A study of AMPTE CCE magnetic field data covering the frequency range 0.1–4.0 Hz using all data obtained during the first complete local time precession of the satellite orbit major axis (7500 hours of observations) has been made to evaluate the occurrence of transverse, narrowband Pc 1–2 emissions, identified as electromagnetic ion cyclotron (EMIC) waves, in the equatorial magnetosphere from L = 3.5 to L = 9 at all local times. We report this work in a pair of papers of which this is the first. A set of example events illustrates the pattern of Pc 1–2 occurrence: events occur primarily for L > 7, and a radial separation of several RE between low (<6) and high L (>6) pulsations is observed. Statistically, the highest concentration of events occurred near apogee in the afternoon. The L > 6 and L < 6 event populations appear to be radially separated in the morning but merge together in the afternoon. We construct a normalized occurrence distribution throughout the equatorial plane from L = 3.5 to L = 9 of Pc 1–2 with peak to peak amplitudes greater than 0.8 nT. The occurrence distribution exhibits a number of properties: for L > 7, Pc 1–2 occur at any given place in the early afternoon (1200–1500 MLT) with 10–20% probability and in the morning (0300–0900 MLT) with ≈3% probability; the L = 6–7 local time distribution reproduces results obtained previously from data at geostationary orbit; L < 5 events occur with a probability of ≤1% and a relatively uniform local time distribution. The predominance of L > 7 events implies that plasma sheet ions develop sufficient temperature anisotropy to generate EMIC waves on a routine basis in their drift from the nightside to the dayside and that plasma sheet ions on open drift paths rather than ring current ions on closed drift paths present the greatest source of equatorially generated EMIC waves. Consideration of the maximum convective growth rate of EMIC waves in a pure proton plasma versus cold plasma density and magnetic field strength illustrates how the predominance of L > 7 events and radial gap structure can be understood in terms of radial profiles of linear convective growth rates.

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.

On 1 Jun 1985 the AMPTE/CCE spacecraft (at a geocentric distance of ∼8.8 RE at the midnight neutral sheet region) observed a dispersionless energetic particle injection and an increase in magnetic field magnitude, which are features commonly attributed to disruption of the near‐earth cross‐tail current sheet during substorm expansion onsets. An analysis based on high time‐resolution measurements from the magnetometer and the energetic particle detector indicates that the current sheet disruption region exhibited localized (< 1 RE) and transient (< 1 min) particle intensity enhancements, accompanied by complex magnetic field changes with occasional development of a southward magnetic field component. Similar features are seen in other current disruption/diversion events observed by the CCE. Our analysis suggests that the current disruption region is quite turbulent, similar to laboratory experiments on current sheet disruption, with signatures unlike those expected from an X‐type neutral line configuration. No clear indication of periodicity in any magnetic field parameter is discernible for this current disruption event.

Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetometer and Medium Energy Particle Analyzer (MEPA) data are used to examine the initial signatures of tail field reconfiguration observed in the near‐Earth magnetotail (< 9 RE). Sixteen events are selected preliminarily from 9 months (January‐September 1985) of magnetometer data according to two criteria, that is, an unambiguous commencement of tail field reconfiguration and a sharp recovery of the north‐south (H) component. The second criterion requires that the satellite was close to the onset region of current disruption. Although these strict criteria result in the small number of events, the magnetic and particle flux signatures of the events are considered to be informative concerning the mechanism of substorm onsets. It is found that these tail reconfiguration events are classified into two types: Type I and Type II. In Type I events a current disruption starts in a flux tube that is inward (earthward/equatorward) of the spacecraft, and consequently, the spacecraft is immersed in a hot plasma region expanding from inward (earthward/equatorward). The other type (Type II) is characterized by a distinctive interval (explosive growth phase) just prior to the local commencement of tail reconfiguration. The duration of this interval is typically 1 min, much shorter than that of the so‐called growth phase. During this interval the north‐south magnetic (H) component is depressed sharply, and the flux of energetic ions increases outward (tailward/poleward) of the spacecraft, suggesting that the cross‐tail current is explosively enhanced. It is also found that the radial magnetic (V) component changes with a distinctive phase relationship relative to the north‐south component, which can also be explained in terms of the explosive enhancement in the cross‐tail current intensity just prior to the current disruption. This enhancement is inferred to be a local process, rather than a result of a current disruption which has occurred somewhere else, although it is possible that the commencement of the H recovery observed is not exactly simultaneous with a substorm onset. The present results contribute significantly to modeling efforts regarding the triggering mechanism of substorms in the magnetotail.