∼500–1000 km/s) in the midnight sector and is generally lower (less than ∼500 km/s) in the dawn and dusk sectors. The flow within a geocentric distance of ∼10 RE is less systematic. The average duration of the flow is ∼12 min. Our result suggests either that the high speed plasma flows are mostly confined near the upper and lower boundaries of the expanding plasma sheet (if it is assumed that the estimation of the distance dZ from the neutral sheet is reasonably accurate) or that the flow lasts for a short duration over the entire plasma sheet (if it is assumed that the dZ estimation is not accurate). The possible implication to the production of bright auroral arcs is discussed.