Survey of caveats in low-energy particle measurements: Ulysses/HI-SCALE and ACE/EPAM Instruments

Highlights

• We analyze measurements recorded by the HI-SCALE and EPAM instruments.

• We achieve a comprehensive quality-assessment on low-energy particle measurements.

• We depict a detailed survey of periods with contamination in low-energy particles.

• We present two types of contamination: solar X-ray and cross-talk contamination.

Abstract

“Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies” (HI-SCALE) onboard the ULYSSES spacecraft, and “Electron, Proton, and Alpha Monitor” (EPAM) onboard the ACE spacecraft, are very similar instruments and were designed to make measurements of ions and electrons over a broad range of energy and intensity. The ions (E_i≥50 keV) and electrons (E_e≥30 keV) are detected by five separate solid-state detector telescopes, oriented to provide essentially complete pitch–angle coverage from the spinning spacecraft. In this work, through detailed data-analysis (i) we perform a comprehensive quality assessment on, and (ii) depict a detailed survey of day-of-year (DOYs) with existing contamination in the high-resolution low-energy particle measurements recorded by the HI-SCALE and EPAM instruments, throughout the years 1991–2009 (i.e. during the total Ulysses mission lifetime) and 1997–2011, respectively. Two major types of contamination were revealed in our analysis: (i) "solar X-ray contamination" due to saturation (by solar photons) in two (of the four) detector sectors, during the telescope's direct exposure to the solar disk, and (ii) "cross-talk contamination" due to electrons being recorded as ions and vice versa. The results presented in this work will prove to be valuable to future users of these unique data sets and to designers of similar future instruments, supported by SEP validation and propagation models.

Introduction

The HI-SCALE and EPAM instruments, onboard the ULYSSES and ACE spacecraft respectively, aside from some minor issues, are identical instruments, and were designed to make measurements of ions (E_i≥50 keV) and electrons (E_e≥30 keV) over a broad range of energy and intensity. The physical parameters (energy passbands, geometric factors, etc.) are nominally identical, while the main difference between the two instruments in terms of data reduction is in the background rate. The nominal background rates for the HI-SCALE detectors are generally greater than those for EPAM and differ from detector head to detector head as a result of their spatial relation to the radiothermal generators (RTG), whereas the EPAM detectors all have the same background rates (Patterson, 2002, Haggerty et al., 2006).

Both instruments were designed and flown with the same goal in mind, investigating the composition of the interplanetary medium. Nevertheless, they are of quite outdated technology and when sent to space there was no method to perfectly simulate and calibrate them (Morgado, 2009).

For example, EPAM measurements include ion intensities for several energy channels. Almost all of the ions measured are protons, but other less abundant species (mostly He²⁺ at the few percent level) are also present. This ion contamination or the occasional electron contamination have been referred to in the scientific literature widely (like Keeney, 1999; Lario et al., 2005; Malandraki et al., 2009; Simnett, 2006; Vandegriff et al., 2005; Costafreda, 2008). In particular, the observations of high energy particles by ACE/EPAM may be affected by saturation effects, especially for the lower energy channels during large Solar Energetic Particle (SEP) events (Shen et al., 2008). Unfortunately, the LEFS60 telescope and the DE30 detector are not calibrated. This makes it difficult to use data from the two electron telescopes in a joint quantitative analysis (Costafreda, 2008). Moreover, a known malfunction of the LEFS150 telescope has been reported since almost the beginning of the ACE mission, while some troublesome solar X-ray influences in the LEMS30 detector are present at times. For channels P1 and P2 (and also for channels P3–P8, in several cases) the sectors on the sunward side of the rotation are omitted from the spin average as these can be contaminated by solar X-rays (http://HI-SCALE.ftecs.com/appendix%2012/HI-SCALE.html). Furthermore, contamination arises from electrons recorded as ions (cross-talk) and vice versa (Lario et al., 2005).

In this work, through energetic particle data analysis, we have performed a comprehensive data quality assessment on high-resolution low-energy particle measurements recorded by the HI-SCALE and EPAM instruments. This has resulted in a detailed survey of day-of-years (DOYs) with existing contamination in these measurements covering the years 1991–2009 (total Ulysses-mission lifetime) and 1997–2011 (ACE). The missions and detectors used in this study are described in Section 2. In the following section results of the data quality assessment are presented. The paper ends with a conclusion and discussion considering how these results can benefit future users of these unique data sets as well as designers of similar future instruments, while being supported by SEP validation and propagation models (e.g. Ng et al., 2003; Zank et al., 2007; Verkhoglyadova et al., 2010; Marsh et al., 2013, 2014).