Electrostatic hiss and the beam driven electron acoustic instability in the dayside polar cusp

In this study it is shown that the upward moving electron beams observed by DE‐1 in the dayside polar cusp at 2 to 5 R E drive an electrostatic electron acoustic mode, rather than the whistler mode near resonance, as previously concluded. The characteristics of this instability are compared with the properties of hiss in the polar cusp observed by DE‐1. The hiss frequencies and funnel shaped frequency‐time spectra are consistent with the electron acoustic mode. However, because the hiss often has a fluctuating magnetic field component near the axis of the funnel shape, the hiss is probably composed of both electron acoustic and whistler mode waves.