Explanation for the simultaneous occurrence of bipolar structures and waves near ion-cyclotron harmonics in the auroral ionosphere

In the downward current region of the auroral ionosphere, the FAST spacecraft has observed bipolar electrostatic structures on Debye length scales and waves at frequencies between the H+ ion cyclotron harmonics. Such bipolar structures have been previously identified with the nonlinearly evolved state of a two-stream electron instability. In this paper, we present the results of long-duration and large-scale particle-in-cell (PIC) simulations which produce, from one set of initial conditions, both bipolar electrostatic structures and, at later times, ion Bernstein waves with peak intensities between multiples of the ion cyclotron frequency. The ion Bernstein waves are driven by a weaker beam instability caused by a residual positive slope in the nonlinearly evolved (nonthermal) electron distribution. Although there are a variety of processes which can produce ion Bernstein modes, we show that a common source (an electron beam) can produce both of these observed phenomena in the downward current region.