1. Ionospheric photoelectrons at Venus: ASPERA-4 observations A.J. Coates 1,2 S.M.E. Tsang 1,2, R.A. Frahm 3, J.D. Winningham 3, S. Barabash 4, R. Lundin 4 and the ASPERA-3 and 4 teams A. Wellbrock 1,2, F.J.Crary 3, D.T.Young 3 and the CAPS team 1. Mullard Space Science Laboratory, UCL, UK 2. Center for Planetary Sciences at UCL/Birkbeck, UK 3. Southwest Research Institite, Texas, USA 4. IRF-Kiruna, Sweden

2. Photoionisation in ionospheres – produces ions and photoelectrons Solar EUV gives peaks: HeII 30.4nm gives ~21-24, 27 eV photoe - at M, E; ~24eV Titan, e.g. Haider 86, Gan et al., 92, Galand et al., 06 Additionally, shoulder at ~50-60eV due to solar decrease <~16nm Measurements in Earth ionosphere give detailed spectra (e.g. Lee et al., 80, Doering et al, 76), also models e.g. Nagy & Banks 70 Peaks and shoulder are ‘fingerprints’ for day side ionosphere Mantas and Hansen, 1979; see also Fox & Dalgarno, 1979 Mars Earth

3. MarsVenusTitan Transition Energy (eV) Transition Energy (eV) Transition Energy (eV) Transition Energy (eV) N 2 X 2 Σ g 25.2CO 2 X 2 Π g 27.02O 2 P22.29N 2 A 2 Π u 24.09 O 2 P22.29CO 2 B 2 Σ u 22.69O 4 S27.17 N 2 A 2 Π u 24.09O 2 P22.29O 2 D23.69 Table 1 – Some dominant photoelectron energies from 30.4 nm solar UV illumination (40.79 eV) Some dominant photoelectron energies from 30.4 nm solar UV illumination (40.79 eV) Ionisation potentials associated with ground and excited states of O, N 2, CO 2 are similar – table shows dominant peaks expected in 20-30eV range From Coates et al., PSS in press, 2010

4. Sensor Reference MeasuresEnergy range (eV/q) Energy resolution (  E/E, %) Angle range (  ) Angle bin (  ) GEOS-1, 2 S-302 Suprathermal Plasma Analyser Wrenn et al., 1981 Energy, direction 0.5-500110, 8018x18, 8x32 Cassini CAPS Electron spectrometer (ELS) Young et al, 04, Linder et al., 98 Energy, direction 0.6-28,00016.7160x520x5 Mars Express ASPERA-3 ELS Barabash et al., 2007a Energy, direction 10-20,0008360x422.5x4 Venus Express ASPERA-4 ELS Barabash et al., 2007b Energy, direction 1-30,0008360x422.5x4 CAPS, ASPERA-ELS can measure ionospheric photoelectron spectrum at Titan, Mars, Venus

5. Ionospheric photoelectrons at Earth, Mars, Titan – seen in ionosphere and remotely (Coates et al., 2010) E: ionosphere E: magnetosphere M: ionosphere M: tail T: ionosphere T: tail Lee et al., 1980 Coates et al., 1985 Coates et al., 2010 Frahm et al., 2006 Coates, 2009 Coates et al., 2007 1 10 100

6. Knudsen et al. (1980) 200-300 km alt 15-40° SZA Spectra from 2 orbits Before Venus Express: PVO/RPA observations Photoelectrons seen in sunlit ionosphere Spectral features not resolved

7. Venus: solar wind interaction – 18 May 06 From Coates et al, PSS 2008 First observation in Venus ionosphere by VEX ASPERA-4 ELS From O rather than CO 2

8. Venus – ionospheric photoelectrons seen in tail up to 1.45 Rv (D), as well as in dayside ionosphere (C) on 3 June 2006, (Tsang et al, 2010, submitted) D C D C

9. AB B A X VSO (R V ) BS Ionospheric photoelectrons Venus Express example 20 April 2008, interval B ends at ~1.5 R v Coates et al., 2010 (PSS in press)

10. 6 October 2010 X VSO (R V )

11. VEX sees photoelectrons Magnetic connection anywhere in sunlit ionosphere, via draped solar wind field, flux rope, etc Sunlit ionosphere – production of photoelectrons Dark ionosphere Photoelectron-drawn escape throughout dayside region – particularly along connected field?

12. Comparison of shapes: Model - VEx/ASPERA4/ELS O + - & He + -related photoelectrons  S/C potential (-4.9 eV) Photoelectron peaks identified by Coates et al. (2008) Newly identified peaks Cui et al., 2010 (JGR, submitted), Galand et al poster

13. Comparison: Model - VEx/ASPERA4/ELS B field line based on Vex/MAG SZA = 87° Magnetic field orientation improves quantitative comparison Cui et al., 2010 (JGR, submitted), Galand et al poster

14. Conclusions on ionospheric photoelectrons (IPE) IPE are seen clearly in the dayside ionospheres with suitable instrumentation The energy spectrum of IPE is distinctive, acting as a ‘fingerprint’ for ionisation processes IPE can, at times, be seen at large distances from the dayside ionospheres, e.g. in Earth’s magnetosphere, and in the tails of Mars, Venus and Titan IPE are a sensitive diagnostic ‘tracer’ of a magnetic connection to the production location, namely the dayside ionospheres IPE may play a role in setting up an ambipolar electric field at the top of the ionosphere which would enhance ionospheric escape in a mechanism analogous to the Earth’s polar wind (e.g. Ganguli, 1996 and refs), enhancing pressure-driven plasma escape. A similar mechanism was suggested at Venus (Hartle and Grebowsky, 1995), and a polar wind- related electric field escape mechanism at Titan was modelled by Keller and Cravens (1994). See Coates et al., PSS in press, 2010 for further details

15. Conclusions (Coates et al., PSS in press, 2010) Photolectrons are a tracer of magnetic connection to ionosphere Similar process at all objects with ambipolar electric field enhancing outflow VenusEarthMarsTitan InteractionSolar wind Saturn magneto- sphere Intrinsic magnetic fieldNoYes - dipoleLocalised crustal magnetization associated with heavily- cratered S. highlands No Species associated with dominant photoelectron peaks (dayside, high altitude) ON 2, OCO 2, ON 2, (CH 4 ) Location of ionospheric photoelectrons seen remotely from dayside ionosphere TailMagneto- sphere Tail Related ion escape?Yes (VEx)Polar windYes (MEx) Yes (T9, T15, T40)