Search for light ion outflow by incoherent scatter radar

Regular observation of the polar wind was one of several measurements considered feasible when the European Incoherent Scatter (EISCAT) auroral zone incoherent scatter facility was first conceived. Whereas most of the other measurements proposed with EISCAT have been successful, direct observations of the polar wind using the standard incoherent scatter radar analysis schemes have proven to be elusive. This paper presents the first direct measurement of the H⁺ velocity by a ground-based instrument situated at high latitudes, where thermal outflow of light ions occurs. We have avoided the difficulties associated with standard analysis by optimizing the detection method to emphasize only those features of the spectrum sensitive to the drift of the O⁺ and H⁺ ions. We have tested the analysis procedure on EISCAT VHF observed spectra. The data analyzed show unmistakable evidence of differences in drift velocities of the two major ion populations. The results indicate considerable variation in the drift velocity of the light ion component. The light ion velocity shows an overall increase with altitude consistent with H⁺ outflows predicted by thermal polar wind models. The observed H⁺ velocities and the prospects for using the procedure on a routine basis are discussed.