Abstract
Abstract. This paper presents a study of nearly field-aligned outflowing ion beams observed on the Cluster satellites over the polar cap. Data are taken at geocentric radial distances of the order of 5–9 RE. The distinction is made between ion beams originating from the polar cusp/cleft and beams accelerated almost along the magnetic field line passing by the spacecraft. Polar cusp beams are characterized by nearly field-aligned proton and oxygen ions with an energy ratio EO+ / EH+, of the order of 3 to 4, due to the ion energy repartition inside the source and to the latitudinal extension of the source. Rapid variations in the outflowing ion energy are linked with pulses/modifications of the convection electric field. Cluster data allow one to show that these perturbations of the convection velocity and the associated ion structures propagate at the convection velocity. In contrast, polar cap local ion beams are characterized by field-aligned proton and oxygen ions with similar energies. These beams show the typical inverted V structures usually observed in the auroral zone and are associated with a quasi-static converging electric field indicative of a field-aligned electric field. The field-aligned potential drop fits well the ion energy profile. The simultaneous observation of precipitating electrons and upflowing ions of similar energies at the Cluster orbit indicates that the spacecraft are crossing the mid-altitude part of the acceleration region. In the polar cap, the parallel electric field can thus extend to altitudes higher than 5 Earth radii. A detailed analysis of the distribution functions shows that the ions are heated during their parallel acceleration and that energy is exchanged between H+ and O+. Furthermore, intense electrostatic waves are observed simultaneously. These observations could be due to an ion-ion two-stream instability.
Highlights
The ionosphere has been shown to be an important source of magnetospheric plasma
The Cluster spacecraft observe O+ and H+ ion beams with energies up to a few keV. They have been separated into two distinct categories: polar cusp ion beams originating from the vicinity of the polar cusp and “locally accelerated” ion beams
The second ones are associated with O+ and H+ ions of similar energies
Summary
The ionosphere has been shown to be an important source of magnetospheric plasma. It acts as a source of hydrogen and heavy ions (mainly O+) for the magnetosphere. After their ejection from the dayside ionosphere, the ions are detected above the polar cap as they undergo the anti-sunward electric convection drift and escape along field lines These cusp ion outflows have been studied using data from different satellites (see reviews by Andreand Yau 1997; Moore et al, 1999) but the properties of the outflow and of the source region are still under discussion. Other polar cap ion outflows have been associated with optical structures caused by electron precipitation These ions, originating from the polar cap ionosphere, are accelerated outward along the magnetic field line in the region corresponding to theta auroras or polar cap arcs This paper presents observations of nearly field-aligned outflowing ion beams made on board the Cluster satellites over the polar cap In this region, ions observed at any given point could originate from different source regions. While the cusp appears to be a continuous source of ionospheric plasma, polar cap outflows are triggered by precipitating electrons during periods of northward IMF
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