Titan is the only Moon in the Solar System with a significant permanent atmosphere. Within this nitrogen–methane atmosphere, an ionosphere forms. Titan has no significant magnetic dipole moment, and is usually located inside Saturn's magnetosphere. Atmospheric particles are ionized both by sunlight and by particles from Saturn's magnetosphere, mainly electrons, which reach the top of the atmosphere. So far, the Cassini spacecraft has made over 45 close flybys of Titan, allowing measurements in the ionosphere and the surrounding magnetosphere under different conditions. Here we review how Titan's ionosphere and Saturn's magnetosphere interact, using measurements from Cassini low-energy particle detectors. In particular, we discuss ionization processes and ionospheric photoelectrons, including their effect on ion escape from the ionosphere. We also discuss one of the unexpected discoveries in Titan's ionosphere, the existence of extremely heavy negative ions up to 10 000 amu at 950 km altitude.
1. Introduction: Titan's space environment
Titan is a body with no significant dipole moment (Ness et al. 1982; Backes et al. 2005). However, it does have a substantial neutral atmosphere and an ionosphere. Other bodies in the Solar System in this category include Mars, Venus, comets, and probably Pluto near its perihelion. Titan is distinct from these other objects, in that for most of the time it is immersed in Saturn's subsonic magnetosphere, rather than in the supersonic solar wind, although at one encounter so far it was observed in Saturn's magnetosheath (Bertucci et al. 2008).
Titan has a solid radius of 2575 km and its atmosphere, which extends considerably above this, consists of mostly N2 and some CH4 (approx. 5% near the surface). There is a significant haze in the atmosphere (e.g. Porco et al. 2005) which obscured the view of the surface in the visible from Voyager (Smith et al. 1991), necessitating different wavelength cameras on Cassini. There is evidence on the surface for lakes (Stofan et al. 2007) and for recent surface modification (Lorenz et al. 2008), indicating a dynamic environment.
Titan's plasma environment provides a source of electrons (e.g. Gan et al. 1992) and ions (Cravens et al. 2008), and of heating for the ionosphere and the atmosphere below (e.g. Gan et al. 1992; Cravens et al. 2008; Sittler et al. submitted and references therein). The magnetospheric upstream conditions are characterized by electron temperatures of approximately 100–1000 eV and densities of approximately 0.1–1 cm−3 at Titan's orbit. The plasma approximately co-rotates with Saturn even at Titan's orbit (approx. 20Rs). Thus, energetic particles in Saturn's magnetosphere can interact with the ionosphere region and inject energy causing ionization and heating; they can also alter the composition by direct injection of particles from the magnetosphere (characterized by an oxygen-rich composition; Young et al. 2005). Similarly, Titan can act as a source of particles for the magnetosphere, e.g. from pick-up ions produced above the exobase and subsequent mass loading. This may contribute to atmospheric escape (see Johnson et al. (submitted) and references therein). Figure 1 summarizes the energy and plasma inputs and outputs relevant to Titan's plasma interaction.
Titan's ionosphere presents a conducting obstacle to the oncoming subsonic flow. Consequently, an induced magnetosphere is expected somewhat similar to that of Venus and Mars (Neubauer et al. 1984; although the upstream conditions are supersonic). Magnetic field draping and a dual magnetic lobe structure are predicted and observed (e.g. Backes et al. 2005).
As Titan orbits Saturn, the same face is always directed at Saturn. There are a number of different relative orientations of the solar and co-rotation wakes (e.g. figure 2, adapted from Ness et al. 1982). This leads to different positions of solar input and magnetospheric input around Titan's orbit. In addition, using the nominal co-rotation plasma velocity and the nominal North–South dipole field orientation as a first approximation, the convection electric field E=−(v×B) is expected to be oriented away from Saturn at all Saturn local times (SLTs) around Titan's orbit. This provides the initial acceleration for pick-up ions produced at Titan, and may be expected to lead to an asymmetry in the mass-loaded flow near Titan (e.g. Hartle et al. 1982).
Of the 44 flybys of Titan during the prime mission, all except one have occurred while Titan was within the magnetosphere. The upstream plasma conditions on these various encounters have been quite different. Recently, it was realized that, at Titan's orbit, Saturn's magnetodisk is distorted by the solar wind (Arridge et al. 2008) and that fluctuations in the plasma environment occur associated with Saturn's rotation period. This makes Titan's upstream plasma environment particularly dynamic.
In this paper we use data from several flybys of Titan taken by the electron spectrometer (ELS; Linder et al. 1998) of the Cassini Plasma Spectrometer (CAPS; Young et al. 2004) to illustrate various features of the interaction between Titan's ionosphere and Saturn's magnetosphere. These include flybys through the sunlit ionosphere, the dark ionosphere and the plasma wake. We also consider the remarkably heavy—and, unexpectedly, negative ions—observed on the encounters. The ELS field of view is fan-like with 160×20 degree sectors; this is sometimes scanned in angle by the CAPS actuator to increase the pitch angle coverage. The ELS energy range is 0.6–28 000 eV, which is covered in 2 s. It should be noted that the populations observed by the ELS at energies less than approximately 5 eV are affected by spacecraft photoelectrons trapped by the spacecraft potential when this is positive. The data shown are not corrected for spacecraft potential except where indicated. In addition, some low-energy populations in Titan's ionosphere are unmeasured by ELS owing to negative spacecraft potential there; in these regions low-energy information can be measured by the radio and plasma wave science (RPWS) Langmuir probe (LP; Wahlund et al. 2005).
2. ELS results from TA, T5 and T9: and negative ion observations
(a) TA: sunlit ionosphere
First, we discuss results from Cassini's first close encounter of Titan, which occurred on 26 October 2004. In the TA encounter (in this nomenclature ‘T’ refers to Titan and ‘A’ the revolution of Saturn by Cassini on which the encounter occurred), the spacecraft flew from day to night relative to Titan, along a trajectory towards Saturn. The encounter was at a SLT of approximately 10.36 LT, at a latitude of 39° N and an altitude of 1175 km. For this encounter, the angle between the Titan–Sun direction and the co-rotation wake can be found using β=90−((12−SLT)×360/24), giving 69° in this case (see table 1).
ELS data from the TA encounter are shown in figure 3. The top panel shows a spectrogram of electron counts (colour scale) as a function of energy (vertical axis) and time (horizontal axis). The strong count level below 5 eV, present for most of the plot, is due to spacecraft photoelectrons. During the region near the closest approach, approximately 15.18–15.40 UT, the spacecraft potential becomes negative owing to the high electron density in Titan's ionosphere and this peak is absent as the spacecraft photoelectrons are lost from the negatively charged spacecraft.
Before and after this period, a population of magnetospheric electrons, with energy a few hundred eV, is visible. This is highly variable in count rate, and there are several possible causes for this variability. The first cause could be the motion of the CAPS actuator with a period of approximately 420 s; in this case, while the magnetospheric electrons are seen (a shorter period is used for approx. 20 min around the closest approach). Second, there are real fluctuations in intensity of the magnetospheric electrons on time scales shorter and longer than the actuator period; these may be spatial or temporal. Third, there are excursions in energy down to approximately 10 eV; these may be associated either with interchange between Saturn's dense, cold inner magnetosphere and the rarer, hot outer magnetosphere (e.g. Burch et al. 2005; Hill et al. 2005), or with ionospheric material from Titan. Fourth, there is a prominent interaction interval of approximately 14 min in this case, where Cassini is within Titan's ionosphere and exosphere region, covering the interval approximately 15.18–15.32 UT.
The density, temperature and energy flux parameters were calculated from spacecraft potential corrected data using a moment integration technique (Lewis et al. 2008) and actuator averaged and noise-subtracted raw data (Arridge et al. submitted). These are shown in figure 3, and indicate the variable upstream electron density and temperature, although the energy flux is relatively constant upstream at approximately 5–7 eV cm−3. This is therefore an estimate of the maximum energy available at the top of the atmosphere for heating of Titan's upper atmosphere from magnetospheric electrons on this encounter. However, complex processes are at work in the atmosphere, including the magnetic field configuration that guides the trajectory of the particles and ionization and absorption processes that affect the magnetospheric particles and their energy spectrum at lower altitudes (e.g. Gan et al. 1992; Cravens et al. 2005; Galand et al. 2006). Note that values near the closest approach are not shown in figure 3 as the spacecraft potential becomes negative in this high-density region and the correction potential cannot be determined from ELS data alone here. In addition, in this region CAPS–ELS measures the non-Maxwellian higher energy electron population including the tail of the bulk thermal, electron population seen by the RPWS LP (Wahlund et al. 2005).
The region near the closest approach begins with a steady, broad decrease in electron energy between approximately 15.08 and 15.18 UT. We interpret this as the entry into a region dominated by Titan's ionospheric plasma. The exit from this region, at 15.40 UT, is much more abrupt. We note that the nominal electric field region would be directed away from Saturn, so this broad cooling region, where CAPS also detects deceleration of ions (e.g. Hartle et al. 2006), is consistent with a region where pick-up ion gyration is important, which may explain the asymmetry. We also note that the magnetic field orientation during this encounter was not nominal with an approximately 23° tilt (e.g. Backes et al. 2005; Neubauer et al. 2006). Therefore, the electric field direction will also be different from the nominal anti-Saturn direction. However, our interpretation of the interval before the closest approach as a region where pick-up ions are in their first phase of acceleration by the electric field is still valid.
Between 15.18 and 15.40 UT, Cassini is in Titan's ionosphere. This region is characterized by an intense, structured peak at energies less than 30 eV and a strong ‘bite out’ of magnetospheric electrons, although this is incomplete particularly early in the period. Such a bite out was also seen by Voyager (Hartle et al. 1982), and we interpret this as associated with absorption of the magnetospheric electrons by Titan's atmosphere; this leads to heating and ionization of the ionosphere by the precipitating electrons.
Also in this region, the main electron peak is observed at approximately 3–4 eV. The negative spacecraft potential in this region, estimated at approximately −0.6 to −1 eV (Wahlund et al. 2005; Coates et al. 2007a), implies that the energy of this peak would be larger by this amount in the undisturbed region a few Debye lengths away from the spacecraft. This potential would preclude the observation of any electrons from the undisturbed region that have initial energies below the spacecraft potential as they would be repelled by the spacecraft potential. We interpret this main 3–4 eV peak as being part of the population of secondary electrons from photoionization in Titan's ionosphere (see also Cravens et al. 2005; Galand et al. 2006). Density calculations on this observed population would therefore underestimate the actual density.
In addition, beginning at approximately 15.22, a sharp, narrow peak is seen at approximately 22–24 eV until about the closest approach distance. This peak is interpreted as photoelectrons from photoionization of nitrogen by the strong HeII solar radiation line at 30.4 nm (cf. Cravens et al. 2005; Galand et al. 2006). Similar narrow electron peaks due to ionization of atmospheric neutrals are also seen at Mars (e.g. Frahm et al. 2006, 2007), Venus (e.g. Coates et al. 2008) and at the Earth (e.g. Coates et al. 1985). As seen in figure 3, this population decreases in intensity as Cassini flies from the sunlit side and passes the terminator near the closest approach; after this time the spacecraft is not actually in Titan eclipse, but the amount of solar radiation reaching the local ionosphere will, however, be attenuated by Titan's atmosphere as observed. This distinctive peak can be used to indicate when Cassini is in Titan's sunlit ionosphere, or when there is a magnetic connection to the ionosphere.
Detailed comparisons of the ionospheric photoelectron spectra in this region with models have been attempted, with quite good agreement between measurements and models. Cravens et al. (2005) presented a model that included the effects of both solar and magnetospheric electrons, while Galand et al. (2006) used an electron transport model including measured ionospheric and thermospheric densities and an magnetohydrodynamics (MHD) model for magnetic field configuration, concluding that the major energy source in the sunlit Titan ionosphere was sunlight. In both the cases, a good quantitative agreement with the observed energetic electron flux was obtained.
Ion observations during Ta were examined by Szego et al. (2005) and Hartle et al. (2006). Hartle et al. interpreted some of the observed ions as pick-up ions and Szego et al. postulated an extended region of disturbance due to Titan's neutral hydrogen cloud. Negative ions were first detected during this encounter as narrow peaks in the ram direction in ELS data; these are discussed further in §2d.
Positive ion observations also reveal distinct ion species, observed both in the CAPS–ion beam spectrometer (IBS) and in the ion and neutral mass spectrometer (INMS) (on other encounters; during Ta, INMS was detecting only neutrals). The maximum peak in the IBS can be used as a measure of spacecraft potential (F. J. Crary et al. 2005, unpublished manuscript; Coates et al. 2007a), assuming that the peak is due to HCNH+ at mass 28, in line with existing models (e.g. Wilson & Atreya 2004; Cravens et al. 2006; Vuitton et al. 2006). This procedure gives a potential of approximately −0.6 eV, in line with RPWS LP estimates (Wahlund et al. 2005).
There have been many other observations at Ta, including MIMI INCA observations consistent with a pick-up ion-related asymmetry (Mitchell et al. 2005); RPWS LP observations of a mass-loaded region and high densities of electrons, covering the energy range missed by ELS due to negative spacecraft potential although without spectral information (Wahlund et al. 2005); and INMS observations revealing a complex hydrocarbon-dominated neutral population (Cravens et al. 2006).
In addition there has been significant modelling activity on Ta. Using an MHD model, Backes et al. (2005) achieved a good comparison with measured magnetometer data by introducing a 23° tilt associated with the magnetic field orientation. Ma et al. (2007) found good agreement between their MHD model with magnetometer observations and also with CAPS–ELS parameters from the magnetosphere and with RPWS LP parameters within the ionosphere. Mondolo & Chanteur (2007) discussed results from their hybrid model in comparison with field and plasma data. Simon et al. (2007) and Sillanpaa et al. (2006) used three-dimensional hybrid modelling to also study the interaction, emphasizing the importance of asymmetries due to kinetic effects from ion pick-up.
(b) T5: dark ionosphere
The T5 encounter was on the part of Cassini's orbit taking the spacecraft outbound from Saturn. It was also a low-altitude encounter at 1025 km, with a latitude of 74 degrees and an earlier morning local time (05.20 LT). In addition, during the passage through the ionosphere, the surroundings were dark.
The ELS data are shown in figure 4. Again, outside of the approximately 30 min of data centred on the closest approach when Cassini is in Titan's ionosphere, the ELS counts at energies below approximately 4 eV are associated with spacecraft photoelectrons and should be ignored. The magnetospheric electrons are much less disturbed, indicating a quieter magnetosphere on this day, and also the CAPS actuator was in a constant position at this flyby. A sudden, strong bite-out of the magnetospheric electrons is seen around the closest approach when Cassini flies through Titan's ionosphere (approx. 19.02–19.18 UT). While some ionospheric electrons are seen, there is no signature of ionospheric photoelectrons—the narrow, sharp 22–24 eV feature. This is because the local ionosphere is dark. There is, however, a blip of 30–40 eV electrons soon after the closest approach, the origin of which is as yet unknown. It is thought that these are not negative ions as there are no distinct peaks. Another possible explanation for this peak could be reconnection in the tail of Titan.
There is evidence for a more gradual cooling on the outbound trajectory, along the nominal electric field. Again, this is consistent with a pick-up ion region accompanied by a cooling of the electrons. Indeed, ion observations from the CAPS ion mass spectrometer (not shown) are consistent with larger numbers of pick-up ions and slowing of the flow on the same side of the encounter. Positive ion peaks are seen, but at lower numbers than during Ta. This may be due to a lower ionospheric density, or the orientation of the sensor slightly away from the ram direction. No negative ions were observed on this encounter, perhaps as there are no ionospheric photoelectrons, or because the fixed orientation of ELS was not quite in the spacecraft ram direction.
Modelling activity by Agren et al. (2007) showed that the night-time observation of a relatively dense ionosphere could be explained by the flux of magnetospheric electrons alone, although the observed intensity of these was higher than that needed to produce a good fit with RPWS LP ionosphere data.
(c) T9: tail pass
The T9 encounter was through the nominal co-rotation wake of Titan, on a trajectory outbound from Saturn. The closest approach occurred at approximately 5RT from the centre of Titan and a SLT of approximately 03.00. Plasma conditions were not nominal: the ion flow velocity had a component away from Saturn (Szego et al. 2007) and the magnetic field orientation was closer to the orbital plane consistent with a location below Saturn's current sheet (Bertucci et al. 2007).
An unusual split signature was seen in the plasma data (Coates et al. 2007b), see figure 5. In interval 1 there was strong evidence of ionospheric photoelectrons at this position well down the tail of Titan, indicative of ionospheric plasma escape and of a magnetic connection to Titan's sunlit ionosphere. Spacecraft potential was also negative in this interval, indicative of high plasma density. The ion data also showed evidence of low-energy ionospheric plasma with mass 16–32 amu q−1, and magnetic data were also consistent with a connection to the sunlit ionosphere (Wei et al. 2007). There were similarities to electron observations at Mars where again photoelectrons were seen in the tail and used as a ‘tracer’ of connection to the ionosphere (Frahm et al. 2006). This has also been observed at Venus (Coates et al. 2008; Tsang et al. submitted). In interval 2, mixed ionospheric and magnetospheric plasma electrons were seen, and escape of lighter ions with mass 2–4. Spacecraft potential was positive in this interval. A magnetic connection to a region above the dark ionosphere was inferred (Coates et al. 2007b).
Coates et al. (2007b) suggested that the photoelectrons, more energetic than the electrons in the surroundings, may set up an ambipolar electric field, which may add to the escape of ions. It was suggested that the mechanism may be similar to that which produces the Earth's polar wind, which also leads to escape of heavy ions.
(d) Negative ions: an unexpected feature in Titan encounters
The high time resolution ELS observations initially made on Ta, and subsequently on 15 other Titan encounters, were analysed by Coates et al. (2007a). In addition to the electron populations discussed above, they presented evidence for negative ions in Titan's ionosphere. This unexpected population was observed near the closest approach and was narrowly confined to the ram direction, and contained distinct peaks. As the spacecraft flies through Titan's cold ionosphere, the spacecraft velocity (approx. 6 km s−1) effectively provides a mass spectrometer for cold, ionospheric ions; assuming singly charged ions, the conversion is mamu=5.32Eev (Coates et al. 2007a).
During Ta, the observed maximum energy of the negative ions was approximately 60 eV, corresponding to approximately 320 amu q−1; in other encounters, notably T16, masses as high as 10 000 amu q−1 are observed (see figure 6). On the various encounters, the ions were observed in rough mass groups at 10–30, 30–50, 50–80, 80–110, 110–200, (200–500, 500+) amu q−1. As an example, the signatures we identify as negative ions can be seen during the T16 encounter as the vertical spikes in the spectrogram (figure 6), each of which contains several mass peaks (see above). The vertical spikes occur as the CAPS actuator moves the ELS field of view through the ram direction. In this case, the structure was asymmetrical about the approach as the ram direction was no longer fully sampled at later times due to spacecraft orientation changes. The shape of each peak with energy is currently unexplained in detail but is likely to be due to a convolution of the instrument response with the cold, but finite, temperature negative ion population. Table 2 contains possible ion identifications, and the density profiles of the different mass groups measured during T16 are also shown in figure 6 (right panel).
Negative ions were not anticipated above approximately 100 km in Titan's atmosphere (Borucki et al. 2006) and were not included in pre-Cassini chemical schemes (e.g. Wilson & Atreya 2004), so this is an important observation requiring new chemical models (e.g. Vuitton et al. submitted).
The relationship between these ions and the heavy positive ion population was discussed by Waite et al. (2007). They suggested that nitrogen and methane in Titan's high atmosphere would be acted on by sunlight and magnetospheric particles forming heavier but relatively simple species by dissociation and ionization processes. Eventually this could lead to the observed benzene and other heavier ions seen by the positive ion instruments up to approximately 350 amu q−1. The process would continue to form heavier positive and negative ions. However, the reason for higher mass negative ions being observed compared with positive ions (which are observed up to 100 amu by INMS and up to approx. 350 amu by CAPS–IBS; Waite et al. (2007)) is not yet understood; but it may be the poorer sensitivity of IBS, or different chemical pathways for positive and negative ions. They suggested that these may be the tholins postulated by Sagan & Khare (1979). Clearly, such large aerosol-like ions may in fact be multiply charged; Coates et al. (2007a) suggest that the charge on such ions in a plasma with densities prevailing in the Titan ionosphere may reach 5 (making assumptions about the density of the aerosols). If that is the case the negative ion mass would be as much as 50 000 amu.
Eventually, the large compounds would become aerosols and drift down towards Titan's surface. This may be the chain of processes by which space physics at the top of the atmosphere eventually affects the surface of Titan. This idea was supported by the more extensive observations of negative ions by Coates et al. (2007a).
3. Summary and discussion
A number of points emerge from this brief comparison of plasma observations at different encounters that are relevant to the interaction of Titan's ionosphere with Saturn's magnetosphere. The Cassini encounters provide a range of geometries, altitudes, latitudes and local times of encounters. Different local times allow a comparison of different conditions of solar illumination and wake geometry.
(a) Asymmetry of the interaction
The ion and electron measurements are asymmetric with respect to the local electric field direction with broader signatures along the field direction. This is consistent with pick-up ion asymmetries that are expected in this direction, indicating a region of enhanced mass loading and associated electron cooling there.
(b) Ionization processes
We have shown examples in which the ionosphere during the encounters is both sunlit (Ta) and dark (T5). The electron results, and modelling activities, show that both solar UV and magnetospheric electrons are important in the ionization of particles in the ionosphere to different extents on the different encounters. Generally, magnetospheric electrons dominate on the night side and solar UV during the day. The data shown here contain evidence for this: for example, T5 shows a pronounced bite-out of magnetospheric electrons, and TA shows evidence for ionospheric photoelectrons. However, a detailed analysis of the relative contributions is beyond the scope of the present paper.
(c) Positive ions
There is a varied and rich population of positive ions. This is not covered in detail in the current paper. However, the combination of INMS with excellent mass resolution up to 100 amu q−1, and CAPS–IBS data with no mass resolution (except that provided by E/q steps), but wider energy coverage, have been used to study the ram positive ion population (e.g. Cravens et al. 2006; Waite et al. 2007, etc.). As expected, the positive ions play an important part in the chemistry, although higher masses and complexity than expected are observed. In addition, we note that penetration of energetic oxygen ions from the magnetosphere may also be important in producing heating and affecting the local chemical composition (Cravens et al. 2008).
(d) Ionospheric photoelectrons
On encounters with the sunlit ionosphere, we always observe ionospheric photoelectrons with ELS. These are distinguished by their characteristic peaks at approximately 22–24 eV together with a related broad but partial (due to spacecraft potential) low-energy spectrum of ionospheric electrons. These are from ionization of N2 by He II 30.4 nm radiation from sunlight. Photoelectrons may be used as a tracer of magnetic connection to the ionosphere (e.g. observation in Titan's tail on T9). As suggested by Coates et al. (2007b), they may have a role in ion escape by producing an ambipolar magnetic field. A similar effect, observation in the tail and associated low-energy ions, is seen at Mars (Frahm et al. 2006) and at Venus (Coates et al. 2008; Tsang et al. submitted).
(e) Negative ions
One of the most unexpected results from the Titan encounters so far has been the observation of negative ions at the lowest altitude encounters (950–1150 km). CAPS–ELS observes an unexpectedly dense population at these flyby altitudes (Coates et al. 2007a). This population, not anticipated in models prior to these encounters, must play an important role in the ion chemistry here. Also the observation of high-mass ions up to greater than 10 000 amu q−1 is important in aerosol formation and may provide the link between space physics, atmospheric physics and surface science as these large molecules and haze components drift towards the surface.
(f) Interaction of Titan's ionosphere with Saturn's magnetosphere
We may also summarize the effects of Saturn's magnetosphere on Titan's ionosphere. This includes ionization of neutrals by the incoming energetic electron population from the magnetosphere, and heating of the upper atmosphere. The importance of these effects compared with solar effects varies as a function of local time, with the magnetospheric effects being particularly important on the co-rotation ram side, and on field lines connected to Saturn's magnetosphere which drape around Titan and may impinge on the top of its atmosphere.
Also, we can summarize the converse—the effect of Titan's ionosphere on Saturn's magnetosphere. Clearly, Titan acts as a source of particles, via ion pick-up and possibly via other mechanisms (e.g. escape of low-energy plasma via ambipolar diffusion, cf. Earth's polar wind). It has also been suggested that, as Venus, Titan may be the source of plumes of dense, cold plasma which emerge from Titan and may even wrap around Saturn in the co-rotating flow. Further study is needed to distinguish the cool, dense population seen at several Cassini encounters from the results of interchange motion in Saturn's magnetosphere.
In conclusion, the interaction of Titan's ionosphere with Saturn's magnetosphere has given us many important elements of our understanding of how unmagnetized objects interact with their surroundings. Future encounters by Cassini may be expected to add further to this understanding. Future missions, e.g. Tandem (Coustenis et al. 2008), will hopefully fill in the gaps in measurement which Cassini can provide (down to 950 km) and where Huygens could measure (less than 450 km). The signs from Cassini–Huygens are that this could finally link the space physics at the top of the atmosphere with the surface.
We thank the Cassini CAPS team (PI D. T. Young) for the success of the CAPS instrument, and the many scientists and engineers at MSSL, RAL and NDRE for making ELS a reality. We thank L. K. Gilbert and G. R. Lewis of MSSL for data display and analysis software, and G. R. Lewis and M. dela Nougerede for help with the diagrams. We thank STFC, UK, for financial support.
One contribution of 14 to a Discussion Meeting Issue ‘Progress in understanding Titan's atmosphere and space environment’.
- © 2008 The Royal Society