The most intense auroral emissions from Earth's polar regions, called discrete for their sharply defined spatial configurations, are generated by a process involving coherent acceleration of ...electrons by slowly evolving, powerful electric fields directed along the magnetic field lines that connect Earth's space environment to its polar regions. In contrast, Earth's less intense auroras are generally caused by wave scattering of magnetically trapped populations of hot electrons (in the case of diffuse aurora) or by the turbulent or stochastic downward acceleration of electrons along magnetic field lines by waves during transitory periods (in the case of broadband or Alfvénic aurora). Jupiter's relatively steady main aurora has a power density that is so much larger than Earth's that it has been taken for granted that it must be generated primarily by the discrete auroral process. However, preliminary in situ measurements of Jupiter's auroral regions yielded no evidence of such a process. Here we report observations of distinct, high-energy, downward, discrete electron acceleration in Jupiter's auroral polar regions. We also infer upward magnetic-field-aligned electric potentials of up to 400 kiloelectronvolts, an order of magnitude larger than the largest potentials observed at Earth. Despite the magnitude of these upward electric potentials and the expectations from observations at Earth, the downward energy flux from discrete acceleration is less at Jupiter than that caused by broadband or stochastic processes, with broadband and stochastic characteristics that are substantially different from those at Earth.
Abstract
Previous Juno mission event studies revealed powerful electron and ion acceleration, to 100s of kiloelectron volts and higher, at low altitudes over Jupiter's main aurora and polar cap (PC; ...poleward of the main aurora). Here we examine 30–1200 keV JEDI‐instrument particle data from the first 16 Juno orbits to determine how common, persistent, repeatable, and ordered these processes are. For the PC regions, we find (1) upward electron angle beams, sometimes extending to megaelectron volt energies, are persistently present in essentially all portions of the polar cap but are generated by two distinct and spatially separable processes. (2) Particle evidence for megavolt downward electrostatic potentials are observable for 80% of the polar cap crossings and over substantial fractions of the PC area. For the main aurora, with the orbit favoring the duskside, we find that (1) three distinct zones are observed that are generally arranged from lower to higher latitudes but sometimes mixed. They are designated here as the diffuse aurora (DifA), Zone‐I (ZI(D)) showing primarily downward electron acceleration, and Zone‐II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. (2) ZI(D) and ZII(B) sometimes (but not always) contain, respectively, downward electron inverted Vs and downward proton inverted Vs, (potentials up to 400 kV) but, otherwise, have broadband distributions. (3) Surprisingly, both ZI(D) and ZII(B) can generate equally powerful auroral emissions. It is suggested but demonstrated for intense portions of only one auroral crossing, that ZI(D) and ZII(B) are associated, respectively, with upward and downward electric currents.
Plain Language Summary
The science objectives of the Juno mission, with its spacecraft now orbiting Jupiter in a polar orbit, include understanding the space environments of Jupiter's polar regions and generation of Jupiter's uniquely powerful aurora. In Jupiter's polar cap regions (poleward of the main auroral oval encircling the northern and southern poles), we find here that (1) beams of electrons aligned with the upward magnetic field direction are ever‐present with energies extended to the 100s to 1,000s of kilo electron volts and (2) downward magnetic field‐aligned electrostatic potentials reaching greater than a million volts occur over broad regions for 80% of the polar cap crossings. For the main auroral oval, we find three distinct zones: designated here as diffuse aurora (DifA), Zone‐I (ZI(D)) showing downward electron acceleration to 100s of kiloelectron volts, and Zone‐II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. ZI(D) sometimes shows upward electrostatic potentials reaching 100s of kilovolts and is associated with upward magnetic field‐aligned electric currents. ZII(B) sometimes shows downward electrostatic potentials reaching 100s of kilovolts and is associated with downward electric currents. Unexpectedly from Earth studies, ZI(D) and ZII(B) are just as likely to generate the most intense auroral emissions.
Key Points
Jupiter's polar caps have upward electron beams essentially everywhere (100s of kiloelectron volts) and often downward megavolt electric potentials
Energetic particles reveal three main auroral acceleration zones: diffuse aurora (DifA), Zone‐I (downward), and Zone‐II (bidirectional)
ZI(D) and ZII(B) sometimes (but not always) contain, respectively, downward electron inverted Vs and downward proton inverted Vs
Juno obtained unique low‐altitude space environment measurements over Jupiter's poles on 27 August 2016. Here Jupiter Energetic‐particle Detector Instrument observations are presented for electrons ...(25–800 keV) and protons (10–1500 keV). We analyze magnetic field‐aligned electron angular beams over expected auroral regions that were sometimes symmetric (bidirectional) but more often strongly asymmetric. Included are variable but surprisingly persistent upward, monodirectional electron angular beams emerging from what we term the “polar cap,” poleward of the nominal auroral ovals. The energy spectra of all beams were monotonic and hard (not structured in energy), showing power law‐like distributions often extending beyond ~800 keV. Given highly variable downward energy fluxes (below 1 RJ altitudes within the loss cone) as high as 280 mW/m2, we suggest that mechanisms generating these beams are among the primary processes generating Jupiter's uniquely intense auroral emissions, distinct from what is typically observed at Earth.
Key Points
Upward, energy‐monotonic energetic electron angular beams are unexpectedly persistent over Jupiter's polar caps
Jupiter's aurora appears not to be associated with monoenergetic electron beams but with other processes
Jupiter's aurora is powered by the downward portion of bidirectional, energy‐monotonic electron angular beams and diffuse precipitation
Magnetospheric Multiscale observations are used to probe the structure and temperature profile of a guide field reconnection exhaust ~100 ion inertial lengths downstream from the X‐line in the ...Earth's magnetosheath. Asymmetric Hall electric and magnetic field signatures were detected, together with a density cavity confined near 1 edge of the exhaust and containing electron flow toward the X‐line. Electron holes were also detected both on the cavity edge and at the Hall magnetic field reversal. Predominantly parallel ion and electron heating was observed in the main exhaust, but within the cavity, electron cooling and enhanced parallel ion heating were found. This is explained in terms of the parallel electric field, which inhibits electron mixing within the cavity on newly reconnected field lines but accelerates ions. Consequently, guide field reconnection causes inhomogeneous changes in ion and electron temperature across the exhaust.
Plain Language Summary
Plasma heating and energization by magnetic reconnection is a fundamental process in space, solar, astrophysical, and planetary plasmas. Most reconnecting current sheets do not exhibit perfectly antialigned magnetic fields and a so‐called guide field is often present. Using new experimental data from NASA's Magnetospheric Multiscale mission, this article shows that far from the X‐line during guide field reconnection, the heating is substantially modified from the typically studied antiparallel case. More specifically, the new multipoint, high time resolution Magnetospheric Multiscale measurements of a guide field reconnection exhaust in the Earth's magnetosheath reveal inhomogenous ion and electron heating and cooling. This uncovers in new detail the structure of the exhaust, including predicted density cavity structure and electron holes, and indicates the importance of the parallel electric field. The results are important for the general understanding of reconnection heating and energization. The results will be of immediate and timely interest to the Geophysical Research Letters (GRL) community and beyond.
Key Points
A guide field reconnection exhaust was encountered by MMS in the magnetosheath ~100 ion inertial lengths downstream from the X‐line
A density cavity forms on one edge of the exhaust with embedded electron jetting toward the X‐line and electron holes on the cavity edge
The parallel electric field causes electron cooling and ion heating in the cavity and inhomogeneous temperature profiles across the exhaust
Two new Juno‐observed particle features of Jupiter's main aurora demonstrate substantial diversity of processes generating Jupiter's mysterious auroral emissions. It was previously speculated that ...sometimes‐observed potential‐driven aurora (up to 400 kV) can turn into broadband stochastic acceleration (dominating at Jupiter) by means of instability. Here direct evidence for such a process is revealed with a “mono‐energetic” electron inverted‐V rising in energy to 200 keV, transforming into a region of broadband acceleration with downward energy fluxes tripling to 3,000 mW/m2, and then transforming back into a mono‐energetic structure ramping down from 200 keV. But a second feature of interest observed nearby is unlikely to have operated in the same way. Here a downward accelerated proton inverted‐V, with inferred potentials to 300–400 kV, occurred simultaneously with downward accelerated broadband electrons with downward energy fluxes as high as any observed (~3,000 mW/m2). This latter feature has no known precedent with Earth auroral observations.
Key Points
Two new particle features are identified within the intense auroral acceleration regions at Jupiter demonstrating great diversity
One feature supports a hypothesis that potential‐driven aurora can become unstable and convert over to broadband, stochastic acceleration
The other feature contradicts that hypothesis and has no qualitative precedent within Earths' auroral acceleration regions
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.
Key Points
Energetic proton fluxes are observed to fall off moving radially inward toward Io's orbit
A radially narrow decrease in energetic electrons within a wider decrease is found along Io's orbit on the wake side of the moon
The mechanism most consistent with the narrow decrease is wave‐particle interactions near Io
In planetary magnetospheres, singly charged energetic particles, trapped by the planet's magnetic field, can steal electrons from cold gas atoms and become neutralized. These now energetic neutral ...atoms (ENAs), no longer confined by the magnetic field, can travel out of the system similar to photons leaving a hot oven. ENAs have been used to image magnetospheric processes at Earth, Jupiter, and Saturn. At Jupiter, the opportunities to image the magnetosphere have been limited and always from the perspective of the near‐equatorial plane at distance >139 RJ. The polar‐orbiting Juno mission carries the Jupiter Energetic particle Detector Instrument that is serendipitously sensitive to ENAs with energies >50 keV, provided that there are no charged particles in the environment to mask their presence. Here we report on the first ENA observations of Jupiter's magnetosphere from a nonequatorial perspective. In this brief report we concentrate on emissions seen during Perijove 22 (PJ22) during very active conditions and compare them with emissions during the inactive Perijove 23 (PJ23). We observe, and discriminate between, distinct ENA signatures from the neutral gases occupying the orbit of Io (away from Io itself), the orbit of Europa (away from Europa), and from Jupiter itself. Strong ENA emissions from Io's orbit during PJ22 are associated with energetic particle injections observed near Io's orbit several hours earlier. Some injections occurred planetward of Io's L‐shell (magnetic position), somewhat of a surprise given that injections are thought to be driven by outward transport of plasmas generated by Io.
Plain Language Summary
In the space environments of magnetized planets (magnetospheres), magnetic fields trap and confine energetic charged particles like protons and singly charged heavier ions. These ions can neutralize themselves by stealing electrons from cold gas atoms within the same environment. They become energetic neutral atoms (ENAs), and no longer confined by the magnetic field, can travel out of the system in a fashion similar to light leaving a hot oven. ENAs have been used to image magnetospheric processes at Earth, Jupiter, and Saturn. At Jupiter, the opportunities to image the magnetosphere have been limited and always from the perspective of the near‐equatorial plane at large distances (>139 RJ). The polar‐orbiting Juno mission carries the Jupiter Energetic particle Detector Instrument that is serendipitously sensitive to ENAs with energies >50 keV, provided that there are no charged particles in the environment to mask their presence. Here we report on the first ENA observations of Jupiter's magnetosphere from a nonequatorial perspective. That perspective allows us to observe distinct ENA signatures from the neutral gases occupying the orbit of the moon Io (away from Io itself), the gases in the orbit of the moon Europa (away from Europa), and from Jupiter itself.
Key Points
The first Jovian off‐equator energetic neutral atom (ENA) viewings reveal distinct emissions from Jupiter and the orbits of Io and Europa
Strong ENA emissions from Io's orbit are associated with energetic particle injections near Io's orbit observed several hours earlier
Energetic particle injections occur inside Io's orbit, a surprise given expectations that outward transport from Io drives injections
In this paper, we exploit the charge‐dependent nature of auroral phenomena in Jupiter's polar cap region to infer the charge states of energetic oxygen and sulfur. To date, there are very limited and ...sparse measurements of the >50 keV oxygen and sulfur charge states, yet many studies have demonstrated their importance in understanding the details of various physical processes, such as X‐ray aurora, ion‐neutral interactions in Jupiter's neutral cloud, and particle acceleration theories. In this contribution, we develop a technique to determine the most abundant charge states associated with heavy ions in Jupiter's polar magnetosphere. We find that O+ and S++ are the most abundant and therefore iogenic in origin. The results are important because they provide (1) strong evidence that soft X‐ray sources are likely due to charge stripping of magnetospheric ions and (2) a more complete spatial map of the oxygen and sulfur charge states, which is important for understanding how the charge‐ and mass‐dependent physical processes sculpt the energetic particles throughout the Jovian magnetosphere.
Key Points
Quasi‐static electric potentials in Jupiter's polar cap region are used to determine the energetic (>hundreds of keV) ion charge states
The most abundant charge states associated with these precipitating ions are O+ and S++ and therefore iogenic in origin
These observations are important for X‐ray auroral and ion‐neutral interaction physics
Juno's Jupiter Energetic particle Detector Instrument often detects energetic electron beams over Jupiter's polar regions. In this paper, we document a subset of intense magnetic field‐aligned beams ...of energetic electrons moving away from Jupiter at high magnetic latitudes both north and south of the planet. The number fluxes of these beams are often dominated by electrons with energies above about 1 MeV. These very narrow beams can create broad angular responses in the Jupiter Energetic particle Detector Instrument with unique signatures in the detector count rates, probably because of >10 MeV electrons. We use these signatures to identify the most intense beams. These beams occur primarily above the swirl region of the polar cap aurora. This polar region is described as being of low brightness and high absorption and the most magnetically “open” at Jupiter.
Plain Language Summary
We find that there are very intense beams of energetic (probably dominated by >1 MeV) electrons moving upward over Jupiter's north and south poles that are likely to be magnetically connected to the swirl region in the aurora. We use data from Juno's Jupiter Energetic particle Detector Instrument to characterize the upward beams and also present Ultraviolet Spectrograph data showing the swirl region in a color ratio. We have created a table of times when these very intense beams are present from the first through the eighth perijove pass of the spacecraft.
Key Points
We survey persistent, intense upward MeV electron beams in Jupiter's polar regions
These intense beams appear to be well correlated with the swirl region of the Jovian aurora
We consider Juno JEDI data from perijove 1 through perijove 8
Many attempts have been made to model X‐ray emission from both bremsstrahlung and ion precipitation into Jupiter's polar caps. Electron bremsstrahlung modeling has fallen short of producing the total ...overall power output observed by Earth‐orbit‐based X‐ray observatories. Heavy ion precipitation was able to reproduce strong X‐ray fluxes, but the proposed incident ion energies were very high (
>1 MeV per nucleon). Now with the Juno spacecraft at Jupiter, there have been many measurements of heavy ion populations above the polar cap with energies up to 300–400 keV per nucleon (keV/u), well below the ion energies required by earlier models. Recent work has provided a new outlook on how ion‐neutral collisions in the Jovian atmosphere are occurring, providing us with an entirely new set of impact cross sections. The model presented here simulates oxygen and sulfur precipitation, taking into account the new cross sections, every collision process, the measured ion fluxes above Jupiter's polar aurora, and synthetic X‐ray spectra. We predict X‐ray fluxes, efficiencies, and spectra for various initial ion energies considering opacity effects from two different atmospheres. We demonstrate that an in situ measured heavy ion flux above Jupiter's polar cap is capable of producing over 1 GW of X‐ray emission when some assumptions are made. Comparison of our approximated synthetic X‐ray spectrum produced from in situ particle data with a simultaneous X‐ray spectrum observed by XMM‐Newton shows good agreement for the oxygen part of the spectrum but not for the sulfur part.
Key Points
Heavy ion precipitation into the Jovian atmosphere can produce the observed auroral X‐ray emission
Using Juno measurements of ion fluxes over Jupiter's pole, we simulate X‐ray spectra
We compare our approximated synthetic X‐ray spectra produced by in situ data to observed emission
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