The arrival of the Juno satellite at Jupiter has led to an increased interest in the dynamics of the Jovian magnetosphere. Jupiter's auroral emissions often exhibit quasiperiodic oscillations with ...periods of tens of minutes. Magnetic observations indicate that ultralow‐frequency (ULF) waves with similar periods are often seen in data from Galileo and other satellites traversing the Jovian magnetosphere. Such waves can be associated with field line resonances, which are standing shear Alfvén waves on the field lines. Using model magnetic fields and plasma distributions, the frequencies of field line resonances and their harmonics on field lines connecting to the main auroral oval have been determined. Time domain simulations of Alfvén wave propagation have illustrated the evolution of such resonances. These studies indicate that harmonics of the field line resonances are common in the 10–40 min band.
Plain Language Summary
The magnetic field lines of planets like Earth and Jupiter can act like the strings of a musical instrument and can support waves at specific frequencies in the same way that a guitar or violin string has a particular frequency. The rapid rotation of Jupiter causes its field line to be stretched out, and Jupiter's moon Io produces a dense cloud of ionized gas (plasma) that populates these stretched field lines. By making a numerical model of Jupiter's magnetic field and plasma, we have calculated the frequencies of these field lines, which have much lower frequencies than a musical instrument so that the periods are tens of minutes. This period corresponds to oscillations in the visible aurora (northern and southern lights) at Jupiter.
Key Points
Alfvén wavefield line resonances are described in a model of Jupiter's magnetosphere based on empirical distributions of the magnetic field and plasma density
These resonances occur with periods of 10–40 min
These waves are consistent with measurements of magnetic fluctuations in Jupiter's magnetosphere, as well as with oscillations in auroral luminosity
Shear mode Alfvén waves are the carriers of field-aligned currents in the auroral zones of Earth and other planets. These waves travel along the magnetic field lines, coupling the outer magnetosphere ...with the ionosphere. However, in ideal magnetohydrodynamic (MHD) theory, the shear mode Alfvén wave does not carry a parallel electric field that could accelerate auroral particles. This can be modified by including kinetic effects, which lead to a parallel electric field when the perpendicular wavelength becomes comparable to the electron inertial length or the ion acoustic gyroradius. These small perpendicular wavelengths can be formed by phase mixing, ionospheric feedback, or nonlinear effects. Kinetic Alfvén waves are further constrained by their interaction with the ionosphere, which acts as a reflector for these waves. In addition, the strong plasma gradients in the topside ionosphere form an effective resonator that leads to fluctuations on time scales of seconds. These rapidly changing parallel electric fields can lead to broadband acceleration of auroral electrons, often called the Alfvénic aurora. Such interactions do not only take place in Earth’s magnetosphere, but have also been observed in Jupiter’s magnetosphere by the Juno satellite.
The interaction of Io with the corotating magnetosphere of Jupiter is known to produce Alfvén wings that couple the moon to Jupiter's ionosphere. We present first results from a new numerical model ...to describe the propagation of these Alfvén waves in this system. The model is cast in magnetic dipole coordinates and includes a dense plasma torus that is centered around the centrifugal equator. Results are presented for two density models, showing the dependence of the interaction on the magnetospheric density. Model results are presented for the case when Io is near the centrifugal and magnetic equators as well as when Io is at its northernmost magnetic latitude. The effect of the conductance of Jupiter's ionosphere is considered, showing that a long auroral footprint tail is favored by high Pedersen conductance in the ionosphere. The current patterns in these cases show a U‐shaped footprint due to the generation of field‐aligned current on the Jupiter‐facing and Jupiter‐opposed sides of Io, which may be related to the structure in the auroral footprint seen in the infrared by Juno. A model for the development of parallel electric fields is introduced, indicating that the main auroral footprints of Io can generate parallel potentials of up to 100 kV.
Plain Language Summary
Jupiter's moon Io generates electrical currents when it passes through Jupiter's magnetic field. These currents take the form of fluctuations in the magnetic field lines, much like the waves on a stringed musical instrument. Due to the motion of Io, these waves follow behind Io and bounce back and forth between Jupiter and the dense ionized gas emitted by Io. This process creates auroral emissions that can be observed, e.g., with the Hubble Space Telescope.
Key Points
The spacing of the main auroral spots in Io's footprint tail depends on the density profile assumed as well as the magnetic latitude of Io
Partial reflections at the boundary of the Io plasma torus lead to secondary reflections and weaker auroral spots between the main spots
The length of the auroral tail depends on the ionospheric conductance at Jupiter, with higher conductances leading to longer tails
The ionospheric Alfvén resonator (IAR) is a structure formed by the rapid decrease in the plasma density above a planetary ionosphere. This results in a corresponding increase in the Alfvén speed ...that can provide partial reflection of Alfvén waves. At Earth, the IAR on auroral field lines is associated with the broadband acceleration of auroral particles, sometimes termed the Alfvénic aurora. This arises since phase mixing in the IAR reduces the perpendicular wavelength of the Alfvén waves, which enhances the parallel electric field due to electron inertia. This parallel electric field fluctuates at frequencies of 0.1–20.0 Hz, comparable to the electron transit time through the acceleration region, leading to the broadband acceleration. The prevalence of such broadband acceleration at Jupiter suggests that a similar process can occur in the Jovian IAR. A numerical model of Alfvén wave propagation in the Jovian IAR has been developed to investigate these interactions, indicating that the IAR resonant frequencies are in the same range as those at Earth. This model describes the evolution of the electric and magnetic fields in the low‐altitude region close to Jupiter that is sampled during Juno's perijove passes. In particular, the model relates measurement of magnetic fields below the ion cyclotron frequency from the MAG and Waves instruments on Juno and electric fields from Waves to the associated parallel electric fields that can accelerate auroral particles.
Plain Language Summary
Just like at Earth, the polar regions of the planet Jupiter are circled by a luminous aurora (northern and southern lights) that can be seen from telescopes like the Hubble Space Telescope near Earth. The aurora on both planets is produced by electrons impacting the upper atmosphere, causing the atoms and molecules in this region to emit light. At Earth, these electrons are mainly produced by large voltages that cause all the electrons to be accelerated to nearly the same energy. However, recent observations from the Juno satellite at Jupiter shows that these electrons are mainly accelerated over a broad range of energies. This suggests that the voltages accelerating these electrons are fluctuating rapidly in time. Such fluctuations can be caused by the strong increase in the effective wave speed due to a rapid decrease in the number of electrons as the altitude is increased. We have developed a computer model to help understand these interactions.
Key Points
Broadband acceleration of auroral particles at Jupiter, can be achieved by Alfvén waves propagating in the ionospheric Alfvén resonator
Numerical results indicate that electrons could be accelerated to the 10–100 keV range for observed levels of Alfvén wave activity
There is also an Alfvén resonator in the high‐Alfvén speed velocity region between the ionosphere and the dense plasma sheet
Juno's highly inclined orbits provide opportunities to sample high‐latitude magnetic field lines connected to the orbit of Io, among the other Galilean satellites. Its payload offers both ...remote‐sensing and in‐situ measurements of the Io‐Jupiter interaction. These are at discrete points along Io's footprint tail and at least one event (12th perijove) was confirmed to be on a flux tube Alfvénically connected to Io, allowing for an investigation of how the interaction evolves down‐tail. Here we present Alfvén Poynting fluxes and field‐aligned current densities along field lines connected to Io and its orbit. We explore their dependence as a function of down‐tail distance and show the expected decay as seen in UV brightness and electron energy fluxes. We show that the Alfvén Poynting and electron energy fluxes are highly correlated and related by an efficiency that is fully consistent with acceleration from Alfvén wave filamentation via a turbulent cascade process.
Plain Language Summary
Io and Jupiter are electrodynamically coupled resulting in the Io footprint tail. This is one of the most persistent, stable, and recognizable features of Jupiter's aurora. The Juno spacecraft routinely samples magnetic field lines connected to Io's orbit, allowing for an investigation of this powerful coupling. We use data recorded by Juno to estimate a proxy for the strength of this interaction, that is, electromagnetic energy, and show its dependence downstream of Io and how the interaction decays. We further show that the available electromagnetic energy and electron energy are intimately linked, suggesting a transfer of energy between wave and particles. This is the basis upon which electrons end up precipitating into Jupiter's upper atmosphere and generate some of the brightest auroras.
Key Points
Alfvénic Poynting fluxes and electron energy fluxes are highly correlated on magnetic field lines connected to Io's orbit
The efficiency in the Main Alfvén Wing is ∼10%, fully consistent with Alfvén wave filamentation via a turbulent cascade process
Field‐aligned current densities are quantified and exhibit a decay in magnitude down‐tail of Io
High‐resolution multispacecraft Swarm data are used to examine magnetosphere‐ionosphere coupling during a period of northward interplanetary magnetic field (IMF) on 31 May 2014. The observations ...reveal a prevalence of unexpectedly large amplitude (>100 nT) and time‐varying magnetic perturbations during the polar passes, with especially large amplitude magnetic perturbations being associated with large‐scale downward field‐aligned currents. Differences between the magnetic field measurements sampled at 50 Hz from Swarm A and C, approximately 10 s apart along track, and the correspondence between the observed electric and magnetic fields at 16 samples per second, provide significant evidence for an important role for Alfvén waves in magnetosphere‐ionosphere coupling even during northward IMF conditions. Spectral comparison between the wave E‐ and B‐fields reveals a frequency‐dependent phase difference and amplitude ratio consistent with interference between incident and reflected Alfvén waves. At low frequencies, the E/B ratio is in phase with an amplitude determined by the Pedersen conductance. At higher frequencies, the amplitude and phase change as a function of frequency in good agreement with an ionospheric Alfvén resonator model including Pedersen conductance effects. Indeed, within this Alfvén wave incidence, reflection, and interference paradigm, even quasi‐static field‐aligned currents might be reasonably interpreted as very low frequency (ω → 0) Alfvén waves. Overall, our results not only indicate the importance of Alfvén waves for magnetosphere‐ionosphere coupling but also demonstrate a method for using Swarm data for the innovative experimental diagnosis of Pedersen conductance from low‐Earth orbit satellite measurements.
Plain Language Summary
The study shows evidence that electromagnetic waves in the ionosphere and currents flowing into the ionosphere can be described by the same physical model. This is important for estimating the total energy going into the ionosphere and potentially allows deriving important high‐resolution information about the ionosphere by studying data recorded when spacecraft fly over the auroral region.
Key Points
Multipoint Swarm 50 sps magnetic field data reveal unexpectedly large amplitude time‐varying fields which evolve on 10 s timescales
The observed E‐ and B‐field spectra are consistent with a continuum of incident, reflected, and interfering Alfvén waves
Alfvén wave dynamics are shown to be of fundamental importance for magnetosphere‐ionosphere coupling and consistent with an IAR model
Recent observations of particle distributions that are narrow in pitch angle and broad in energy have suggested that kinetic Alfvén waves are a significant contributor to auroral particle ...acceleration. Oscillations at frequencies near 1 Hz are a natural consequence of the propagation of Alfvén waves in the strongly varying Alfvén speed profile above the auroral ionosphere, the so‐called ionospheric Alfvén resonator. These waves often propagate in the presence of perpendicular density gradients at various spatial scales. Simulations have been performed to study the evolution of these fields including both parallel and perpendicular inhomogeneity. Phase mixing at the boundaries of the density cavity leads to small‐scale Alfvén waves, which can develop the parallel electric fields needed to accelerate the Alfvénic aurora. These simulations verify the kinetic Alfvén wave dispersion relation in the electron inertial limit, which predicts that the perpendicular phase and group velocity of these waves are in the opposite direction. In addition, the results show that narrow spatial scales are favored by high ionospheric conductance.
The electrodynamics associated with dual discrete arc aurora with antiparallel flow along the arcs were observed nearly simultaneously by the enhanced Polar Outflow Probe (e‐POP) and the Swarm A and ...C spacecraft. Auroral imaging from e‐POP reveals 1–10 km structuring of the arcs, which move and evolve on second timescales and confound the traditional single‐spacecraft field‐aligned current algorithms. High‐cadence magnetic data from e‐POP show 1–10 Hz, inferred Alfvénic, perturbations coincident with and at the same scale size as the observed dynamic auroral fine structures. High‐cadence electric and magnetic field data from Swarm A reveal nonstationary electrodynamics involving reflected and interfering Alfvén waves and modulation consistent with trapping in the ionospheric Alfvén resonator (IAR). These observations suggest a role for Alfvén waves, perhaps also the IAR, in discrete arc dynamics on 0.2–10 s timescales and ~1–10 km spatial scales and reinforce the importance of considering Alfvén waves in magnetosphere‐ionosphere coupling.
Plain Language Summary
An ongoing question in space physics is whether the energy that powers the vibrant and dynamic aurora is the result of static electric fields or magnetic waves. We address this question using data from three observational satellites, e‐POP, Swarm A, and Swarm C. We compare electric and magnetic field measurements at the locations of the spacecraft to high‐speed images of the aurora below as the satellites traveled over northern Canada. We show that as the satellites traveled over the aurora they detected magnetic waves known as Alfvén waves. We argue that these waves play an important and underappreciated role in transporting energy from near‐Earth space to the atmosphere in order to power the aurora.
Key Points
High‐resolution e‐POP magnetic and auroral data reveal coherent, inferred Alfvénic, dynamics and structuring of discrete auroral arcs
Multipoint high‐cadence Swarm E‐ and B‐ field data reveal nonstationary electrodynamics involving reflected and interfering Alfvén waves
Suggests a role for Alfvén waves and possibly ionospheric Alfvén resonator (IAR) in discrete arc dynamics on 0.2–10 s and 1–10 km scales
The Juno spacecraft's polar orbits have enabled direct sampling of Jupiter's low‐altitude auroral field lines. While various data sets have identified unique features over Jupiter's main aurora, they ...are yet to be analyzed altogether to determine how they can be reconciled and fit into the bigger picture of Jupiter's auroral generation mechanisms. Jupiter's main aurora has been classified into distinct “zones”, based on repeatable signatures found in energetic electron and proton spectra. We combine fields, particles, and plasma wave data sets to analyze Zone‐I and Zone‐II, which are suggested to carry upward and downward field‐aligned currents, respectively. We find Zone‐I to have well‐defined boundaries across all data sets. H+ and/or H3+ cyclotron waves are commonly observed in Zone‐I in the presence of energetic upward H+ beams and downward energetic electron beams. Zone‐II, on the other hand, does not have a clear poleward boundary with the polar cap, and its signatures are more sporadic. Large‐amplitude solitary waves, which are reminiscent of those ubiquitous in Earth's downward current region, are a key feature of Zone‐II. Alfvénic fluctuations are most prominent in the diffuse aurora and are repeatedly found to diminish in Zone‐I and Zone‐II, likely due to dissipation, at higher altitudes, to energize auroral electrons. Finally, we identify significant electron density depletions, by up to 2 orders of magnitude, in Zone‐I, and discuss their important implications for the development of parallel potentials, Alfvénic dissipation, and radio wave generation.
Key Points
We discuss how the various fields, particles, and plasma wave phenomena of Jupiter's low‐altitude auroral zones are related
We confirm that Zone‐I and Zone‐II carry upward and downward field‐aligned currents, respectively
We identify large‐scale electron density depletions over the auroral zones and discuss the implications for auroral acceleration processes
Due to differences in solar illumination, a geomagnetic field line may have one foot point in a dark ionosphere while the other ionosphere is in daylight. This may happen near the terminator under ...solstice conditions. In this situation, a resonant wave mode may appear, which has a node in the electric field in the sunlit (high conductance) ionosphere and an antinode in the dark (low conductance) ionosphere. Thus, the length of the field line is one quarter of the wavelength of the wave, in contrast with half‐wave field line resonances in which both ionospheres are nodes in the electric field. These quarter waves have resonant frequencies that are roughly a factor of 2 lower than the half‐wave frequency on the field line. We have simulated these resonances using a fully three‐dimensional model of ULF waves in a dipolar magnetosphere. The ionospheric conductance is modeled as a function of the solar zenith angle, and so this model can describe the change in the wave resonance frequency as the ground magnetometer station varies in local time. The results show that the quarter‐wave resonances can be excited by a shock‐like impulse at the dayside magnetosphere and exhibit many of the properties of the observed waves. In particular, the simulations support the notion that a conductance ratio between day and night foot points of the field line must be greater than about 5 for the quarter waves to exist.
Key Points
Quarter‐wave modes can be excited on magnetic field lines with one foot point in a sunlit ionosphere and the other foot points in darkness
These wave modes are strongly coupled with cavity resonances in the plasmasphere and magnetosphere
The quarter‐wave modes are found when the ratio of Pedersen conductances between the sunlit and dark ionospheres is greater than 5
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