The effect of the convection electric field in accelerating ions that escape from the polar ionosphere is investigated. It is shown that at high altitudes the velocity component of the ions along the magnetic field may be increased by more than an order of magnitude. The highest velocities are acquired by ions that escape from the region of the ionosphere that is connected along magnetic field lines to the dayside cusps. During disturbed times, ions from that region intercept the center plane of the magnetotail in one to two hours at radial distances exceeding about 6 RE. Investigation of the resulting O+ properties in the center plane, viz., their locations, number densities, and energies, indicates that the polar ionosphere near the cusps is the principal source of the O+ observed in the plasma sheet. Moreover, a study of the ion motion at quiet and disturbed times indicates that the increase of O+ in the plasma sheet with increasing AE values [Lennartsson and Shelley, 1986] is due mainly to an increase in the source of O+ rather than alteration of its transport path.

The transport of O+ ions from the cusp/cleft ionosphere to the magnetotail during highly disturbed times was determined by computing the guiding‐center trajectories of the ions to a distance of 6 RE from the ionosphere and the full‐motion trajectories at later times. Case histories were tallied in six planes perpendicular to the XGSM axis, three planes perpendicular to the YGSM axis, and in the center plane of the tail. At various times relative to the enhancement of the convection electric field, the following ion properties were constructed from the case histories: number density, mean energy, energy and pitch angle distributions of the flux, and ion pressure components parallel and perpendicular to the magnetic field. It was found that after about 1.7 hours the ion flux in the near‐Earth magnetotail increased dramatically and the spectrum hardened, much as observed during periods just preceding substorms. This increase is attributed to (1) the increase in the O+ outflux from the ionosphere, (2) the increased energization of the ions by the convection electric field, and (3) ion trapping, which generally occurs because the ion magnetic moments generally increase after the ions first cross the geomagnetotail center plane. Moreover, the parallel pressure of the ions exceeds the energy density of the magnetic field at XGSM < −8 RE. On the basis of the expected alterations of the magnetic and electric fields in response to this O+ pressure, a substorm trigger mechanism is suggested.

Abstract. Centrifugal acceleration of escaping ionospheric plasma is one of the important and fundamental processes responsible for energizing and transporting ionospheric plasma to all regions of the magnetosphere. Normally this mechanism operates over extremely large distances. Because of this feature and limitations in particle instrumentation, it has not been possible to directly confirm from insitu plasma measurements the very specific predictions made by basic plasma theory about centrifugal acceleration. We report here data obtained near Polar satellite apogee over the northern polar cap during a magnetospheric compression event that occurred on September 24, 1998.

The transport of ions from the polar ionosphere to the inner magnetosphere during storm time conditions has been computed using a Monte Carlo diffusion code. The effect of the electrostatic turbulence assumed to be present during the substorm expansion phase was simulated by a process that accelerated the ions stochastically perpendicular to the magnetic field with a diffusion coefficient proportional to the rate of energization of the ions by the induced electric field. This diffusion process was continued as the ions were convected from the plasma sheet boundary layer to the double-spiral injection boundary. Inward of the injection boundary the ions were convected adiabatically. By using as input an O + flux of 2.8 x 108 cm -2 s -1 (w>10 eV) and an H + flux of 5.5 x 108 cm -2 s -1 (w>.63 eV) the computed distribution functions of the ions in the ring current were found to be in good agreement, over a wide range in L (4-8), with measurements made with the ISEE 1 satellite during a storm. This O + flux and a large part of the H + flux appear to be consistent with the DE-1 and DE-2 satellite measurements of the polar ionospheric outflow during disturbed times.

Abstract.On September 24, 1998 at 2345 UT the magnetosphere was suddenly compressed as the dynamic pressure of the solar wind rapidly rose from 2 to 15 nPa. At the Polar spacecraft, at high altitudes above the center of the northem polar cap, a remarkably smooth increase in the field strength occurred while the plasma properties changed abruptly, as described in an accompanying paper. Comparisons with models and an examination of the wave amplitudes during the compression indicate that the initial change in plasma properties was most probably due to convection of pre-existing boundary layer plasma to the location of Polar rather than due to local heating by betatron acceleration and ion cyclotron waves. The smoothness of the increase in field strength is attributed to the very high velocity of compressional waves in the tail that outrun the advancing solar wind disturbance. The signatures as measured by GOES 10 at 1444 LT and at GOES 8 at 1846 LT in low latitude geosynchronous orbit are the more familiar sudden jump on the dayside, where the density is high and the compressional wave velocity low, and a weak change on the nightside, where tail current changes oppose the effects of the dayside magnetopause currents. This event is an ideal candidate for collaborative investigation of the effects of a classical sudden storm commencement on the magnetosphere.