Modeling transverse heating and outflow of ionospheric ions from the dayside cusp/cleft. 1 A parametric study

Abstract. The transport patterns of non-thermal H + and O + field-aligned flows from the dayside cusp/cleft, associated with transverse heating by means of wave-particle interactions and in combination with the poleward motion due to the magnetospheric convection are investigated. This has been accomplished by developing a steady-state, two-dimensional, trajectory-based code. The ion heating is modelled by means of a Monte Carlo technique, via the process of ion cyclotron resonance (ICR), with the electromagnetic left-hand circular polarized component of a broad-band, extremely low-frequency (BBELF) turbulence. The altitude dependence of ICR heating from 1000 km to 3 Earth radii (R_E) is modelled by a power law spectrum, with an index a, and a parameter w₀ that is proportional to the spectral density at a referenced gyrofrequency. Because of the finite latitudinal extent of the cusp/cleft, the incorporation of the horizontal convection drift leads to a maximum residence time t_D of the ions when being energized. A large set of simulations has been computed so as to study the transport patterns of the H + and O + bulk parameters as a function of t_D , a, and w₀. Residence time effects are significant in O + density patterns while negligible for H +. When comparing the results with analytical one-dimensional theories (Chang et al., 1986; Crew et al., 1990), we find that mean ion energies and pitch angles at the poleward edge of the heating region are slightly influenced by t_D and may be used as a probe of ICR parameters ( a, w₀). Conversely, poleward of the heating region, upward velocity and mean energy dispersive patterns depend mainly on t_D (e.g. the magnitude of the convection drift) with latitudinal profiles varying versus t_D . In short, the main conclusion of the paper is that any triplet (t_D , a, w₀) leads to a unique transport-pattern feature of ion flows associated with a cusp/cleft ionospheric source. In a companion paper, by using high-altitude (1.5–3 R_E) ion observations as constraints, the results from the parametric study are used to determine the altitude dependence of transverse ion heating during a significant number of passes of the Interball-2 satellite.

Key words. Magnetospheric physics (auroral phenomena) – Space plasma physics (numerical simultation studies; wave-particle interactions)