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 Astrid satellite in the polar cap at 1000 km during a moderate magnetic storm (Dst ≥ -80 nT) on February 8, 1995, are simulated using a parameterized model of the equatorial ion distribution and a six-component Chamberlain exosphere with parameters from the Mass Spectrometer Incoherent Scatter Extended (MSISE-90) model. By changing the ion parameters until a matching ENA image is obtained, one can extract the equatorial ion distribution. Four consecutive images from different view points several minutes apart are simulated assuming H⁺ and O⁺, respectively, as parent ions. The optimal set of parameters is extracted by minimizing the χ² difference between simulated and observed ENA images using Powell's minimization algorithm. The optimal equatorial model ion distribution consists of O⁺ peaked 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 distribution being empty.