Recent studies of wideband plasma wave data from the ISEE 1 and ISEE 2 spacecraft have revealed that whistler mode chorus emissions in the earth's outer magnetosphere are often accompanied by high-frequency bursts of electrostatic waves with a frequency slightly below the electron plasma frequency. Investigations have shown that in some cases the electrostatic waves are modulated at the chorus frequency. Further studies using the plasma analyzer (LEPEDEA) data on ISEE 1 indicate that these bursts are produced by a 'beam' of electrons in Landau (longitudinal) resonance with the chorus wave and thus moving at the chorus phase velocity. A threshold exists in the chorus intensity below which the electrostatic bursts do not appear. The high-frequency electrostatic waves are believed to be caused by a type of two-stream instability called the resistive medium instability. The resistive medium instability is characterized by a reduction in the electrostatic burst frequency below the electron plasma frequency. The instability is applicable only in the regime where V0/VT is on the order of 1, where V0 is the velocity of the beam and VT is the average thermal velocity of the plasma electrons. Our derivation assumes cold ions but warm electrons. The instability requires Landau damping to operate. Thus the beam velocity must be in the region of steep slope on the electron distribution function rather than in the high-velocity tail region. In the cases examined from the LEPEDEA data the electron thermal energies are on the order of a few hundred eV. The beam velocities in the observed cases were ≈400 eV and ≈630 eV, thus verifying that the electrostatic bursts are in the proper regime for the resistive medium instability.