Abstract
Abstract. Data from the Cluster CIS instrument have been used for studying proton and heavy ion (O+ and He+) characteristics of the sub-keV ring current. Thirteen events with dispersed heavy ions (O+ and He+) were identified out of two years (2001 and 2002) of Cluster data. All events took place during rather geomagnetically quiet periods. Three of those events have been investigated in detail: 21 August 2001, 26 November 2001 and 20 February 2002. These events were chosen from varying magnetic local times (MLT), and they showed different characteristics. In this article, we discuss the potential source for sub-keV ring current ions. We show that: (1) outflows of terrestrial sub-keV ions are supplied to the ring current also during quiet geomagnetic conditions; (2) the composition of the outflow implies an origin that covers an altitude interval from the low-altitude ionosphere to the plasmasphere, and (3) terrestrial ions are moving upward along magnetic field lines, at times forming narrow collimated beams, but frequently also as broad beams. Over time, the ion beams are expected to gradually become isotropised as a result of wave-particle interaction, eventually taking the form of isotropic drifting sub-keV ion signatures. We argue that the sub-keV energy-time dispersed signatures originate from field-aligned terrestrial ion energising and outflow, which may occur at all local times and persist also during quiet times.
Highlights
The ring current consists of particles trapped in the geomagnetic field
We show that: (1) outflows of terrestrial sub-keV ions are supplied to the ring current during quiet geomagnetic conditions; (2) the composition of the outflow implies an origin that covers an altitude interval from the low-altitude ionosphere to the plasmasphere, and (3) terrestrial ions are moving upward along magnetic field lines, at times forming narrow collimated beams, but frequently as broad beams
We observed three events during relatively quiet geomagnetic periods and different magnetic local time periods, 14.0–13.2 magnetic local times (MLT) at L∼17.4–5.1, 7.1–6.9 MLT at L∼6.5–4.5 and 0.6–0.7 MLT at L∼5.9–4.6, i.e., two events were observed on the dayside and one event on the nightside
Summary
The ring current consists of particles trapped in the geomagnetic field. The ring current varies with time and causes a slight decrease of the Earth’s surface magnetic field during magnetic storms (Williams, 1983). Yamauchi et al (1996) observed energy-latitude dispersive structures of trapped ions in the sub-keV energy range inside the ring current region. Of 2–3) than in the simulation by Ebihara et al (2001) with and H+ ions originate, we studied in more detail CIS-data nearly zero lag time from substorm onset to the appearance of the wedge-like structure in the morning sector (Yamauchi 23 from three Cluster orbits (two dayside and one nightside, see Fig. 2) during relatively quiet times when the convection and Lundin, 2006). Yamauchi and Lundin (2006) found that the wedge- investigated possible plasma source regions for the sub-keV like structures seen in the evening sector have travelled from ring current ions by analysing in more detail the pitch-angle www.ann-geophys.net/27/839/2009/.
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