The cusp as a source of magnetospheric energetic particles, currents, and electric fields: A new paradigm

Energetic particle instrumentation on the Polar satellite has discovered th at significant fluxes of energetic particles are continuously present in th e region of the dayside magnetosphere where they cannot be stably trapped. This region is associated with either open magnetic field lines or a magnet ic topology associated with pseudo-trapping. Two distinct features [Time-En ergy Dispersion (TED) signatures and Cusp Energetic Particle (CEP) events] are observed in these energetic particle fluxes that strongly suggest a loc al acceleration of mostly shocked solar wind particles. As the solar wind p articles ram themselves into the cusp geometry, they form diamagnetic cavit ies with strong turbulence that are capable of accelerating particles to en ergies of 100s and 1000s of kiloelectronvolts. This process forms a layer o f energetic particles on the magnetopause as well as permits such particles to enter via drift the equatorial nightside magnetosphere to distances as close as six Earth radii under the influence of gradient and curvature effe cts in the local magnetic field. The fluxes of these particles have all of the properties associated with the ring current and can supply the magnitud e of the cross tail current required. ISEE-1 energetic particle data and th eir pitch angle distributions [PAD] are examined at the magnetic equatorial plane on the night side to investigate and possibly validate the insights gains from the Polar data and energetic particle trajectory tracing in a re alistic magnetic field. The existence and properties of butterfly-type PADs strongly supports the concept of a dayside high latitude source of energet ic particle fluxes. Because the CEP process is impulsive and time variable the charge separation produced by the drifting electrons (eastward) and ion s (westward) on the magnetospheric nightside may be responsible for the cro ss tail electric field that has been ascribed to the reconnection/convectio n process.