Abstract.Particles with different energies produce varying contributions to the total ring current energy density as the storm progresses. Ring current energy densities and total ring current energies were obtained using particle data from the Polar CAMMICE/MICS instrument during several storms observed during the years 1996-1998. Four different energy ranges for particles are considered: total (1-200 keV), low (1-20 keV), medium (20-80 keV) and high (80-200 keV). Evolution of contributions from particles with different energy ranges to the total energy density of the ring current during all storm phases is followed. To model this evolution we trace protons with arbitrary pitch angles numerically in the drift approximation. Tracing is performed in the large-scale and small-scale stationary and time-dependent magnetic and electric field models. Small-scale time-dependent electric field is given by a Gaussian electric field pulse with an azimuthal field component propagating inward with a velocity dependent on radial distance. We model particle inward motion and energization by a series of electric field pulses representing substorm activations during storm events. We demonstrate that such fluctuating fields in the form of localized electromagnetic pulses can effectively energize the plasma sheet particles to higher energies (>80 keV) and transport them inward to closed drift shells. The contribution from these high energy particles dominates the total ring current energy during storm recovery phase. We analyse the model contributions from particles with different energy ranges to the total energy density of the ring current during all storm phases. By comparing these results with observations we show that the formation of the ring current is a combination of large-scale convection and pulsed inward shift and consequent energization of the ring current particles.

The Cluster satellites observed a distinct pattern in the evolution of energetic electron pitch angle distributions on November 13, 2003, in the mid‐tail (radius >10RE) associated with intensifications of these electron fluxes that accompanied an observed local dipolarization of the magnetic field. The intensity of electrons (20–200 keV) were first observed to increase for perpendicular pitch angles (pancake distribution), then evolve into isotropic distributions, then further evolve into mixed distributions ‐ a combination of perpendicularly peaked distributions and beamlike distributions (cigar distribution), and at the end of each of three episodes always evolved into a beamlike or field‐aligned distribution. This evolution pattern seen at a relative fixed observing (satellite) location usually developed within the locally observed dipolarization time scale. We have interpreted this pattern to indicate that electrons were coming from different regions along the magnetotail and have gotten accelerated differently. This has led us to suggest and present a model in which Fermi acceleration dominates for electrons that are accelerated from the reconnection region, while betatron acceleration dominates for electrons that are accelerated from the mid‐tail region that is very close to the satellite location.

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Abstract. Energetic electrons (E≥30 keV) travelling along and perpendicular to the magnetic field lines have been observed in the magnetotail at L∼17:00 and 22:00 MLT during the recovery phase of a storm-time substorm on 7 October 2002. Three-dimensional electron distributions of the full unit sphere obtained from the IES/RAPID sensor system demonstrated a rather complicated and random behavior of the energetic electrons. Occasionally these electrons were appearing to travel parallel, perpendicular, or in both directions, relative to the magnetic field direction, forming in this way bi-directional, perpendicular-peaked, and mixed distributions. The electron enhancements occurred while the Cluster spacecraft were on closed field lines in the central plasma sheet approaching the neutral sheet from the northern tail lobe. Magnetic field and energetic particle measurements have been used from geosynchronous and Cluster satellites, in order to describe the general context of the event and then give a possible interpretation regarding the occurrence of the electron anisotropies observed by the IES/RAPID spectrometer on board Cluster. According to geosynchronous measurements an electron dispersionless ejection is very well correlated with a dipolar re-configuration of the magnetic field. The latter fact supports the idea that electrons and, in general, particle ejections at geosynchronous altitude are directly related to electric fields arising from field dipolarization caused by current disruption. Also, having as a main objective the understanding of the way 3-D electron distributions are formed, we have analyzed electron energy spectra along and perpendicular to the magnetic field direction, demonstrating the fact that the electron population consists of two distinct components acting independently and in a random manner relative to each other. This leads to the conclusion that these two electron populations along and perpendicular to the field are generated at different remote locations at Correspondence to: I. I. Vogiatzis (ivogiatz@ee.duth.gr) different rates. The main conclusion of the present paper is that the perpendicular-peaked electron enhancements (electrons with pitch angle around 90 degrees, subjected mainly to curvature drift) observed by Cluster are produced in a remote location duskward of the satellite location, due to the longitudinal and tailward expansion of a current disruption region, and subsequently transported to the Cluster location by means of curvature drift. On the other hand, bi-directional electrons (electrons with pitch angle around 0 and 180 degrees, bouncing mainly along the field lines) are believed to be generated in the vicinity of the neutral sheet or around an X-type region, as suggested by a plethora of previous studies. Finally, in the Discussion section, we make an attempt to present in a more thorough way the substorm model developed by Vogiatzis et al. (2005), which is intimately related to the importance of X-line formation for the initiation of a substorm.