A statistical study of narrowband transverse 0.1‐ to 4.0‐Hz magnetic pulsations, essentially Pc 1–2 (0.1–5.0 Hz), occurring from L=3.5 to L=9, |MLAT| < 16°, and all local times, has been made using data from the AMPTE CCE satellite. This work is reported in a pair of papers of which this is the second. In the first paper (Anderson et al., this issue) the occurrence distributions of Pc 1–2 were reported, and here we present statistical distributions of normalized frequency, X = F/FH+ (F and FH+ are the wave frequency and proton gyrofrequency, respectively); ellipticity, ε, and spectral power. Events occurring from 1000 to 1800 MLT are left‐hand to linearly polarized and exhibited properties corresponding to previous reports. The normalized frequency was found to depend systematically on MLT and L, decreasing from 0.5–0.45 at 1000 MLT to 0.30 at 1800 MLT and increasing from 0.28 at L = 3–5 to 0.39 at L = 8–9 (for 12–15 MLT). The average X dependence on MLT and L can be accounted for by several mechanisms including variations in cold or hot plasma density, He+ concentration, and hot proton parameters T⊥ and T⊥/T∥. A decrease (increase) in T⊥/T∥ with MLT (L) is shown to be necessary, and increases (decreases) in either T⊥ or plasma densities with MLT (L) are also required to reproduce the observed X variation. The role of He+ in causing the X variations is unclear. Events occurring at dawn, 0300–0900 MLT, and L > 7 have not previously been discussed and are found to exhibit remarkable polarization behavior. The A.M. events have the highest X observed in the data base, with X averaging 0.4 to 0.5, but most remarkably, they are linearly polarized at all magnetic latitudes sampled. Given the high normalized frequency, the equatorial linear polarization of the A.M. events cannot be explained in terms of a crossover from left‐ to right‐hand polarization occurring during propagation from low to high magnetic field strengths. Oblique propagation or the effects of multiple reflection through the wave growth region might lead to linear polarizations. The results suggest that a new examination of EMIC wave generation specifically addressing the properties of this A.M. population may be needed.

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

We have conducted a statistical study of large‐scale Birkeland currents, hot plasma, and the By component of the interplanetary magnetic field. Forty‐eight Viking orbits from July 19, 1986, to September 2, 1986, were used in the study during a time period when the 3‐RE geocentric apogee was located near the dayside high‐latitude region usually associated with the polar cusp. We first compared the location of magnetosheathlike electrons to the region 1 and “traditional cusp” Birkeland current systems near noon. It was found that the region 1 Birkeland current system was colocated with the region of most intense magnetosheathlike electron flux. We therefore infer that the “traditional cusp” current system, located poleward of the region 1 system, is associated with field lines that extend to the plasma mantle in the outer magnetosphere. Using this same data base a statistical study between the By component of the IMF and the flow direction of the region 1 Birkeland current system as a function of magnetic local time was performed. We determined that the flow direction of the region 1 current system near noon depends on the By component of IMF. The meridian that separates the dawnside and duskside region 1 Birkeland currents shifted to magnetic local times before noon when By was less than 0 nT, and to the afternoon when By was greater than 0 nT. The extent of the shift away from noon was dependent on the magnitude of the IMF By component.

Magnetic and electric field fluctuations in the Pc 1 frequency range (0.2-5 Hz) have been observed by the polar-orbiting Viking satellite. The fluctuations, interpreted here as electromagnetic ion cyclotron (EMIC) waves, were observed during 21 of 450 orbits surveyed between 0900 and 1400 MLT, near 3 RE geocentric altitude, and at invariant latitudes from 59° to 77°. The frequency structure of the waves is investigated, using spectral analysis and by determining the distribution of the wave frequency as a function of invariant latitude. At in variant latitudes from 59° to 72°, EMIC waves were observed in the frequency range below the equatorial He + gyrofrequency, while from 70° to 77° invariant latitude, EMIC waves were observed in the frequency range above the equatorial He + gyrofrequency. This latitude structure of the wave frequency is discussed in terms of the linear growth rate dependence of the waves on the heavy ion density, ion anisotropy, and ion energy. The propagation characteristics of these waves were also investigated, using minimum variance analy sis and polarization analysis, and by estimating the Poynting flux based on the observed magnetic and electric field. The waves had Poynting vectors directed downward toward Earth and reached magnitudes between 0.01 and 0.1 erg/cm2 s. The polarization of the waves was found to vary between linear, left-hand, and right-hand as a function of time or latitude. This variation is interpreted as the structure of spatially localized Pc 1 waves at high latitudes above the ionosphere.