The calibration of the Cassini–Huygens CAPS Electron Spectrometer

doi:10.1016/j.pss.2009.11.008

Planetary and Space Science

Volume 58, Issue 3, February 2010, Pages 427-436

https://doi.org/10.1016/j.pss.2009.11.008 Get rights and content

Abstract

We present the two-stage method used to calibrate the electron spectrometer (ELS), part of the plasma spectrometer (CAPS) on board the Cassini spacecraft currently in orbit around Saturn. The CAPS-ELS is a top-hat electrostatic analyser designed to measure electron fluxes between 0.5 eV and 26 keV. The on-ground calibration method described here includes the production of photoelectrons, which are energised and passed into the CAPS-ELS in a purpose designed calibration facility. Knowledge of the intensity of these incident electrons and the subsequent instrument output provides an on-ground calibrated geometric factor. Comparative studies of physical quantities such as plasma density and electron differential flux calculated using on-ground calibration factor with the quantities deduced from the wave experiment and high energy electron detector provide in-flight calibration. The results of this are presented together with a comparison of the experimentally calibrated values with simulated calibration values.

Introduction

The Cassini–Huygens spacecraft has now concluded its primary mission lifetime and has begun the extended mission. It went into orbit around Saturn on 1 July 2004 and over the last five years has returned an unparalleled dataset covering the planet's space environment. The suite of instruments carried by the spacecraft has provided a unique, in-depth exploration of Saturn's plasma and fields environment. In particular, using the magnetospheric and plasma science instruments it has been possible to study the physical and chemical processes in the different regions of Saturn's complex and dynamic environment. We have been able to investigate the strong interactions between the different components of the planetary system: the planet itself, the rings, numerous satellites (the icy moons and Titan) and various dust, neutral, and plasma populations (Blanc et al., 2002, André et al., 2008).

The Electron Spectrometer (ELS), together with the Ion Mass Spectrometer (IMS) and the Ion Beam Spectrometer (IBS), collectively make up the Cassini Plasma Spectrometer (CAPS) (Young et al., 2004). Electrons are good tracers of magnetic field line topology due to their large thermal velocity and they provide an indication of the total plasma density when the ion instruments are unable to view the ion distributions. Measurements of electron spectra and pitch angle distributions reveal acceleration and absorption processes at work in the magnetosphere as well as the plasma environments of icy moons, including Titan and Enceladus. These measurements are key to the study of wave-particle interactions. The CAPS-ELS is designed to study the low-energy electron populations observed in situ in Saturn's magnetosphere, from the micro-physical to the global scale (Young et al., 2004, Linder et al., 1998). Such studies require the characterisation of the electron velocity distributions and the derivation of various bulk plasma parameters, namely density and temperature. There are many papers and textbooks giving details of this process. Lewis et al. (2008) and Arridge et al. (2009) give details particular to the CAPS-ELS. In order to use the CAPS-ELS instrument to obtain these data products, the returned electron counts must be converted to a physical quantity, such as phase space density which requires the instrument's geometric factor. This geometric factor, as the name suggests, is dependent on the geometry of the instrument, and can be thought of as the relationship between the number of electrons entering the instrument and the number of electrons being measured by the instrument. As well as the geometry of the instrument, the geometric factor defined in this way also includes the effects of the instrument's intrinsic efficiencies.

The purpose of this paper is to describe in full the calibration of the CAPS-ELS. The paper is organised in the following way. In Section 2, we provide a brief description of the CAPS-ELS instrument. Section 3 gives a detailed account of the laboratory calibration procedure and the calculation of the terms required to calculate the geometric factor. In Section 4, we compare the densities obtained using the geometric factor derived here with the CAPS-ELS data and the total electron densities determined from measurements of the upper hybrid frequency line obtained from the Radio and Plasma Wave Science data (RPWS) (Gurnett et al., 2004). We then use these comparisons to uniformly scale the geometric factor and so complete the calibration. In Section 5 we compare our results with those from the Low-Energy Magnetospheric Measurement System (LEMMS) of the Magnetospheric Imaging Instrument (MIMI) (Krimigis et al., 2004). This instrument measures electrons over the energy range 20 keV to 5 MeV. Since the CAPS-ELS energy range is 0.58 eV to 26 keV there is an overlap at the higher end of the CAPS-ELS energy range and the lower end of the MIMI-LEMMS energy range. Our results are also compared with those from the Voyager electron plasma instruments (Maurice et al., 1996). We complete our discussion by comparing the experimentally derived geometric factor with a computer simulated geometric factor. We then conclude that the CAPS-ELS provides reliable plasma parameter data that can be used in all manner of scientific studies with a high level of confidence. Together with Linder et al. (1998), Lewis et al. (2008), and Arridge et al. (2009), this paper will serve as a reference point for the CAPS-ELS data products and calibration procedures.

Section snippets

The CAPS-ELS instrument

A full description of the CAPS-ELS instrument is given by Linder et al. (1998) and Young et al. (2004). However, for completeness a brief description is given here. The CAPS-ELS is a hemispherical top-hat electrostatic analyser (Young et al., 2004, Linder et al., 1998, Coates et al., 1992). Electrons enter a baffled collimator structure and undergo electrostatic analysis between two concentric spherical plates. The outer plate is grounded and a variable, positive, high voltage is applied to the

The geometric factor

To obtain plasma bulk parameters such as electron density, velocity and pressure from top-hat analysers, the geometric factor of the instrument must first be obtained. The derivation and explanation of this quantity can be found in many text books and papers (e.g. Wurz et al., 2007, Marshall et al., 1986, Kessel et al., 1989). The standard equation for the geometric factor, G, in summation form for an electrostatic top-hat analyser is $G_{ijk} = \sum_{i} \sum_{j} \sum_{k} \frac{N_{ijk}}{t_{a} η} (\frac{δ E}{E_{0}}) Δ θ Δ ϕ,$ where ijk subscripts refer to the

In-flight calibration

In order to test that the derived absolute geometric factor values are correct we calculate an electron density value using the 3d method presented in Lewis et al. (2008). We then compare this value with the electron densities derived by the Radio Plasma Wave Science (RPWS) instrument on board Cassini (Gurnett et al., 2004).

RPWS electron density values derived from the upper hybrid resonance frequency provide the total electron density, (Persoon et al., 2005). The RPWS, however, can only make

Comparison with MIMI-LEMMS

The CAPS-ELS has a logarithmically spaced energy table ranging between 0.58 eV and 26 keV. The MIMI-LEMMS instrument also on board Cassini (Krimigis et al., 2004) records electrons between energies of 26 keV and 5 MeV. As the two instruments overlap at their upper and lower energies we can make a useful comparison between the two instruments. Differential number flux calculated by the two instruments is shown in Fig. 10. Here we can see that the values show a good agreement at the overlap energy of

Conclusion

Previous studies using calibrated data from CAPS-ELS have used an on-ground calibrated geometric factor. Such studies have involved the analysis of moments (Lewis et al., 2008) and spectra (presented in calibrated physical units). The reliability of the conclusions of these studies is limited by the accuracy of the geometric factor used to convert measured counts/s into differential fluxes and phase space densities. Previous comparative studies of the differential (number) fluxes of electrons

Acknowledgements

The authors wish to acknowledge STFC for supporting the work carried out at MSSL-UCL. An acknowledgment is also made to, N. Krupp, T. Armstrong, J. Vandegriff, S. Taherion from the Cassini MIMI team for their work in calibrating the MIMI instrument. G.H.Jones is supported by an STFC Advanced Fellowship. The authors also wish to acknowledge A.J. Fazakerley and I. Dandouras for useful comments and discussions.

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The calibration of the Cassini–Huygens CAPS Electron Spectrometer

Abstract

Introduction

Section snippets

The CAPS-ELS instrument

The geometric factor

In-flight calibration

Comparison with MIMI-LEMMS

Conclusion

Acknowledgements

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Science

Plasma observations near Saturn: initial results from Voyager 1

Science

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