Particle acceleration and transport in the CME breakout model

Abstract:

We examine particle energization in CMEs generated via the breakout mechanism. In the breakout scenario, reconnection at a breakout current sheet (CS) destabilizes the force balance and initiates the flux rope eruption. Reconnection at the flare CS triggers the fast acceleration of the CME, which forms flare loops below. The test-particle studies in 2.5D/3D configuration are conducted at both the impulsive and decay phases of the eruption. We find that particles are accelerated more efficiently in the flare CS than in the breakout CS even in the presence of large magnetic islands. The maximum particle energy gain can be estimated from the energization terms based on the guiding-centre approximation. Particles are first accelerated in the CSs (with or without magnetic islands) where particle curvature drift dominates. Accelerated particles can (1) escape to the interplanetary space along open field lines rather than trapped in flux ropes, (2) move into the chromosphere along the flare loops, or (3) become trapped in the flare loop top due to the magnetic mirror structure. Some trapped particles are re-accelerated, either via re-injection to the flare CS or through a local Betatron-type acceleration associated with compression of the magnetic field. The energy gains of particles result in relatively hard energy spectra during the impulsive phase. During the gradual phase, particle energization efficiency drops due to the relaxation of the shear in the magnetic field, which reduces the guiding magnetic field in the flare CS.