Two types of ion spectral gaps in the quiet inner magnetosphere: Interball-2 observations and modeling

Abstract. We analyse measurements of ion spectral gaps (ISGs) observed by the ION particle spectrometer on board the Interball-2 satellite. The ISG represents a sharp decrease in H⁺ flux at a particular narrow energy range. ISGs are practically always observed in the inner magnetosphere in a wide MLT range during quiet times. Clear examples of ISG in the morning, dayside, evening and nightside sectors of the magnetosphere are selected for detailed analysis and modeling. To obtain a model ISG, the trajectories of ions drifting in the equatorial plane from their nightside source to the observation point were computed for the energy range 0.1–15 keV. Three global convection models (McIlwain, 1972, 1986; Volland, 1973; Stern, 1975) were tested to reproduce the observed ISGs in all MLT sectors. Qualitative agreement is obtained for all three models, but the better agreement for quiet times is reached with the McIlwain (1972) convection model. It is shown that the ISGs observed by the ION spectrometer throughout the inner magnetosphere are the result of super-position of the two effects, already described in the literature (e.g. McIlwain, 1972; Shirai et al., 1997), but acting under different conditions. Also, the role of particle source location on the model gaps is investigated. It may be concluded that despite the evidence of large amplitude and directional local fluctuations of electric fields in the inner magnetosphere (Quinn et al., 1999), the existence of a stationary average convection pattern is confirmed by this modeling. This fact directly follows from observations of ISGs and from a good agreement of observations with modeled gaps calculated in the frames of adiabatic theory for a stationary (average) convection pattern.

Key words. Magnetospheric physics (plasma convection; electric fields)