Abstract. The term "dispersionless injection" refers to a class of events which show simultaneous enhancement (injection) of electrons and ions with different energies usually seen at or near geosynchronous orbit. We show that dispersionless injections can be understood as a consequence of changes-in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test-particle simulation. The model background magnetic field is a basic dipole field made asymmetrical by a compressed dayside and a weakened nightside. The transient fields are modeled with only one component of the electric field which is westward and a consistent magnetic field. These fields are used to model the major features of a dipolarization process during a substorm onset. We follow the electrons using a relativistic guiding center code. Our simulation results, with an initial kappa electron energy flux spectrum, reproduce the observed electron injection and subsequent drift echoes and show that the energization of injected electrons is mainly due to betatron acceleration of the preexisting electron population at larger radial distances in the magnetotail by transient fields.

Abstract. The substorm-associated behavior of the thermal plasma (30eV < E < 40keV) in the plasma sheet is examined by means of a superposed epoch analysis, using a full year of data from a spacecraft in geosynchronous orbit. The zero epoch time is taken to be substom onset as indicated by a dispersionless energetic particle injection observed on the same spacecraft. Five classes of injection events are found to be well ordered by their average local times. These range from pure ion injections ~3 hours prior to local midnight, to ion injections followed a few minutes later by an electron injection ~2 hours before midnight, to simultaneous ion and electron injections close to midnight, to electron injections that are followed by an ion injection ~ 1 hour postmidnight, and finally to pure electron injections ~2 hours postmidnight. The thermal electrons show a significant increase in temperature (from ~ 1 to ~2 keV) and pressure at substorm onset, while the density and the thermal ion signatures (below ~30 keV) are typically weak and may even vary with local time. However, energetic ions (above ~30 keV), which contribute significantly to the total ion pressure, show clear flux enhancements, leading to a rise in the total temperature from ~ 10 to ~ 16 keV. Preexisting perpendicular anisotropies in the thermal electrons are reduced during the substorm growth phase but become enhanced again after onset, sometimes after a brief period of parallel anisotropy. Similar anisotropy signatures are found for the thermal ions, although somewhat less pronounced.

Particle‐injection events are monitored on three geosynchronous satellites to determine the occurrences of magnetospheric substorms: for every consecutive pair of substorms found, the time interval Δt between substorm onsets is determined. In this manner, 1001 values of Δt are obtained. A statistical analysis of the Δt values finds that the most‐probable time between substorms onsets is Δt ≈ 2.75 hours: this is interpreted to be the period between substorms when substorms occur cyclically. The statistical analysis of the Δt values also finds a random probability for the occurrence of substorms with a mean time between random substorms of about 5 hours: it is speculated that this random occurrence may be caused by a property of the solar wind that varies randomly with an approximately 5‐hour time scale. About 1500 substorms occur per year: about half are periodic and about half occur randomly.

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Abstract. The advanced energetic particle spectrometer RAPID on board Cluster can provide a complete description of the relevant particle parameters velocity, V , and atomic mass, A, over an energy range from 30 keV up to 1.5 MeV. We present the first measurements taken by RAPID during the commissioning and the early operating phases. The orbit on 14 January 2001, when Cluster was travelling from a perigee near dawn northward across the pole towards an apogee in the solar wind, is used to demonstrate the capabilities of RAPID in investigating a wide variety of particle populations. RAPID, with its unique capability of measuring the complete angular distribution of energetic particles, allows for the simultaneous measurements of local density gradients, as reflected in the anisotropies of 90 • particles and the remote sensing of changes in the distant field line topology, as manifested in the variations of loss cone properties. A detailed discussion of angle-angle plots shows considerable differences in the structure of the boundaries between the open and closed field lines on the nightside fraction of the pass and the magnetopause crossing. The 3 March 2001 encounter of Cluster with an FTE just outside the magnetosphere is used to show the first structural plasma investigations of an FTE by energetic multi-spacecraft observations.Correspondence to: U. Mall (mall@linmpi.mpg.de) Key words. Magnetospheric physics (energetic particles, trapped; magnetopause, cusp and boundary layers; magnetosheath) The instrumentThe RAPID spectrometer (Research with Adaptive Particle Imaging Detectors), described in detail by Wilken et al. (1995), is an advanced particle detector for the analysis of suprathermal plasma distributions in the energy range from 20-400 keV for electrons, 30 keV-1500 keV for hydrogen, and 10 keV/nucleon-1500 keV for heavier ions. Innovative detector concepts, in combination with pinhole acceptance, allow for the measurement of angular distributions over a range of 180 • in the polar angle for electrons and ions. Identification of the ion species is based on a two-dimensional analysis of the particle's velocity and energy. Electrons are identified by the well-known energy-range relationship. Table 1 list the main parameters of the RAPID instrument.The energy signals in RAPID are analyzed in 8 bit ADCs. With a mapping process the 256 channels are reduced to 8 channels in the case of the ion sensor and into 9 channels in the case of the electron sensor. The resulting energy channel limits are listed in Table 2.