Characteristics of field-aligned currents have been determined during a large number of substorms from the magnetic field observations acquired with the Triad satellite. The statistical features of field-aligned currents include the following: (1) The large-scale regions of field-aligned currents determined previously by the authors (Iijima and Potemra, 1976a) persist during all phases of substorm activity, namely, region 1, located near the poleward boundary of the field-aligned current region, and region 2, located near the equatorward boundary. Field-aligned currents flow into region I on the morningside and away from region 1 on the eveningside. The current flow in region 2 is reversed to region I at any given local time except in the Harang discontinuity region (42000-2400 MLT), where the flow patterns are more complicated. (2) During active periods (IAL[ > 100 -•) the average latitude width of regions I and 2 increases by 20-30%, and the centers of these regions shift equatorward by 2o-3 ø with respect to the quiet time values. (3) The current density in region I is statistically larger than the current density in region 2 at all local times except during active periods and in the midnight to morning local time sector. In this region, where the westward electrojet is most active, the current density in region 2 can exceed the current density in region 1. (4) During relatively quiet conditions ([ALl < 100 '•) the largest field-aligned current densities occur in two areas of region I near noon (near 41030 MLT and 41300 MLT) with an average value of 41.6 t•A/m •. During active periods (IAL] > 100-•) the regions of peak current density shift toward the nightside (the region near 1030 MLT shifts to 40730 MLT, and the region near 41300 MLT shifts to •-, 1430 MLT), and the average current density increases to •2.2 t•A/m •'. (5) The average total amount of field-aligned current flowing into the ionosphere always equals the current flow away from the ionosphere during a wide range of quiet and disturbed conditiong. The average total current during quiet periods is 42.7 X 10 • A and during disturbed periods is 45.2 X 10 • A. (6) A three-region pattern of'field-aligned current flow persists in the Harang discontinuity region (42000-2400 MLT) during undisturbed and disturbed periods, when the westward auroral electrojet does not intrude into this sector. This flow pattern consists of an upward flowing field-aligned current surrounded to the north and south by downward flowing currents. During periods when the westward auroral electrojet has intruded deeply into the evening sector the Triad magnetometer data exhibit complicated and fine-structured variations indicating the presence of complex field-aligned currents in this sector. (7) The alignment of current sheets is generally along the boundary of the auroral oval (rather than in the east-west direction), but noticeable distortions of this alignment occur during very disturbed periods. The alignment of fieldaligned currents is different in region I and region 2 durin...

Stable, well‐defined patterns of transverse magnetic disturbances have been observed in the polar regions that persist during periods of strongly positive Bz (≥ 5 nT) and that increase in amplitude as Bz increases. This has been determined from an examination of magnetic field data acquired during 146 orbits of MAGSAT over the south polar regions during November 1979 to January 1980 and supplemented by four consecutive orbits of TRIAD over the north polar region in July 1977. The characteristics of these polar disturbances include the following: (1) they occur at latitudes poleward of the Region 1 Birkeland current system at daytime magnetic local times (0600 MLT through noon to 1800 MLT); (2) the spatial distribution along the dawn‐dusk direction resembles the “W”‐shaped distribution of electric fields observed in the polar cap during periods of positive Bz (Burke et al. [1979]); (3) these patterns show remarkable stability, showing little change from orbit to orbit, up to seven orbits of MAGSAT (equivalent to a period of 10 hours); and (4) the magnitude of the peak disturbance, ΔB, correlates with a “complementary” magnetospheric transmission function of the form: ϵ* = (By² + Bz²)½ cos θ/2, where θ is the angle between the positive z axis and the interplanetary magnetic field (IMF). If the magnetic disturbances are interpreted in terms of Birkeland currents, they flow downward on the duskside and flow away on the morningside (identical to the cusp current flow reported by Iijima and Potemra [1976b]). During periods of negative By the region of morningside (upward flowing) currents is much larger than the eveningside (downward flowing) current region in the southern hemisphere. The density of the currents in the smaller spatial region is larger than the density of the currents in the larger region. This pattern systematically reverses during periods of positive By. We interpret these observations as evidence for a large‐scale, stable Birkeland current system in the polar region that is associated with merging on field lines in the geomagnetic tail. This current system intensifies and is more stable as Bz becomes more northward (reminiscent of the behavior of the Region 1 current system with increasing southward values of Bz). The new stable polar cap current system described here is referred to as the “NBZ” Birkeland current system for “northward Bz” and is important because of its relationship to a variety of other northward By phenomena such as polar cap auroral arcs (“theta aurora”) and multicell convective flow patterns. The existence of these stable NBZ currents and the correlation of their amplitudes with the IMF substantiate the fact that energy continues to flow to the earth's polar regions during periods of strongly northward IMF.

We elaborate upon the leakage model for the escape of energetic magnetospheric particles into the magnetosheath. Unlike the merging model, no interconnection (or merging) of magnetospheric and magnetosheath magnetic field lines is required. Because outer magnetospheric energetic particle drift paths intersect the magnetopause, the leakage model requires the continual escape of ions at postnoon local times and electrons at prenoon local times, regardless of solar wind conditions. It also predicts a division between dawnward and duskward streaming ions at the point where most magnetosheath magnetic field lines make their closest approach to the magnetopause, typically near 1500 LT. Like the merging model, the leakage model predicts equatorward streaming just inside the magnetopause. We study the motion of an escaping energetic ion at a planar magnetopause to show that, without scattering, ions must move dawnward and northward in a duskward magnetosheath magnetic field and dawnward and southward in a dawnward magnetosheath magnetic field. Scattering permits some ions to move duskward. We present new observations of streaming ions outside the dayside magnetopause made by the Charge Composition Explorer satellite, a part of the Active Magnetospheric Particle Tracer Explorers program. To place these observations in context, we have performed a statistical study of previous particle observations both inside and outside the dayside magnetopause. The ensemble of observations indicates that energetic magnetospheric ions of all species continually escape from the dayside magnetosphere and stream along magnetosheath magnetic field lines, even when no merging is expected. The magnetosheath magnetic field controls the direction in which the ions stream: they move away from the magnetosphere. The results of this work indicate that energetic particle observations at the dayside magnetopause need not be taken as evidence for merging of magnetosheath and magnetospheric magnetic field lines. 12,097 12,098 SIBECK ET AL.: ENERGETIC MAGNETOSPHERIC IONS AT THE DAYSIDE MAGNETOPAUSE

Spectrograms of ULF waves in the 0 to 80 mHz frequency range have been prepared from magnetic field data obtained by the elliptically orbiting AMPTE/CCE satellite (with an apogee of approximately 8.8 Re). The most prominent feature of these spectrograms (which cover a full 15.6 hour orbit) is the presence of harmonically structured, azimuthally polarized pulsations in the outer magnetosphere during daytime hours. The frequencies of these pulsations decrease with increasing radial distance from the earth, indicating that they represent independent resonances of local magnetic flux tubes. The latitudinal structure of these harmonic pulsations, observed as AMPTE/CCE traveled to + 16 ø magnetic latitude, is consistent with accepted field line resonance models. Correlations with IMP 8 satellite data confirm a close connection between appearance of these harmonic pulsations in the dayside magnetosphere and interplanetary magnetic field conditions favorable for the generation of upstream waves. Although we also observe pulsations associated with ring current injections, the monochromatic pulsations predicted by some theoretical models are extremely rare in the dayside AMPTE/CCE data set.

During the later stages of the auroral substorm the luminosity distribution frequently resembles a double oval, one oval lying poleward of the normal or main UV auroral oval. We interpret the double oval morphology as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low‐latitude boundary layers, then the double oval configuration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discrete. This transition is associated with the substorm process and is fundamental to understanding the near‐Earth character of substorm onset. On the other hand, the poleward arc system in the nightside ionosphere occurs adjacent to or near the open‐closed field line boundary. This system activates at the end of the optical expansion phase and is a part of the recovery phase configuration in substorms where it occurs. These two source regions for nightside discrete auroral arcs are important in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ considerably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near‐Earth substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temporal development of features within the double oval. Using “variance images,” a new technique for the investigation of these dynamics is outlined.