The spatial distribution and magnitudes of field‐aligned currents at 800‐km altitude over northern high latitudes were determined from Triad magnetometer data recorded at College, Alaska, during the period from July 1973 to October 1974. The characteristics that were determined include the following: (1) Large‐scale field‐aligned currents are concentrated in two principal areas encircling the geomagnetic pole: region 1, located near the poleward part of the field‐aligned current region; and region 2, located near the equatorward part. (2) In region 1 during moderately disturbed conditions (2− ≤ Kp ≤ 4+) the largest current densities occur on the forenoon sector, with currents flowing into the ionosphere (with a peak value of ∼2 µA/m² between 0700 and 0800 MLT), and in the afternoon sector, with currents flowing away from the ionosphere (with a peak value of ∼1.8 µA/m² between 1500 and 1600 MLT). These areas of maximum current density in region 1 are approximately coincident with the location of the foci of the Sqp current system. (3) In region 2 for 2− ≤ Kp ≤ 4+ the largest current densities occur on the night side where auroral electrojets are usually most active, namely, in the evening to premidnight sector, with currents flowing into the ionosphere (with a peak value of ∼ 1 µA/m² between 2100 and 2300 MLT), and in the midnight to morning sector, with currents flowing away from the ionosphere (with a peak value of ∼ 1.3 µA/m² between 0200 and 0300 MLT). (4) The currents in region 1 are statistically larger than the currents in region 2 at all local times except for the sector near midnight (∼2100–0300 MLT), where the region 2 currents are comparable to or slightly larger than the region 1 currents. (5) The region 2 currents in the midnight to morning sector are correlated with the intensity of the westward electrojet, and the region 2 currents in the evening to premidnight sector are correlated with the intensity of the eastward electrojet. (6) The region 1 currents appear to persist, especially on the day side, even during very low geomagnetic activity with a value of current density ≳0.6 µA/m² for Kp = 0. We suggest that the magnetosphere‐ionosphere current system which contains field‐aligned currents consists of two distinct parts: a permanent part with field‐aligned currents in region 1 and the other part having field‐aligned currents in region 2, which is an important element of the auroral electrojets.

The characteristics of field-aligned currents at an altitude of 800 km in the dayside high-latitude region over the northern hemisphere were determined from the Triad satellite magnetometer data recorded at College, Alaska, from January 1973 to October 1974. The field-aligned currents discussed here are located poleward of the large-scale field-aligned currents reported earlier and referred to as 'region 1 field-aligned currents' by the authors (Iijima and Potemra, 1976). These high-latitude field-aligned currents are most often observed in the dayside sector between 0930 and 1430 MLT and are statistically distributed between 78 ø and 80 ø invariant latitude during weakly disturbed conditions as indicated by westward electrojet activity (IALI < 100 •). Although these high-latitude field-aligned currents show complicated variations, they generally flow away from the ionosphere in the forenoon hours (0930-1200 MLT) and into the ionosphere in the afternoon hours (1200-1430 MLT). These flow directions are opposite to the quasipermanent region 1 field-aligned currents related to the Sq p currents previously discussed by the authors. The directions and spatial distribution of these high-latitude field-aligned currents are consistent with the antisolarward equivalent ionospheric current near 1200 MLT deduced from simultaneous ground-based magnetograms at •81ø invariant latitude. The intensity of these high-latitude field-aligned currents increases as the interplanetary magnetic field increases in the southward direction. These field-aligned currents are located within the region associated with the dayside magnetospheric cusp, and their relationship to geomagnetic activity, especially interplanetary magnetic field variations, suggests that they may play an important role in the coupling between the interplanetary medium and the magnetosphere.

Three‐component dynamic spectrograms (0–80 mHz) of AMPTE/CCE magnetic field data from August 24, 1984, through December 7, 1985, have been used to survey ULF pulsation activity occurring from L = 5 to 9 in the equatorial magnetosphere (±16° magnetic latitude) at all local times. The data were scanned visually, and each half‐hour interval was categorized by spectral type, approximate polarization, spectral intensity, and spacecraft location to produce a data base representing 7231 hours of observations. Coherent pulsations are divided into nine categories which fall into four basic classes: (1) harmonic toroidal resonances (17.9% of the data), (2) fundamental mode toroidal resonances (11.0%), (3) radially polarized pulsations (5.2%), and (4) compressional low‐frequency pulsations (10.6%). Classes (1) and (2) are not mutually exclusive. Intervals devoid of coherent pulsations are divided into five levels according to noise intensity and account for 56.8% of the observations. Uncategorized events represented 4.2% of the data. The spatial occurrence distributions of each pulsation category and the latitude distribution of selected categories have been examined. The basic conclusions of the study are as follows: (1) Harmonic toroidal resonances are found to be the dominant coherent activity on the dayside, particularly in the prenoon hours, where they occur 60% of the time. Their region of excitation is uniformly distributed in radial distance and extends in local time from 0600 to 1600 but cuts off sharply at the local time boundaries. The pattern is consistent with excitation by a dayside energy source, possibly upstream waves, but the spatial distribution places constraints on models of energy transmission. (2) Fundamental toroidal resonances are observed with an occurrence rate of 40% to 50% for L > 8 at dawn but were observed less than 10% of the time at dusk for L > 8. Dusk is also the site of several types of pulsations of sufficient average intensity to obscure fundamental mode resonances, so this trend may be exaggerated by masking effects. Nonetheless, the dawn/dusk asymmetry is sufficiently remarkable to warrant further study. The occurrence distribution exhibits a pronounced node at the magnetic equator, and the occurrence rate for MLAT > 13° at dawn is ≈80%, suggesting that the fundamental mode resonances are present continually at the dawn flank. The preference for occurrence at the flank suggests that the Kelvin‐Helmholtz instability is ultimately responsible for driving the resonances. (3) Radially polarized pulsations are observed ≈10% of the time except in the 0400–1100 MLT, sector where their occurrence rate is less than 5%. The occurrence distribution of radially polarized waves suggests that they are driven by wave‐particle interactions but not by freshly injected particles. (4) Storm time Pc 5 type waves were observed most often at dusk for L > 8 where they occurred ≈30% of the time. A secondary maximum in storm time Pc 5 type occurrence near dawn was also found. The spatial distribution of storm...

Observations from the Charge Composition Explorerin 1985 and 1986 revealed fifteen current disruption events in which the magnetic field fluctuations were large and their onsets coincided well with ground onsets of substorm expansion or intensification. These events are of short durations locally (∼1–5 min). They are mostly confined to within ∼0.5 RE of the neutral sheet and 1 hour local time from the magnetic midnight. Over the disruption interval, the local magnetic field can change by as much as a factor of ∼7. In general, the stronger the current buildup and the closer to the neutral sheet, the larger the resultant field change. There is also a tendency for a larger subsequent enhancement in the AE index with a stronger current buildup prior to current disruption. For events with good pitch angle coverage and extended observation in the neutral sheet region we find that the particle pressure increases toward the disruption onset and decreases afterward. Just prior to disruption, either the total particle pressure is isotropic, or the perpendicular component (P⊥) dominates the parallel comment (P∥), the plasma beta is seen to be as high as ∼70, and the observed plasma pressure gradient at the neutral sheet is large along the tail axis. The deduced local current density associated with pressure gradient is ∼27–80 nA/m² and is ∼85–105 mA/m when integrated over the sheet thickness. We infer from these results that just prior to the onset of current disruption, (1) an extremely thin current sheet requiring P∥ > P⊥ for stress balance does not develop at these distances, (2) the thermal ion orbits are in the chaotic or Speiser regime while the thermal electrons are in the adiabatic regime and, in one case, exhibit peaked fluxes perpendicular to the magnetic field, thus implying no electron orbit chaotization to possibly initiate ion tearing instability, and (3) the neutral sheet is in the unstable regime specified by the cross‐field current instability. Subsequent to the disruption onset, enhancement of magnetic noise over a broad frequency range, magnetic field aligned counterstreaming electron beams, ion energization perpendicular to the magnetic field, and current reduction in the amount similar to that of current buildup during the growth phase are observed. These features seem to be compatible with the predicted development of the cross‐field current instability.

The AMPTE CCE spacecraft observed a transverse Pc 5 magnetic pulsation (period --• 200 s) at 2155-2310 UT on November 20 (day 324), 1985, at a radial distance of 5.7-7.0 Re, at a magnetic latitude of 1.2ø-1.9 ø , and near 1300 magnetic local time. The magnetic field perturbation was observed primarily in the radial component with an amplitude of 15 nT peak to peak. Ion fluxes (energy > 50 keV) measured by the medium energy particle analyzer (MEPA) on board CCE were also observed to oscillate at the frequency of the magnetic pulsation. The wide range of energy and pitch angle of ions covered by the MEPA allowed us to study the ion flux oscillations in great detail. It is found that (1) regardless of energy the oscillation amplitude tends to maximize near the field-aligned directions while it is essentially zero at 90 ø pitch angle, (2) for a given energy and the given location (east or west) of ion guiding centers, flux oscillations at pitch angle a and at its conjugate, 180 ø-a, are 180 ø out of phase,for a given look direction, the oscillation phase changes with energy, and (4) for a given pitch angle and energy, the eastside flux oscillation leads the westside flux oscillation. These observations can be explained by the adiabatic theory of ion flux pulsations with finite Larmor radius effects included (Southwood and Kivelson, 1981; Kivelson and Southwood, 1983), if we assume an antisymmetric standing wave on the field line, westward propagation of the wave, and a large azimuthal wave number Iml • 110. These properties of the wave are consistent with a second-harmonic standing Alfv6n wave excited in the region where the ring current ions have an inward density gradient. ]. Hughes et al. [ 1979] studied ion flux oscillations observed with the University of California, San Diego (UCSD), plasma analyzer on board ATS 6 in association with a meridionally polarized Pc 4 magnetic pulsation.They found that the low-energy ions (energy <2 keV) oscillated in quadrature with the radial magnetic field oscillation, whereas the high-energy ions (energy -7-30 keV) oscillated in antiphase with the magnetic oscillation. They explained these observations with electric field acceleration for the low-energy ions and with pressure balance between magnetic field and particles for the high-energy ions.