Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Extraction of the equatorial ion distribution

Energetic neutral atom (ENA) images obtained by the imager on board the Ast rid satellite in the polar cap at 1000 km during a moderate magnetic storm (Dst greater than or equal to -80 nT) on February 8, 1995, are simulated us ing a parameterized model of the equatorial ion distribution and a six-comp onent Chamberlain exosphere with parameters from the Mass Spectrometer Inco herent Scatter Extended (MSISE-90) model. By changing the ion parameters un til a matching ENA image is obtained, one can extract the equatorial ion di stribution. Four consecutive images from different view points several minu tes apart are simulated assuming H+ and O+, respectively, as parent ions. T he optimal set of parameters is extracted by minimizing the chi (2) differe nce between simulated and observed ENA images using Powell's minimization a lgorithm. The optimal equatorial model ion distribution consists of O+ peak ed around dusk. The lower intensity of fluxes obtained from vantage points closer to the pole is an effect of the loss cone of the parent ion distribu tion being empty.