The Quasi-Perpendicular Bow Shock as a Temporal Trapping Barrier and Accelerator of Magnetospheric Particles

Abstract

The bow shock has been studied so far, mostly as a boundary that influences the particles that are incident from the upstream region (of solar origin). In this paper, we provide, for the first time, observational evidence from the energetic-particle-experiment and the charged-particle-measurement-experiment instruments onboard the IMP-8 spacecraft, showing that ions leaking from the magnetosphere, during storms or substorms, are affected by the quasi-perpendicular (Q-P) bow shock and can be temporally trapped just downstream from the shock in a magnetic configuration opposite to the magnetic mirror. The observations from three representative event periods examined in this paper suggest that magnetospheric energetic ( keV) ions show general flows along the field lines in the direction from the magnetosheath toward the interplanetary space, in both sides of the shock (up- and downstream), and characteristic cross-field anisotropic distributions just downstream from the shock, consistent with a ldquotrapped population.rdquo In a series of successive bow-shock crossings within several hours, the IMP-8 spacecraft observed intensity gradients toward the magnetosheath, which strongly suggest a spatial modulation of magnetospheric ion fluxes at the Q-P bow shock. Highest peaks of ion intensities and a very hard spectrum in the energy range between 100 and 400 keV were observed just at the shock front, which suggest acceleration of magnetospheric particles during their temporal trapping at the bow shock. The observations near the Earth's bow shock examined in this paper are well explained in terms of the shock drift acceleration (SDA) theory for magnetospheric particles reaching the bow shock. The phenomenon discussed here, an apparent temporal trapping-and acceleration-of escaped magnetospheric ions at the Earth's bow shock, appears to be an important mechanism that influences the particle distributions near the Earth's bow shock and may have important applications to other shocks in our solar system (planetary, heliospheric, interplanetary, and corotating shock waves) and the interstellar medium (supernova shocks).