[1] Examination of Geotail measurements in the near-tail (X > À30 R E ) has revealed the presence of small flux ropes in the plasma sheet. A total of 73 flux rope events were identified in the Geotail magnetic field measurements between November 1998 and April 1999. This corresponds to an estimated occurrence frequency of $1 flux rope per 5 hours of central plasma sheet observing time. All of the flux ropes were embedded within high-speed plasma sheet flows with 35 directed Earthward, hV x i = 431 km/s, and 38 moving tailward, hV x i = À451 km/s. We refer to these two populations as ''BBF-type'' and ''plasmoid-type'' flux ropes. The flux ropes were usually several tens of seconds in duration, and the two types were readily distinguished by the sense of their quasisinusoidal ÁB z perturbations, i.e., Ç for the ''BBF'' events and ± for the ''plasmoid'' events. Most typically, a flux rope was observed to closely follow the onset of a high-speed flow within $1-2 min. Application of the Lepping-Burlaga constant-a flux rope model (i.e., J = aB) to these events showed that approximately 60% of each class could be acceptably described as cylindrical, force-free flux ropes. The modeling results yielded mean flux rope diameters and core field intensities of 1.4 R E and 20 nT and 4.4 R E and 14 nT for the BBF and plasmoid-type events, respectively. The inclinations of the flux ropes were small relative to the GSM X-Y plane, but a wide range of azimuthal orientations were determined within that plane. The frequent presence of these flux ropes in the plasma sheet is interpreted as strong evidence for multiple reconnection X-lines (MRX) in the near-tail. Hence, our results suggest that reconnection in the near-tail may closely resemble that at the dayside magnetopause where MRX reconnection has been hypothesized to be responsible for the generation of flux transfer events.

No abstract

Abstract. In a two-fluid picture of magnetic reconnection, inflow electrons flow with the magnetic field line to the diffusion region, whereas inflow ions cannot reach the diffusion region and rest a. round a distance of the ion inertial length. The relative motion of electrons and ions results in electric currents, that is, the Hall currents. The Hall current system produces a quadrupole structure in the cross-tail component of the magnetic field near the magnetic reconnection region. Furthermore, this relative motion forms the electric field, whose direction is toward the equatorial plane (midplane). We have investigated the plasma and magnetic field structure near the magnetic reconnection region in the magnetotail with the Geotail spacecraft. We commonly observed inflowing low-energy (less than 5 keV) electrons in the outermost layer of the plasma sheet in magnetic reconnection events, where accelerated ions and electrons flow away from the magnetic reconnection region. These electrons can carry currents to form part of the Hall current system. The observed east-west variations in the magnetic field are consistent with the quadrupole structure produced by the Hall current system. ¾Ve also noted that inflowing ions have consistently a dawnward motion, almost perpendicular to the magnetic field. These ions indicate the presence of the electric field toward the equatorial pla, ne. The present observations demonstrate the ion-electron decoupling processes for magnetic reconnection in the magnetotail.

Abstract. Poleward boundary intensifications are nightside geomagnetic disturbances that have an auroral signature that moves equatorward from the poleward boundary of the auroral zone. They occur repetitively, so that many individual disturbances can occur during time intervals of-1 hour, and they appear to be the most intense auroral disturbance at times other than the expansion phase of substorms. We have used data from three nightside conjunctions of the Geotail spacecraft in the magnetotail with the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground-based array in central Canada to investigate the relation between the poleward boundary intensifications and bursty plasma sheet flows and to characterize the bursty flows associated with the disturbances. We have found a distinct difference in plasma sheet dynamics between periods with, and periods without, poleward boundary intensifications. During periods with identifiable poleward boundary intensifications, the plasma sheet has considerable structure and bursty flow activity. During periods without such poleward boundary intensifications, the plasma sheet was found to be far more stable with fewer and weaker bursty flows. This is consistent with the intensifications being the result of the mapping to the ionosphere of the electric fields that give rise to bursty flows within the plasma sheet. Two different types of plasma sheet disturbance have been found to be associated with the poleward boundary intensifications. The first consists of plasma sheet flows that appear to be the result of Speiser motion of particles in a localized region of thin current sheet. The second, seen primarily in our nearest-to-the-Earth example, consists of energy-dispersed ion structures that culminate in bursts of low-energy ions and isotropic low-energy electrons and are associated with minima in magnetic field and temperature and maxima in ion density and pressure. Both types of plasma sheet disturbance are associated with localized regions of enhanced dawn-to-dusk electric fields and appear to be associated with localized enhanced reconnection. Our analysis has shown that poleward boundary intensifications are an important aspect of geomagnetic activity that is distinct from substorms. In addition to their very distinct auroral signature, we have found them to be associated with a prolonged series of ground magnetic Pi 2 pulsations and ground X component perturbations, which peak at latitudes near the ionospheric mapping of the magnetic separatrix, and with a series of magnetic B z oscillations near synchronous orbit. Like substorms, the tail dynamics associated with the poleward boundary intensifications can apparently extend throughout the entire radial extent of the plasma sheet. Color versions of figures are available at http ://www' atmøs'ucla'edu/-larry/geøtail'html'

Magnetic field data from the geostationary satellites GOES 2 and GOES 3 are examined to study the development of substorm activity in the near‐earth nightside magnetosphere (around a radial distance of 6.6 RE). Substorm events are those in which a well‐defined single onset is seen at low latitudes on the ground. The field configuration change from more taillike to more dipolelike starts initially in a longitudinally localized region in association with the ground onset, and it develops westward and eastward, even when the simultaneous onset of the low‐latitude positive bay is recorded in a wide longitudinal region on the ground. It is also found that the variation caused by the field‐aligned current starts at the ground onset and reaches a peak at the time of the field change. The present results are consistent with the view that a substorm is associated with a disruption and subsequent conversion of the cross‐tail current to the field‐aligned current connected with the polar ionosphere. It is indicated that the cross‐tail current near synchronous orbit and its disruption are important in producing the field configuration change at synchronous orbit.