Control of plasma waves associated with the space shuttle by the angle between the orbiter's Velocity vector and the magnetic field

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Abstract
The interaction between water outgassed from the space shuttle and the ionospheric plasma leads to production of water ions by charge exchange and an active and complex plasma wave environment for the space shuttle. We show that the amplitude and spectral character of some of these waves are controlled by the angle between the magnetic field and the shuttle's velocity vector VT relative to the ionospheric plasma. When the flow is approximately perpendicular to the magnetic field (V∥/VT ∼ 0), large wave amplitudes and characteristic “mushroom” wave structures are observed, whereas more nearly parallel flows |V∥| ∼ V⊥ are characterized by low wave levels. We show that linear instability theory predicts the growth of Doppler‐shifted lower hybrid waves in the observed frequency range when driven by the ring and/or beam distributions of water ions produced by charge exchange in the vicinity of the space shuttle. Two mutually compatible interpretations for the V∥/VT effect exist. The first interpretation involves the path lengths available for growth of waves driven by pickup ions varying with the quantity V∥/VT and being limited by spatial variations in the water ion distribution. The second interpretation follows directly from the linear theory: decreasing the ring/beam speed V⊥ of the pickup ions driving the waves (increasing V∥/VT) results in smaller growth rates, with zero growth rate below some threshold value of V⊥. The linear theory shows that decreased growth lengths or growth rates should naturally produce the observed amplitude and bandwidth changes constituting the V∥/VT effect. These results have immediate implications for future shuttle missions and orbiting platforms subject to outgassing of water. If these facilities are used for ionospheric plasma studies or active experiments involving plasma waves, the plasma wave background due to pickup ions associated with the orbiter should be minimized. This requires orbits with large |V∥| ≥ V⊥; that is, orbital velocities within about 45° of the magnetic field over as much of the orbit as possible. These constraints favor more nearly polar orbits and argue strongly against equatorial orbits. Alternatively, free‐flying spacecraft situated upstream from the orbiter's water cloud should be used.