P. Louarn

Magnetic signatures of plasma-depleted flux tubes in the Saturnian inner magnetosphere

Initial Cassini observations have revealed evidence for interchanging magnetic flux tubes in the inner Saturnian magnetosphere. Some of the reported flux tubes differ remarkably by their magnetic signatures, having a depressed or enhanced magnetic pressure relative to their surroundings. The ones with stronger fields have been interpreted previously as either outward moving mass-loaded or inward moving plasma-depleted flux tubes based on magnetometer observations only. We use detailed multi-instrumental observations of small and large density depletions in the inner Saturnian magnetosphere from Cassini Rev. A orbit that enable us to discriminate amongst the two previous and opposite interpretations. Our analysis undoubtedly confirms the similar nature of both types of reported interchanging magnetic flux tubes, which are plasma-depleted, whatever their magnetic signatures are. Their different magnetic signature is clearly an effect associated with latitude. These Saturnian plasma-depleted flux tubes ultimately may play a similar role as the Jovian ones.

Emission and propagation of Saturn kilometric radiation: Magnetoionic modes, beaming pattern, and polarization state

The Cassini mission crossed the source region of the Saturn kilometric radiation (SKR) on 17 October 2008. On this occasion, the Radio and Plasma Wave Science (RPWS) experiment detected both local and distant radio sources, while plasma parameters were measured in situ by the magnetometer (MAG) and the Cassini Plasma Spectrometer (CAPS). A goniopolarimetric inversion was applied to RPWS 3-antenna electric measurements to determine the wave vector k and the complete state of polarization of detected waves. We identify broadband extraordinary (X) as well as narrowband ordinary (O) mode SKR at low frequencies. Within the source region, SKR is emitted just above the X mode cutoff frequency in a hot plasma, with a typical electron-to-wave energy conversion efficiency of 1% (2% peak). The knowledge of the k-vector is then used to derive the locus of SKR sources in the kronian magnetosphere, that shows X and O components emanating from the same regions. We also compute the associated beaming angle at the source theta=(k,-B) either from (i) in situ measurements or a model of the magnetic field vector or from (ii) polarization measurements. Obtained results, similar for both modes, suggest quasi-perpendicular emission for local sources, whereas the beaming pattern of distant sources appears as a hollow cone with a frequency-dependent constant aperture angle: theta=75{deg}+/-15{deg} below 300kHz, decreasing at higher frequencies to reach theta(1000kHz)=50{deg}+/-25{deg}. Finally, we investigate quantitatively the SKR polarization state, observed to be strongly elliptical at the source, and quasi-purely circular for sources located beyond approximately 2 kronian radii. We show that conditions of weak mode coupling are achieved along the ray path, under which the magneto-ionic theory satisfactorily describes the evolution of the observed polarization.

Electron beams and loss cones in the auroral regions of Jupiter

We report on the first observations of 100 eV to 100 keV electrons over the auroral regions of Jupiter by the Jovian Auroral Distributions Experiment (JADE) onboard the Juno mission. The focus is on the regions that were magnetically connected to the main auroral oval. Amongst the most remarkable features, JADE observed electron beams, mostly upward going but also some downward going in the south, at latitudes from ~69° to 72° andu2009~u2009−66° to −70° corresponding to M-shells (“M” for magnetic) from ~18 to 54 and ~28 to 61, respectively. The beams were replaced by upward loss cones at lower latitudes. There was no evidence of strongly accelerated downward electrons analogous to the auroral “inverted Vs” at Earth. Rather, the presence of upward loss cones suggests a diffuse aurora process. The energy spectra resemble tails of distributions or power laws (suggestive of a stochastic acceleration process), but can also have some clear enhancements or even peaks generally between 1 and 10 keV. Electron intensities change on time scales of a second or less at times implying that auroral structures can be of the order of a few tens of km.

Accelerated flows at Jupiter's magnetopause: Evidence for magnetic reconnection along the dawn flank

We report on plasma and magnetic field observations from Junos Jovian Auroral Distributions Experiment and Magnetic Field Investigation at eighteen magnetopause crossings when the spacecraft was located at ~6u2009h magnetic local time and 73 – 114 jovian radii from Jupiter. Several crossings showed evidence of plasma energization, accelerated ion flows, and large magnetic shear angles, each representing a signature of magnetic reconnection. These signatures were observed for times when the magnetosphere was in both compressed and expanded states. We compared the flow change magnitudes to a simplified Walen relation and found ~60% of the events to be 110% or less of the predicted values. Close examination of two magnetopause encounters revealed characteristics of a rotational discontinuity and an open magnetopause. These observations provide compelling evidence that magnetic reconnection can occur at Jupiters dawn magnetopause and should be incorporated into theories of solar wind coupling and outer magnetosphere dynamics at Jupiter.

A new view of Jupiter's auroral radio spectrum

Junos first perijove science observations were carried out on 27 August 2016. The 90° orbit inclination and 4163u2009km periapsis altitude provide the first opportunity to explore Jupiters polar magnetosphere. A radio and plasma wave instrument on Juno called Waves provided a new view of Jupiters auroral radio emissions from near 10u2009kHz to ~30u2009MHz. This frequency range covers the classically named decametric, hectometric, and broadband kilometric radio emissions, and Juno observations showed much of this entire spectrum to consist of V-shaped emissions in frequency-time space with intensified vertices located very close to the electron cyclotron frequency. The proximity of the radio emissions to the cyclotron frequency along with loss cone features in the energetic electron distribution strongly suggests that Juno passed very close to, if not through, one or more of the cyclotron maser instability sources thought to be responsible for Jupiters auroral radio emissions.

Identification of Saturn's magnetospheric regions and associated plasma processes: Synopsis of Cassini observations during orbit insertion

[1]xa0Saturns magnetosphere is currently studied from the microphysical to the global scale by the Cassini-Huygens mission. During the first half of 2004, in the approach phase, remote sensing observations of Saturns magnetosphere gave access to its auroral, radio, UV, energetic neutral atom, and dust emissions. Then, on 1 July 2004, Cassini Saturn orbit insertion provided us with the first in situ exploration of Saturns magnetosphere since Voyager. To date, Saturn orbit insertion is the only Cassini orbit to have been described in common by all field and particle instruments. We use the comprehensive suite of magnetospheric and plasma science instruments to give a unified description of the large-scale structure of the magnetosphere during this particular orbit, identifying the different regions and their boundaries. These regions consist of the Saturnian ring system (region 1, within 3 Saturn radii (RS)) and the cold plasma torus (region 2, within 5–6 RS) in the inner magnetosphere, a dynamic and extended plasma sheet (region 3), and an outer high-latitude magnetosphere (region 4, beyond 12–14 RS). We compare these observations to those made at the time of the Voyager encounters. Then, we identify some of the dominant chemical characteristics and dynamical phenomena in each of these regions. The inner magnetosphere is characterized by the presence of the dominant plasma and neutral sources of the Saturnian system, giving birth to a very special magnetosphere dominated by water products. The extended plasma sheet, where the ring current resides, is a variable region with stretched magnetic field lines and contains a mixture of cold and hot plasma populations resulting from plasma transport processes. The outer high-latitude magnetosphere is characterized by a quiet magnetic field and an absence of plasma. Saturn orbit insertion observations enabled us to capture a snapshot of the large-scale structure of the Saturnian magnetosphere and of some of the main plasma processes operating in this complex environment. The analysis of the broad diversity of these interaction processes will be one of the main themes of magnetospheric and plasma science during the Cassini mission.

Plasma environment at the dawn flank of Jupiter's magnetosphere: Juno arrives at Jupiter

This study examines the first observations from the Jovian Auroral Distributions Experiment (JADE) as the Juno spacecraft arrived at Jupiter. JADE observations show that Juno crossed the bow shock at 08:16u2009UT on 2016 day of year (DOY) 176 and magnetopause at 21:20 on DOY 177, with additional magnetopause encounters until 23:39 on DOY 181. JADE made the first detailed observations of the plasma environment just inside the dawn flank of the magnetopause. We find subcorotational ions and variable electron beaming, with multiple flux tubes of varying plasma properties. Ion composition shows a dearth of heavy ions; protons dominate the plasma, with only intermittent, low fluxes of O+/S++, along with traces of O++ and S+++. We also find very little H3+ or He+, which are expected for an ionospheric plasma source. A few heavy ion bursts occur when the radial field nears reversal, but many other such reversals are not accompanied by heavy ions.

Plasma measurements in the Jovian polar region with Juno/JADE

Jupiters main auroral oval provides a window into the complex magnetospheric dynamics of the jovian system. The Juno spacecraft entered orbit about Jupiter on 5 July 2016 and carries onboard the Auroral Distributions Experiment (JADE) that can directly sample the auroral plasma structures. Here, we identify five distinct regimes in the JADE data based on composition/energy boundaries and magnetic field mappings, which exhibit considerable symmetry between the northern and southern passes. These intervals correspond to periods when Juno was connected to the Io torus, inner plasma sheet, middle plasma sheet, outer plasma sheet, and the polar region. When connected to the torus and inner plasma sheet, the heavy ions are consistent with a corotating pickup population. For Junos first perijove, we do not find evidence for a broad auroral acceleration region at Jupiters main auroral oval for energies below 100u2009keV.

Generation of the Jovian hectometric radiation: First lessons from Juno

Using Juno plasma and wave and magnetic observations (JADE and Waves and MAG instruments), the generation mechanism of the Jovian hectometric radio emission is analyzed. It is shown that suitable conditions for the cyclotron maser instability (CMI) are observed in the regions of the radio sources. Pronounced loss cone in the electron distributions are likely the source of free energy for the instability. The theory reveals that sufficient growth rates are obtained from the distribution functions that are measured by the JADE-Electron instrument. The CMI would be driven by upgoing electron populations at 5–10u2009keV and 10–30° pitch angle, the amplified waves propagating at 82°–87° from the B field, a fraction of a percent above the gyrofrequency. Typical e-folding times of 10−4u2009s are obtained, leading to an amplification path of ~1000u2009km. Overall, this scenario for generation of the Jovian hectometric waves differs significantly from the case of the auroral kilometric radiation at Earth.

What is the nature of magnetosheath FTEs

Cluster multipoint measurements are used to study two successive magnetosheath flux transfer events (FTEs). Magnetic field lines in the leading region are found to be closed magnetospheric field lines. For event 1 these field lines are wounded up by a large current structure oriented eastward and moving poleward. Conversely, the trailing region corresponds to opened magnetic field lines. For both events the leading edge of the FTEs is a tangential discontinuity separating the magnetosheath from closed field lines. In the case of event 1 magnetosheath ions are accelerated through the FTE trailing edge via a rotational discontinuity and penetrate on closed field lines through a second discontinuity. Thus, the ion jet is accelerated equatorward of the spacecraft but the backtracking of the discontinuities and the lack of dispersion show that ion acceleration occurs at less than 2u2009RE from Cluster. On the other hand the extrapolation forward indicates that the FTE bulge steepens as in simulations of Dorelli and Bhattacharjee (2009). Evidence is given for the penetration of magnetosheath ions inside the core of the FTE, on closed field lines. Magnetosheath electrons are accelerated in parallel and antiparallel directions on open and on closed field lines, thus breaking the frozen-in condition. Event 2 is also split in two distinct regions but no evidence is found for accelerated bidirectional magnetosheath electrons. For event 2 the two discontinuities at the trailing region are stacked together when they are crossed by the spacecraft, suggesting that the current splitting is a reconnection signature.