Upstream of Saturn and Titan Arridge, C. S.; André, N.; Bertucci, C. L. ...
Space science reviews,
12/2011, Volume:
162, Issue:
1-4
Journal Article, Conference Proceeding
Peer reviewed
The formation of Titan’s induced magnetosphere is a unique and important example in the solar system of a plasma-moon interaction where the moon has a substantial atmosphere. The field and particle ...conditions upstream of Titan are important in controlling the interaction and also play a strong role in modulating the chemistry of the ionosphere. In this paper we review Titan’s plasma interaction to identify important upstream parameters and review the physics of Saturn’s magnetosphere near Titan’s orbit to highlight how these upstream parameters may vary. We discuss the conditions upstream of Saturn in the solar wind and the conditions found in Saturn’s magnetosheath. Statistical work on Titan’s upstream magnetospheric fields and particles are discussed. Finally, various classification schemes are presented and combined into a single list of Cassini Titan encounter classes which is also used to highlight differences between these classification schemes.
Observations from the Cassini spacecraft have shown that Saturn's small icy moon Enceladus ejects a plume of water vapor and small ice particles into Saturn's rapidly co‐rotating magnetosphere. In ...this paper we show that the interaction of the moon with the magnetospheric plasma produces a number of electrodynamics effects that are remarkably similar to those observed in Earth's auroral regions and near Jupiter's moon Io. These include whistler‐mode emissions similar to terrestrial auroral hiss, magnetic‐field‐aligned electron beams, and currents associated with a standing Alfvén wave excited by the moon. Ray path analyses of the auroral hiss show that the electron beams responsible for the emissions are accelerated very close to the moon, most likely by parallel electric fields associated with the Alfvén wave. However, other possibilities such as electric fields due to electrostatic charging of the moon's surface or of particles in the water vapor plume should be considered.
Key Points
Enceladus has a strong electrodynamic interaction with Saturn's magnetosphere
The interaction involves acceleration of electrons and emission of auroral hiss
Most likely the electron acceleration is caused by a standing Alfven wave
Negative ions in the Enceladus plume Coates, A.J.; Jones, G.H.; Lewis, G.R. ...
Icarus (New York, N.Y. 1962),
04/2010, Volume:
206, Issue:
2
Journal Article, Conference Proceeding
Peer reviewed
During Cassini’s Enceladus encounter on 12th March 2008, the Cassini Electron Spectrometer, part of the CAPS instrument, detected fluxes of negative ions in the plumes from Enceladus. It is thought ...that these ions include negatively charged water group cluster ions associated with the plume and forming part of the ‘plume ionosphere’. In this paper we present our observations, argue that these are negative ions, and present preliminary mass identifications. We also suggest mechanisms for production and loss of the ions as constrained by the observations. Due to their short lifetime, we suggest that the ions are produced in or near the water vapour plume, or from the extended source of ice grains in the plume. We suggest that Enceladus now joins the Earth, Comet Halley and Titan as locations in the Solar System where negative ions have been directly observed although the ions observed in each case have distinctly different characteristics.
One of the unexpected results of the Cassini mission was the discovery of negative ions at altitudes between 950 and 1400
km in Titan's ionosphere with masses up to 10,000
amu/q Coates, A.J., Crary, ...F.J., Lewis, G.R., Young, D.T., Waite Jr., J.H., Sittler Jr., E.C., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett., 34, L22103,
doi:10.1029/2007GL030978; Waite Jr., J.H., Young, D. T., Coates, A. J., Crary, F. J., Magee, B. A., Mandt, K. E., Westlake, J. H., 2008. The Source of Heavy Organics and Aerosols in Titan's Atmosphere, submitted to Organic Matter in Space, Proceedings IAU Symposium no. 251. These ions are detected at low altitudes during Cassini's closest Titan encounters by the Cassini plasma spectrometer (CAPS) electron spectrometer. This result is important as it is indicative of complex hydrocarbon and nitrile chemical processes at work in Titan's high atmosphere. They may play a role in haze formation and ultimately in the formation of heavy particles (tholins), which fall through Titan's atmosphere and build up on the surface. During Cassini's prime mission negative ions were observed on 23 Titan encounters, including 7 in addition to those reported by Coates et al. Coates, A.J., Crary, F.J., Lewis, G.R., Young, D.T., Waite Jr., J.H., Sittler Jr., E.C., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett., 34, L22103,
doi:10.1029/2007GL030978. Here, we also examine the altitude and latitude dependence of the high-mass negative ions observed in Titan's ionosphere, and we examine the implications of these results. We find that the maximum negative ion mass is higher at low altitude and at high latitudes. We also find a weaker dependence of the maximum mass on solar zenith angle.
The Cassini Langmuir Probe (LP) onboard the Radio and Plasma Wave Science experiment has provided much information about the Saturnian cold plasma environment since the Saturn Orbit Insertion in ...2004. A recent analysis revealed that the LP is also sensitive to the energetic electrons (250–450 eV) for negative potentials. These electrons impact the surface of the probe and generate a current of secondary electrons, inducing an energetic contribution to the DC level of the current‐voltage (I‐V) curve measured by the LP. In this paper, we further investigated this influence of the energetic electrons and (1) showed how the secondary electrons impact not only the DC level but also the slope of the (I‐V) curve with unexpected positive values of the slope, (2) explained how the slope of the (I‐V) curve can be used to identify where the influence of the energetic electrons is strong, (3) showed that this influence may be interpreted in terms of the critical and anticritical temperatures concept detailed by Lai and Tautz (2008), thus providing the first observational evidence for the existence of the anticritical temperature, (4) derived estimations of the maximum secondary yield value for the LP surface without using laboratory measurements, and (5) showed how to model the energetic contributions to the DC level and slope of the (I‐V) curve via several methods (empirically and theoretically). This work will allow, for the whole Cassini mission, to clean the measurements influenced by such electrons. Furthermore, the understanding of this influence may be used for other missions using Langmuir probes, such as the future missions Jupiter Icy Moons Explorer at Jupiter, BepiColombo at Mercury, Rosetta at the comet Churyumov‐Gerasimenko, and even the probes onboard spacecrafts in the Earth magnetosphere.
Key Points
The impact of energetic electrons on the Langmuir probe is shown and modeled
We show the existence of critical/anticritical temperatures
The secondary yield of the probe surface is estimated from onboard measurements
•Power observed in SKR during the Saturn auroral campaign of 2013.•There is a good correlation between SKR power and power input to the UV aurora.•SKR intensifies and extends to lower frequencies in ...response to CIRs.•The UV aurora brighten and expand poleward in response to CIRs.
The Saturn auroral campaign carried out in the spring of 2013 used multiple Earth-based observations, remote-sensing observations from Cassini, and in situ-observations from Cassini to further our understanding of auroras at Saturn. Most of the remote sensing and Earth-based measurements are, by nature, not continuous. And, even the in situ measurements, while continuously obtained, are not always obtained in regions relevant to the study of the aurora. Saturn kilometric radiation, however, is remotely monitored nearly continuously by the Radio and Plasma Wave Science instrument on Cassini. This radio emission, produced by the cyclotron maser instability, is tightly tied to auroral processes at Saturn as are auroral radio emissions at other planets, most notably Jupiter and Earth. This paper provides the time history of the intensity of the radio emissions through the auroral campaign as a means of understanding the temporal relationships between the sometimes widely spaced observations of the auroral activity. While beaming characteristics of the radio emissions are known to prevent single spacecraft observations of this emission from being a perfect auroral activity indicator, we demonstrate a good correlation between the radio emission intensity and the level of UV auroral activity, when both measurements are available.
We present results from the CAPS electron spectrometer obtained during the downstream flyby of Titan on 26 December 2005, which occurred during a period of enhanced plasma pressure inside the ...magnetosphere. The electron data show an unusual split signature with two principal intervals of interest outside the nominal corotation wake. Interval 1 shows direct evidence for ionospheric plasma escape at several RT in Titan's tail. Interval 2 shows a complex plasma structure, a mix between plasma of ionospheric and magnetospheric origin. We suggest a mechanism for plasma escape based on ambipolar electric fields set up by suprathermal ionospheric photoelectrons.
During a close pass of Cassini through the plasma wake of Saturn's moon Dione on April 7, 2010 the Cassini Plasma Spectrometer (CAPS) detected molecular oxygen ions (O
2
+
) on pick up ring velocity ...distributions, thus providing the first in situ detection of a neutral exosphere surrounding the icy moon. The density of O
2
+
determined from the CAPS data ranges from 0.01 to 0.09 /cm
3
and is used to estimate the exosphere O
2
radial column density, obtaining the range 0.9 to 7 × 10
11
/cm
2
. CAPS was unable to directly detect pick up H
2
O
+
from the exosphere but the observations can be used to set an upper limit to their density of ∼10 times the O
2
+
density.
Key Points
Dione exosphere inferred via Cassini detection of molecular oxygen pick‐up ions
Column density of exosphere calculated with range 0.9 to 7 X 10^11 / cm^2
Density of molecular oxygen ions calculated with range 0.01 to 0.09 per cc
Measurements by the Cassini spacecraft during a close flyby of Saturn's moon Rhea on March 2, 2010, show the presence of intense plasma waves in the magnetic flux tube connected to the surface of the ...moon. Three types of waves were observed, (1) bursty electrostatic waves near the electron plasma frequency, (2) intense whistler‐mode emissions below one half of the electron cyclotron frequency, and (3) broadband electrostatic waves at frequencies well below the ion plasma frequency. The waves near the electron plasma frequency are believed to be driven by a low energy (∼35 eV) electron beam accelerated away from Rhea. Their bursty structure is believed to be due to a nonlinear process similar to the three‐wave interaction that occurs for Langmuir waves in the solar wind. The whistler‐mode emissions are propagating toward Rhea and are shown to be generated by the loss‐cone anisotropy (at parallel cyclotron resonance energies around 230 eV) caused by absorption of electrons at the surface of the moon. Scattering by these whistler‐mode waves may be able to explain previously reported depletions of energetic electrons in the vicinity of the moon. The low‐frequency waves may play a role in nonlinear three‐wave interactions with the bursty electrostatic waves.
Key Points
Very intense plasma waves are observed in the vicinity of Saturn's moon Rhea
These waves coincide with unstable electron distributions generated by Rhea
Large‐amplitude whistler‐mode waves can have similar effects as chorus emissions
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.
PDF
