Observations of the artificially injected Porcupine xenon ion beam in the ionosphere

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Abstract
A heavy (xenon) ion beam injected approximately perpendicular to the ambient magnetic field into the collisionless ionospheric plasma during the Porcupine campaign in March 1979 was observed over a range of distances from the ion source and of time by instruments on board the main payload and on a spatially separated probe. Plasma and field measurements from these instruments are presented for different phases of beam propagation. After an initial development of a diamagnetic cavity around the ion source, one finds a buildup of the beam current in the near zone of the beam for distances of less than R = 15 m. At larger distances the beam represents a current carrying about 90% of the initial beam current of 4 A. The signature of the transverse magnetic field is consistent with a strong current flowing in the vicinity of the ion source at R < 3 m along the magnetic field into the source. This current is carried by the escaping beam electrons. In the near zone the electric field is saturated (>0.1 V m−1), indicating that part of the electron population follows the beam by E Λ B drift motion. In the intermediate and far zones the electric field E⊥ is only about 10% of the expected maximum polarization field. Neutralization of the beam has to be provided by different processes and constitutes a major problem. From current conservation it is concluded that field‐aligned electron fluxes contribute to depolarization of the beam, thereby closing the beam current system. No significant deceleration or scattering of the beam ions is observed over distances up to half of the ion gyroradius, so there is no violent scattering of ions on self‐generated ion plasma waves on the time scale of observation. This is in agreement with one‐dimensional numerical simulations (Roth et al., 1983). The strong parallel electric fields observed in the beam are attributed either to density gradients or to anomalous resistivity. In future missions it will be most important to resolve the electron distribution function on a time scale short enough to obtain information about the field‐aligned electron fluxes contributing to the overall and local charge neutralization and current closure in the beam.