A multi-satellite study of accelerated ionospheric ion beams above the polar cap

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 R E . 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 E O+ /E H+ , 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 fieldaligned 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 Correspondence to: R. Maggiolo (maggiolo@cesr.fr) 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.

Outflow of low-energy O<sup>+</sup> ion beams observed during periods without substorms

“…2a, b and c. The IMF B z component on 31 October was predominantly negative but small; on 19 March 2001 it was also negative from 04:00 to 06:00 UT after which it became positive (data missing from 07:30 to 09:00 UT); and on 19 November 2001 it fluctuated between positive and negative values. The IMF behavior here indicates that the events we discuss are different from periods when the polar cap beams flow out (Maggiolo et al, 2006;Nilsson et al, 2006). with very low density ∼ 0.01 cm −3 after ∼ 05:40 UT.…”

“…By comparing our results with those in Nilsson et al (2010) and with the discussion in Nilsson et al (2013), one may find that not much heating and centrifugal acceleration is expected along these outflow paths in the near-Earth lobes to the plasma sheet. The auroral ions have similar and different features from the ions in the polar wind, upwelling ions (cleft ion fountain) and polar cap (Yan and Maggiolo et al, 2006Maggiolo et al, , 2011Nilsson et al, 2006). In this first report, we will show three examples from periods with different geomagnetic disturbance levels under which the ion beams were observed flowing out of the ionosphere.…”

“…Ions flowing out with transpolar arcs (TPAs, also known as theta auroras) are field-aligned and observed when the interplanetary magnetic field (IMF) is northward (Kullen, 2012). Case events and statistical studies have shown the ion beams flowing out of the polar cap are similar to auroral beams accelerated by fieldaligned potentials (Maggiolo et al, 2006(Maggiolo et al, , 2011Nilsson et al, 2004Nilsson et al, , 2006Kronberg et al, 2014). However, cold O + beams may form from velocity dispersion, and the beams can look similar to those accelerated by an electric field (Horwitz, 1984;Liao et al, 2010).…”

“…2b, 3b, 4b). (06:00-10:00 UT Maggiolo et al, 2006 . Of the three examples, this day (19 November 2001) was the most disturbed with the average integrated photon fluxes starting out around 1.25 kR until ∼ 10:30 UT, when a substorm onset occurred (also identified by Frey et al (2004)).…”

“…These escaping ions have been observed by experiments from radars on the ground (Wahlund et al, 1992) and on numerous satellites including DE1, Polar, Geotail and Cluster (Yan and André and Yau, 1997;Moore et al, 1999;Maggiolo et al, 2006Maggiolo et al, , 2011Nilsson et al, 2006;Seki et al, 2001). The polar cap ions include the polar wind (Engwall et al, 2009;Li et al, 2012) and cusp origin O + Liao et al, 2010Liao et al, , 2012.…”