Relatively few studies have focused on the cold electron and cold ion populations of the magnetosphere due to difficulties associated with obtaining measurements using instruments mounted on charged spacecraft. The cold particle populations are defined as those with total energy approximately <100 eV which is an energy range for which measurements are difficult (regardless of species), but which often make up the bulk of the plasma density. These populations have a number of known impacts on the structure and dynamics of the magnetosphere (Delzanno et al., 2021) but to date have not yet been measured extensively. In sunlight, spacecraft in the magnetosphere typically float from a few volts positive to several tens of volts positive (Thomsen, et al., 2007), preventing the bulk of the cold ion spectrum from reaching spacecraft instruments (Genestreti, et al., 2017). This effectively limits the energy measurement range to just the ion population with energy above the spacecraft potential. Simultaneously, sunlit spacecraft surfaces emit a constant flux of low-energy electrons (DeForest 1972) which contaminate or completely mask instrument measurements of ambient, low-energy magnetospheric electrons (Scime et al., 1994; Gershman, et al., 2017). These difficulties and subsequent scarcity of measurements have been well described in the literature (Andre and Cully 2012; Lee and Angelopoulos 2014; Haaland, et al., 2015; Delzanno, et al., 2021) leading to the colloquial term for low-energy magnetospheric ions as the “hidden populations” (Olsen 1982). This paper seeks to describe the instruments and techniques that have been used to date and present options for future missions.

There have been many theories, studies, and debates regarding the processes of magnetosphere-ionosphere coupling (MIC), currently focused on the location and driving conditions for magnetic reconnection and how the ionosphere regulates reconnection and convection (Lotko 2007; Welling, et al., 2015). An improved understanding of the complex plasma dynamics is needed to answer these long-standing mysteries and requires appropriate instrumentation for obtaining in-situ observations. Low-energy ions originating from the ionosphere dominate the magnetosphere 50%–70% of the time (Andre and Cully 2012). To accurately model the impact of various plasma sources on magnetospheric composition, distributed in-situ measurements of cold ion and cold electron populations are needed from the outflow regions through the magnetosphere. As described in detail by Delzanno et al. (2021), the primary sources of plasma in the Earth’s magnetosphere are attributed to ion and electron outflow at the low and high latitude ionosphere in addition to contributions from the solar wind. The outflow populations can get strongly energized as they journey through the magnetosphere and contribute to the warm plasma cloak, ring current and plasma sheet (C. R. Chappell 1982; Chappell et al., 2008).

Since the initial observations of heavy ions in the magnetosphere by Shelley et al. (1972), the dynamics leading to the ionospheric outflow of O⁺ ions and their impact on the evolution of the magnetosphere-ionosphere system have been the subject of numerous studies. At low latitudes, charged particles travel along closed field lines and create the cold (<eV) protons and electrons that make up the plasmasphere (Carpenter 1962; Lemaire, et al., 1998; Kotova 2007). Magnetospheric cold electron sources include outflow of low-energy photoelectrons from the sunlit upper atmosphere (Kitamura, et al., 2012). This creates a spatial separation between the light electrons and heavier ions resulting in the ambipolar electric field which aids in the acceleration of ions upwards in the polar wind (Khazanov et al., 1997; Haaland, et al., 2015; Glocer et al., 2017). The high latitude auroral outflow is composed of hydrogen and other heavy ions (O⁺, He⁺, N⁺, N₂ ⁺, and NO⁺) which flow from the high-latitude ionosphere along or near open magnetic field lines and into the low-pressure magnetosphere (Moore et al., 2005a; Ilie and Liehohn 2016). Outflow is commonly observed at high latitudes (Moore, et al., 1999a; Andre 2015) and is generally divided into three categories: 1) cold-temperature ion flows with energies of a few eV in which all ions acquire a bulk flow velocity such as the polar wind and auroral bulk O⁺ up-flow from the topside auroral and polar-cap ionosphere; 2) ions with suprathermal energies above the ionosphere (Eliasson, et al., 1994) or with higher energies at higher altitude (Möbius, et al., 1998) and 3) heated ion outflows in the cusp and auroral zone in which a fraction of the ions have been energized to much greater energies including transversely accelerated ions, upwelling ions, ion conics, and ion beams (Yau and Andre 1997; Welling, et al., 2015). The low-energy outflowing ions provide cold plasma filling to the plasmasphere (Welling, et al., 2015) and cold supersonic outflow to the inner magnetosphere and magnetotail such as from the polar wind (Su, et al., 1998; Engwall, et al., 2006a; Engwall, et al., 2009). A depiction of the ion outflow process as a function of ion energy category and destination as described by Yamauchi (2019) is shown in Figure 1. Consequently, the outflow flux of ions and electrons are highly dynamic and vary with the solar magnetospheric activity, including solar extreme ultra-violet (EUV) radiation, interplanetary magnetic fields, solar wind pressure, geomagnetic indices, precipitating electron fluxes and Poynting fluxes into the ionosphere (Yau, Shelley, et al., 1985; Barghouthi et al., 1998; Tam et al., 1998; Moore et al., 1999b; Barakat and Schunk 2001; Cully, et al., 2003; Strangeway, et al., 2005; Kitamura et al., 2011; Delzanno, et al., 2021).

The solar wind is composed of predominantly hydrogen, alpha particles, and high charge-state heavy ions (Bame, Hundhausen, et al., 1968; Geiss et al., 1995; Zurbuchen et al., 2002). While not traditionally considered a source of cold ions or cold electrons, the solar wind does contribute significantly to the overall magnetospheric plasma populations. The solar wind protons and electrons travel outward at ∼400 km/s with temperature on the order of 10 eV (Feldman, et al., 1974; McComas et al., 1998). Upon crossing the bow shock the solar wind temperature in the magnetosheath increases to ∼20–60 eV for electrons and ∼100 eV–1 keV for ions on the dayside (Wang, et al., 2012; Delzanno, et al., 2021). As the magnetosheath plasma moves to the nightside the temperature decreases to ∼10–30 eV for electrons and ∼30–300 eV for ions (Wang, et al., 2012; Dimmock and Nykyri 2013). This results in a total energy of the magnetosheath ∼1 keV for ions while the electrons remain relatively cold (Delzanno, et al., 2021).

As a result of the ionospheric outflow and incident solar wind, the magnetospheric environment consists of multiple ion and electron populations with a broad range of energies, from sub-eV particles of the ionosphere to the relativistic energies of the radiation belts (Singh and Horowitz 1992; Zhao, et al., 2015; Genestreti, et al., 2017). Traditional ion mass spectrometer instruments are designed to detect the hot tenuous magnetospheric ion species and are typically not as effective at measuring cold, low energy (<100 eV) plasmas. This is due to the trade-off in the instrument design’s objective between having a large dynamic energy range from a few eV to tens of keV, versus focusing on low-energy sensitivity.

The amount of coupling of the solar wind to the Earth’s magnetosphere-ionosphere system is controlled by magnetic field-line reconnection between the solar wind and the magnetosphere, which is a function of local plasma parameters at the reconnection site (Cassak and Shay 2007; Borovsky et al., 2008; Borovsky et al., 2008). The magnetic field strength and mass density of the magnetospheric and magnetopause plasma populations at the magnetopause determine the reconnection rate between the solar wind and the magnetosphere. Ionospheric outflows, which can control the magnetospheric mass density, can have a direct role in controlling the dayside reconnection rate; therefore ionospheric outflows potentially have a significant role in controlling solar wind/magnetosphere coupling through mass loading (Borovsky et al., 2013; Fuselier et al., 2019a; Toledo-Redondo et al., 2021). Plasmaspheric plumes and the warm plasma cloak (whose energy spectrum partially overlaps with our definition of cold plasma) also affect solar wind/magnetospheric coupling during geomagnetically active times. Mass loading could also operate in the magnetotail although to what extent particles remains cold as they cross the tail remains an open question [see the discussion in Delzanno et al. (2021)].

The diverse magnetospheric particle populations are co-located and interact by means of plasma waves. For instance, acceleration and loss of radiation belt electrons is thought to be due to wave-particle interactions, with the relevant waves modulated by lower energy plasma particles (Summers et al., 2007; Reeves et al., 2009). In addition to mass loading and dayside magnetopause reconnection, the cold magnetospheric plasma impact the generation of whistler and electromagnetic ion cyclotron (EMIC) waves by the cyclotron instability (de Soria-Santacruz et al., 2013) and their subsequent propagation in the inhomogeneous magnetospheric plasma. The cold particle populations affect the frequencies and amplitudes of ultra-low-frequency (ULF) waves (Fraser, et al., 2005), whistler waves (Thorne 2010), whistler-mode hiss waves (Dunckel and Helliwell 1969; Russell et al., 1969; Bortnik et al., 2008; Hartley, et al., 2018), and EMIC waves (Blum et al., 2009; Chen et al., 2011; Blum and Breneman 2019), which control scattering and energization rates of the higher-energy populations, impacting the dynamics of the plasma sheet, ring current and radiation belts. Note also that accurate knowledge of the ion composition of the cold plasma in the magnetosphere is critical to determine the properties of EMIC waves. The mass density of cold ions also affects the magnetopause stability to Kelvin-Helmholtz waves and the transport of solar wind into the magnetosphere (Melander and Parks 1981; Hasegawa, et al., 2004).

To date, the majority of cold plasma research efforts have focused on the cold ion populations leaving a considerable gap in the understanding of cold electron populations in the magnetosphere. Cold electrons are considered to play an important role in wave-particle interactions such as chorus and hiss waves. Recent work by Roytershteyn and Delzanno (2021) suggests that the presence of cold electron population introduces coupling of the whistler modes to short-wavelength, oblique, electrostatic instabilities. This can subsequently lead to cold-electron heating and damping of the primary whistler waves. EMIC waves can also interact with cold electrons resulting in cold-electron heating (Zhou, et al., 2013; Yuan, et al., 2014). This mechanism is thought to be responsible for stable auroral arcs (Chandra et al., 1971; Chandra et al., 1972) through the enhancement of ionospheric electron temperature. A proposed mechanism for the subauroral heating is energy transfer from the ring current to magnetospheric electrons and the downward heat conduction into the ionosphere (Brace et al., 1967, Kozyra, et al., 1986, Khazanov et al., 1992).

The lack of accurate measurements of low-energy or “cold” ion and electron populations through the magnetosphere makes closure of these science questions extremely difficult if not impossible. Therefore, to significantly advance our understanding of magnetospheric plasma physics, the full phase-space densities of the cold plasma population must be thoroughly measured. This will require innovations in plasma spectrometers and associated techniques required to obtain high-fidelity measurements of the cold ion and electron populations in the magnetosphere.

One of the primary challenges related to making cold ion and cold electron measurements in the magnetosphere is due to spacecraft charging. In sunlight, the spacecraft floating potential is typically a few volts or less in the plasmasphere and can be tens of volts or more in the tenuous outer magnetosphere (Thomsen, et al., 2007; Ferguson et al., 2015; Sarno-Smith, et al., 2016). The potential difference between the charged spacecraft and the ambient plasma will result in either an increase or decrease to the particle energy and total density measured by electrostatic analyzers depending on the charge of the incident particle. For example, ions impinging on negatively charged spacecraft surfaces will be accelerated and experience an increase in measured energy, while electrons will experience a decrease in measured energy. When the polarity of the spacecraft potential and the particle charge are the same, the resulting electrostatic repulsion results in truncation of the energy distributions at low energies and the resulting loss in density. A number of studies have examined the deleterious effects of spacecraft charging on charged particle measurements when comparing particle trajectories measured at the spacecraft with the actual particle distributions in the ambient plasma (Sojka et al., 1984, W. C; Knudsen 1966; Parker and Whipple 1970; Toledo-Redondo et al., 2019).

Measurements of low-energy electrons present additional challenges due to a combination of spacecraft charging effects and the presence of photoelectrons continuously emitted from sunlit spacecraft surfaces (DeForest 1972). The photoelectron population is emitted at ∼ eV energy and can contaminate the low-energy channels of instruments measuring ionospheric and magnetospheric electrons (Mozer, et al., 2017). Secondary electrons emitted by impact of high energy ambient electrons and ions create similar problems. Spacecraft with positive potentials accelerate both ambient low-energy electrons and photoelectrons alike into detector apertures resulting in an indiscernible measured distribution. The difficulty associated with deconvolving the two electron distributions has led to a lack of observations and understanding of the cold-electron impact in the magnetosphere (Gershman, et al., 2017).

Multiple instruments have made partial measurements of the cold ion populations in the magnetosphere with the difficulties discussed in Section 2. Traditional ion mass spectrometer instruments are designed to detect the hot tenuous magnetospheric ion species and are typically not as effective at measuring cold low-energy (<100 eV) plasmas. This is due to the trade-off in instrument design between having a large dynamic energy range from a few eV to tens of keV, versus focusing on low-energy sensitivity. Traditionally, space plasma instruments have focused on measuring large dynamic ranges due to limited spacecraft resources and mission opportunities.

An overview of ion mass spectrometers is captured (see Table 1) by briefly reviewing ion composition studies of the terrestrial magnetosphere by the Hot Plasma Composition Analyzers (HPCA) (Young, Burch, et al., 2016), the Helium, Oxygen, Proton, and Electron (HOPE) (Funsten, et al., 2013) mass spectrometer, the Cluster Ion Spectrometry (CIS) Experiment (Reme, et al., 1997), and the Retarding Ion Mass Spectrometer (RIMS) (Chappell, et al., 1981). These ion composition instruments and subsequent observations have provided the steppingstone for some of the most exciting magnetospheric science discoveries of the modern space science era. For example, the data from these instruments have been used to characterize the equatorial plasma environment and ion composition (Fernandes, et al., 2017; Yue, et al., 2018), evolution of the ring current during geomagnetic storms (Zhao, et al., 2015), initial observations of the “oxygen plasma torus” (Chappell 1982), magnetospheric ion influence on magnetic reconnection at the duskside and dayside magnetopause (Fuselier et al., 2016; Fuselier et al., 2019b), and the confirmation that wave-particle interactions can play a key role in acceleration within the radiation belts (Thorne 2010; Turner, et al., 2014; Baker 2021). These examples are a very limited sampling of the literature and by no means an exhaustive discussion of the total scientific advancements enabled by these instruments. For context, the Cluster mission alone has generated more than 2,587 refereed publications and 142 PhD/thesis (Escoubet, et al., 2020). However, while the HOPE, CIS/CODIF, and HCPA ion mass spectrometers have provided invaluable insights into the plasma source and transport within the terrestrial magnetosphere, these instruments have only made limited observations of the cold ions which often constitute the bulk of the magnetospheric plasma density.

The difficulties associated with measuring low-energy magnetospheric ions have been addressed with limited success, i.e., measurements of partial distributions and limited time-series data sets, by earlier missions through the use of active spacecraft control (Riedler, et al., 1997; Torkar, et al., 2016) and electrostatic biasing of entrance apertures (Chappell, et al., 1981) or the entire instrument (Knudsen, et al., 2015). These active spacecraft potential controls reduced the spacecraft potential, but did not reduce the potential to zero for all measurements, resulting in portions of the distributions unmeasured.

The RIMS instrument on-board the Dynamics Explorer 1 (DE-1) mission has provided some of the earliest and most robust observations of the cold ion populations in the magnetosphere to date. The sensor design utilized a front aperture capable of being commanded to any of four values as a bias for a non-zero spacecraft potential (Chappell, et al., 1981). This bias created an electrostatic potential to steer low-energy ions through the spacecraft potential sheath to the detector aperture. The sensor had the ability to resolve H⁺, He⁺, O⁺, and O⁺⁺ within a narrow energy range of 0–50 eV. This allowed for the initial observations of the O⁺ torus, warm plasma cloak, thermal composition of the plasmapause, and the ionospheric outflow or polar wind (Chappell, et al., 1981, C. R; Chappell 1982, C. R; Chappell 1988, Chappell et al., 2008). However, due to a lack of spacecraft potential measurements the ion densities derived from the RIMS observations have considerable uncertainties. Initial density values were arrived at through an iterative method involving estimating the spacecraft potential as a function of geocentric distance and the electron density profile (Persoon et al., 1983), applying the spacecraft potential estimates to an empirical relation (Comfort et al., 1988), comparing to modeled total density (Ho and Horwitz 1993), and iterating until the spacecraft potential and density converged (Ho and Horwitz 1994). The temperatures obtained from the instrument measurements consist of a high degree of uncertainty due to the energy spacing of the RPA steps being larger than the thermal energies (Ho and Horwitz 1994). The temperature is also derived using a thin sheath approximation for the spacecraft potential sheath (Comfort et al., 1985) which has been shown to introduce significant error (W. C. Knudsen 1966; Parker and Whipple 1970; Sojka et al., 1984; Marchand et al., 2010; Lavraud and Larson 2016). Lastly, the radial retarding potential analyzer (RPA) head of the RIMS failed, limiting the DE-1 data set (Ho and Horwitz 1994). Although there are some limitations of the RIMS instrument, it has provided a wealth of data and contributed to our understanding of the ionosphere-magnetosphere system and provided a crucial steppingstone for cold ion and cold electron observations in the magnetosphere.

A statistical study of the spacecraft potential on Van Allen Probes demonstrated that the satellites charged to between 0 and 10 V for much of the mission (Sarno-Smith, et al., 2016; Denton, et al., 2017). Consequently, the low-energy portion of the magnetospheric cold ion population has not been measured by the HOPE instrument due to the positive spacecraft potential. The CIS/CODIF instrument demonstrated the ability to measure ∼0–25 eV ions (O⁺, H⁺, He⁺) while emitting indium ions to control the spacecraft potential (Reme, et al., 1997; Riedler, et al., 1997). However, while the active spacecraft control used for Cluster and the Magnetospheric Multiscale (MMS) missions (Burch, et al., 2016) does reduce the spacecraft potential it does not eliminate it. The potential is reduced to ∼1–2 V, thus leaving a portion of the cold ion energy distribution unobserved. ∼1 The Thermal Ion Dynamics Experiment (TIDE) on POLAR was designed to measure the low-energy polar winds at very high altitudes. The TIDE ion mass spectrometer was used in combination with the Plasma Source Instrument (PSI) to measure to overcome the effects of spacecraft potential on low-energy ion measurements (Moore et al., 1997). The PSI could actively control the spacecraft potential using emitted xenon ions to reduce the spacecraft potential to 1.8–2.2 V (Su, et al., 1998), resulting in a portion of the cold ion population remaining unmeasured. Unfortunately, TIDE lost its mass resolution capability in 1996, then returning total ion results. A critical distinction must also be made regarding the capabilities of the instruments listed in Table 1. The HOPE sensor is the only instrument that was designed to be insensitive to energetic particle contamination background counts as a result of penetrating radiation and capable of making measurements throughout the inner magnetosphere. The HOPE data were also used to estimate the temperature of plasmaspheric ions using novel techniques required to extract the low-energy values while correcting for charged spacecraft moving through the ambient plasma (Genestreti, et al., 2017).

While these methods have allowed for limited measurements of low-energy ions, there were also impacts to the measured distributions as a result of the non-uniform sheath near the spacecraft (Scime et al., 1994; Marchand et al., 2010). These effects can be mitigated by operating the plasma instrument outside of the spacecraft sheath, at least for conditions when the local plasma Debye length is not too large, thus minimizing ion trajectory distortions due to asymmetric spacecraft potential sheaths. As has been done for ionospheric missions, this requires either a low-resource instrument or an engineering solution to mount a larger instrument to a boom. Several thermal ion and electron instruments have been successfully developed as low-resource payloads for sounding rocket missions (Frederick-Frost, et al., 2007; Fernandes et al., 2016; Fraunberger, et al., 2020) even utilizing aperture biasing techniques to accelerate target populations in the presence of spacecraft potential (Cohen, et al., 2016). As opposed to driving the spacecraft potential to near 0 V relative to the plasma potential, as done with ion beams or plasma contactors (Torkar et al., 2016), an alternate method of making these low-energy measurements would be to actively bias the spacecraft potential to focus on either cold ions or cold electrons. A sufficient negative potential will enable transport of low-energy ions to detector apertures and thus be observed. This is the case of the LANL-GEO spacecraft, which tend to charge negatively (Thomsen et al., 2013). Conversely, a large positive potential will enable low-energy electrons to be distinguishable from contaminating photoelectrons as discussed in Section 5.

More recently, the Swarm (Friis-Christensen, et al., 2008) and Magnetospheric Multiscale (Burch, et al., 2016) missions have utilized computational methods to assess spacecraft charging effects with to analyze and develop correction relations for instrument data interpretation (Marchand et al., 2010; Barrie, et al., 2019). Importantly, these studies have demonstrated that thicker plasma sheaths tend to induce more distortions in particle trajectories. Therefore, spacecraft sheath effects are most pronounced in the low density, high temperature plasmas of the magnetosphere and are less pronounced in the high density, low temperature plasmas of the ionosphere (Marchand et al., 2010). Additionally, both Marchand et al. (2010) and Barrie et al. (2019) observed that spacecraft geometry, most notably spacecraft booms, produce asymmetrical spacecraft sheaths which cause additional trajectory perturbations. These previous studies have demonstrated the clear need for spacecraft charging corrections for all low-energy particle measurements intended for operation in the ionosphere to magnetosphere.

It is well established that our understanding of the terrestrial magnetosphere can be significantly advanced by flying suitable ion mass spectrometers. Ion mass spectrometers need to be designed for the appropriate energy range, be able to mitigate the effects of penetrating radiation, and have sufficient mass resolution to resolve at a minimum (O⁺, H⁺, He⁺, N⁺). If not operated in conjunction with active spacecraft potential control, it would be beneficial for the instrument to be mounted on a biasable panel or of such low-resource constraints as to enable mounting on a boom outside of the spacecraft sheath. While the instruments described here and listed in Table 1 are advanced in both design and capability at the time of their build, the measurement sensitivity comes at a high cost that precludes them from missions operating under ever increasing fiscal constraints or for implementation as science payloads of large constellations required for ubiquitous multi-point space plasma measurements. Specifically, the ion mass spectrometers listed in Table 1 come with such high monetary costs that they cannot be used in large constellations without incurring an exorbitant mission cost.

While the cold plasma populations of the magnetosphere have been termed a “hidden population”, the cold electron populations have received even less attention when compared to the cold ion populations. This can largely be attributed to the challenges associated with the contaminating photoelectrons in combination with the effects of spacecraft charging. An overview of state-of-the-art plasma spectrometers for measuring electron populations in the magnetosphere is briefly presented in Table 2. These spectrometers are based on conventional spherical electrostatic analyzer (ESA) designs and have provided observations of electron energy spectra data in the magnetosphere to advance our understanding of Whistler anisotropy instabilities as the source of banded chorus (Fu, et al., 2014), the contribution of precipitating electrons to pulsating aurora (Miyosh, et al., 2015), and spacecraft charging effects (Thomsen et al., 2013; Sarno-Smith, et al., 2016). Again, these are but a few of the many scientific discoveries enabled by the electron spectrometers described in this section.

While the instruments in Table 2 have provided invaluable insights regarding the electron populations within the terrestrial magnetosphere, these instruments have made extremely limited observations of the cold electrons which often constitute the bulk of the magnetospheric plasma density. This is due in part to the effects of spacecraft potential but predominantly due to the difficulties associated with contamination of low-energy channels due to photoelectrons. The effects of spacecraft potential on the three-dimensional measurements of electron distributions in the solar wind were observed in the electron spectrometer data aboard the Ulysses spacecraft. It was observed that the electron distributions varied with the look angle of the instrument prompting the development of a correction factor to recover more gyrotropic distributions (Scime et al., 1994). The same effect is observed in the Solar Wind Electron Proton Alpha Monitor (SWEPAM) data which are subsequently corrected for spacecraft potential prior to dissemination to the broader space physics community (Koning, et al., 2006; Skoug, et al., 2006). It is important to note that the SWEPAM electron (SWEPAM-E) spectrometer is one of three nearly identical instruments flown, in chronological order, on the Ulysses (Bame et al., 1992), Advanced Composition Explorer (ACE) (McComas et al., 1998), and Genesis missions (Barraclough, et al., 2003). The initial observations of the Ulysses low-energy electron measurements near the spacecraft potential were identified as contaminated with photoelectrons (Bame et al., 1992) and subsequently the Genesis instrument avoided photoelectron contamination by setting the minimum measurable energy band at 61 eV (Barraclough, et al., 2003). The Los Alamos National Laboratory (LANL) Magnetospheric Plasma Analyzer (MPA) was also subject to background counts caused by photoelectrons believed to be created on nearby spacecraft surfaces within the instrument field-of-view (McComas et al., 1993). Further LANL/MPA observations concluded that the relatively high count rates at electron energies below 20 eV were caused by photoelectron contamination (McComas et al., 1993). The Plasma Electron And Current Experiment (PEACE) electron spectrometers (Johnstone, et al., 1997), on-board the Cluster satellites, also demonstrate that low-energy electron measurements (particularly below the spacecraft potential) are contaminated by photoelectrons and secondaries originating at the spacecraft (Chaston, et al., 2005). The Cluster satellites used active spacecraft potential control to reduce the potential to a few volts. However, the low-energy electron population below the controlled spacecraft potential could still not be differentiated from the contaminating background counts (Szita, et al., 2001).

More recent instruments include the HOPE on board the Van Allen Probes and the ESAs on board the Time History of Events and Macroscale Interactions during Substorms (THEMIS) and MMS satellites. The HOPE electron data go down to only 15 eV due to photoelectron contamination at the lowest energies (Goldstein et al., 2014). Note, the HOPE instrument is the only design capable of measuring the TOF of negatively charged particles, thus it can simultaneously measure electron and negative ion populations. During the in-flight calibration of the THEMIS ESA, the cold electron data with energies ∼ eΦ _SC were avoided due to the difficulty associated with separating the photoelectron and cold electron populations (McFadden, et al., 2008). The THEMIS ESA data with spacecraft potential and contaminating photoelectron population are shown in Figure 5 (Walsh, et al., 2020). The Dual Electron Spectrometers (DES), designed for electron measurements to support the Fast Plasma Investigation (FPI) on-board the MMS satellites (C. Pollock, et al., 2016), have also been shown to be susceptible to photoelectron contamination. Therefore, the DES plasma moments are determined by excluding the electron energy ranges below the spacecraft potential (Gershman, et al., 2017).

In summary, cold electron measurements have been severely hindered by the inability to resolve ambient magnetospheric electrons from photoelectrons and this holds back our full understanding of the magnetosphere. Indeed, cold electrons play an important role in a variety of critical magnetospheric processes, including setting the ambipolar electric field and taking part in heating processes that drives some of the ionospheric outflow, controlling aspects of wave-particle interactions and magnetosphere-ionosphere coupling (such as the structuring of the pulsating aurora) (Delzanno, et al., 2021). The following section will discuss viable techniques to allow for the measurement of the elusive cold electron population within the magnetosphere.

As discussed in Sections 3 and Section 4, there have been limited successes at measuring the cold ion populations but there are essentially no measurements of cold electrons. Outside of ionospheric sounding rocket missions and POLAR, there has yet to be a large magnetospheric spacecraft mission dedicated to measuring the source of both the low energy electron and ion particle populations robustly.

While there are challenges to making measurements of the cold magnetospheric ions and electrons, a number of viable techniques exist for cold ions that have been conducted for sounding rocket missions such as 1) mounting a low-resource (mass and volume) instrument on a boom (L. Brace 1998, C. J; Pollock, et al., 1996), and 2) biasing the electrostatic plasma spectrometer mounted to a boom or plate thus allowing for differential biasing of the instrument and/or spacecraft (Chappell, et al., 1981, C. J; Pollock, et al., 1996). Measuring cold ion bulk properties through the properties of the wake of the spacecraft (Engwall et al., 2009) has also been performed successfully, but this technique does not yield the full energy distributions.

In-situ cold ion and cold electron measurements are key to understand the dynamics of the Earth magnetosphere. There are two main challenges to overcome to achieve the measuring of the full particle distribution of the cold plasma populations in the magnetosphere: spacecraft potential and contamination of low-energy spacecraft-generated electrons in the particle detector.

Cold ion and electron measurements are both affected by the sheath electric field on and surrounding the spacecraft which alters the trajectories of the particle measured by a detector. Depending on the polarity at which the spacecraft charges with respect to the ambient plasma, which depends on the environment and spacecraft materials, some plasma species will be attracted and some other repelled. For the repelled species, depending on the mean energy compared to the spacecraft potential, the particle distribution will be partially measured. In this case the full distribution can be approximated by extrapolating from the measured, truncated distribution. This approach, however, could yield large uncertainties in the parameter estimation. As for the attracted species, the problem comes from accounting for the acceleration that measured particles experiment inside the spacecraft sheath. It is mandatory to correct for the spacecraft potential effect on the particle counts to accurately estimate the plasma moments that characterize the unperturbed magnetospheric cold plasma. Given the complex spacecraft geometries on which the structure of the sheath depends, this is a challenging task.

On the other hand, the contamination of low-energy spacecraft-generated electrons only affects the cold electron measurements. Because the cold electrons and the spacecraft-generated electrons can have similar energies (a few eV), it is very difficult to distinguish one species from the other in the detector particle count. Moreover, the density of the spacecraft-generated electrons can be orders of magnitude larger than the density of magnetospheric cold electrons resulting in a complete masking of the ambient electron signal. This is the main reason why cold electron measurements of the lowest energies have not been accomplished yet.

Fortunately, there are promising techniques to enable robust cold ion and cold electron measurements in the Earth’s magnetosphere. Those discussed herein include, active spacecraft potential control, biasing of aperture surfaces, and low-resource instruments mounted to booms. Each of these methods have been demonstrated sparsely in space and some have been supported by numerical simulations. One possibility to measure the full ion distribution is to use differential biasing of the detector and mounting face to a sufficiently negative voltage with respect to the ambient plasma. This will eliminate any potential barrier due to the spacecraft sheath, giving free access for the cold ions to reach the detector. As for the cold electron measurements, this can be achieved by active spacecraft potential control realized by an electron beam. By biasing the entire spacecraft to sufficiently positive voltage, the spacecraft-generated electrons are clearly differentiated from the ambient cold electron signal in the detector count. Note that although these approaches (differential biasing for ions and active spacecraft potential control for electrons) enable robust measurement of the full particle distributions, the effect of the spacecraft sheath in the particle counts still needs to be corrected in order to compute accurate ambient plasma parameters.

In a complex system of interconnected parts like the Earth’s magnetosphere, a lack of understanding of one part of the system (the cold electron and ion populations) is disabling. Hence, until cold electron and cold ion measurements are obtained robustly with every mission, the magnetospheric plasma environment cannot be completely understood.