Abstract
AbstractBaryonic matter in geospace is almost exclusively in a plasma state, with protons (H ) and to some extent ionized helium (He) and oxygen (O) being the dominant ion species. But also other heavier ion species and even molecular ions are present in geospace. The Research with Adaptive Particle Imaging Detectors (RAPID) on board the Cluster satellites can identify and characterize some of these ions by utilizing their measured time of flight and energy. Usually, the measurements are then assigned into three discrete species channels; protons (H ), helium (He), and a common channel for carbon, nitrogen, and oxygen (CNO), each with flux, energy, and angular information. But RAPID also has a Direct Event (DE) diagnostic mode in which the full time of flight and energy information for a limited number of incident particles are stored. With knowledge about energy losses in the various detector parts, it is then possible to derive the atomic mass of the incident particle. In this paper we report on results from a study of Cluster DE events during the years 2001–2018, with a particular emphasis of iron (Fe) ions. We show that suprathermal Fe ions can be found all over geospace covered by Cluster, and that the time variation is consistent with modulation by geomagnetic disturbances and solar activity. We do not find any clear correlations between detection of suprathermal Fe and meteor showers or sputtering off the moon.
Highlights
In geospace, protons and to some extent helium (He) and oxygen (O) ions are the dominant ion species and most relevant for the electrodynamics of the magnetosphere and ionosphere
We show that suprathermal Fe ions can be found all over geospace covered by Cluster, and that the time variation is consistent with modulation by geomagnetic disturbances and solar activity
We present Cluster observations of iron (Fe), which has an atomic mass around 56 atomic mass units (AMU) in its most frequently occurring isotope
Summary
Protons and to some extent helium (He) and oxygen (O) ions are the dominant ion species and most relevant for the electrodynamics of the magnetosphere and ionosphere. Observations have shown that Fe of solar wind origin can enter the magnetosphere directly via the cusp (Fritz et al, 2003) and mix with any preexisting Fe. Outflow from the terrestrial ionosphere is another important source of ions in geospace (Shelley et al, 1972; Yau & Andre, 1997). Whereas the presence of Fe ions in the atmosphere is established, there are few studies on how these ions can be extracted from the mesospheric layers through the upper ionosphere into space Due to their mass, Fe is stronger gravitational bound, and many of the classical outflow mechanisms, for example, acceleration by ambient electric fields, transverse heating, and subsequent increase in the mirror force or other thermal forces (Yau & Andre, 1997), are less effective.
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