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, UCL3 Centre for Planetary Sciences, UCL 4 Space Science and Engineering, SwRI5 NASA/GSFC

UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICSMULLARD SPACE SCIENCE LABORATORY

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

Altitude (km): 2242 1264 999 1590 2755

UT time: 01:40 01:45 01:50 01:55 02:00

Neg ion spikesEnerg

y (eV)

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 relativespeed -> 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

T57 encounter geometry

In Titan’s shadow from 18:11 – 18:31UT

Closest approach (CA): 18:32 UT, at 955 km

22 June 2009

Towards sun Towards Saturn Nominal corotation wake Cassini flyby trajectory Closest approach ± 60 min

6/19

Actuator fixed in ram direction:

Boundary between central anodes (4 & 5) is aligned with ram direction ~18:22 – 18:42 UT

-> continuous negative ion data instead of individual spikes

-> increases available data significantly

Bottom plot: Angle between ram direction and anode 4 & 5 central lines

In shadow CA

Angle (deg)

ELS spectrogram of T57 – an actuator fixed flyby

Anode 5

energy (eV)

Anode 4

energy (eV)

Time (hours)

7/19

Titan – Solar Zenith Angle – s/c trajectory plot

SZA=0°

SZA=90°

SZA=135°

Altitude range 950 – 1050 km

Titan atmosphere < 950 km

Titan solid body

sun

T57 flyby trajectory

8/19

Mass group 1: 10-30 amuSZA dependence at altitudes 950-1050 km

• Data from 22 encounters (between T16 and T49)

sun

SZA=90°

SZA=180° SZA (°)

n_i, group1, (cm^-3)

9/19

Mass group 1: 10-30 amuDay/night side dependence at altitudes 950-1050 km

• Data from actuator fixed encounter T57

n_i, group1, (cm^-3)

sun

SZA=90°

SZA=180°

Alt

itud

e (

km)

* In

shadow

∆ not in shadow

10/19

Mass group 5: 110-200 amuSZA dependence at altitudes 950-1050 km

• From 22 encounters (between T16 and T49)

n_i, group5, (cm^-3)

sun

SZA=90°

SZA=180° SZA (°)

11/19

Mass group 5: 110-200 amuDay/night side dependence at altitudes 950-1050 km

• Data from actuator fixed encounter T57

Alt

itud

e (

km)

n_i, group5, (cm^-3)

* In

shadow

∆ not in shadow

12/19

Density results summary + discussion (1)

Photochemistry yields highest group 1 densities

Nightside reactions yield highest group 5 densities

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 photo-dissociation

Mass group 1 (10-30 amu)

Mass group 5 (110-200 amu)

Dayside Highest n_i at lower SZA

Wide n_i range

Near terminator region

Lowest n_i Lowest n_i

Nightside Decreased n_i range

Highest n_i

13/19

Why are densities lowest near terminator region (both mass groups)?

Density results summary + discussion (2)

Altitude range 950 – 1050 km

Titan atmosphere < 950 km

Titan solid body

sun

•Insufficient solar flux for photochemical reactions?•but enough for some photo-dissociation and no recombination reactions?

SZA=0°

SZA=90°

SZA=135°

14/19

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)

ConclusionThis study helps constrain the chemical formation and destruction processes of negative ions in Titan’s ionosphere

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

Fraser et al, 2002: Calculated variation with mass of the channel efficiencyof an MCP of the Oberheide et al. geometry [22]. Individualcurves labelled with ion energies in keV. Open area fraction indicatedby the broken horizontal line. Squares: experimental fit tomacromolecular ion relative efficiency data of Twerenbold et al.[28]; triangle: data point (5% at 30 keV acceleration for a 66,400 ubovine albumine ion) from Frank et al. [54].

MCP efficiency from Fraser et al, 2002

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

First chemical model including negative ions, ELS spectrum at 1015 km (T40)(Vuitton et al PSS 2009)

Now look at density trends of different masses

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)

C/s

amu/q

CN- C3N- C5N

-

19/19

Mass group 1: 10 to 30 amuLatitude dependence at altitudes 950km to 1050km

• From 22 encounters (between T16 and T49)

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Mass group 5: 110 to 200 amuLatitude dependence at altitudes 955km to 1050km

• From 22 encounters (between T16 and T49)

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• From 22 encounters (between T16 and T49)

Mass group 2: 30 to 50 amuSZA dependence at altitudes 955km to 1050km

22/19

Altitude range 950 –1050 km

Titan atmosphere < 950 km

Titan solid body

sun

SZA=0°

SZA=90° SZA=135°

SZA

* In shadow

∆ not in shadow

Alt

itu

de

(km

)

10-30 amuMass group

110-200 amuMass group

•Data from 22 flybys •Data from T57 ( around 135°)

0°

180°

SZA

0°

180°

Poster P29 A. Wellbrock, A.J. Coates et al aw2@mssl.ucl.ac.uk

Negative ions measured by Cassini CAPS Electron Spectrometer:

Effects of Solar Zenith Angle (SZA) on relative densities (horizontal axes)

Alti

tude

950 km

1050 km

Alti

tude

950 km

1050 km

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

• Preference for larger mass at lower altitude• Error bars refer to energy resolution (16.7%)

Altitude (km)

Ion mass (amu/q)

27/19

Dependence of max ion mass with latitude

• Larger masses at higher latitudes?

Latitude (degrees)

Ion mass (amu/q)

28/19

Dependence of max ion mass with SZA

• Some tendency to detect highest masses near terminator

Ion mass (amu/q)

SZA (degrees)

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