In situ observations of particle acceleration in shock-shock interaction

We use detailed multispacecraft observations to study the interaction of an interplanetary (IP) shock with the bow shock of the Earth on August 9-10, 1998. We can distinguish four different phases of particle acceleration in the shock-shock interaction: (1) formation of magnetic contact with IP shock and the seed population of energetic particles accelerated by it, (2) reacceleration of this population by the bow shock, (3) first order Fermi acceleration as the two shocks approach each other, and (4) particle acceleration and release as the shocks collide. Such a detailed analysis was made possible by the particularly advantageous quasi-radial interplanetary magnetic field configuration. To our knowledge this is the first time the last phase of acceleration at a shock-shock collision has been reported using in situ space plasma observations.