  In plasma wakefield accelerators (e.g. AWAKE) the proton bunch
self-modulation is seeded by the ionization front of a high-power laser pulse
ionizing a vapour and by the resulting steep edge of the driving bunch profile
inside the created plasma. In this paper, we present calculations in 2D linear
theory for a concept of a different self-modulation seeding mechanism based on
electron injection. The whole proton bunch propagates through a preformed
plasma and the effective beam current is modulated by the external injection of
a short electron bunch at the centre of the proton beam. The resulting sharp
edge in the effective beam current in the trailing part of the proton bunch is
driving large wakefields that can lead to a growth of the seeded
self-modulation (SSM). Furthermore, we discuss the feasibility for applications
in AWAKE Run 2.
