¹Department of Physical Sciences, University of Oulu, Linnanmaa, FIN-90570 Oulu, Finland.
²Institute of Physics, University of St. Petersburg, St. Petersburg, 198904 Russia
³NOAA R/E/SE, 325 Broadway, Boulder, CO 80303-3328, USA
⁴Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau, Germany

High-time resolution observations of energetic particles and the magnetic field by the CRRES spacecraft show that short-time (1 min) depressions of the H-component of the magnetic field accompanied by sudden changes of the particle flux often occur prior to the sudden field dipolizations. Such events have the following characteristics: (1) They are observed at sudden substorm onsets or activations and sometimes start shortly after the onset of Pi1B pulsations on the ground. (2) They are accompanied by large variations of the D-component of the magnetic field indicative of enhanced field-aligned currents. (3) They appear also in strong B-field (100-300 nT) regions. These signatures are always associated with non-dispersive variations of energetic particle fluxes, although their characteristic features differ from case to case. Sometimes sudden increases (decreases) of the proton flux occur at energies above (below) about 50 keV, which may imply local energization of particles. Our observations show that this category of events includes a variety of different phenomena including previously described signatures of 'injection fronts' and 'explosive growth phase events'. We present examples of the events and discuss their physical interpretation.