Polar cap ion beams during periods of northward IMF: Cluster statistical results

Abstract

Abstract. Above the polar caps and during prolonged periods of northward IMF, the Cluster satellites detect upward accelerated ion beams with energies up to a few keV. They are associated with converging electric field structures indicating that the acceleration is caused by a quasi-static fieldaligned electric field that can extend to altitudes higher than 7 R E (Maggiolo et al., 2006;Teste et al., 2007).Using the AMDA science analysis service provided by the Centre de Données de la Physique des Plasmas, we have been able to extract about 200 events of accelerated upgoing ion beams above the polar caps from the Cluster database. Most of these observations are taken at altitudes lower than 7 R E and in the Northern Hemisphere.We investigate the statistical properties of these ion beams. We analyze their geometry, the properties of the plasma populations and of the electric field inside and around the beams, as well as their dependence on solar wind and IMF conditions. We show that ∼40 % of the ion beams are collocated with a relatively hot and isotropic plasma population. The density and temperature of the isotropic population are highly variable but suggest that this plasma originates from the plasma sheet. The ion beam properties do not change significantly when the isotropic, hot background population is present. Furthermore, during one single polar cap crossing by Cluster it is possible to detect upgoing ion beams both with and without an accompanying isotropic component.The analysis of the variation of the IMF B Z component prior to the detection of the beams indicates that the delay between a northward/southward turning of IMF and the appearance/disappearance of the beams is respectively ∼2 h and 20 min. The observed electrodynamic characteristics of high altitude polar cap ion beams suggest that they are closely connected to polar cap auroral arcs. We discuss the impliCorrespondence to: R. Maggiolo (romain.maggiolo@aeronomie.be) cations of these Cluster observations above the polar cap on the magnetospheric dynamics and configuration during prolonged periods of northward IMF.

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…It is an intermittent source of O + ions active only during prolonged periods of northward IMF and the total outflow flux associated with polar cap ion beams is low compared to other sources. However, it may provide a source of O + ions to the far tail regions because of the significant field aligned acceleration and low convection electric field during periods of northward IMF (Maggiolo et al 2011). …”

Polar cap arcs from the magnetosphere to the ionosphere: kinetic modelling and observations by Cluster and TIMED

“…These so-called polar cap ion beams (PCIB) also show clear signatures of field-aligned acceleration by quasi-static electric fields (Maggiolo et al, 2006). A statistical analysis of 185 PCIB detected by Cluster between 2001 and 2006 revealed that PCIB have properties similar to the polar cap arcs (Maggiolo et al, 2011). PCIB are elongated in the Sun-Earth direction, are associated with upward current sheets and have long lifetimes.…”

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

“…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).…”

“…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.…”