Non stationarity of a collision-less quasi-perpendicular shock: consequences for particle acceleration
Résumé
Among several mechanisms issued from simulation and theoretical studies proposed to account for the nonstationarity of quasi-perpendicular supercritical shocks, one process - the so-called self-reformation - driven by the accumulation of reflected ions at a foot distance from the ramp has been intensively analyzed with simulations. Present results based on experimental CLUSTER mission clearly evidence signatures of this self-reformation process for the terrestrial bow shock. The study based on magnetic field measurements includes two parts: (i) a detailed analysis of two typical shock crossings for almost perpendicular shock directions where the risk of pollution by other nonstationarity mechanisms is minimal. A special attention is drawn on non appropriate treatment of data which could lead to wrong interpretation. One key signature of this self-reformation is that the ramp width can reach a very narrow value covering a few electron inertial lengths only; (ii) a statistical analysis based evidences the signatures of this nonstationarity versus different plasma conditions and shock regimes. Present results are compared with previous works and implications for particle accelaeration are driven.