Energy spectral properties of the twin-CME driven shocks

More and more observations show that large ground-level enhancement (GLE) events are often associated with twin coronal mass ejections (CMEs). In the twin-shock scenario, the preceding CME-driven shock provides an enhanced-level turbulence at the upcoming CME-driven shock. In this work, we focus on the GLE event on 2012 May 17, which would be associated with the twin-shock interactions. Using the particle simulation method to examine the effect of the preceding CMEs being surrounded by the forthcoming CMEs on the energy spectrum. We use the preceding CMEs-driven shock to produce an enhanced seed population injecting into the later CME-driven shock for particle re-acceleration. As results, we simulate the twin-shock interactions and obtain the compound proton spectrum. Therefore we suggest that the twin-shock interactions would produce an GLE event with an ``ankle-like'' proton spectral slope.