Suprathermal Fe in the Earth's Plasma Environment: Cluster RAPID Observations

Abstract

Baryonic 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.

Heavy Metal and Rock in Space: Cluster RAPID Observations of Fe and Si

“…We now investigate how Si and Fe are distributed across the regions of space traversed by Cluster over the last 19 years. As in Christon et al (2017) and Haaland et al (2020), we define regions and plasma regimes in space by spacecraft location rather than local plasma properties. Table 2 shows the definition of the regions and Figure 4 shows spectra of Si and Fe as well as the locations of individual measurements for each of these regions.…”

“…This product, termed Direct Events (DE), was utilized in Haaland et al. (2020) to study the distribution of energetic Fe in geospace.…”

“…A prioritization scheme ensures that information about ions with high masses are transmitted first, while ions mapping to E-TOF combinations corresponding to hydrogen, helium, and oxygen come later in the output queue. This product, termed Direct Events (DE), was utilized in Haaland et al (2020) to study the distribution of energetic Fe in geospace.…”

“…Energy thresholds for the five species and their eight energy channels of the RMTRX product are shown in Table 1. We primarily focus the spatial distribution and long term variations of Si ("Rock") and Fe ("Heavy Metal") from RMTRX, but we will also dwell on abundance ratios between species, e.g., [Si] As in Haaland et al (2020), we internally store the elements of the RMTRX in a table in a relational database. The time-stamp of each observation (i.e., center time of the 64 spin accumulation period) serves as a key to join the RAPID composition data with other tables, for example, spacecraft position, solar wind measurements as well as geomagnetic activity indices obtained from other data sources.…”

“…Occurrence rates for Si and Fe are percentages of observation time where count rates are above our noise threshold. Definition of regions is identical to Haaland et al (2020). ( * ) See notes about the possible presence of molecular ions in Section 3.2.3.…”