  The second brightest GRB in history, GRB230307A provides an ideal laboratory
to study the details of GRB prompt emission thanks to its extraordinarily high
photon statistics and its single broad pulse overall shape characterized by an
energy-dependent fast-rise-exponential-decay (FRED) profile. Here we
demonstrate that its broad pulse is composed of many rapidly variable short
pulses, rather than being the superposition of many short pulses on top of a
slow component. Such a feature is consistent with the picture of many mini-jets
due to local magnetic reconnection events in a large emission zone far from the
GRB central engine, as envisaged in the internal-collision-induced magnetic
reconnection and turbulence (ICMART) model, but raises a great challenge to the
internal shock models that attribute all variability components to collisions
among different shells. Since relativistic mini-jets demand strong
magnetization in the outflow, this work provides strong evidence for a
Poynting-flux-dominated jet composition of this bright GRB.
