With data from Van Allen Probes, we investigate electromagnetic ion cyclotron (EMIC) wave excitation using simultaneously observed ion distributions. Strong He band waves occurred while the ...spacecraft was moving through an enhanced density region. We extract from helium, oxygen, proton, and electron mass spectrometer measurement the velocity distributions of warm heavy ions as well as anisotropic energetic protons that drive wave growth through the ion cyclotron instability. Fitting the measured ion fluxes to multiple sinm‐type distribution functions, we find that the observed ions make up about 15% of the total ions, but about 85% of them are still missing. By making legitimate estimates of the unseen cold (below ∼2 eV) ion composition from cutoff frequencies suggested by the observed wave spectrum, a series of linear instability analyses and hybrid simulations are carried out. The simulated waves generally vary as predicted by linear theory. They are more sensitive to the cold O+ concentration than the cold He+ concentration. Increasing the cold O+ concentration weakens the He band waves but enhances the O band waves. Finally, the exact cold ion composition is suggested to be in a range when the simulated wave spectrum best matches the observed one.
Key Points
Study of EMIC wave excitation using directly measured ion measurements
Integrated analysis of observation, linear theory, and hybrid simulations
Magnetospheric banded chorus is enhanced whistler waves with frequencies ωr<Ωe, where Ωe is the electron cyclotron frequency, and a characteristic spectral gap at ωr≃Ωe/2. This paper uses spacecraft ...observations and two‐dimensional particle‐in‐cell simulations in a magnetized, homogeneous, collisionless plasma to test the hypothesis that banded chorus is due to local linear growth of two branches of the whistler anisotropy instability excited by two distinct, anisotropic electron components of significantly different temperatures. The electron densities and temperatures are derived from Helium, Oxygen, Proton, and Electron instrument measurements on the Van Allen Probes A satellite during a banded chorus event on 1 November 2012. The observations are consistent with a three‐component electron model consisting of a cold (a few tens of eV) population, a warm (a few hundred eV) anisotropic population, and a hot (a few keV) anisotropic population. The simulations use plasma and field parameters as measured from the satellite during this event except for two numbers: the anisotropies of the warm and the hot electron components are enhanced over the measured values in order to obtain relatively rapid instability growth. The simulations show that the warm component drives the quasi‐electrostatic upper band chorus and that the hot component drives the electromagnetic lower band chorus; the gap at ∼Ωe/2 is a natural consequence of the growth of two whistler modes with different properties.
Key PointsThe frequency gas of banded chorus is explained by linear dispersion theoryBanded chorus is excited by two distinct anisotropic electron componentsTheory, simulations, and observations agree
With the aid of the radio and plasma wave (Waves) instrument on board the Juno spacecraft, the first scientific close encounter to Jupiter (Perijove 1) of Juno led to an opportunity to perform ...direction‐finding measurements of the intense Jovian broadband kilometric (bKOM) radiation at 10 to 142 kHz, two escaping continuum radiation (ECR) events at 9 to 22 kHz, and two narrowband kilometric (nKOM) radiation events at 45–112 kHz. We conclude that the northern bKOM radio sources are localized on M‐shell = 50–60 field lines where M‐shell is similar to L‐shell for nondipolar fields. The beam cone half‐angle varies from 40° to 55°. By intersecting the wave k vector with the Jovian centrifugal equator, two ECR sources are located inside and outside of 11–12 RJ, and two nKOM sources are found between 11 and 20 RJ. These source frequencies and locations can be used for plasma diagnostics in Jupiter's inner magnetosphere.
Key Points
We perform direction‐finding analysis for Jupiter's low‐frequency radio emissions
Northern bKOM radio sources are located along high‐latitude auroral magnetic field lines
Four smooth components provide a proxy of the electron density profiles in Jovian inner magnetosphere
We expand on previous observations of magnetic reconnection in Jupiter's magnetosphere by constructing a survey of ion‐inertial scale plasmoids in the Jovian magnetotail. We developed an automated ...detection algorithm to identify reversals in the Bθ ${B}_{\theta }$ component and performed the minimum variance analysis for each identified plasmoid to characterize its helical structure. The magnetic field observations were complemented by data collected using the Juno Waves instrument, which is used to estimate the total electron density, and the JEDI energetic particle detectors. We identified 87 plasmoids with “peak‐to‐peak” durations between 10 and 300 s. Thirty‐one plasmoids possessed a core field and were classified as flux‐ropes. The other 56 plasmoids had minimum field strength at their centers and were termed O‐lines. Out of the 87 plasmoids, 58 had in situ signatures shorter than 60 s, despite the algorithm's upper limit being 300 s, suggesting that smaller plasmoids with shorter durations were more likely to be detected by Juno. We estimate the diameter of these plasmoids assuming a circular cross section and a travel speed equal to the Alfven speed in the surrounding lobes. Using the electron density inferred by Waves, we contend that these plasmoid diameters were within an order of the local ion‐inertial length. Our results demonstrate that magnetic reconnection in the Jovian magnetotail occurs at ion scales like in other space environments. We show that ion‐scale plasmoids would need to be released every 0.1 s or less to match the canonical 1 ton/s rate of plasma production due to Io.
Key Points
We identify and analyze 87 ion‐inertial scale plasmoids (56 O‐lines, 31 flux‐ropes) in the Jovian magnetotail using an automated algorithm
North‐South field reversals with peak‐to‐peak durations less than 60 s are more common than those with durations between 60 and 300 s
Ion‐inertial scale plasmoids alone cannot account for the >500 kg/s loss‐rate deficit unless they are being produced every ∼0.1 s or less
Non-thermal radio emissions from Saturn, known as Saturn Kilometric Radiation (SKR), are analyzed for the Faraday rotation effect detected in Cassini RPWS High Frequency Receiver (HFR) observations. ...This phenomenon, which mainly affects the lower-frequency part of SKR below 200 kHz, is characterized by a rotation of the semi-major axis of the SKR polarization ellipse as a function of frequency during wave propagation through a birefringent plasma medium. Faraday rotation is found in 4.1% of all HFR data recorded by Cassini above 20 degrees northern and southern magnetic latitude, from mid-2004 to late 2017. A statistical visibility analysis shows that elliptically polarized SKR from the dawn source regions, when beamed toward high latitudes into the noon and afternoon local time sectors, is most likely to experience Faraday rotation along the ray path. The necessary conditions for Faraday rotation are discussed in terms of birefringent media and sharp plasma density gradients, where SKR (mostly R-X mode) gets split into the two circularly polarized modes R-X and L-O. By means of a case study we also demonstrate how Faraday rotation provides an estimate for the average plasma density along the ray path.
•First statistical analysis of Faraday rotation in Saturn Kilometric Radiation.•Modeling of the source–observer geometry during Faraday rotation events.•Evidence for plasma density structures in the high-latitude magnetosphere of Saturn.•Application of Faraday rotation as a tool for probing the plasma density.
We investigate the structure of Jupiter's dawnside magnetopause using observations obtained by particle and fields instrumentation on the Juno spacecraft. Characterization of Jupiter's magnetopause ...is critical for the understanding of mass and energy transport between the solar wind and the magnetosphere. We find an extended magnetopause boundary layer (MPBL) during a magnetopause crossing on 14 July 2016. This thick MPBL, in combination with a large magnetic field component normal to the magnetopause boundary, suggests that strong magnetospheric compression enhances mass transport across the magnetopause via magnetic reconnection. We further identify ~2 h increases in the total magnetospheric pressure adjacent to the magnetopause on 14 July 2016 and 1 August 2016. These large‐scale structures provide evidence of focused energy transport into the magnetosphere via magnetohydrodynamic structures.
Key Points
Jupiter's dawnside magnetosphere is highly compressible and subject to strong Alfvén‐magnetosonic mode coupling
Magnetospheric compressions may enhance reconnection rates and increase mass transport across the magnetopause
Total pressure increases inside the magnetopause with durations of hours are indicative of strong solar wind‐magnetosphere energy transport
The Galileo spacecraft probed the Io atmosphere/torus interaction region along six flybys between 1995 and 2001. The instruments on board provided measurements of the plasma density, average ion ...temperature, composition, flow, and magnetic perturbations to which we compare models and from which we constrain Io's atmosphere asymmetries. We have developed a model of the local interaction at Io that couples an MHD model of the flow and magnetic perturbations around the moon with a multispecies chemistry model that includes the physical chemistry of the main species: atoms, molecules, and ions derived from Io's volcanic gases (S, O, SO2, SO). We prescribe several scenarios of the multicomponent neutral atmosphere of Io based on observations and compare our results with the plasma properties inferred from the Galileo measurements. Owing to limited data on the I25 flyby, it is excluded to make a total of five flybys for our comparisons. We show that the ion average temperature profile is a key quantity to constrain the radial extension and longitudinal asymmetries of the atmosphere of Io. We propose that the atmosphere has longitudinal asymmetries: its radial extension is limited upstream and significantly larger on the anti‐Jovian downstream side. A very extended corona of SO2and SO is present mainly downstream of Io. We discuss the possible temporal variability of Io's plasma‐atmosphere interaction by comparing two flybys at different times at approximately the same location in the wake. We also discuss the plasma composition in the wake and the O1356A auroral emissions observed on the flanks of Io.
Key Points
The interaction of Io's atmosphere is modeled with MHD and chemistry codes
Comparison to Galileo data suggests Io's atmosphere is longitudinally asymmetric
Interaction generates substantial corona of SO2
Two well‐defined Jovian decametric radio arcs were observed at latitudinal separations of 11°–16° from the Juno spacecraft near Jupiter and the Nançay Decameter Array (NDA) at Earth on 17 May and 25 ...August 2016. These discrete arcs are from the so‐called A source covering both Io‐related and non‐Io‐related emissions. By measuring the wave arrival time at two distant observers with propagation time correction, the remaining delay times are 92.8 ± 1.3 min for the first arc and 116.0 ± 1.2 min for the second arc. This implies that both radio sources are not controlled by the orbital motion of Io but Jupiter's rotation itself. The geometrical information for Juno and NDA and the loss cone‐driven electron cyclotron maser instability theory provide these radio sources that are located at about 173° ± 10° in system III longitude projected onto Jupiter's north surface and imply resonant electron energy ranges from 0.5 to 11 keV.
Key Points
Two decametric (DAM) arcs were observed from widely spaced latitudes by Juno and NDA
Direct evidence of long‐lasting DAM arcs corotating with Jupiter is presented
The loss cone‐driven CMI theory of fundamental X‐mode emission provides a constraint of the concurrent arcs
We report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's ...northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant timevariability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m−2 are required for its OI 1,356 Å emission making Ganymede a very poor auroral emitter.
Plain Language Summary
Jupiter's moon Ganymede is the largest moon in the solar system and the only known moon with an intrinsic magnetic field and two auroral ovals around its north and south poles. Earth also possesses two auroral ovals, which are bands of emission around its poles. This emission is also referred to as northern and southern lights. We use the Hubble Space Telescope to observe Ganymede's aurora around the time when NASA's Juno spacecraft had a close flyby at Ganymede. We find that the brightness of the northern and southern ovals alternate in intensity with a period of 10 hr. Additionally, we derive that an energy flux of several tens of milli‐Watt per square meter is necessary to power the auroral emission. This energy flux comes from energetic electrons accelerated in the vicinity of Ganymede.
Key Points
Hubble Space Telescope observations of Ganymede's orbitally trailing hemisphere on 7 June 2021 in support of Juno flyby
Brightness ratio of northern and southern auroral ovals oscillates such that the oval facing the Jovian plasma sheet is brighter
Oscillation suggests the aurora is driven by magnetic stresses coupling the moon's magnetic field to the surrounding Jovian plasma sheet
PDF
