Factors controlling ionospheric outflows as observed at intermediate altitudes

Data acquired by the Fast Auroral Snapshot (FAST) Small Explorer during the 24–25 September 1998 geomagnetic storm have been used to determine the controlling parameters for ionospheric outflows. The data were restricted to dayside magnetic local times. Two primary sources of ion outflows are considered: ion heating through dissipation of downward Poynting flux and electron heating through soft electron precipitation. Ion outflows are shown to be correlated with both, although ion outflows have a higher correlation with soft electrons, measured by the density of precipitating electrons. At 4000 km altitude it is found that fi = 1.022 × 109±0.341nep2.200±0.489, where fi is the ion flux in cm−2 s−1 and nep is precipitating electron density, with a correlation coefficient r = 0.855, based on log‐log regression. This scaling law can be mapped to other altitudes by scaling the flux and density with the magnetic field magnitude. The ion flux is also correlated with the Poynting flux, fi = 2.142 × 107±0.242S1.265±0.445, where S is the Poynting flux at 4000 km altitude in mW m−2 and r = 0.721. Either of these two scaling laws can be used specify ion outflow fluxes, since there is a strong intercorrelation between the various parameters. In particular the present study cannot completely eliminate either of the two candidate processes (ion versus electron heating in the ionosphere, corresponding to Poynting flux versus soft electron precipitation). Soft electron precipitation does have a higher correlation coefficient, however, and if possible the precipitating electron density scaling law should be used. Since Poynting flux may be more easily specified in global simulations, for example, this scaling law is a useful alternate. For the interval under study the ion outflows were dominated by oxygen ions, predominantly in the form of ion conics, with a characteristic energy of order 10–30 eV.