Intense electromagnetic impulses induced by Jupiter's lightning have been recognised to produce both low-frequency dispersed whistler emissions and non-dispersed radio pulses. Here we report the ...discovery of electromagnetic pulses associated with Jovian lightning. Detected by the Juno Waves instrument during its polar perijove passes, the dispersed millisecond pulses called Jupiter dispersed pulses (JDPs) provide evidence of low density holes in Jupiter's ionosphere. 445 of these JDP emissions have been observed in snapshots of electric field waveforms. Assuming that the maximum delay occurs in the vicinity of the free space ordinary mode cutoff frequency, we estimate the characteristic plasma densities (5.1 to 250 cm
) and lengths (0.6 km to 1.3 × 10
km) of plasma irregularities along the line of propagation from lightning to Juno. These irregularities show a direct link to low plasma density holes with ≤250 cm
in the nightside ionosphere.
Jupiter’s unique x-ray aurora is driven by the same processes as Earth’s: Heavy ion scattering by electromagnetic waves.
Jupiter’s rapidly rotating, strong magnetic field provides a natural ...laboratory that is key to understanding the dynamics of high-energy plasmas. Spectacular auroral x-ray flares are diagnostic of the most energetic processes governing magnetospheres but seemingly unique to Jupiter. Since their discovery 40 years ago, the processes that produce Jupiter’s x-ray flares have remained unknown. Here, we report simultaneous in situ satellite and space-based telescope observations that reveal the processes that produce Jupiter’s x-ray flares, showing surprising similarities to terrestrial ion aurora. Planetary-scale electromagnetic waves are observed to modulate electromagnetic ion cyclotron waves, periodically causing heavy ions to precipitate and produce Jupiter’s x-ray pulses. Our findings show that ion aurorae share common mechanisms across planetary systems, despite temporal, spatial, and energetic scales varying by orders of magnitude.
We analyze data acquired by the Van Allen Probes on 8 November 2012, during a period of extended low geomagnetic activity, to gain new insight into plasmaspheric ultralow frequency (ULF) waves. The ...waves exhibited strong spectral power in the 5–40 mHz band and included multiharmonic toroidal waves visible up to the eleventh harmonic, unprecedented in the plasmasphere. During this wave activity, the interplanetary magnetic field cone angle was small, suggesting that the waves were driven by broadband compressional ULF waves originating in the foreshock region. This source mechanism is supported by the tailward propagation of the compressional magnetic field perturbations at a phase velocity of a few hundred kilometers per second that is determined by the cross‐phase analysis of data from the two spacecraft. We also find that the coherence and phase delay of the azimuthal components of the magnetic field from the two spacecraft strongly depend on the radial separation of the spacecraft and attribute this feature to field line resonance effects. Finally, using the observed toroidal wave frequencies, we estimate the plasma mass density for L = 2.6–5.8. By comparing the mass density with the electron number density that is estimated from the spectrum of plasma waves, we infer that the plasma was dominated by H+ ions and was distributed uniformly along the magnetic field lines. The electron density is higher than the prediction of saturated plasmasphere models, and this “super saturated” plasmasphere and the uniform ion distribution are consistent with the low geomagnetic activity that prevailed.
Key Points
The Van Allen Probes observed ULF waves in the plasmasphere
The waves were driven by ULF waves in the foreshock region
Plasmaspheric mass density is estimated from the frequency of the waves
Jupiter lightning discharges produce various kinds of phenomena including radio wave pulses at different frequencies. On 6 April 2019, the Juno Waves instrument captured an extraordinary series of ...radio pulses at frequencies below 150 kHz on timescales of submilliseconds. Quasi‐simultaneous multi‐instrument data show that the locations of their magnetic footprints are very close to the locations of ultrahigh frequency (UHF) sferics recorded by the Juno MWR instrument. Hubble Space Telescope images show that the signature of active convection includes cloud‐free clearings, in addition to the convective towers and deep water clouds that were also recognized in previous spacecraft observations of lightning source regions. Furthermore, the detections of 17 very low frequency/low‐frequency (VLF/LF) radio pulses suggest a minimum duration of lightning processes on the order of submilliseconds. These observations provide new constraints on the physical properties of Jupiter lightning.
Plain Language Summary
Jupiter lightning illuminates clouds and produces a strong pulse at radio wavelengths. Juno's radio observatory (consisting of two onboard instruments) in a broad radio range made several detections of extraordinary radio pulses on 6 April 2019. The high‐temporal observations of such radio pulses detected below 150 kHz indicate variations of the lightning related processes on the order of submilliseconds. Observations of these radio pulses and direct lightning‐induced radio emissions at 600 MHz come from the same area, very close to deep water clouds detected by the Hubble Space Telescope (HST) in the Jovian atmosphere. The coordinated Juno‐HST lightning observations provide a new way of understanding the lightning processes and lightning source regions associated with the cloud features at Jupiter.
Key Points
First common observations of VLF/LF radio pulses, UHF sferics, and thunderstorms were carried out using Juno and the Hubble Space Telescope
New high‐time resolution measurements of radio pulses associated with Jovian lightning processes resolve submillisecond variations
Cloud structure with juxtaposed deep water clouds, convective towers, and clearings is the signature of active convection
Observations of Jovian broadband kilometric (bKOM) radiation and ultraviolet (UV) auroras were acquired with the Waves and Juno‐UVS instruments for ∼2 hr over the northern and southern polar regions ...during Juno's perijoves 4, 5, and 6 passes (PJ4, PJ5, and PJ6). During all six time periods, Juno traversed auroral magnetic field lines connecting to the UV main auroral ovals, matching the estimates of bKOM radio source footprints. The localized bKOM radio sources for the PJ4 north pass map to magnetic field lines having distances of 10 to 12 Jovian radii (RJ) at the magnetic equator, whereas the extended bKOM radio sources for the other events map to field lines extending to 20–61 RJ. We found the peak bKOM intensities during Juno's potential radio source crossings show positive, negative, and no correlations with the UV main oval brightness and color ratio. Only the positive correlations suggest wave‐particle energy transport.
Plain Language Summary
Jupiter's polar auroras show complex emissions at radio and ultraviolet (UV) wavelengths. However, understanding these two phenomena and how they interact has been hindered by poor visibility of these observations from Earth or near‐equator spacecraft and insufficient spatial resolution of the Jovian radio images. Since 5 July 2016, the Juno spacecraft has toured Jupiter as its first polar explorer in a 53‐day eccentric orbit. One of Juno's main goals is to survey Jupiter's complex auroral regions at both poles. Using the first concurrent radio‐UV aurora observations of both north and south hemispheres during three closest approaches to the planet, we found that the source locations of Jovian auroral radiation at the kilometer wavelengths (broadband kilometric radiation) are magnetically connected to regions of bright UV emissions on the main auroral oval. For some events, the UV auroral intensity positively correlates with broadband kilometric radio intensity. Juno's vantage point of Jovian auroras at radio and UV wavelengths yields a better understanding of the auroral processes at different altitudes.
Key Points
First simultaneous observations of Jovian broadband kilometric radio source footprints and ultraviolet auroras were carried out using Juno
Jovian broadband kilometric radio source footprints correspond to the UV main oval locations
Link of Jovian radio intensity during Juno's near‐source crossing with both brightness and color ratio of the UV main oval was investigated
In the dawn sector, L ∼ 5.5 and MLT ∼ 4–7, from 01:30 to 06:00 UT during the 14 November 2012 geomagnetic storm, both Van Allen Probes observed an alternating sequence of locally quiet and disturbed ...intervals with two strikingly different power fluctuation levels and magnetic field orientations: either small (∼10−2 nT2) total power with strong GSM Bx and weak By or large (∼10 nT2) total power with weak Bx and strong By and Bz components. During both kinds of intervals the fluctuations occur in the vicinity of the local ion gyrofrequencies (0.01–10 Hz) in the spacecraft frame, propagate oblique to the magnetic field, (θ ∼ 60∘), and have magnetic compressibility C=|δB∥|/|δB⊥|∼1, where δB∥ (δB⊥) are the average amplitudes of the fluctuations parallel (perpendicular) to the mean field. Electric field fluctuations are present whenever the magnetic field is disturbed, and large electric field fluctuations follow the same pattern for quiet and disturbed intervals. Magnetic frequency power spectra at both spacecraft correspond to steep power laws ∼f−α with 4 < α < 5 for
f≲2 Hz, and 1.1 < α < 1.7 for
f≳ 2 Hz, spectral profiles that are consistent with weak kinetic Alfvén wave (KAW) turbulence. Electric power is larger than magnetic power for all frequencies above 0.1 Hz, and the ratio increases with increasing frequency. Vlasov linear analysis is consistent with the presence of compressive KAW with
k⊥ρi≲1, right‐handed polarization and positive magnetic helicity, in the plasma frame, considering a multiion plasma. All these results suggest the presence of weak KAW turbulence which dissipates the energy associated with the intermittent sudden changes in the magnetic field during the main phase of the storm.
Key Points
Observation of striking magnetic fluctuations during the 14 November 2012 magnetic storm
Spectral analysis suggests the presence of kinetic Alfvén waves turbulence
The interpretation is consistent with Vlasov linear theory analysis
During the Juno perijove explorations from 27 August 2016 through 1 September 2017, strong electromagnetic impulses induced by Jupiter lightning were detected by the Microwave Radiometer (MWR) ...instrument in the form of 600‐MHz sferics and recorded by the Waves instrument in the form of Jovian low‐dispersion whistlers discovered in waveform snapshots below 20 kHz. We found 71 overlapping events including sferics, while Waves waveforms were available. Eleven of these also included whistler detections by Waves. By measuring the separation distances between the MWR boresight and the whistler exit point, we estimated the distance whistlers propagate below the ionosphere before exiting to the magnetosphere, called the coupling distance, to be typically one to several thousand of kilometers with a possibility of no subionospheric propagation, which gives a new constraint on the atmospheric whistler propagation.
Plain Language Summary
Lightning at Jupiter produces a strong electromagnetic impulse, which can escape the Jovian atmosphere and enter the inner magnetosphere. Among the lightning, microwave‐frequency sferics come from lightning spots, and audio‐frequency whistlers propagate away from the spots below the ionosphere. If certain plasma conditions are met, these whistlers can leak into the magnetosphere. Estimates of whistler propagation distances at the planet have not been previously performed. Since the arrival at Jupiter on 5 July 2016, the Juno spacecraft has provided the opportunity to monitor the two kinds of lightning activity with two onboard instruments during its closest approach to Jupiter. This opportunity happens every 53.6 day in the eccentric, polar orbit of Juno. Using data collected during Juno's closest approaches to Jupiter, the whistler propagation distance was estimated to be approximately one to several thousand kilometers, which may be comparable to the terrestrial equivalent. This new approach provides the benefit of understanding multidimensional structures of lightning at Jupiter.
Key Points
First multi‐instrument investigation for Jupiter whistler and sferic events was carried out using Juno
Whistler and sferic observations were compared on timescales of the order of a few milliseconds
A new constraint on the whistler‐sferic coupling distances was estimated
Plasmaspheric hiss was observed by Van Allen Probe B in association
with energetic electron injections in the outer plasmasphere. The energy of
injected electrons coincides with the minimum resonant ...energy calculated for
the observed hiss wave frequency. Interestingly, the variations in hiss wave
intensity, electron flux and ultra low frequency (ULF) wave intensity exhibit remarkable
correlations, while plasma density is not correlated with any of these
parameters. Our study provides direct evidence for the first time that the
injected anisotropic electron population, which is modulated by ULF waves,
modulates the hiss intensity in the outer plasmasphere. This also implies
that the plasmaspheric hiss observed by Van Allen Probe B in the outer
plasmasphere (L > ∼ 5.5) is locally amplified.
Meanwhile, Van Allen Probe A observed hiss emission at lower L shells
(< 5), which was not associated with electron injections but
primarily modulated by the plasma density. The features observed by Van Allen
Probe A suggest that the observed hiss deep inside the plasmasphere may have
propagated from higher L shells.
Observations by the Pioneer, Voyager, Ulysses, and Galileo spacecraft in Jupiter's dayside magnetosphere revealed a cushion region, where the magnetic field became increasingly dipolar and the 10‐hr ...periodicity associated with rotation of the magnetodisc was no longer visible. Focused observations at the dawn terminator by the Juno spacecraft provide critical constraints on the formation physics of the dayside cushion. We observe a persistent 10‐hr periodicity at dawn with only minor distortions of the field near the magnetopause boundary, indicating the absence of a systematic dawn cushion region. These data suggest that the dayside cushion is not formed via mass loss associated with magnetic reconnection along a localized X line but rather may be due to the gradual compression of the dawnside magnetic field as it rotates toward local noon.
Plain Language Summary
Jupiter's space environment is strongly influenced by its fast rotation period of ~10 hr. Dipolar magnetic field lines are stretched out by centrifugal forces out to distances up to 100 times Jupiter's radius. Observations of Jupiter's dayside magnetosphere from several spacecraft revealed a cushion region that was thought to contain an absence of plasma, constraining how mass and magnetic field are circulated around the planet. This cushion was thought to extend to dawn local times, but there have been very few spacecraft observations of this region until Juno. The Juno data presented here demonstrates that no systematic cushion region exists near the dawn terminator such that flux circulation models need to be reassessed in order to account for a cushion region that is confined closer towards noon local times.
Key Points
Jupiter's magnetosphere was likely in a compressed state during the first several Juno perijove passes
Jupiter's magnetodisc extends to the magnetopause along the dawn terminator, indicating the absence of a thick dawn cushion region
The dayside cushion likely forms as a result of the gradual dipolarization of magnetodisc field lines
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