G. Clark

Johns Hopkins University Applied Physics Laboratory

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B. H. Mauk

Johns Hopkins University Applied Physics Laboratory

S. J. Bolton

Southwest Research Institute

F. Allegrini

The University of Texas at San Antonio

J. E. P. Connerney

Goddard Space Flight Center

W. S. Kŭrth

University of Iowa

F. Bagenal

Laboratory for Atmospheric and Space Physics

R. W. Ebert

Southwest Research Institute

C. Paranicas

Johns Hopkins University Applied Physics Laboratory

P. Kollmann

Johns Hopkins University Applied Physics Laboratory

G. R. Gladstone

Southwest Research Institute

Cooperative Institutions

Goddard Space Flight Center

166

Southwest Research Institute

151

Johns Hopkins University Applied Physics Laboratory

140

Charles River Laboratories (Netherlands)

133

John Wiley & Sons (United States)

University of Colorado Boulder

Planetary Science Institute

Johns Hopkins University

Jet Propulsion Laboratory

Centre National de la Recherche Scientifique

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Internal Versus External Sources of Plasma at Saturn: Overview From Magnetospheric Imaging Investigation/Charge‐Energy‐Mass Spectrometer Data

Journal of Geophysical Research Space Physics (2018)

Robert C. Allen D. G. Mitchell C. Paranicas D. C. Hamilton G. Clark

Abstract Plasma composition observations provide a useful mechanism for investigating plasma sources and subsequent evolution within planetary magnetospheres. While He ++ is the second most abundant ion species in the solar wind, there are no known sources of He ++ ions within the magnetosphere of Saturn, allowing He ++ to serve as a tracer of solar wind ions within the Kronian magnetosphere. Meanwhile, water group ions (W + , consisting of O + , OH + , H 2 O + , and H 3 O + ) and H 2 + , known to originate within the magnetosphere of Saturn, serve as a tracer of internally ionized plasma. In this paper, we investigate the relative abundances and properties of energetic (32–220 keV) ion species originating from sources within the magnetosphere and from the solar wind. Solar wind‐originating ions are observed to have significant relative abundance (up to ~0.05) in the midnight, dawn, and noon local time quadrants at high radial distances (~40, ~45, and ~20 R S , respectively). Several possible entry processes, such as reconnection and Kelvin‐Helmholtz instabilities, are outlined in this paper as well as a discussion of subsequent transport.

Magnetosphere of Saturn

Polar wind

Noon

10.1029/2018ja025262

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Citations (17)

Revealing the source of Jupiter’s x-ray auroral flares

Science Advances (2021)

Zhonghua Yao W. R. Dunn E. E. Woodfield G. Clark B. H. Mauk

Jupiter’s unique x-ray aurora is driven by the same processes as Earth’s: Heavy ion scattering by electromagnetic waves.

Jupiter (rocket family)

Solar flare

10.1126/sciadv.abf0851

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Citations (36)

Evidence for Magnetic Reconnection at Ganymede's Upstream Magnetopause During the PJ34 Juno Flyby

Geophysical Research Letters (2022)

R. W. Ebert S. A. Fuselier F. Allegrini F. Bagenal S. J. Bolton

Abstract Juno made a close flyby of Ganymede and flew through its magnetosphere on 7 June 2021, including an outbound crossing of Ganymede's upstream magnetopause. We present plasma and magnetic field observations near the upstream magnetopause from Juno's Jovian Auroral Distributions Experiment (JADE) and magnetometer. JADE observed enhanced electron fluxes, including field‐aligned electrons accelerated up to 2–3 keV/q, some having bidirectional pitch angle distributions, as Juno crossed Ganymede's magnetopause. Energy enhancements of cold protons and heavy ions originating from Ganymede were also observed on approach to the magnetopause. We interpret the presence of accelerated, field‐aligned electrons as indicating that magnetic reconnection is occurring on magnetic field lines that connect the spacecraft to Ganymede's magnetopause at that time. Counter‐streaming electrons observed on both sides of the magnetopause suggest the presence of multiple reconnection sites, both north and south of the spacecraft.

Magnetosheath

10.1029/2022gl099775

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Citations (17)

Characterizing cometary electrons with kappa distributions

Journal of Geophysical Research Space Physics (2016)

T. W. Broiles G. Livadiotis J. L. Burch K. Chae G. Clark

Abstract The Rosetta spacecraft has escorted comet 67P/Churyumov‐Gerasimenko since 6 August 2014 and has offered an unprecedented opportunity to study plasma physics in the coma. We have used this opportunity to make the first characterization of cometary electrons with kappa distributions. Two three‐dimensional kappa functions were fit to the observations, which we interpret as two populations of dense and warm (density = 10 cm −3 , temperature = 2 × 10 5 K, invariant kappa index = 10−>1000), and rarefied and hot (density = 0.005 cm −3 , temperature = 5 × 10 5 K, invariant kappa index = 1–10) electrons. We fit the observations on 30 October 2014 when Rosetta was 20 km from 67P, and 3 AU from the Sun. We repeated the analysis on 15 August 2015 when Rosetta was 300 km from the comet and 1.3 AU from the Sun. Comparing the measurements on both days gives the first comparison of the cometary electron environment between a nearly inactive comet far from the Sun and an active comet near perihelion. We find that the warm population density increased by a factor of 3, while the temperature cooled by a factor of 2, and the invariant kappa index was unaffected. We find that the hot population density increased by a factor of 10, while the temperature and invariant kappa index were unchanged. We conclude that the hot population is likely the solar wind halo electrons in the coma. The warm population is likely of cometary origin, but its mechanism for production is not known.

Kappa

Coma (optics)

10.1002/2016ja022972

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Citations (67)

Comparing Electron Energetics and UV Brightness in Jupiter's Northern Polar Region During Juno Perijove 5

Geophysical Research Letters (2018)

R. W. Ebert T. K. Greathouse G. Clark F. Allegrini F. Bagenal

We compare electron and UV observations mapping to the same location in Jupiter's northern polar region, poleward of the main aurora, during Juno perijove 5. Simultaneous peaks in UV brightness and electron energy flux are identified when observations map to the same location at the same time. The downward energy flux during these simultaneous observations was not sufficient to generate the observed UV brightness; the upward energy flux was. We propose that the primary acceleration region is below Juno's altitude, from which the more intense upward electrons originate. For the complete interval, the UV brightness peaked at ~240 kilorayleigh (kR); the downward and upward energy fluxes peaked at 60 and 700 mW/m2, respectively. Increased downward energy fluxes are associated with increased contributions from tens of keV electrons. These observations provide evidence that bidirectional electron beams with broad energy distributions can produce tens to hundreds of kilorayleigh polar UV emissions.

Energetics

Jupiter (rocket family)

Polar cap

10.1029/2018gl081129

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Citations (22)

Currents and associated electron scattering and bouncing near the diffusion region at Earth's magnetopause

Geophysical Research Letters (2016)

B. Lavraud Y. C. Zhang Y. Vernisse D. J. Gershman J. Dorelli

Abstract Based on high‐resolution measurements from NASA's Magnetospheric Multiscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earth's magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20 eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of inflowing electrons, associated with field‐aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90° away from the center (where it mirrors owing to higher magnetic field and probable field‐aligned potentials).

Pitch angle

Current sheet

Guiding center

Electron scattering

10.1002/2016gl068359

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Citations (87)

Jupiter's X‐ray Emission During the 2007 Solar Minimum

Journal of Geophysical Research Space Physics (2020)

W. R. Dunn G. Branduardi‐Raymont V. Carter‐Cortez Ashley Campbell Ronald F. Elsner

Abstract The 2007–2009 solar minimum was the longest of the space age. We present the first of two companion papers on Chandra and XMM‐Newton X‐ray campaigns of Jupiter through February–March 2007. We find that low solar X‐ray flux during solar minimum causes Jupiter's equatorial regions to be exceptionally X‐ray dim (0.21 GW at minimum; 0.76 GW at maximum). While the Jovian equatorial emission varies with solar cycle, the aurorae have comparably bright intervals at solar minimum and maximum. We apply atomic charge exchange models to auroral spectra and find that iogenic plasma of sulphur and oxygen ions provides excellent fits for XMM‐Newton observations. The fitted spectral S:O ratios of 0.4–1.3 are in good agreement with in situ magnetospheric S:O measurements of 0.3–1.5, suggesting that the ions that produce Jupiter's X‐ray aurora predominantly originate inside the magnetosphere. The aurorae were particularly bright on 24–25 February and 8–9 March, but these two observations exhibit very different spatial, spectral, and temporal behavior; 24–25 February was the only observation in this campaign with significant hard X‐ray bremsstrahlung from precipitating electrons, suggesting this may be rare. For 8–9 March, a bremsstrahlung component was absent, but bright oxygen O 6+ lines and best‐fit models containing carbon, point to contributions from solar wind ions. This contribution is absent in the other observations. Comparing simultaneous Chandra ACIS and XMM‐Newton EPIC spectra showed that ACIS systematically underreported 0.45‐ to 0.6‐keV Jovian emission, suggesting quenching may be less important for Jupiter's atmosphere than previously thought. We therefore recommend XMM‐Newton for spectral analyses and quantifying opacity/quenching effects.

Jupiter (rocket family)

Solar minimum

Magnetosphere of Jupiter

Solar maximum

10.1029/2019ja027219

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Citations (21)

Io's Effect on Energetic Charged Particles as Seen in Juno Data

Geophysical Research Letters (2019)

C. Paranicas B. H. Mauk D. K. Haggerty G. Clark P. Kollmann

Abstract On 12 February 2019, the Juno spacecraft crossed the particle drift shells (L shells) of the moon Io. The energetic particle detector, Jupiter Energetic Particle Detector Instrument (JEDI), found very low fluxes of energetic protons when the spacecraft was inward of about L~6.5. Recent modeling suggests wave‐particle interactions may explain why energetic proton fluxes measured at these radial distances are low. JEDI also measured both a wide and a narrow decrease in the energetic electron count rate in Io's wake. At the time of this decrease, the JEDI detectors were dominated by 0.42 to 10‐MeV electrons. The dimensions of the narrow count rate decrease are about three Io diameters and are unlikely to be caused by absorption by moon itself.

Jupiter (rocket family)

Particle (ecology)

Wave–particle duality

10.1029/2019gl085393

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Citations (18)

Proton Acceleration by Io's Alfvénic Interaction

Journal of Geophysical Research Space Physics (2019)

J. R. Szalay F. Bagenal F. Allegrini Bertrand Bonfond G. Clark

Abstract The Jovian Auroral Distributions Experiment aboard Juno observed accelerated proton populations connected to Io's footprint tail aurora. While accelerated electron populations have been previously linked with Io's auroral footprint tail aurora, we present new evidence for proton acceleration due to Io's Alfvénic interaction with Jupiter's magnetosphere. Separate populations were accelerated above the Io torus and at high latitudes near Jupiter. The timing suggests the acceleration is due to Alfvén waves associated with Io's Main Alfvén Wing. The inferred high‐latitude proton acceleration region spans 0.9–2.5 Jovian radii in altitude, comparable to the expected location for electron acceleration, and suggests the associated Alfvén waves are able to accelerate electrons and protons in similar locations. The proton populations magnetically connected to Io's orbit are recently perturbed, equilibrating with the nominal torus plasma population on a timescale smaller than Io's System III orbital period of ~13 h, likely due to wave‐particle interactions. The tail populations are split into a wake‐like structure with distinct inner and outer regions, where the inner region maps to an equatorial width nearly identical to the diameter of Io. The approximately symmetric surrounding outer regions are each slightly smaller than the central region and may be related to Io's atmospheric extent. The nominal, corotational torus proton population exhibits energization throughout all regions, peaking at the anti‐Jovian flank of the inner core region mapping to Io's diameter. These proton observations suggest Alfvén waves are capable of accelerating protons in multiple locations and provide further evidence that Io's Alfvénic interaction is bifurcated.

Jupiter (rocket family)

10.1029/2019ja027314

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Citations (27)

Spectra of Saturn’s proton belts revealed

Icarus (2021)

P. Kollmann E. Roussos G. Clark J. F. Cooper Steven J. Sturner

Saturn is permanently surrounded by 6 discrete proton radiation belts that are rigidly separated by the orbits of its inner moons and dense rings. These radiation belts are ideal environments to study the details of radial diffusion and the CRAND source process, yet progress has been hindered by the fact that the energy spectra are not known with certainty: Reanalysis of the response functions of the LEMMS instrument on-board the Cassini orbiter has shown that measurements of ≲10MeV protons may be easily contaminated by ≳10MeV protons and that many available measurements characterize a very broad energy range, so that the calculation of an energy-resolved spectrum is not as straightforward as previously assumed. Here we use forward modeling of the measurements based on the instrument response and combine this technique where useful with numerical modeling of the proton belt physics in order to determine Saturn's spectra with higher certainty. We find significant proton intensities up to ≈1 GeV. While earlier studies reported on proton spectra roughly following a power law with exponent ≈−2, our more advanced analysis shows harder spectra with exponent ≈−1. The observed spectra provide independent confirmation that Saturn's proton belts are sourced by CRAND and are consistent with the provided protons being subsequently cooled in the tenuous gas originating from Saturn or Enceladus. The intensities at Saturn are found to be lower than at Jupiter and Earth, which is also consistent with the source of Saturn being exclusively CRAND, while the other planets can draw from additional processes. Our new spectra can be used in the future to further our understanding of Saturn's proton belts and the respective physical processes that occur at other magnetized planets in general. Also, the spectra have applications for several topics of planetary science, such as space weathering of Saturn's moons and rings, and can be useful to constrain properties of the main rings through their production of secondary particles.

Enceladus

Jupiter (rocket family)

Magnetosphere of Saturn

10.1016/j.icarus.2021.114795

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Citations (8)