Bursty energetic electrons confined in flux ropes in the cusp region

Bursty energetic electron events with energies of about 20–400 keV have been observed in the cusp region when the CLUSTER S/C were travelling outbound on 5 April 2001. These electron events are characterized by strong impulsive increases in the flux whereas energetic ions (>30 keV) show no obvious changes. The enhanced energetic electron flux is more than 10 times higher than the background level whereas the energetic ions show no significant changes. The bursts of energetic electrons occurred mainly in the time interval when the northward IMF had a significant By component and were associated with the rotation of the local Bz (bi-polar signature) and with a dramatic change of the plasma flow (<30 keV) from tailward to dawnward. These bursts of electrons are well confined in a small scale flux rope structure as implied by the magnetic field and plasma measurements. Energetic electrons were flowing dawnward from the cusp region along (but anti-parallel to) the magnetic field. No stable energetic electron flux that indicates the cusp has a general open field line geometry has been observed. The spike-like electron bursts could be explained by energetic electrons leaking out of the magnetosphere into the cusp when the flux ropes are formed by a transient reconnection in the cusp stagnation region. Further, we propose that the flux ropes are most likely formed in dawn (dusk) side of the cusp region during northward IMF with a significant positive (negative) By component.