Abstract
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 (RE) 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 tD 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 tD , 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 tD 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 tD (e.g. the magnitude of the convection drift) with latitudinal profiles varying versus tD . In short, the main conclusion of the paper is that any triplet (tD , 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 RE) 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)
Highlights
The energization and outflow of ionospheric ions at auroral latitudes have been an area of active research over the past three decades, since it was first observed by Shelley et al (1972)
We have studied the spatial structure, from 1000 km up to 20 000 km in altitude, of dayside ionospheric ion outflow as a function of ion heating parameters and the convection drift using a two-dimensional (2-D), Monte Carlo, trajectory-based code
In contrast to previous work based on 1-D simulation, the introduction of a poleward convection drift leads to a limited residence time tD of ions when being heated in the dayside cusp/cleft
Summary
The energization and outflow of ionospheric ions at auroral latitudes have been an area of active research over the past three decades, since it was first observed by Shelley et al (1972). When observed at high altitudes, outflowing ion distributions are conical at high energies in the heating region, and field-aligned at lower energies poleward to the heating region (Horwitz, 1986; Knudsen et al, 1994; Dubouloz et al, 1998) This picture can be formed by ion transverse heating in a region of finite latitudinal extent, followed by adiabatic convective flow to the satellite orbit. A large set of simulations is computed so as to study the effects of different geophysical parameters, such as the altitude dependence of the ion heating rate or the magnetospheric convection drift, on the transport patterns of H+ and O+ ions The aim of this parametric study is focussed on determining the altitude dependence of ion transverse heating for a specific event, providing that highaltitude (below 3 RE) ion observations inside and poleward of the cusp/cleft are available.
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