UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICS
MULLARD SPACE SCIENCE LABORATORY
Density trends of negative ions at Titan
A. Wellbrock1,2,3, A. J. Coates1,3, G. H. Jones1,3, G. R. Lewis1,3, C. S.
Arridge1,3, D. T. Young4, B. A. Magee4, F. J. Crary4, J. H. Waite4, E. C.
Sittler5
1 Mullard Space Science Laboratory, Department of Space and Climate Physics,
UCL
2 Department of Physics and Astronomy, UCL
3 Centre for Planetary Sciences, UCL
4 Space Science and Engineering, SwRI
5 NASA/GSFC
2/19
1. Introduction
• Instrumentation:
Cassini CAPS Electron Spectrometer (CAPS-ELS)
Hemispherical top-hat electrostatic analyser
Energy range 0.6eV – 27,000eV
Energy resolution 16.7%
Mounted on an actuator; can sweep the instrument at
a nominal rate of 1/s
[Linder et al, 1998, Young et al, 2004].
3/19
Electron spectrogram, T26, 01:40UT – 02:00UT
Neg ion spikes
Energy
(eV)
Altitude (km):
2242
1264
999
1590
2755
UT time:
01:40
01:45
01:50
01:55
02:00
How do we know that spikes are due to negative ions?
•Ion velocities are small compared to spacecraft velocity (~ 6 km/s)
•CAPS-ELS only sees spikes when actuator points in ram direction
– because negative ions are too slow to enter instrument when actuator points
in a different direction
•The electron distribution is isotropic - hence seen from any actuator position
•Energy can be converted into mass using kinetic energy associated with relative
speed -> spikes represent mass distribution
•Max mass observed: 13,800 amu/q
4/19
Negative ions at Titan
• Discussed by Coates et al (2007, 2009)
and Waite et al (2007)
• Expected at lower altitudes but not at
Cassini altitudes – necessitates
reconsideration of chemical processes
(e.g. Vuitton et al, 2009)
• Observed at altitudes < 1400 km when
pointing in ram direction
• Waite et al (2007, 2008), Coates et al
(2007) suggested they are aerosol
precursors. Tholins eventually
precipitate towards surface (heavy
products from energetic processing of
mixtures of gases such as CH4, N2, and
H2O)
• An additional hypothesized chemical
pathway involving the heavy ions as
fullerenes is presented by Sittler et al
(PSS in press)
Waite et al, 2007
5/19
Towards sun
Towards Saturn
Nominal corotation
wake
Cassini flyby trajectory
Closest approach ± 60
min
T57 encounter
geometry
In Titan’s shadow
from 18:11 –
18:31UT
Closest approach
(CA): 18:32 UT, at
955 km
22 June 2009
6/19
ELS spectrogram of T57 – an actuator fixed flyby
In shadow
CA
Actuator fixed in
ram direction:
Boundary between
central anodes (4
& 5) is aligned
with ram direction
~18:22 – 18:42 UT
Anode
4
energy
(eV)
-> continuous
negative ion data
instead of
individual spikes
Anode
5
energy
(eV)
-> increases
available data
significantly
Angle
(deg)
Bottom plot: Angle
between ram
direction and
anode 4 & 5
central lines
Time (hours)
7/19
Titan – Solar Zenith Angle – s/c trajectory plot
Altitude range 950 –
1050 km
SZA=90
°
Titan atmosphere
< 950 km
Titan solid body
sun
SZA=0°
T57 flyby trajectory
SZA=135
°
8/19
Mass group 1: 10-30 amu
SZA dependence at altitudes 950-1050 km
SZA=90°
sun
SZA
(°)
SZA=180°
n_i, group1, (cm^-3)
• Data from 22 encounters (between T16 and T49)
9/19
Mass group 1: 10-30 amu
Day/night side dependence at altitudes 950-1050 km
SZA=90°
* In
shadow
Altitude (km)
∆ not in
shadow
sun
n_i, group1, (cm^-3)
• Data from actuator fixed encounter T57
SZA=180°
10/19
Mass group 5: 110-200 amu
SZA dependence at altitudes 950-1050 km
SZA=90°
SZA
(°)
sun
n_i, group5, (cm^-3)
• From 22 encounters (between T16 and T49)
SZA=180°
11/19
Mass group 5: 110-200 amu
Day/night side dependence at altitudes 950-1050 km
* In
∆ not in
shadow
Altitude (km)
shadow
n_i, group5, (cm^-3)
• Data from actuator fixed encounter T57
12/19
Density results summary + discussion (1)
Photochemistry
yields highest
group 1
densities
Mass
group 1
(10-30
amu)
Highest
n_i at
lower SZA
Mass
group 5
(110-200
amu)
Wide n_i
range
Near
terminator
region
Lowest n_i
Lowest n_i
Nightside
Decreased
n_i range
Highest
n_i
Dayside
Due to enhanced recombination rates and/or
lack of photo-dissociation on nightside?
Photochemistry
reaction yield
lower than
nightside
reactions
-or similar yield
but limited by
photodissociation
Nightside
reactions yield
highest group
5 densities
13/19
Density results summary + discussion (2)
Why are densities lowest near terminator region
(both mass groups)?
•Insufficient solar flux for photochemical reactions?
•but enough for some photo-dissociation
and no recombination reactions?
SZA=90
°
SZA=135
°
Altitude range 950 –
1050 km
SZA=0°
Titan atmosphere
< 950 km
Titan solid body
sun
14/19
Conclusion
This study helps constrain the chemical formation and
destruction processes of negative ions in Titan’s ionosphere
Future work
• Many possible controlling parameters that affect densities:
Latitude, SZA, Titan local time (TLT), altitude, seasonal
effects/winds/orbital and spin period, the angle between
magnetospheric co-rotation and solar ionisation sources/SLT
• Increase data set with future flybys
• Obtain information about ion temperature using attitude
changes
• Subtract electron background for actuator-fixed flybys
• Assign formal uncertainties to densities (large due to e.g.
background subtraction, MCP efficiency)
15/19
Supplementary slides
16/19
Ion densities
• Count rates converted to ion densities by assuming they represent a
current of ions in the ram direction
• n_ion density = C/(A_eff*epsilon*v)
• where C is the count rate,
• A_eff is the effective area of the instrument estimated from aperture size
and ground calibration data,
• epsilon is the estimated microchannel plate efficiency for ions at this bias
voltage and v is the spacecraft velocity.
• CAPS-ELS was designed to measure electrons;
• negative ions were not expected at these altitudes before the Cassini
mission
• the instrument response was not characterized in the laboratory for ions,
• -> micro-channel plate efficiency has so far been estimated based on
nominal ion efficiencies
• -> uncertainties in the densities are currently difficult to quantify but may
be large
17/19
MCP efficiency from Fraser et al, 2002
Fraser et al, 2002: Calculated variation with mass of the channel efficiency
of an MCP of the Oberheide et al. geometry [22]. Individual
curves labelled with ion energies in keV. Open area fraction indicated
by the broken horizontal line. Squares: experimental fit to
macromolecular ion relative efficiency data of Twerenbold et al.
[28]; triangle: data point (5% at 30 keV acceleration for a 66,400 u
bovine albumine ion) from Frank et al. [54].
18/19
Coates et al 2009 PSS:
At which altitudes, latitudes and SZAs do the highest masses appear?
Looked at max mass for each negative ion spike:
• Clear preference for larger mass at lower altitude
• Some tendency for larger masses at higher latitudes
• Some tendency to detect highest masses near terminator
Now look at density trends of different
masses
CN- C3N- C5NC/s
Mass groups: 10-30 amu, 30-50 amu,
50-80 amu, 80-110 amu, 110-200
amu, 200+ amu
(Coates et al, 2007)
Vuitton et al 2009: Identified peaks of
first 3 groups: CN , C3N , C5N
Many possible controlling parameters;
start with SZA dependence, 2 mass
groups (1 & 5) and a fixed altitude
range (950 – 1050 km)
amu/q
First chemical model including
negative ions, ELS spectrum at
1015 km (T40)
(Vuitton et al PSS 2009)
19/19
Mass group 1: 10 to 30 amu
Latitude dependence at altitudes 950km to
1050km
• From 22 encounters (between T16 and T49)
20/19
Mass group 5: 110 to 200 amu
Latitude dependence at altitudes 955km to
1050km
• From 22 encounters (between T16 and T49)
21/19
Mass group 2: 30 to 50 amu
SZA dependence at altitudes 955km to 1050km
• From 22 encounters (between T16 and T49)
22/19
Poster
SZA=90°
P29 A. Wellbrock, A.J. Coates et al
[email protected]
SZA=135°
Altitude range 950 –
1050 km
Negative ions measured by Cassini CAPS
Electron Spectrometer:
Titan atmosphere
< 950 km
SZA=0°
Titan solid body
sun
Effects of Solar Zenith Angle (SZA) on relative
densities (horizontal axes)
SZA
0
°
180
°
110-200 amu
Mass group
SZA
0
°
Altitude (km)
10-30 amu
Mass group
*shadow
Altitude
180
°
•Data from T57 ( around 135°)
1050 km
In
950 km
1050 km
Altitude
•Data from 22 flybys
950 km
∆ not in
shadow
23/19
Increasing data set available
- by using spacecraft attitude changes
There are 8 anodes in ELS
Usually the central anode (5) points in
the ram direction
-> neg ion spikes can only be seen in
this anode
Sometimes the spacecraft’s attitude
changes
-> other anodes briefly point in ram
direction
Data from such observations can be
used to increase the data used to look
for trends
8 ELS
anodes
24/19
2. Increasing data set available
- by using spacecraft attitude changes
Energy (eV)
Anode 3
Anode 4
Anode 5
Anode 6
12 minutes
25/19
• Data set: Use max neg ion mass of each spike
from 23 encounters
Energy (eV)
Anode 3
Anode 4
Anode 5
Anode 6
12 minutes
26/19
Dependence of max ion mass with altitude
Altitude
(km)
Ion mass (amu/q)
• Preference for larger mass at lower altitude
• Error bars refer to energy resolution (16.7%)
27/19
Dependence of max ion mass with latitude
Latitude
(degrees)
Ion mass (amu/q)
• Larger masses at higher latitudes?
28/19
Dependence of max ion mass with SZA
SZA
(degrees)
Ion mass (amu/q)
• Some tendency to detect highest masses near
terminator
29/19
•Spot radius proportional to log of ion mass
•One spot per neg ion spike
•Colour scale -> altitude
30/19
•Spot radius linearly proportional ion mass
•One spot per neg ion spike
•Colour scale -> altitude
31/19
Contents
1. Introduction
2. T57: An actuator fixed encounter
3. Mass group density dependence on solar
zenith angle (SZA) – preliminary results
4. Summary, discussion, future work