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Phil. Trans. R. Soc. A (2009) 367, 773–788
doi:10.1098/rsta.2008.0248
Published online 20 November 2008
REVIEW
Interaction of Titan’s ionosphere with
Saturn’s magnetosphere
B Y A NDREW J. C OATES *
Mullard Space Science Laboratory, University College London,
Holmbury St Mary, Dorking RH5 6NT, UK
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.
Keywords: Titan; ionosphere; magnetosphere; Saturn; photoelectrons; negative ions
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.
*[email protected]
One contribution of 14 to a Discussion Meeting Issue ‘Progress in understanding Titan’s
atmosphere and space environment’.
773
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A. J. Coates
Saturn
draped
magnetic field
(nominally out
of plane)
hot
magnetospheric
ions
hot magnetospheric
electrons
atmosphere and
ionosphere
corotation wake
magnetospheric
plasma flow
ion escape
pick-up ions
(nominal E direction)
solar
radiation
Figure 1. Titan plasma environment illustrating energy and plasma flow. bZ0 corresponds to 06.00
SLT and increases clockwise in this illustration with increasing local time, becoming negative at
SLT earlier than 06.00. The b value shown corresponds to Ta conditions.
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 cmK3 at Titan’s orbit. The plasma approximately co-rotates with Saturn even at Titan’s orbit (approx. 20R s). 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.
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Review. Ionosphere–magnetosphere interaction
E = –(v×B)
Figure 2. Wake orientations at different local times around Saturn’s orbit (adapted from Ness
et al. 1982).
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 EZK(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 massloaded 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.
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A. J. Coates
Table 1. Parameters of Titan encounters discussed here.
TA
T5
T9
T16
Titan latitude
Saturn local time, (8N, at the closest
approach)
SLT (hh:mm)
Titan altitude
(km, at the closest
approach)
angle Sun to
co-rotation (8)
10:36
05:20
03:00
02:27
1175
1025
4RT
950
69
K10
K45
K53.25
39
74
0
85
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 398 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 bZ90K((12KSLT)!360/24), giving 698 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.
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Review. Ionosphere–magnetosphere interaction
777
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 cmK3. 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 238 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
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778
A. J. Coates
CA
1174 km
Pe
(eV cm–3)
Te
(eV)
ne
(cm–3)
(b)
10 4
10 3
10 2
10
1
10
1
10–1
10–2
10 3
10 2
10
1
10 2
Saturn
E
log counts sec–1
energy
(eV)
(a)
(c)
4.6
Saturn
E
1.6
6
4
(i)
z
(ii)
2
0
–2
–4
Sun
corotation
flow
–6
4
(iii)
2
y
10
0
–2
–4
–4
–2
0
2
4
x
1
15 : 00
20.20
– 0.4
10 : 34
16 : 00
19.80
–0.2
10 : 37
UT (hh : mm)
R (Rs)
Lat (deg.)
LT (hh : mm)
Figure 3. CAPS–ELS results from TA. (a) A spectrogram from anode 5. (b(i)–(iii)) Bulk parameter
estimates and (c) encounter geometry with respect to the nominal co-rotation direction.
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 K0.6 to K1 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
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(a)
(b)
CA
1025 km
Saturn
E
4.6
104
log counts s–1
energy
(eV)
103
102
10
6
4
2
z 0
–2
–4
–6
1.6
1
18 : 00
20.40
0.20
05 : 15
19 : 00
20.70
0.10
05 : 17
20 : 00
20.90
0.00
05 : 19
UT (hh : mm)
R (Rs)
Lat (deg.)
LT (hh : mm)
E
Saturn
corotation
flow
4
y
2
0
–2
–4
– 4 –2
0
2
4
x
Figure 4. (a,b) CAPS–ELS results from T5, and encounter geometry.
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 HCNHC 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 K0.6 eV, in line with
RPWS LP estimates (Wahlund et al. 2005).
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A. J. Coates
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 238 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.
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Review. Ionosphere–magnetosphere interaction
781
In other observations at T5, the INMS observed a complex set of ion species
again as a result of the complex organic chemistry at work in the ionosphere
(Cravens et al. 2006; Vuitton et al. 2006).
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 5R T 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 qK1, 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 sK1) effectively provides a
mass spectrometer for cold, ionospheric ions; assuming singly charged ions, the
conversion is mamuZ5.32Eev (Coates et al. 2007a).
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A. J. Coates
(b)
1.00
4.6
1.6
10
3.5
5
Saturn
Sun
corotation
flow
z 0
1.5
–5
–10
10
0.10
5
10–2
103
y
102
1
2
0
–5
–10–10
–5
5
0
10
x
10
10
1
18 : 00
20.06
0.1
03 : 02
19 : 00
20.09
0.1
03 : 04
20 : 00
21.20
0.1
03 : 06
energy
(eV)
10 4
10 3
10 2
10
1
18 : 20 : 00
20.07
0.1
003 : 02 : 49
counts s–1
log counts s–1
2
log counts s–1
Pe
(eVcm–3)
Te
(eV)
ne
(cm–3)
energy
(eV)
energy
(eV)
1
104
103
102
10
1
104
103
102
10
log counts s–1
(a)
18 : 30 : 00
20.07
0.1
03 : 03 : 12
UT (hh : mm)
R (Rs)
Lat (deg.)
LT (hh : mm)
4.6
1.6
18 : 40 : 00 18 : 50 : 00 UT (hh : mm : ss)
R (Rs)
20.9
20.80
Lat (deg.)
0.1
0.1
03 : 03 : 34 03 : 03 : 56 LT (hh : mm : ss)
10 4
Ionospheric
photoelectrons
10 3
10 2
1 10 10 210 310 4 1 10 10 210 310 4 1 10 10 210 310 4
energy (eV q–1)
Figure 5. (a,b) CAPS–ELS results from T9.
During Ta, the observed maximum energy of the negative ions was
approximately 60 eV, corresponding to approximately 320 amu qK1; in other
encounters, notably T16, masses as high as 10 000 amu qK1 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, 500C) amu qK1. 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
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Review. Ionosphere–magnetosphere interaction
(b) 1150 (i)
1100
1050
1000
950
1150
(ii)
1100
1050
1.6
1000
950
UT (hh : mm)
900
R (Rs)
10
102
10–1 1.0
Lat (deg.)
–3
ni (cm )
LT (hh : mm)
103
102
10
1
00 : 20
20.86
0.32
02 : 26
00 : 25
20.83
0.31
02 : 26
00 : 30
20.80
0.31
02 : 26
altitude (km)
4.6
104
log counts s–1
energy (eV)
(a)
783
103
Figure 6. (a) CAPS–ELS data from T16 showing negative ion signatures observed at this
encounter. (b(i) and (ii)) Density as a function of altitude, for the mass groups indicated, during
T16. (i) Diamond, 10–30 amu; triangle, 30–50 amu; square, 50–80 amu. (ii) Down triangle, 80–
110 amu; circle, 110–200 amu; right triangle, 200C amu.
Table 2. Possible negative ion identifications (after Coates et al. 2007a).
mass group (amu qK1)
possible identification
10–30
30–50
50–80
80–110
K
CNK, NHK
2, O
K
NCN , HNCNK, C3HK
K
K
C5 HK
5 , C6H , C6 H5
110–200
200–500
500–10000
9
>
>
>
>
=
>
>
>
>
;
polyynes, high-order nitriles, PAHs, cyano-aromatics (aerosols)
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
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A. J. Coates
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 qK1. 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.
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(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 qK1, 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 qK1 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 pickup and possibly via other mechanisms (e.g. escape of low-energy plasma via
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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.
References
Agren, K. et al. 2007 On magnetospheric electron impact ionisation and dynamics in Titan’s ramside & polar ionosphere—a Cassini case study. Ann. Geophys. 25, 2359–2369.
Arridge, C. S., Khurana, K. K., Russell, C. T., Southwood, D. J., Achilleos, N., Dougherty, M. K.,
Coates, A. J. & Leinweber, H. K. 2008 Warping of Saturn’s magnetospheric and magnetotail
current sheets. J. Geophys. Res. 113, A08217. (doi:10.1029/2007JA012963)
Arridge, C. S., Gilbert, L. K., Lewis, G. R., Sittler, E. C., Jones, G. H., Kataria, D. O., Coates, A.
J. & Young, D. T. Submitted. The effect of a penetrating background on numerically integrated
moments from the Cassini CAPS electron spectrometer.
Backes, H. et al. 2005 Titan’s magnetic field signature during the first Cassini encounter. Science
308, 992–995. (doi:10.1126/science.1109763)
Bertucci, C., Neubauer, F. M., Szego, K., Wahlund, J.-E., Coates, A., Dougherty, M. K., Young,
D. & Kurth, W. 2007 Structure of Titan’s mid-range magnetic tail. Cassini magnetometer
observations during the T9 flyby. Geophys. Res. Lett. 34, L24S10. (doi:10.1029/2007GL031627)
Bertucci, C. et al. 2008 The magnetic memory of Titan’s ionized atmosphere. Science 321,
1475–1478. (doi:10.1126/science.1159780)
Borucki, W. J., Whitten, R. C., Bakes, E. L. O., Barth, E. & Tripathi, S. 2006 Predictions of the
electrical conductivity and charging of the aerosols in Titan’s atmosphere. Icarus 181, 527–544.
(doi:10.1016/j.icarus.2005.10.030)
Burch, J. L., Goldstein, J., Hill, T. W., Young, D. T., Crary, F. J., Coates, A. J., André, N., Kurth,
W. S. & Sittler Jr, E. C. 2005 Properties of local plasma injections in Saturn’s magnetosphere.
Geophys. Res. Lett. 32, L14S02. (doi:10.1029/2005GL022611)
Coates, A. J., Johnstone, A. D., Johnson, J. F. E., Sojka, J. J. & Wrenn, G. L. 1985 Ionospheric
photoelectrons observed in the magnetosphere at distances of up to 7 Earth radii. Planet. Space
Sci. 33, 1267. (doi:10.1016/0032-0633(85)90005-4)
Coates, A. J., Crary, F. J., Lewis, G. R., Young, D. T., Waite Jr, J. H. & Sittler Jr, E. C. 2007a
Discovery of heavy negative ions in Titan’s ionosphere. Geophys. Res. Lett. 34, L22103. (doi:10.
1029/2007GL030978)
Coates, A. J., Crary, F. J., Young, D. T., Szego, K., Arridge, C. S., Bebesi, Z., Sittler Jr, E. C.,
Hartle, R. E. & Hill, T. W. 2007b Ionospheric electrons in Titan’s tail: plasma structure during
the Cassini T9 encounter. Geophys. Res. Lett. 34, L24S05. (doi:10.1029/2007GL030919)
Phil. Trans. R. Soc. A (2009)
Downloaded from http://rsta.royalsocietypublishing.org/ on June 16, 2017
Review. Ionosphere–magnetosphere interaction
787
Coates, A. J. et al. 2008 Ionospheric photoelectrons at Venus: initial observations by ASPERA-4
ELS. Planet. Space Sci. 56, 802–806. (doi:10.1016/j.pss.2007.12.008)
Coustenis, A. et al. 2008 TandEM: Titan and Enceladus mission. Exp. Astron. (doi:10.1007/
s10686-008-9103-z)
Cravens, T. E. et al. 2005 Titan’s ionosphere: model comparisons with Cassini Ta data. Geophys.
Res. Lett. 32, L12108. (doi:10.1029/2005GL023249)
Cravens, T. E. et al. 2006 Composition of Titan’s ionosphere. Geophys. Res. Lett. 33, L07105.
(doi:10.1029/2005GL025575)
Cravens, T. E., Robertson, I. P., Ledvina, S. A., Mitchell, D., Krimigis, S. M. & Waite Jr, J. H. 2008
Energetic ion precipitation at Titan. Geophys. Res. Lett. 35, L03103. (doi:10.1029/2007GL032451)
Frahm, R. A. et al. 2006 Carbon dioxide photoelectron peaks at Mars. Icarus 182, 371–382. (doi:10.
1016/j.icarus.2006.01.014)
Frahm, R. A. et al. 2007 Locations of atmospheric photoelectron energy peaks within the Mars
environment. Space Sci. Rev. 126, 389–402. (doi:10.1007/s11214-006-9119-5)
Galand, M., Yelle, R. V., Coates, A. J., Backes, H. & Wahlund, J.-E. 2006 Electron temperature of
Titan’s sunlit ionosphere. Geophys. Res. Lett. 33, L21101. (doi:10.1029/2006GL027488)
Gan, L., Keller, C. N. & Cravens, T. E. 1992 Electrons in the ionosphere of Titan. J. Geophys. Res.
97, 12 137–12 151. (doi:10.1029/92JA00300)
Hartle, R. E., Sittler, E. C., Ogilvie, K. W., Scudder, J. D., Lazarus, A. J. & Atreya, S. K. 1982
Titan’s ion exosphere observed from Voyager 1. J. Geophys. Res. 87, 1383–1394. (doi:10.1029/
JA087iA03p01383)
Hartle, R. E. et al. 2006 Initial interpretation of Titan plasma interaction as observed by the
Cassini plasma spectrometer: comparisons with Voyager 1. Planet. Space Sci. 54, 1211–1224.
(doi:10.1016/j.pss.2006.05.029)
Hill, T. W. et al. 2005 Evidence for rotationally-driven plasma transport in Saturn’s magnetosphere. Geophys. Res. Lett. 32, L14S10. (doi:10.1029/2005GL022620)
Johnson, R. E., Tucker, O. J., Michael, M., Sittler, E. C., Young, D. T. & Waite, J. H. Submitted.
Mass loss processes in Titan’s upper atmosphere, ch. 18. Titan after Cassini–Huygens.
Lewis, G. R., André, N., Arridge, C. S., Coates, A. J., Gilbert, L. K., Linder, D. R. & Rymer, A. M.
2008 Derivation of density and temperature from the Cassini–Huygens CAPS electron
spectrometer. Planet. Space Sci. 56, 901–912. (doi:10.1016/j.pss.2007.12.017)
Linder, D. R. et al. 1998 The Cassini CAPS electron spectrometer. In Measurement techniques in
space plasmas: particles (eds R. E. Pfaff, J. E. Borovsky & D. T. Young). AGU Geophysical
Monograph, no. 102. pp. 257–262. Washington, DC: American Geophysical Union.
Lorenz, R. D. et al. 2008 Titan’s inventory of organic surface materials. Geophys. Res. Lett. 35,
L02206. (doi:10.1029/2007GL032118)
Ma, Y. et al. 2007 3D global multi-species Hall-MHD simulation of the Cassini T9 flyby. Geophys.
Res. Lett. 34, L24S10. (doi:10.1029/2007GL031627)
Mitchell, D. G., Brandt, P. C., Roelof, E. C., Dandouras, J., Krimigis, M. & Mauk, B. H. 2005
Energetic neutral atom emissions from Titan interaction with Saturn’s magnetosphere. Science
308, 989–992. (doi:10.1126/science.1109805)
Mondolo, R. & Chanteur, G. 2007 A global hybrid model for Titan’s interaction with the Kronian
plasma: application to the Cassini Ta flyby. J. Geophys. Res. 113, A01317. (doi:10.1029/
2007JA012453)
Ness, N. F., Acuna, M. H., Behannon, K. W. & Neubauer, F. M. 1982 The induced magnetosphere
of Titan. J. Geophys. Res. 87, 1369–1381. (doi:10.1029/JA087iA03p01369)
Neubauer, F. M., Gurnett, D. A., Scudder, J. D. & Hartle, R. E. 1984 Titan’s magnetospheric
interaction. In Saturn (eds T. Gehrels & M. S. Matthews), pp. 760–787. Tucson, AZ: University
of Arizona Press.
Neubauer, F. M. et al. 2006 Titan’s near magnetotail from magnetic field and electron plasma
observations and modeling: Cassini flybys TA, TB, and T3. J. Geophys. Res. 111, A10220.
(doi:10.1029/2006JA011676)
Phil. Trans. R. Soc. A (2009)
Downloaded from http://rsta.royalsocietypublishing.org/ on June 16, 2017
788
A. J. Coates
Porco, C. C. et al. 2005 Imaging of Titan from the Cassini spacecraft. Nature 434, 159–168. (doi:10.
1038/nature03436)
Sagan, C. & Khare, B. N. 1979 Tholins—organic chemistry of interstellar grains and gas. Nature
277, 102–107. (doi:10.1038/277102a0)
Sillanpaa, I., Kallio, E., Janhunen, P., Schmidt, W., Mursula, K., Vilppola, J. & Tanskanen, P.
2006 Hybrid simulation study of ion escape at Titan for different orbital positions. Adv. Space
Res. 38, 799–805. (doi:10.1016/j.asr.2006.01.005)
Simon, S., Boesswetter, A., Bagdonat, T., Motschmann, U. & Schuele, J. 2007 Three dimensional
multispecies hybrid simulation of Titan’s highly variable plasma environment. Ann. Geophys.
25, 117–144.
Sittler, E., Bertucci, C., Coates, A., Cravens, T., Dandouras, I. & Shemansky, D. Submitted.
Energy deposition processes in Titan’s upper atmosphere, ch. 18. In Titan after Cassini–
Huygens.
Smith, B. A. et al. 1991 Encounter with Saturn: Voyager 1 imaging science results. Science 212,
163–191. (doi:10.1126/science.212.4491.163)
Stofan, E. R. et al. 2007 The lakes of Titan. Nature 445, 61–64. (doi:10.1038/nature05438)
Szego, K. et al. 2005 The global plasma environment of Titan as observed by Cassini plasma
spectrometer during the first two close encounters with Titan. Geophys. Res. Lett. 32, L20S05.
(doi:10.1029/2005GL022646)
Szego, K., Bebesi, Z., Bertucci, C., Coates, A. J., Crary, F., Erdos, G., Hartle, R., Sittler, E. C. &
Young, D. T. 2007 The charged particle environment of Titan during the T9 flyby. Geophys.
Res. Lett. 34, L24S03. (doi:10.1029/2007GL030677)
Tsang, S. M. E., Coates, A. J., Jones, G. H., Frahm, R. A., Winningham, J. D., Barabash, S.,
Lundin, R. & Fedorov, A. Submitted. Ionospheric photoelectrons at Venus: case studies and
first observations in the tail.
Vuitton, V., Yelle, R. V. & Anicich, V. G. 2006 The nitrogen chemistry of Titan’s upper
atmosphere revealed. Astrophys. J. 647, L175–L178. (doi:10.1086/507467)
Vuitton, V., Lavvas, P., Yelle, R. V., Galand, M., Wellbrock, A., Lewis, G. R., Coates, A. J. &
Wahlund, J.-E. Submitted. Negative ion chemistry in Titan’s upper atmosphere.
Wahlund, J. E. et al. 2005 Cassini measurements of cold plasma in the ionosphere of Titan. Science
308, 986–989. (doi:10.1126/science.1109807)
Waite Jr, J. H., Young, D. T., Cravens, T. E., Coates, A. J., Crary, F. J., Magee, B. & Westlake, J.
2007 The process of Tholin formation in Titan’s upper atmosphere. Science 316, 870. (doi:10.
1126/science.1139727)
Wei, H. Y., Russell, C. T., Wahlund, J.-E., Dougherty, M. K., Bertucci, C., Modolo, R., Ma, Y. J.
& Neubauer, F. M. 2007 Cold ionospheric plasma in Titan’s magnetotail. Geophys. Res. Lett.
34, L24S06. (doi:10.1029/2003GL019389)
Wilson, E. H. & Atreya, S. K. 2004 Current state of modeling the photochemistry of Titan’s
mutually dependent atmosphere and ionosphere. J. Geophys. Res. 109, E06002. (doi:10.1029/
2003JE002181)
Young, D. T. et al. 2004 Cassini plasma spectrometer investigation. Space Sci. Rev. 114, 1–112.
(doi:10.1007/s11214-004-1406-4)
Young, D. T. et al. 2005 Composition and dynamics of plasma in Saturn’s magnetosphere. Science
307, 1262–1265. (doi:10.1126/science.1106151)
Phil. Trans. R. Soc. A (2009)