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
Electron Acceleration to MeV Energies at Jupiter and Saturn Kollmann, P.; Roussos, E.; Paranicas, C. ...
Journal of geophysical research. Space physics,
November 2018, 2018-Nov, 2018-11-00, 20181101, Letnik:
123, Številka:
11
Journal Article
Recenzirano
Odprti dostop
The radiation belts and magnetospheres of Jupiter and Saturn show significant intensities of relativistic electrons with energies up to tens of megaelectronvolts (MeV). To date, the question on how ...the electrons reach such high energies is not fully answered. This is largely due to the lack of high‐quality electron spectra in the MeV energy range that models could be fit to. We reprocess data throughout the Galileo orbiter mission in order to derive Jupiter's electron spectra up to tens of MeV. In the case of Saturn, the spectra from the Cassini orbiter are readily available and we provide a systematic analysis aiming to study their acceleration mechanisms. Our analysis focuses on the magnetospheres of these planets, at distances of L > 20 and L > 4 for Jupiter and Saturn, respectively, where electron intensities are not yet at radiation belt levels. We find no support that MeV electrons are dominantly accelerated by wave‐particle interactions in the magnetospheres of both planets at these distances. Instead, electron acceleration is consistent with adiabatic transport. While this is a common assumption, confirmation of this fact is important since many studies on sources, losses, and transport of energetic particles rely on it. Adiabatic heating can be driven through various radial transport mechanisms, for example, injections driven by the interchange instability or radial diffusion. We cannot distinguish these processes at Saturn with our technique. For Jupiter, we suggest that the dominating acceleration process is radial diffusion because injections are never observed at MeV energies.
Plain Language Summary
Space is filled with a radiation of protons and electrons moving with almost light speed. While in free space this is cosmic radiation of extreme energies, magnetized planets trap radiation particles of lower energies, typically in the megaelectronvolt range. These are the so called radiation belts that are found, for example, around Jupiter, Saturn, and Earth. All radiation particles initially start out from being almost at rest and are over time accelerated to almost light speed. The physical mechanisms responsible for this are the subject of ongoing research. Here we focus on the processes accelerating electrons around Jupiter and Saturn based on data from the Galileo and Cassini orbiters. During their life, electrons change their orbits around a planet, get closer to the planetary surface, and are exposed to stronger magnetic fields that these planets are producing. It is known that such changes in magnetic field exposure are in principle able to accelerate particles. Here we find that this exposure is indeed the main reason for acceleration of electrons at relatively large distances from Jupiter and Saturn, not other candidate processes that could in principle also have been responsible.
Key Point
Electrons in Saturn's and Jupiter's magnetosphere, outside the most intense radiation belts, are accelerated through adiabatic transport
The Jupiter Energetic Particle Detector Instruments (JEDI) on the Juno Jupiter polar-orbiting, atmosphere-skimming, mission to Jupiter will coordinate with the several other space physics instruments ...on the Juno spacecraft to characterize and understand the space environment of Jupiter’s polar regions, and specifically to understand the generation of Jupiter’s powerful aurora. JEDI comprises 3 nearly-identical instruments and measures at minimum the energy, angle, and ion composition distributions of ions with energies from H:20 keV and O: 50 keV to >1 MeV, and the energy and angle distribution of electrons from <40 to >500 keV. Each JEDI instrument uses microchannel plates (MCP) and thin foils to measure the times of flight (TOF) of incoming ions and the pulse height associated with the interaction of ions with the foils, and it uses solid state detectors (SSD’s) to measure the total energy (
E
) of both the ions and the electrons. The MCP anodes and the SSD arrays are configured to determine the directions of arrivals of the incoming charged particles. The instruments also use fast triple coincidence and optimum shielding to suppress penetrating background radiation and incoming UV foreground. Here we describe the science objectives of JEDI, the science and measurement requirements, the challenges that the JEDI team had in meeting these requirements, the design and operation of the JEDI instruments, their calibrated performances, the JEDI inflight and ground operations, and the initial measurements of the JEDI instruments in interplanetary space following the Juno launch on 5 August 2011. Juno will begin its prime science operations, comprising 32 orbits with dimensions 1.1×40 RJ, in mid-2016.
The Juno spacecraft acquired direct observations of the jovian magnetosphere and auroral emissions from a vantage point above the poles. Juno’s capture orbit spanned the jovian magnetosphere from bow ...shock to the planet, providing magnetic field, charged particle, and wave phenomena context for Juno’s passage over the poles and traverse of Jupiter’s hazardous inner radiation belts. Juno’s energetic particle and plasma detectors measured electrons precipitating in the polar regions, exciting intense aurorae, observed simultaneously by the ultraviolet and infrared imaging spectrographs. Juno transited beneath the most intense parts of the radiation belts, passed about 4000 kilometers above the cloud tops at closest approach, well inside the jovian rings, and recorded the electrical signatures of high-velocity impacts with small particles as it traversed the equator.
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
A physical model of the Jovian trapped protons with kinetic energies higher than 1 MeV inward of the orbit of the icy moon Europa is presented. The model, named Salammbô, takes into account the ...radial diffusion process, the absorption effect of the Jovian moons, and the Coulomb collisions and charge exchanges with the cold plasma and neutral populations of the inner Jovian magnetosphere. Preliminary modeling of the wave‐particle interaction with electromagnetic ion cyclotron waves near the moon Io is also performed. Salammbô is validated against in situ proton measurements of Pioneer 10, Pioneer 11, Voyager 1, Galileo Probe, and Galileo Orbiter. A prominent feature of the MeV proton intensity distribution in the modeled area is the 2 orders of magnitude flux depletion observed in MeV measurements near the orbit of Io. Our simulations reveal that this is not due to direct interactions with the moon or its neutral environment but results from scattering of the protons by electromagnetic ion cyclotron waves.
Key Points
A global physical model of the proton radiation belts of Jupiter inward of the orbit of Europa is presented
Observed 2 orders of magnitude flux depletions in MeV proton fluxes near Io are not from direct interactions with the moon or its torus
Resonant interaction with low‐frequency electromagnetic waves is modeled and likely to be dominant near Io's orbit
Pitch 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.
Key Points
Energetic oxygen and sulfur charge states inferred from ion dispersion theory
Sulfur is predominantly multiply charged, whereas oxygen is both singly and doubly charged
Charge gain plays an important role on energetic oxygen‐neutral interactions in the Europa torus
The magnetic field observations from the Galileo, Ulysses, Voyager 1 and 2, and Pioneer 10 and 11 spacecraft are analyzed to understand the structures and thickness of the Jovian magnetospheric ...current sheet. Using a new technique that can determine the motion of the spacecraft relative to the current sheet, we have built the first global map of the thickness of the Jovian magnetospheric current sheet. We show that the current sheet thickness in Jupiter's magnetosphere is not equal everywhere but changes systematically with both radial distance and local time. The extremely thin dawnside current sheet appears to be linked to the stresses imposed by the solar wind on the open magnetic flux housed in the lobes of the magnetosphere in that sector. We also show evidence of a mostly closed magnetosphere in the dusk sector. We show that the current sheet thickness asymmetries likely arise from a field and plasma convection system set up by the reconnection of the interplanetary magnetic field with that of Jupiter.
Plain Language Summary
The Jovian magnetospheric current sheet located close to the equatorial plane of Jupiter is the main repository of its corotating plasma derived from Io. Using a new technique that can determine the relative motions between the current sheet and the spacecraft that have traversed it, we have developed a detailed map of the current sheet thickness. Surprisingly, the thickness of the current sheet is small in the dawn sector and very large in the dusk sector of the magnetosphere. We show that the current sheet thickness asymmetries likely arise from a field and plasma convection system set up by the reconnection of the interplanetary magnetic field with that of Jupiter.
Key Points
We have developed the first map of the thickness of the Jovian magnetospheric current sheet which shows strong local time asymmetries
The observed thickness asymmetries may arise from the dawn/dusk differences in the interconnectedness of the Jovian field to the interplanetary magnetic field
A new technique was developed that can determine the motion of the spacecraft relative to the instantaneous location of the current sheet
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.
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