Characteristics of high altitude oxygen ion energization and outflow as observed by Cluster: a statistical study

Abstract

Abstract.The results of a statistical study of oxygen ion outflow using Cluster data obtained at high altitude above the polar cap is reported. Moment data for both hydrogen ions (H + ) and oxygen ions (O + ) from 3 years (2001)(2002)(2003) of spring orbits (January to May) have been used. The altitudes covered were mainly in the range 5-12 R E geocentric distance. It was found that O + is significantly transversely energized at high altitudes, indicated both by high perpendicular temperatures for low magnetic field values as well as by a tendency towards higher perpendicular than parallel temperature distributions for the highest observed temperatures. The O + parallel bulk velocity increases with altitude in particular for the lowest observed altitude intervals. O + parallel bulk velocities in excess of 60 km s −1 were found mainly at higher altitudes corresponding to magnetic field strengths of less than 100 nT. For the highest observed parallel bulk velocities of O + the thermal velocity exceeds the bulk velocity, indicating that the beam-like character of the distribution is lost. The parallel bulk velocity of the H + and O + was found to typically be close to the same throughout the observation interval when the H + bulk velocity was calculated for all pitch-angles. When the H + bulk velocity was calculated for upward moving particles only the H + parallel bulk velocity was typically higher than that of O + . The parallel bulk velocity is close to the same for a wide range of Correspondence to: H. Nilsson (hans.nilsson@irf.se) relative abundance of the two ion species, including when the O + ions dominates. The thermal velocity of O + was always well below that of H + . Thus perpendicular energization that is more effective for O + takes place, but this is not enough to explain the close to similar parallel velocities. Further parallel acceleration must occur. The results presented constrain the models of perpendicular heating and parallel acceleration. In particular centrifugal acceleration of the outflowing ions, which may provide the same parallel velocity increase to the two ion species and a two-stream interaction are discussed in the context of the measurements.

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.…”

“…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.…”

“…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).…”

“…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.…”

Escape of high-energy oxygen ions through magnetopause reconnection under northward IMF

“…It is known that oxygen ions of low charge state are originated from the terrestrial ionosphere (cf. Abe et al, 1993;André and Yau, 1997;Nilsson et al, 2006) and that they follow various fates; for instance, some of them escape into the distant tail, and…”