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In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 astronomical units (au) of interstellar space, revealing a large-scale density gradient and compressive turbulence outside the heliopause. Previous studies of the plasma density relied exclusively on the detection of discrete plasma oscillation events that are triggered ahead of shocks propagating outwards from the Sun, which were used to infer the plasma frequency and hence density. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 Plasma Wave System data that persists from 2017 onwards and enables a steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling time of 3 days, or 0.03 au. We find au-scale density fluctuations that trace turbulence in the interstellar medium between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and could be clarified by new findings from Voyager or a future interstellar mission. The persistence of the emission suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events., Comment: This is a preprint of an article published in Nature Astronomy. The final authenticated version is available online at: https://doi.org/10.1038/s41550-021-01363-7

Due to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind at the ionopause at Mars, with only 13% of the cases where the ionospheric thermal pressure plays a more important role in pressure balance. This percentage at Venus, however, is up to 65%. We also find that the ionopause altitude at Mars decreases as the normal component of the solar wind dynamic pressure increases, similar to the altitude variation of the ionopauses at Venus. Moreover, our results show that the ionopause thickness at Mars and Venus is mainly determined by the ion gyromotion and is equivalent to about 5 ion gyroradii.

Previous observations have shown that, during Martian dust storms, the peak of the ionosphere rises in altitude. Observational studies of this type, however, have been extremely limited. Using 13 years of ionospheric peak altitude data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on Mars Express, we study how the peak altitude responded to dust storms during six different Mars Years (MY). The peak altitude increased $\sim$10-15 km during all six events, which include a local dust storm (MY 33), three regional regional dust storms (MYs 27, 29, and 32), and two global dust storms (MYs 28 and 34). The peak altitude's orbit-to-orbit variability was exceptionally large at the apexes of the MY 29 and MY 32 dust seasons, and dramatically increased during the MY 28 and MY 34 global dust storms. We conclude that dust storms significantly increase upper atmospheric variability, which suggests that they enhance dynamical processes that couple the lower and upper atmospheres, such as upward propagating gravity waves or atmospheric tides.

We use Mars Atmosphere and Volatile EvolutioN observations of the upstream solar wind, and Mars Express observations of ionospheric electron densities and magnetic fields, to study how the topside ionosphere ($>$ 320 km) of Mars is affected by variations in solar wind dynamic pressure. We find that high solar wind dynamic pressures result in the topside ionosphere being depleted of plasma at all solar zenith angles, coincident with increased induced magnetic field strengths. The depletion of topside plasma in response to high solar wind dynamic pressures is observed in both weak and strong crustal magnetic field regions. Taken together, our results suggest that high solar wind dynamic pressures lead to ionospheric compression, increased ion escape, and reduced day-to-night plasma transport in the high-altitude nightside ionosphere.

We study the ionopause of Mars using a database of 6,893 ionopause detections obtained over 11 years by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) experiment. The ionopause, in this work, is defined as a steep density gradient that appears in MARSIS remote sounding ionograms as a horizontal line at frequencies below 0.4 MHz. We find that the ionopause is located on average at an altitude of $363 \pm 65$ km. We also find that the ionopause altitude has a weak dependence on solar zenith angle and varies with the solar extreme ultraviolet (EUV) flux on annual and solar cycle time scales. Furthermore, our results show that very few ionopauses are observed when the crustal field strength at 400 km is greater than 40 nT. The strong crustal fields act as mini-magnetospheres that alter the solar wind interaction and prevent the ionopause from forming., Comment: 22 pages, 5 figures

The interaction between the solar wind and a magnetosphere is fundamental to the dynamics of a planetary system. Here, we address fundamental questions on the large-scale magnetosheath flow around Saturn using a 3D magnetohydrodynamic (MHD) simulation. We find Saturn's polar-flattened magnetosphere to channel ~20% more flow over the poles than around the flanks at the terminator. Further, we decompose the MHD forces responsible for accelerating the magnetosheath plasma to find the plasma pressure gradient as the dominant driver. This is by virtue of a high-beta magnetosheath, and in turn, the high-MA bow shock. Together with long-term magnetosheath data by the Cassini spacecraft, we present evidence of how nonaxisymmetry substantially alters the conditions further downstream at the magnetopause, crucial for understanding solar wind-magnetosphere interactions such as reconnection and shear flow-driven instabilities. We anticipate our results to provide a more accurate insight into the global conditions upstream of Saturn and the outer planets., Comment: Accepted for publication in Journal of Geophysical Journal: Space Physics

The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW-LISM interaction. By performing three-dimensional, MHD plasma / kinetic neutral atom simulations, we determine the width of the outer heliosheath - the LISM plasma region affected by the presence of the heliosphere - and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by IBEX., Comment: Accepted for publication in Astrophysical Journal

In a two-week period between February and March of 2015, a series of interplanetary coronal mass ejections (ICMEs) and (solar energetic particles) SEPs made contact with Mars. The interactions were observed by several spacecraft, including Mars Express (MEX), Mars Atmosphere and Volatile Evolution Mission (MAVEN), and Mars Odyssey (MO). The ICME disturbances were characterized by an increase in ion speed, plasma temperature, magnetic field magnitude, and energetic electron flux. Furthermore, increased solar wind density and speeds, unusually high local electron densities and high flow velocities were detected on the nightside at high altitudes during the March 8th event. These effects are thought to be due to the transport of ionospheric plasma away from Mars. The peak electron density at periapsis shows a substantial increase, reaching number densities about 2.7 x 104 cm-3 during the second ICME in the deep nightside, which corresponds to an increase in the MOHigh-Energy Neutron Detector flux, suggesting an increase in the ionization of the neutral atmosphere due to the high intensity of charged particles. SEPs show a substantial enhancement before the shock of fourth ICME causing impact ionization and absorption of the surface echo intensity which drops to the noise levels. Moreover, the peak ionospheric density exhibits a discrete enhancement over a period of about 30 hrs around the same location, which maybe due to impact ionization. Ion escape rates at this time are calculated to be in the order of 1025 - 1026 s-1, consistent with MAVEN results, but somewhat higher.

We present initial analysis and conclusions from plasma observations made during the reported "Mars plume event" of March - April 2012. During this period, multiple independent amateur observers detected a localized, high-altitude "plume" over the Martian dawn terminator [Sanchez-Lavega et al., Nature, 2015, doi:10.1038/nature14162], the cause of which remains to be explained. The estimated brightness of the plume exceeds that expected for auroral emissions, and its projected altitude greatly exceeds that at which clouds are expected to form. We report on in-situ measurements of ionospheric plasma density and solar wind parameters throughout this interval made by Mars Express, obtained over the same surface region, but at the opposing terminator. Measurements in the ionosphere at the corresponding location frequently show a disturbed structure, though this is not atypical for such regions with intense crustal magnetic fields. We tentatively conclude that the formation and/or transport of this plume to the altitudes where it was observed could be due in part to the result of a large interplanetary coronal mass ejection (ICME) encountering the Martian system. Interestingly, we note that the only similar plume detection in May 1997 may also have been associated with a large ICME impact at Mars., Comment: 21 pages, 6 figures, article accepted for publication in J. Geophys. Res

The lightning activity in Saturn's atmosphere has been monitored by Cassini for more than six years. The continuous observations of the radio signatures called SEDs (Saturn Electrostatic Discharges) combine favorably with imaging observations of related cloud features as well as direct observations of flash-illuminated cloud tops. The Cassini RPWS (Radio and Plasma Wave Science) instrument and ISS (Imaging Science Subsystem) in orbit around Saturn also received ground-based support: The intense SED radio waves were also detected by the giant UTR-2 radio telescope, and committed amateurs observed SED-related white spots with their backyard optical telescopes. Furthermore, the Cassini VIMS (Visual and Infrared Mapping Spectrometer) and CIRS (Composite Infrared Spectrometer) instruments have provided some information on chemical constituents possibly created by the lightning discharges and transported upward to Saturn's upper atmosphere by vertical convection. In this paper we summarize the main results on Saturn lightning provided by this multi-instrumental approach and compare Saturn lightning to lightning on Jupiter and Earth., Comment: 10 pages, 6 figures, 2 tables; Proc. PRE VII conference Graz Sept. 2010

On 17 October 2008, the Cassini spacecraft crossed the southern sources of Saturn kilometric radiation (SKR), while flying along high-latitude nightside magnetic field lines. In situ measurements allowed us to characterize for the first time the source region of an extra-terrestrial auroral radio emission. Using radio, magnetic field and particle observations, we show that SKR sources are surrounded by a hot tenuous plasma, in a region of upward field-aligned currents. Magnetic field lines supporting radio sources map a continuous, high-latitude and spiral-shaped auroral oval observed on the dawnside, consistent with enhanced auroral activity. Investigating the Cyclotron Maser Instability (CMI) as a mechanism responsible for SKR generation, we find that observed cutoff frequencies are consistent with radio waves amplified perpendicular to the magnetic field by hot (6 to 9 keV) resonant electrons, measured locally.

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.

We describe the statistical properties of narrowband drifting auroral kilometric radiation ('striated' AKR) based on observations from the Cluster wideband receiver during 2002-2005. We show that the observed characteristics, including frequency drift rate and direction, narrow bandwidth, observed intensity, and beaming angular sizes are all consistent with triggering by upward traveling ion solitary structures (`ion holes'). We calculate the expected perturbation of a horseshoe electron distribution function by an ion hole by integrating the resonance condition for a cyclotron maser instability (CMI) using the perturbed velocity distribution. We find that the CMI growth rate can be strongly enhanced as the horseshoe velocity distribution contracts inside the passing ion hole, resulting in a power gain increase greater than 100 dB. The gain curve is sharply peaked just above the R-mode cut-off frequency, with an effective bandwidth ~50 Hz, consistent with the observed bandwidth of striated AKR emission. Ion holes are observed in situ in the acceleration region moving upward with spatial scales and speeds consistent with the observed bandwidth and slopes of SAKR bursts. Hence, we suggest that SAKR bursts are a remote sensor of ion holes and can be used to determine the frequency of occurrence, locations in the acceleration region, and lifetimes of these structures., Comment: 10 pages, 10 figures. J .Geophys. Res. (in press)

Isolated electrostatic structures are observed throughout much of the 4 Re by 19.6 Re Cluster orbit. These structures are observed in the Wideband plasma wave instrument's waveform data as bipolar and tripolar pulses. These structures are observed at all of the boundary layers, in the solar wind and magnetosheath, and along auroral field lines at 4.5-6.5 Re. Using the Wideband waveform data from the various Cluster spacecraft we have carried out a survey of the amplitudes and time durations of these structures and how these quantities vary with the local magnetic field strength. Such a survey has not been carried out before, and it reveals certain characteristics of solitary structures in a finite magnetic field, a topic still inadequately addressed by theories. We find that there is a broad range of electric field amplitudes at any specific magnetic field strength, and there is a general trend for the electric field amplitudes to increase as the strength of the magnetic field increases over a range of 5 to 500 nT. We provide a possible explanation for this trend that releates to the structures being Bernstein-Greene-Kruskal mode solitary waves. There is no corresponding dependence of the duration of the structures on the magnetic field strength, although a plot of these two quantities reveals the unexpected result that with the exception of the magnetosheath, all of the time durations for all of the other regions are comparable, wheras the magnetosheath time durations clearly are in a different category of much smaller time duration. We speculate that this implies the structures are much smaller in size., Comment: 24 pages plus 8 figures; paper presented at Spatio-Temporal Analysis and Multipoint Measurements in Space (STAMMS) conference, held in Orleans, France on 12-16 May 2003

The Cluster Wideband Data instrument has been used to determine the locations of auroral kilometric radiation (AKR) using very long baseline interferometry. The technique involves cross-correlating individual AKR bursts from all six Cluster baselines using time and frequency filtered waveforms. We report the locations of over 1,700 individual AKR bursts during six observing epochs between 10 July 2002 and 22 January 2003 when the Cluster constellation was high above the southern or northern hemisphere. In general we find that the AKR burst locations lie along magnetic field lines which map onto the nighttime auroral zone as expected from previous AKR studies. The distribution of AKR auroral footprint locations at each epoch had a overall spatial scale between 1000 - 2000 km, much larger than the positional uncertainty of an individual AKR burst location magnetic footprint, but a small fraction of the auroral oval. For two of the six epochs, there was a significant drift in the mean location of AKR activity over a period of 1-2 hours. The drift was predominantly in latitude at one epoch and in longitude at the other, with average drift speed V ~ 80-90 m s-1 at the AKR emission location., Comment: 31 pages, 9 figures, accepted for publication 19 June 2003 in JGR Space Physics. accepted for publication

Auroral hiss emissions are ubiquitous in planetary magnetospheres, particularly in regions where electric current systems are present. They are generally diagnostic of electrodynamic coupling between conductive bodies, thus making auroral and moon-connected magnetic field lines prime locations for their detection. However, the role of Saturn's rings as a dynamic conductive body has been elusive and of great interest to the community. Cassini's Grand Finale orbits afforded a unique opportunity to directly sample magnetic field lines connected to the main rings. Here we provide strong evidence for the persistent and organized presence of auroral hiss demonstrably associated with the main rings. This is in contrast to recent observations suggesting that Saturn's rings may be barriers to field-aligned currents. Our results provide a new view of Saturn's rings as a dynamic system that is in continuous and ordered electrodynamic coupling with the planet.

We report on a set of clear and abrupt decreases in the high-frequency boundary of whistlerode emissions detected by Cassini at high latitudes (about ±40°) during the low-altitude proximal flybys f Saturn . These abrupt decreases or dropouts have start and stop locations that correspond to L shells at the dges of the A and B rings. Langmuir probe measurements can confirm, in some cases, that the abrupt decrease in the high-frequency whistler mode boundary is associated with a corresponding abrupt electron density dropout over evacuated field lines connected to the A and B rings. Wideband data also reveal electron plasma oscillations and whistler mode cutoffs consistent with a low-density plasma in the region. he observation of the electron density dropout along ring-connecting field lines suggests that strong ambipolar forces are operating, drawing cold ionospheric ions outward to fill the flux tubes. There is an analog with the refilling of flux tubes in the terrestrial plasmasphere. We suggest that the ring-connected electron density dropouts observed between 1.1 and 1.3 R(sub s) are connected to the low-density ring plasma cavity observed overtop the A and B rings during the 2004 Saturn orbital insertion pass.

We re-examine the radio and plasma wave observations obtained during the Cassini Saturn orbit insertion period, as the spacecraft flew over the northern ring surface into a radial distance of 1.3 Rs (over the C-ring). Voyager era studies suggest the rings are a source of micro-meteoroid generated plasma and dust, with theorized peak impact-created plasma outflows over the densest portion of the rings (central B-ring). In sharp contrast, the Cassini Radio and Plasma Wave System (RPWS) observations identify the presence of a ring-plasma cavity located in the central portion of the B-ring, with little evidence of impact-related plasma. While previous Voyager era studies have predicted unstable ion orbits over the C- ring, leading to field-aligned plasma transport to Saturns ionosphere, the Cassini RPWS observations do not reveal evidence for such instability-created plasma fountains. Given the passive ring loss processes observed by Cassini, we find that the ring lifetimes should extend >10(exp 9) years, and that there is limited evidence for prompt destruction (loss in <100 Myrs).