  We theoretically demonstrate that dc electron flow across the junction of
two-dimensional electron systems leads to excitation of edge magnetoplasmons.
The threshold current for such plasmon excitation does not depend on contact
effects and approaches zero for ballistic electron systems, which makes a
strong distinction from the well-known Dyakonov-Shur and Cerenkov-type
instabilities. We estimate the competing plasmon energy gain from dc current
and loss due to electron scattering. We show that plasmon self excitation is
feasible in GaAs-based heterostructures at $T\lesssim 200$ K and magnetic
fields $B \lesssim 10$ T.
