Galileo Energetic Particle Detector Research Articles

On the Formation of Trapped Electron Radiation Belts at Ganymede

AbstractThis study presents evidence of stably trapped electrons at Jupiter's moon Ganymede. We model energetic electron pitch angle distributions and compare them to observations from the Galileo Energetic Particle Detector to identify signatures of trapped particles during the G28 encounter. We trace electron trajectories to show that they enter Ganymede's mini‐magnetospheric environment, become trapped, and drift around the moon for up to 30 min, in some cases stably orbiting the moon multiple times. Conservation of the first adiabatic invariant partially contributes to energy changes throughout the electrons' orbits, with additional acceleration driven by local electric fields, before they return to Jupiter's magnetosphere or impact the surface. These trapped particles manifest as an electron population with an enhanced flux compared to elsewhere within the mini‐magnetosphere that are detectable by future spacecraft.

Comment on “An Active Plume Eruption on Europa During Galileo Flyby E26 as Indicated by Energetic Proton Depletions” by Huybrighs et al.

AbstractThe Galileo spacecraft passed close to Europa on 11 encounters, two of which (E12 and E26) came within 400 km of the surface. In E12 data, there are perturbations in field and plasma data consistent with effects of a nearby plume (Jia et al., 2018). Huybrighs et al. (2020, https://doi.org/10.1029/2020GL087806) report depletions of proton flux in one channel of the Galileo Energetic Particle Detector (EPD) as Galileo passed close to Europa on E26. They trace particle trajectories in the magnetic field provided by a magnetohydrodynamic simulation and conclude that the spacecraft probably also passed through or close to a vapor plume on E26. However, the absence of a related signature in the measured magnetic field led us to question this conclusion. Examination of the EPD data remote from Europa on the E26 flyby reveals that the putative plume signature in the EPD data is an artifact.

Monte Carlo Evaluation of the Europa Clipper TID Margin Based on the Variability of the Jovian Radiation Environment With Application for Mission Design

AbstractThe Europa Clipper mission will repeatedly pass in and out of the Jovian radiation belts as it explores the icy moon. In order to evaluate the effectiveness of any safety margins for total ionizing dose (TID) ratings on Europa Clipper spacecraft components, the variability of the expected radiation environment must be considered. This study outlines a Monte Carlo calculation of the cumulative probability that the TID will deviate from the nominal value of 150 krad (Si) based on the lognormal distributions of electron fluxes observed in the Jupiter environment by the Galileo Energetic Particle Detector instrument. Our model shows that parts hardened to 300 krad (Si) for a radiation design factor of 2 are expected to withstand the measured variations of the Jupiter radiation environment with margin. This result can be scaled to apply to all parts and materials with different TID levels for the Europa Clipper mission.

Using the Galileo Solid-State Imaging Instrument as a Sensor of Jovian Energetic Electrons

We develop a technique to quantitatively characterize the high-energy electron environment using the Galileo spacecraft solid-state imaging (SSI) instrument and results from 3-D Monte Carlo particle transport simulations in Geant4, validating our findings with the Galileo energetic particle detector (EPD). We postprocess raw SSI images to obtain frames with only the radiation contribution. The camera settings (gain state, filter, and so on) are used to compute the intensity of the observed radiation hits. Simulating the response of the SSI instrument to mono-energetic electron environments, we find that the SSI is capable of detecting ≥10-MeV electrons (>90% of 11-MeV integral flux channel. We find agreement within $3\sigma $ of the EPD data for 43 out of 43 (100%) of the SSI images evaluated. 62% of fluxes are also within $1\sigma $ of the EPD data. This approach can be applied to other sets of imaging data (star trackers) in energetic electron environments, such as those found in geostationary Earth orbit.

The Latest Jovian-Trapped Proton and Heavy Ion Models

Jupiter possesses the harshest trapped radiation environment in the solar system. Models of the Jovian-trapped radiation environment are a prerequisite for planning missions to this extreme environment. The high energy (>0.1 MeV for electrons and >1 MeV/nuc for protons and heavier ions) particle environment drives the shielding and microelectronic parts design for a spacecraft. Recently, models based on the measurements by the Galileo energetic particle detector and heavy ion counter have been updated for the trapped protons and heavy ions. The purpose of this paper is to describe these changes to the proton and heavy ion models and demonstrate their application to and relative impact on mission design. While trapped electrons at Jupiter dominate for total ionizing dose and displacement damage dose for typically shielded parts/materials (e.g., with ~100 mil or 2.54 mm of aluminum shielding), trapped protons can be a significant contributor to lightly shielded components (e.g., solar cells under 10–20 mil of cover glass). In contrast, although they will be described here for completeness, the Jovian-trapped heavy ions are typically not a driving environmental consideration for single event effects evaluations (their energies are so low that a small amount of shielding will attenuate the fluxes to levels below the interplanetary solar energetic particle and galactic cosmic rays).

An empirical model of the high‐energy electron environment at Jupiter

AbstractWe present an empirical model of the energetic electron environment in Jupiter's magnetosphere that we have named the Galileo Interim Radiation Electron Model version‐2 (GIRE2) since it is based on Galileo data from the Energetic Particle Detector (EPD). Inside 8RJ, GIRE2 adopts the previously existing model of Divine and Garrett because this region was well sampled by the Pioneer and Voyager spacecraft but poorly covered by Galileo. Outside of 8RJ, the model is based on 10 min averages of Galileo EPD data as well as on measurements from the Geiger Tube Telescope on board the Pioneer spacecraft. In the inner magnetosphere the field configuration is dipolar, while in the outer magnetosphere it presents a disk‐like structure. The gradual transition between these two behaviors is centered at about 17RJ. GIRE2 distinguishes between the two different regions characterized by these two magnetic field topologies. Specifically, GIRE2 consists of an inner trapped omnidirectional model between 8 to 17RJ that smoothly joins onto the original Divine and Garrett model inside 8RJ and onto a GIRE2 plasma sheet model at large radial distances. The model provides a complete picture of the high‐energy electron environment in the Jovian magnetosphere from ∼1 to 50RJ. The present manuscript describes in great detail the data sets, formulation, and fittings used in the model and provides a discussion of the predicted high‐energy electron fluxes as a function of energy and radial distance from the planet.

Charge states of energetic oxygen and sulfur ions in Jupiter's magnetosphere

AbstractPitch angle distributions of proton and energetic heavy ion fluxes near Europa's orbit have been measured by the Galileo Energetic Particles Detector (EPD). At similar energies, these distributions have important differences. If their source and transport processes are similar, as we hypothesize here, then it is difficult to reconcile their different pitch angle distributions. By looking at the same question, other researchers have proposed that the heavies are multiply charged, leading to differences in how the particles are lost. This could not be confirmed directly with EPD because that detector does not separate heavy ion measurements by charge state. However, indirect analyses of the data have extracted the charge state of a few sulfur events. We present here a complete list of ion injections observed with EPD over the whole mission. Energetic sulfur and oxygen charge states can be inferred through a dispersion analysis of dynamic injections that makes use of the charge‐dependent nature of the gradient‐curvature azimuthal drift. We find that sulfur is predominantly multiply charged, whereas oxygen is more evenly distributed between singly and doubly charged states. In addition to current theories on energetic heavy ion transport near the Europa region, we propose that charge gain for the oxygen ions (electron stripping) may play an important role in the character of energetic particles in that region.

Three‐dimensional convective flows of energetic ions in Jupiter's equatorial magnetosphere

AbstractFrom 1995 to 2003, the Galileo Energetic Particles Detector (EPD) measured the three‐dimensional distribution of protons, oxygen, and sulfur ions with total energies between 0.1 and 1 MeV throughout the equatorial Jovian magnetosphere. We perform a spherical harmonics expansion of the measured distributions through the second order and use the resulting anisotropy coefficients to identify purely convecting distributions and derive ion flow velocities via the Compton‐Getting effect. We demonstrate that the second‐order harmonic terms are an essential diagnostic in excluding spurious gradient anisotropies in the velocity derivation. This analysis unambiguously confirms that energetic ion flows in the azimuthal direction are significantly slower than rigid planetary corotation by an amount that is local time dependent, a phenomenon that is qualitatively consistent with expectations of plasma mass loading within an asymmetric magnetic field configuration. However, both the polar and radial components of the ion flows exhibit unexpected and poorly understood global morphology. Consistently northward and inward flows are observed near the dayside and predusk sectors of the equatorial inner magnetosphere, while southward and outward flows are observed within the plasma sheet in the predawn middle magnetosphere. The persistence of southward convection in this region, which is operative regardless of whether the spacecraft was transiting the plasma sheet from the northern magnetic lobe or from the southern lobe, is inconsistent with contemporary models of dynamical plasma sheet motion, while the distinctive local time asymmetries imply that the solar wind is a significant driver of plasma convection at radial distances as small as 15 RJ.

Global magnetodisk disturbances and energetic particle injections at Jupiter

AbstractMeasurements made by the Galileo Energetic Particles Detector and the plasma wave/radio instrument are analyzed to establish relationships between dynamic processes observed independently in the distant and the inner Jovian disk (at 80–120 Jovian radii (RJ) and 10–25 RJ, respectively). It is first shown that global magnetospheric disturbances identified from the radio emissions (the “energetic events” are well correlated with reconnection/reconfiguration events observed at the outer edge of the disk. Then, considering all Galileo perijoves, it is also demonstrated that the energetic events occurring as Galileo was at less than ~25 RJ are systematically associated with new injections of energetic particles seen from ~10 to 25 RJ. This demonstrates that major disturbances commonly affect the whole magnetodisk, from 10 to 80–120 RJ. Overall, their phenomenology involves simultaneous auroral activation, formation of new sources of radio emission (narrow‐band kilometric radiation) and particle injections in the Io torus, magnetic reconfigurations, and radial flow bursts in the distant disk, over time scale of a few hours.

Whistler mode instability and pitch-angle diffusion near Ganymede

The present study is aimed at resolving the issue of pitch-angle diffusion by whistler mode waves near Ganymede and to understand the interaction of Ganymede with magnetosphere of Jupiter. A new electron distribution function, which is a combination of Maxwellian and kappa loss-cone distribution, is used to calculate whistler mode wave growth rates. The distribution function is fitted to observed distribution near Ganymede. Wave growth rates have been used to obtain bounce-averaged pitch-angle diffusion coefficients along the field line connecting Ganymede and Jupiter. Variations of electron density and magnetic field along the particle bounce trajectory and a non dipolar field have been taken into account. Pitch-angle variation of diffusion coefficients is parameterized in a simple form ~Dο(sinα)p. Do is diffusion coefficient (independent of pitch angle). The calculated energy dependence of diffusion coefficients (Do) is in good agreement with observed energy dependence of coefficient Do. Wave magnetic field ~16pT is required to match observed and calculated diffusion coefficients. The exponent p takes values from ~1 to 4 for electron energies 15keV to 1MeV, respectively. It is found that contribution of Ganymede interaction region to pitch-angle scattering is quite small (~4% to 20%). Further analysis of Galileo energetic particle detector (EPD) and plasma wave data or new particle and wave data near Ganymede from JUNO mission to Jupiter are necessary to confirm the findings of present work.

Location and spatial shape of electron beams in Io's wake

The Galileo spacecraft observed energetic field‐aligned electron beams very close to Io during several flybys. We apply a three‐dimensional magnetohydrodynamic (MHD) model of the far‐field Io‐Jupiter interaction to simulate for the first time the location and spatial shape of field‐aligned electron beams. Io continuously generates MHD waves by disturbing the Jovian magnetoplasm. Currents carried by Alfvén waves propagate predominantly along the magnetic field lines. As the number of charge carriers decreases along the travel path, electrons are accelerated toward Jupiter. These energetic electrons precipitate into the Jovian ionosphere, visible as prominent Io footprint emission. Electrons are also accelerated toward Io and form the equatorial beams observed by the Galileo spacecraft. Unlike the beam formation, the position and spatial structure of these beams have not been addressed in detail before. We use a 3‐D MHD model with initial conditions corresponding to the individual Galileo flyby and determine the spatial morphology of the beams in Io's orbital plane. Our results for the beam locations are in good agreement with the Galileo Energetic Particle Detector observations. We find that the ratio of the one‐way travel time of the Alfvén wave from Io to Jupiter and the convection time of the plasma past the obstacle controls the location of the beam. This leads to the conclusion that at other satellites with other plasma environments, the electrons might not be close to the satellite but can be shifted significantly downstream along its plasma wake. Thus, the future search for electron beams near a satellite should be further extended to the wake region.

Equatorial electron beams and auroral structuring at Jupiter

It has been reported that low‐altitude regions of downward electric current on auroral magnetic field lines are sites of dramatic upward magnetic field‐aligned electron acceleration that generates intense magnetic field‐aligned electron beams within Earth's equatorial middle magnetosphere. Field‐aligned equatorial electron beams also are observed within Jupiter's middle magnetosphere. The mystery about these Jovian beams is that they are observed in a region thought to map to Jupiter's brightest aurora and on field lines that generally carry electric current away from Jupiter's atmosphere rather than toward the atmosphere as anticipated at Earth. Here we develop procedures for quantifying the character of the Jupiter electron beams (for example, how confined they are with respect to the magnetic field‐aligned directions). We apply the procedures to the highest time resolution electron data available from the Galileo Energetic Particle Detector (EPD). We find that the Jupiter equatorial electron beams are spatially and/or temporally structured (down to <20 km at auroral altitudes, or less than several minutes), with regions of intense beams intermixed with regions absent of such beams. We suggest that, as with the situation at Earth, Jupiter's circuit of electric currents that supports its brightest aurora is structured, with regions of upward current intermixed with regions of downward current.

Europa's near‐surface radiation environment

As flux tubes are carried over Europa, hundreds of keV to tens of MeV electrons are absorbed by the moon due to their short bounce times. This results in an asymmetry in energetic charged particles in the near surface environment and in the bombardment of Europa's surface. For example, electrons between about 0.1 and 50 MeV do not have direct access to more than 50% of the sphere at 100 km Europa altitude. Moreover, a hypothetical orbiting spacecraft at this altitude would see a reduction of about 33% of the sky, based on Europa's obstructing its field‐of‐view. We find that the predicted radiation reduction due to the moon's presence is consistent with data from Galileo's Energetic Particles Detector (EPD). In this paper, we present data that illustrate this consistency and use standard models of the radiation environment to evaluate the energetic electron fluence at 100 km.

Radial diffusion of geomagnetically trapped protons observed by the Galileo Energetic Particle Detector

The Galileo spacecraft encountered the Earth once on 8 December 1990 (Earth‐I) and again on 8 December 1992 (Earth‐II). These flybys provided excellent opportunities to evaluate the performance of the Energetic Particle Detector (EPD) and establish analysis procedures in a relatively well‐known environment. Further, because Galileo's Earth flyby trajectories were very rapid and nearly radial, the radiation belt measurements provided an excellent “snapshot” of trapped radiation. Because of the rapid flyby and the 20‐s spin period of Galileo, great care had to be taken to remove time aliasing from the pitch angle distributions. Large anisotropies were also present due to intrinsic density gradients. Spherical harmonics were fitted to the pitch and phase distributions in order to obtain fluxes from which phase space densities could be computed. The phase space density (PSD) was calculated from the fitted count rate for the particles (protons) that conserve the first and second adiabatic invariants. The values of 10.0, 15.0, 20.0, 25.0 30.0, 35.0, 40.0, 45.0, and 50.0 MeV/G were used for the first adiabatic invariant, and the values of 0.10, 0.15, 0.20, 0.25, 0.30, and 0.50 G0.5 RE were used for the second adiabatic invariant to determine the PSD from Earth‐I and Earth‐II observations. The extracted PSDs were examined for radial diffusion. Results show there is no unique global dependency of the diffusion coefficient to L, except for a limited region of the first and second adiabatic invariants.

Galileo energetic particle detector observations of the spatial distributions and energy spectra of >1 and >11 MeV electrons in the 10–40 [formula omitted] region of the jovian magnetosphere

The intensities and distribution of >1 MeV electrons in Jupiter's mid-magnetosphere have been characterized using the observations of the Galileo Energetic Particle Detector (EPD) instrument. Because of the more than 6 years of observation made by the EPD instrument in this intense radiation environment, and the inherent difficulties of measuring electrons with energies capable of penetrating shielding, it was necessary to check the performance of the instrument. Fortunately, in the mid-magnetosphere region particle intensities are such that the EPD instrument operated within its range of accurate counting. Salient results include: 1. Intensities that diminish exponentially with radial distance from 10 to approximately 18 R J , with >1 MeV electrons e-folding in about 1.9 R J and >10 MeV electrons in about 1.2 R J . 2. A much more gradual decrease beyond 18 R J (e-folding in about 9.6 R J for >1 MeV and about 5.3 R J for >10 MeV. 3. A persistent intensity modulation at the planetary rotation period most strongly evident in the 10–18 R J region. 4. Secular variations that can be as large as factors of 100 most strongly evident in the 18–40 R J region. 5. Near order of magnitude variations of flux with magnetic latitude in the mid-magnetosphere, 24–25 R J between latitudes 0° and 15°. This manuscript describes the analysis and provides further detail on the results.

Diffuse auroral precipitation in the jovian upper atmosphere and magnetospheric electron flux variability

The deposition of energetic electrons in Jupiter's upper atmosphere provides a means, via auroral observations, of monitoring electron and plasma wave activity within the magnetosphere. Not only does particle precipitation indicate a potential change in atmospheric chemistry, it allows for the study of episodic, pronounced flux enhancements in the energetic electron population. A study has been made of the effects of such electron injections into the jovian magnetosphere and of their ability to provide the source population for variations in diffuse auroral emissions. To identify the source region of precipitating auroral electrons, we have investigated the pitch-angle distributions of high-resolution Galileo Energetic Particle Detector (EPD) data that indicate strong flux levels near the loss cone. The equatorial source region of precipitating electrons has been determined from the locations of Galileo's in situ measurements by tracing magnetic field lines using the KK97 model. The primary source region for Jupiter's diffuse aurora appears to lie in the magnetic equator at 15 – 40 R J , with the predominant contribution to precipitation flux (tens of ergs cm −2 s −1 sr −1) stemming from < 30 R J . Variability of flux for energetic electrons in this region is also important to the irradiation of surfaces and atmospheres for the Galilean moons: Europa, Ganymede, and Callisto. The average diffuse auroral precipitation flux has been shown to vary by as much as a factor of six at a given radial location. This variability appears to be associated with electron injection events that have been identified in high-resolution Galileo EPD data. These electron flux enhancements are also associated with increased whistler-mode wave activity and magnetic field perturbations, as detected by the Galileo Plasma Wave Subsystem (PWS) and Magnetometer (MAG), respectively. Resonant interactions with the whistler-mode waves cause electron pitch-angle scattering and lead to pitch-angle isotropization and precipitation.

Ion abundance ratios in the Jovian magnetosphere

Galileo, as the first orbiting spacecraft around Jupiter, provides the opportunity to study globally the composition of the energetic ion population in the equatorial plane of the Jovian magnetosphere. This enables us to derive the relative importance of the various sources and sinks of plasma and energetic particles in the largest magnetosphere of our solar system. In this paper we use data from the Energetic Particles Detector (EPD) on board Galileo and derive relative ion abundance ratio maps of S/O, S/He, O/He, and p/He. We extend the previous work in terms of global coverage, species, and energy range. In addition we compare them with previous results and especially with those derived on board Voyager 2. We found that the S/O abundance ratio is relatively constant throughout the magnetosphere decreasing slightly with radial distance. Within the error bars the S/O ratios could be reproduced. Only a minor energy dependence is observed for this ratio pointing to a common source for both ions. The S/He‐, O/He‐, and p/He‐ratios decrease with increasing radial distance, furthering the notion that sulfur, oxygen, and protons originate mainly from a source within the Jovian system, in contrast with helium, which originates from the solar wind. A spectral kink observed at energies of several 100 keV/nuc for all species and most pronounced for helium explains the observed energy dependence of the ratios relative to helium. Differences in the abundance ratios up to more than one order of magnitude between sequential orbits are evidence of large temporal variations taking place in the Jovian magnetosphere. These variations and the strong energy dependence can explain the differences between the results derived from Galileo EPD measurements and those from Voyager data.

Energetic ion characteristics and neutral gas interactions in Jupiter's magnetosphere

Spectra, integral moments, and composition (H, He, O, S) of energetic ions (50 keV to 50 MeV) are presented for selected Jupiter magnetospheric positions near the equator between radial distances of ∼6 to ∼46 Jupiter radii (RJ), as revealed by analysis of the Galileo Energetic Particle Detector data. These characteristics are then used as the basis of interpreting and modeling reported signatures of energetic ion/neutral gas interactions within Jupiter's inner magnetosphere, particularly energetic neutral atom emissions measured during the Cassini spacecraft flyby of Jupiter. Key findings include the following: (1) sulfur ions significantly dominate the energetic (≥50 keV) ion density and pressure at all radial distances &gt;7 RJ; (2) protons dominate integral number and energy intensity planetward of 20–25 RJ; (3) a distinct signature of local, equatorial acceleration of energetic protons is revealed between Io (5.9 RJ) and Europa (9.4 RJ); (4) significant spectral and compositional signatures of neutral gas interactions are also revealed between the orbits of Io and Europa; (5) a previously reported significant depletion of ring current ion populations between Io and Europa during the early‐phase operation of Galileo (∼1995), as compared with observations obtained during the Voyager epoch (1979), has persisted and probably deepened during later Galileo phases (1999); and (6) detailed energetic neutral atom emission modeling, based on the in situ results reported here, further constrains recent estimates of the contents of the neutral gas torus of Europa.

Global flows of energetic ions in Jupiter's equatorial plane: First‐order approximation

Galileo, as the first orbiting spacecraft in an outer planet's magnetosphere, provides the opportunity to study global energetic ion distributions in Jupiter's magnetosphere. We present directional anisotropies of energetic ion distributions measured by the Galileo Energetic Particles Detector (EPD). The EPD measurements of proton (80–1050 keV), oxygen (26–562 keV/nucleon), and sulfur (16–310 keV/nucleon) distributions cover a wide energy range. Spatially, the data set includes measurements from 6 to 142 Jovian radii (RJ) and covers all local times inside the Jovian magnetosphere. For each species a single detector head scans almost the entire sky (≈ 4π sr), producing the three‐dimensional angular distributions from which the anisotropies are derived. Consequently, the resulting anisotropy estimates are both global and robust. Such anisotropies, generally produced by convective flow, ion intensity gradients, and field‐aligned components, have long been used to estimate flow velocities and to locate spatial boundaries within magnetospheres. They can therefore provide vital information on magnetospheric circulation and dynamics. We find that the EPD measured anisotropies in the Jovian magnetosphere are dominated by a component in the corotational direction punctuated by episodic radial components, both inward and outward. Under the assumption that anisotropies are produced predominantly by convective flow, we derive flow velocities of protons, oxygen ions, and sulfur ions. The validity of that approach is supported by the fact that these three independently derived flow velocities agree, to a large extent, in this approximation. Thus, for the first time, we are able to derive the global flow pattern in a magnetosphere of an outer planet. In a comparison between the first‐order EPD flow velocities and those predicted by a magnetohydrodynamic (MHD) simulation of the Jovian magnetosphere, we find that qualitatively the directions appear similar, although no firm evidence of steady outflow of ions has been observed at distances covered by Galileo. A first rough comparison indicates that the measured first‐order flow velocities are higher by at least a factor of 1.5 than the MHD simulation results.

Spectroscopic Evidence for High-Altitude Aurora at Jupiter from Galileo Extreme Ultraviolet Spectrometer and Hopkins Ultraviolet Telescope Observations

The Galileo Extreme Ultraviolet Spectrometer (EUVS) and the Hopkins Ultraviolet Telescope (HUT) acquired UV spectra of Jupiter Aurora in the period from 1995 through 1997. The EUVS spectra spanned the wavelength range 540–1280 Å and the HUT spectra measured the extreme ultraviolet and far ultraviolet (EUV+FUV) wavelength range 830–1850 Å. Both sets of spectra present evidence of high-altitude, optically thin H 2 band emissions from the exobase region. The analysis of the UV spectra with a two-stream electron transport model and a jovian model auroral atmosphere indicates that the primary electron flux is composed of both soft and hard electrons with characteristic energies in the soft electron energy range of 20–200 eV and the hard electron range of 5–100 keV. The soft electron flux causes enhanced EUV emission intensities below 1100 Å. The soft electron flux may explain the high temperature of the upper atmosphere above the homopause as measured from H 3 + rovibrational temperatures in the IR. For the deep aurora, a high primary characteristic energy above 5 keV is known to be present. The Galileo Energetic Particle Detector (EPD) has measured the electron distribution functions for energies above 15 keV in the middle magnetosphere. The high-energy distribution functions can be modeled by a combination of Maxwellian and kappa distributions. However, the EUV (800–1200 Å) portion of the HUT spectrum cannot be modeled with a single distribution of hard electrons as was possible in the past for the FUV (1200–1650 Å) spectrum measured by itself. The combination of EUV and FUV spectral observations by HUT serves to identify the amount of soft electron flux relative to the hard primary flux required to produce the high-altitude aurora in the neighborhood of the exobase.