[Show abstract][Hide abstract] ABSTRACT: The Mars Atmosphere and Volatile Evolution (MAVEN) mission, during the second of its Deep Dip campaigns, made comprehensive
measurements of martian thermosphere and ionosphere composition, structure, and variability at altitudes down to ~130 kilometers
in the subsolar region. This altitude range contains the diffusively separated upper atmosphere just above the well-mixed
atmosphere, the layer of peak extreme ultraviolet heating and primary reservoir for atmospheric escape. In situ measurements
of the upper atmosphere reveal previously unmeasured populations of neutral and charged particles, the homopause altitude
at approximately 130 kilometers, and an unexpected level of variability both on an orbit-to-orbit basis and within individual
orbits. These observations help constrain volatile escape processes controlled by thermosphere and ionosphere structure and
[Show abstract][Hide abstract] ABSTRACT: Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed
to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile
Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with
the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the
bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models
both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history
may have been a major contributor to the long-term evolution of the Mars atmosphere.
[Show abstract][Hide abstract] ABSTRACT: With attention turned to Europa as a target for exploration, we focus on the space environment in which Europa is embedded. We review remote and in situ observations of plasma properties at Europa's orbit, between Io's dense, UV-emitting plasma torus and Jupiter's dynamic plasma sheet. Where observations are limited (e.g. in plasma composition), we supplement our analysis with models of the neutral and plasma populations from Io to Europa. We evaluate variations and uncertainties in plasma properties with radial distance, latitude, longitude and time.
[Show abstract][Hide abstract] ABSTRACT: We model the interaction of the jovian magnetospheric plasma with the atmosphere of Europa using a multi-species chemistry model where the atmospheric distributions of H2 and O2 are prescribed. The plasma flow is idealized as an incompressible flow around a conducting obstacle. We compute changes in plasma composition resulting from this interaction as well as the reaction rates integrated over the simulation domain for several upstream plasma conditions (ion density, ion temperature and flow velocity). We show that for all cases, the main atmospheric loss process is a cascade of symmetrical charge exchanges on O2, which results in the ejection of neutrals. The production rate of ejected neutrals is about an order of magnitude larger than the production of ions. This conclusion is relevant to future missions to Europa that aim to detect fast neutrals. The neutral ejection resulting from this charge exchange creates an oxygen cloud around the orbit of the moon that is very extended radially but also very tenuous, and has not yet been directly detected.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We investigate the complex interaction between Saturn's magnetosphere and Titan's upper ionosphere using ion data acquired by the Cassini Plasma Spectrometer (CAPS) during the T40 encounter. Bounds on ion-group abundances at altitudes between ~2733 and ~12,541 km are determined by fitting mass spectra with model functions derived from instrument calibration data. The spectra are dominated by H+, H2+, H3+, and two hydrocarbon groups with mass ranges 12–19 and 24–32 amu, respectively. Notably, this constitutes the first reported observation of H3+ in Titan's exosphere. These measurements are discussed in the context of data from the CAPS Electron Spectrometer (ELS) and Cassini's Ion and Neutral Mass Spectrometer (INMS), which fortuitously sampled the ionospheric outflow during the T40 encounter at altitudes between ~2225 and ~3034 km [Westlake et al., 2012]. The CAPS data reveal a composition that is constitutively similar to that sampled by INMS, with hydrocarbon ions first observed as far as ~11,000 km from Titan and increasing by more than an order of magnitude along Cassini's inbound trajectory. In addition, we juxtapose the CAPS ion data with numerical results from three different interaction models and show that it is consistent with the location of the field-draping boundary described by Ulusen et al.  and the Saturnward ion tail predicted by Sillanpää et al. .
No preview · Article · Nov 2014 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract] ABSTRACT: Interchange injections events are commonly observed by the Cassini spacecraft in the region between about 6 and 12 Rs (1Rs = 60268 km) and even frequently beyond. In this study, thirteen examples of interchange injection events are identified in Cassini/CAPS data under special conditions such that time-of-flight (TOF) mass spectra could be obtained from entirely within the events. Using the TOF data to separate the main ion species H+, H2+, and W+, approximate densities of each species are calculated under the assumption that all distributions were isotropic. The light-ion density ratios, H2+/H+, in the injection events are not discernibly different from those ratios in control intervals from the ambient plasma. However, the water-group ratio, W+/H+, is significantly lower than ambient. Comparison of the measured density ratios with the range of values observed throughout Saturn's magnetosphere indicates that values of W+/H+ that are as low as those observed within the injection events are found primarily beyond L ~ 14 (where L is the equatorial crossing distance, in Rs, of a dipole field line), indicating that the injection events are delivering plasma from the outer magnetosphere, at times traveling at least 6 Rs.
No preview · Article · Nov 2014 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract] ABSTRACT: We present observations of significant dynamics within two UV auroral storms observed on Saturn using the Hubble Space Telescope in April/May 2013. Specifically, we discuss bursts of auroral emission observed at the poleward boundary of a solar wind-induced auroral storm, propagating at ~330% rigid corotation from near ~01 h LT toward ~08 h LT. We suggest these are indicative of ongoing, bursty reconnection of lobe flux in the magnetotail, providing strong evidence that Saturn's auroral storms are caused by large-scale flux closure. We also discuss the later evolution of a similar storm, and show that the emission maps to the trailing region of an energetic neutral atom enhancement. We thus identify the auroral form with the upward field-aligned continuity currents flowing into the associated partial ring current.
[Show abstract][Hide abstract] ABSTRACT:  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.
Full-text · Article · Nov 2013 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract] ABSTRACT: Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan's upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.
No preview · Article · Feb 2013 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: There have been three Cassini encounters with the south-pole eruptive plume of Enceladus for which the Cassini Plasma Spectrometer (CAPS) had viewing in the spacecraft ram direction. In each case, CAPS detected a cold dense population of heavy charged particles having mass-to-charge (m/q) ratios up to the maximum detectable by CAPS (similar to 10(4) amu/e). These particles are interpreted as singly charged nanometer-sized water-ice grains. Although they are detected with both negative and positive net charges, the former greatly outnumber the latter, at least in the m/q range accessible to CAPS. On the most distant available encounter (E3, March 2008) we derive a net (negative) charge density of up to similar to 2600 e/cm(3) for nanograins, far exceeding the ambient plasma number density, but less than the net (positive) charge density inferred from the RPWS Langmuir probe data during the same plume encounter. Comparison of the CAPS data from the three available encounters is consistent with the idea that the nanograins leave the surface vents largely uncharged, but become increasingly negatively charged by plasma electron impact as they move farther from the satellite. These nanograins provide a potentially potent source of magnetospheric plasma and E-ring material.
No preview · Article · May 2012 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: We use measurements of the Cassini Plasma Spectrometer, the
Magnetospheric Imaging Instrument and the Magnetometer of the Cassini
spacecraft to analyse the structure and composition of the tail section
of the induced magnetosphere of Saturn's moon, Titan. The orbital
positions of the Titan flybys of Cassini are distributed over various
Saturn Local Time regions, so the effects of multiple-source ionization
can be studied. We included flybys TA-T78 into the analysis. For many
of the encounters the position of the center of the tail differed from
that derived from the nominal flow direction, depending on SLT. We
compared this with hybrid simulations. We also examined how the
different mass particles (m/q=1, 2, 16-19) were distributed in the tail
section. During the many tail flybys Titan was close to the plasma sheet
of Saturn (for example during T11, T15, T29, T36, T44) hence embedded in
a tilted magnetic field and higher density plasma. The possible effects
of the Kronian magnetodisk on Titan's tail are discussed.
[Show abstract][Hide abstract] ABSTRACT: Magnetic reconnection is an important process that occurs at the
magnetopause boundary of Earth's magnetosphere because it leads to
transport of solar wind energy into the system, driving magnetospheric
dynamics. However, the nature of magnetopause reconnection in the case
of Saturn's magnetosphere is unclear. Based on a combination of Cassini
spacecraft observations and simulations we propose that plasma β
conditions adjacent to Saturn's magnetopause largely restrict
reconnection to regions of the boundary where the adjacent magnetic
fields are close to anti-parallel, severely limiting the fraction of the
magnetopause surface that can become open. Under relatively low
magnetosheath β conditions we suggest that this restriction becomes
less severe. Our results imply that the nature of solar
wind-magnetosphere coupling via reconnection can vary between planets,
and we should not assume that the nature of this coupling is always
Earth-like. Studies of reconnection signatures at Saturn's magnetopause
will test this hypothesis.
No preview · Article · Apr 2012 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: We present observations of CAPS electron and ion spectra during Titan
distant tail crossings by the Cassini spacecraft. In common with closer
tail encounters, we identify ionospheric plasma in the tail. Some of the
electron spectra indicate a direct magnetic connection to Titan's
dayside ionosphere due to the presence of ionospheric photoelectrons.
Ion observations reveal heavy and light ion populations streaming into
the tail. Using the distant tail encounters T9, T75 and T63, we estimate
total plasma loss rates from Titan via this process.
Full-text · Article · Apr 2012 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: During three Cassini encounters with Saturn's satellite, Enceladus, the
spacecraft crossed through the plume of water vapor and dust south of
the satellite with a spacecraft orientation which allowed the Cassini
Plasma Spectrometer (CAPS) to observe ions and nanograin dust particles
associated with the plume. These measurements were made on the E3 (March
12, 2008), E5 (October 9, 2008) and E7 (November 2, 2009) encounters.
Analysis of these results, using data from the CAPS ion mass
spectrometer (IMS) and electron spectrometer (ELS), found cold ions at
rest with respect to Enceladus , negative water group and water
cluster ions , and both positively and negatively charged dust
particles in the 0.5 to 2 nm (1000 to 20,000 AMU) size range [3,4]. We
present previously unreported observations from the third CAPS sensor,
the ion beam spectrometer (IBS). This sensor measured the flux of
positive ion and nonoparticle on the same three encounters discussed,
above. The IBS measurements differ from those of IMS in several,
complementary respects. The IBS data covers a more limited energy range
(a factor of 67, rather than nearly four decades) and does not, in the
mode used during the encounters, provide any angular resolution. It
does, however, acquire energy spectra with 1.4% rather than 17% energy
resolution at 2-second rather than 4-second time resolution. The IMS and
IBS sensors are co-aligned, but the IBS sensor's field of view is a
factor of five narrower. The IBS data allows us to estimate the
temperature and flow speed of the cold ions, and characterize structure
which is not resolved by in the IMS data.
[Show abstract][Hide abstract] ABSTRACT: Magnetic reconnection is an important process that occurs at the magnetopause boundary of Earth's magnetosphere because it leads to transport of solar wind energy into the system, driving magnetospheric dynamics. However, the nature of magnetopause reconnection in the case of Saturn's magnetosphere is unclear. Based on a combination of Cassini spacecraft observations and simulations we propose that plasma βconditions adjacent to Saturn's magnetopause largely restrict reconnection to regions of the boundary where the adjacent magnetic fields are close to anti-parallel, severely limiting the fraction of the magnetopause surface that can become open. Under relatively low magnetosheathβconditions we suggest that this restriction becomes less severe. Our results imply that the nature of solar wind-magnetosphere coupling via reconnection can vary between planets, and we should not assume that the nature of this coupling is always Earth-like. Studies of reconnection signatures at Saturn's magnetopause will test this hypothesis.
No preview · Article · Apr 2012 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: Discrete peaks near 24.1 eV are seen in electron spectra measured in Titan’s
ionosphere by the ELS (Electron Spectrometer), part of the Cassini Plasma Spectrometer
(CAPS), and are interpreted as photoelectrons. These photoelectrons are generated
as a result of ionization of N2 by the strong solar He II (30.4 nm) line. They are generally
observed in the dayside ionosphere, because this is where neutral N2 particles can be
ionized by solar radiation. Coates et al. (2007) discussed initial observations of
photoelectrons in Titan’s distant tail during the T9 encounter. Here, we describe additional
photoelectron peak observations at large distances from Titan, where they are unlikely
to have originated because of low neutral N2 densities. We consider the tail structures
during the encounters T15, T17, and T40. We infer that the distant photoelectrons
may have traveled to the observation sites by means of a magnetic connection from lower
altitudes in the dayside ionosphere, where they could have been produced. This idea
is supported by results of hybrid modeling. Thus photoelectrons may be used as tracers
of magnetic field lines and further improve our understanding of Titan’s complex
No preview · Article · Mar 2012 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: The Cassini-Huygens mission to Saturn launched in October 1997. The Cassini Plasma Spectrometer (CAPS) is an in-situ instrument and is one of 12 instruments on the orbiter. Cruise science (beyond instrument checkout periods and periodic maintenance) was approved, and over 870 days of data were collected prior to the start of the prime mission. Prime mission operations started in January 2004 and continued through end of June 2008. Given the success of the Cassini mission at Saturn, an extended mission, the Equinox Mission, was approved for an additional two years, ending September 2010. The continuing success of the Equinox mission led to approval of a second extended mission called the Solstice Mission. The Solstice mission has approval through September 2012 with pending approval through July 2017. The Cassini Plasma Spectrometer (CAPS) has generated a wealth of science data and we have over 240 publications. In addition, the CAPS scientists participate in conferences, inter-team collaborations, and community outreach. Collection of data has not been without instrument related challenges. This paper will discuss a few of the in-flight instrument related anomalies experienced by CAPS including timing difference between the engineering model and the flight model, actuator related anomalies, and a latched bit. The analysis will cover how the anomalies were discovered, the techniques used to diagnose the problems in-flight, fixes that were implemented, how the anomalies affected operation of the instrument and collection of science, and lessons learned from the process.
No preview · Article · Mar 2012 · IEEE Aerospace Conference Proceedings
[Show abstract][Hide abstract] ABSTRACT: 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 (O2+) on pick up ring
velocity distributions, thus providing the first in situ detection of a
neutral exosphere surrounding the icy moon. The density of
O2+ determined from the CAPS data ranges from 0.01
to 0.09 /cm3 and is used to estimate the exosphere
O2 radial column density, obtaining the range 0.9 to 7
× 1011/cm2 . CAPS was unable to directly
detect pick up H2O+ from the exosphere but the
observations can be used to set an upper limit to their density of
˜10 times the O2+ density.
Full-text · Article · Feb 2012 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: NASA's Juno mission, which is scheduled to launch this August, will be
the first spacecraft to achieve polar orbit around Jupiter. The Jovian
Auroral Distributions Experiment (JADE), onboard Juno, measures the full
pitch angle distribution of electrons (JADE-E) and the 3D velocity-space
distribution of ions and ion composition (JADE-I). JADE-E consists of
three identical energy analyzers covering the range from ~0.1-100 keV.
Below ~5keV and in the relatively strong Jovian magnetic field (up to ~8
G along the orbit) the gyro-radius of electrons is comparable to the
radius of curvature of the analyzers. Therefore, the response at low
energies will be strongly affected by the magnetic field. To better
understand, predict, and correct for the response of JADE-E in the
presence of Jupiter's strong magnetic field we are characterizing its
response in the laboratory. A set of Helmholtz coils capable of
producing a magnetic field up to about 9 G with three axis control was
built around the vacuum test chamber. We also compare the laboratory
measurements with electro-optics simulations. We will present both
simulations and measurement results taken with the engineering model of
JADE-E and discuss expected performance of the flight instruments when
they arrive at Jupiter.