An investigation is made into the stability of electrostatic hydrogen ion cyclotron and ion acoustic waves in a model plasma where an ion beam, population 2, and oppositely directed drifting electrons pass through a stationary ion background, population 1. The excited wave properties are then compared with the characteristics of the unstable modes observed on the S3-3 satellite. Three temperature regimes are studied: (1) T e > Ti2 >> Til , (2) Ti2 > T e >/ Til, and (3) T e ----. Til > Ti2. It is found that the ion beam acts as a free energy source only in regime 1. This regime is also highly unstable to the electrons as a free energy source. Unstable modes in regimes 2 and 3 seem to best satisfy the electrostatic hydrogen cyclotron wave (EHC) properties at 1R E. For these cases the electrons are the free energy source, the beam supplies damping. source the ion beam drive waves by n -0 inverse Landau damping. Alternately, when the electrons are the free energy source the beam interacts with the waves by n = 1 or 2 cyclotron damping. This is a non-resonant interaction in that there are no beam ions at the phase velocity of the wave. Thus, the evolution of the ion beam distribution function may be expected to depend upon the free energy source of the waves.The EHC waves observed on S3-3 have several properties that may help identify the plasma parameters in the excitation region and, as a consequence, the free energy source of the waves. These properties are 953 1. The waves are often coherent. The power spectrum will exhibit a spread of <• 10% about the peak near the first cyclotron harmonic. The sinusoidal wave form seen in the broadband data allows one to determine the frequency of the wave by counting the number of wave oscillations over a certain time period. A histogram of 124 such calculations is shown in Figure 1 (M. Temerin, private communication, 1983). The average of these measurements is 1.2 11ci. Note that with two exceptions the frequency falls in the range 11 ci •< 6o •< 2 11 c•.2. There is little Doppler shift of the wave frequency. That is, the Doppler shift due to the satellite should not be so large as to shift the generated frequency out of the first harmonic range described in 1.3. Since the frequency is measured in the moving satellite frame, the apparent frequency is 6o'--6o -k ß Vs, where Vs is the satellite velocity. The parallel components k, and Vs, are much smaller than their respective perpendicular components, so 6o'-6o-ki Vsi. A narrowbanded spectrum requires that the excited waves cover a small range in 6o, and either a small range in k•, or that k• be small enough so 11 c• •< 6o' •< 211 ci. 4. The ratio k,/k• is less than 0.2 (K. Cerny and M. Temerin, private communication, 1983). Figure 2a shows a histogram of the percentage of k,/ki measured during several time intervals from data taken during orbit 757. The data used to generate the histogram was taken during a nightside auroral pass near 6000 km altitude. To determine the k,/k• of an EHC wave, the electric field amplitude from e...
