Factors controlling ionospheric outflows as observed at intermediate altitudes

Abstract

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.

Mechanisms of ionospheric mass escape

“…The ionospheric flow is powered by the Poynting energy flux from the driver plasma into the convecting ionospheric plasma. Thus, either Poynting flux (F P ) or plasma flow can be regarded as the driver, but we adopt F P as the driver to facilitate comparison with the results of Strangeway et al [2005]. Given fixed Fp, ionospheric convection speed depends on the ionospheric conductance.…”

“…These parameters are illustrated in Figure 2. Here the Poynting Flux at 4000 km altitude has been set to 1, 10, and 100 mW/m 2 so the system is driven across the effective range observed by the Strangeway et al [2005] study, for a typical value (8 S) of dayside auroral ionospheric (R. J. Strangeway, private communication, 2009).…”

“…[10] For present purposes, we assume that the precipitating electron density identified by Strangeway et al [2005] characterizes the auroral secondary electron population and is proportional to the fractional superthermal electron density from Khazanov et al [1997], with the constant of proportionality regarded as a free parameter to be determined. Electron precipitation was evaluated for E > 50 eV in the former study, while the latter study considered the entire superthermal electron distribution, taken to have a characteristic energy of 20-60 eV.…”

“…[6] Strangeway et al [2005] and work cited therein also identified Alfvén waves, generated in the magnetosphere and dissipated in the ionosphere, as agents of plasma outflow, and showed that outflow flux is well correlated with the Poynting flux of AC or wave energy propagating into the ionosphere along magnetic flux tubes. Such waves may exist in the ion resonant frequency range, or may nonresonantly power outflows via the ponderomotive effect [Khazanov et al, 1998;Guglielmi and Lundin, 2001].…”

“…[3] Strangeway et al [2005] and Zheng et al [2005] found that two local magnetospheric factors are well correlated empirically with outflow flux: (1) the precipitating magnetospheric electron density and (2) the DC electromagnetic (or Poynting) flux into the F region ionosphere. Plasma outflow resulting from soft electron precipitation can be understood [Caton et al, 1996] as the effect of ambipolar coupling of ions and electrons.…”