Long-wavelength velocity oscillations in the outer heliosphere

Voyager 2 detected unusual long-wavelength, low frequency velocity oscillations in the solar wind with a period of 2.3 days and peak to trough amplitudes that ranged from 10 km/s - 20 km/s [Paularena et al., 1996]. Since the observed oscillatory features had not yet merged, Paularena et al concluded that the waves had been generated comparatively near Voyager 2 in the outer heliosphere. Three possibilities for the excitation of these waves were suggested, none of which were convincing except that it was somehow associated with pickup ions. We present a simple model for wave excitation driven by pickup ion pressure in the outer heliosphere that appears to account for the observations. Examples of the theoretical model are compared directly to the observed velocity oscillations and we find very reasonable agreement.