1. Cluster Reveals Properties of Cold Plasma Flow May 15, 2009 Erik Engwall

2. May 15, 2008 Outflows from the ionosphere Vsc ~ 20-40 V > E ion ~ 0-10 eV Chappell et al. [1987,2000]

3. Erik Engwall May 15, 2008 Outflows from the ionosphere Chappell et al. [1987,2000]

4. Erik Engwall May 15, 2008 Model Wake formation in flowing plasmas

5. Erik Engwall May 15, 2008 Studies of wake formation Ion density (normalized) Potential [V] 100 m (Engwall et. al, 2006) 200 m 150 m100 m 50 m 150 m 0 m 100 m 50 m 150 m 0 m 100 m 250 m300 m50 m0 m 200 m 150 m100 m 250 m300 m50 m0 m 0.8 0.6 0.4 1 0.2

6. Erik Engwall May 15, 2008 Model Wake formation in flowing plasmas EFW & EDI EDI FGM Wake field assumed to be in flow direction: E wake = g(u,…) u Velocity calculation cross-validated with CIS in low-energy mode and ASPOC operating.

7. Erik Engwall May 15, 2008 Model Wake formation in flowing plasmas EFW & EDI EDI FGM Wake field assumed to be in flow direction: E wake = g(u,…) u The plasma density is obtained from the spacecraft potential (Pedersen et al., 2008) The outward flux, nu //, is now given! (We cannot separate different ion species, but we see mainly H+) Velocity calculation cross-validated with CIS in low-energy mode and ASPOC operating.

8. Erik Engwall May 15, 2008 Statistical study 1 s/c, July - October 2001-2005, 765.000 data points

9. Erik Engwall May 15, 2008 Statistical study 1 s/c, July - October 2001-2005, 765.000 data points

10. Erik Engwall May 15, 2008 Geographical distribution

11. Erik Engwall May 15, 2008 Geographical distribution cm -3 (Engwall et. al, [2006], Paper V)

12. Erik Engwall May 15, 2008 Outflow properties

13. Erik Engwall May 15, 2008 Solar and magnetic activity  Clear dependence on solar radiation and geomagnetic activity  Driving solar wind parameters are – Magnitude of the magnetic field – Solar wind dynamic pressure, nmv 2

14. Erik Engwall May 15, 2008 Comparison to previous results Polar @ 8 RE (Su et al. [1998]) Cluster study

15. Erik Engwall May 15, 2008 Comparison to previous results Very good agreement to previous values: Confirms continuation of ionospheric outflows far out in the magnetotail lobes Ionosphere supplies plasma to magnetosphere Cold ion outflow dominates (Engwall et al., 2009)

16. Erik Engwall May 15, 2008 Conclusions  Powerful new method: cold plasma flows inferred from spacecraft wakes.  Cold ions dominate in large parts of the magnetosphere, both in flux and density  Cold plasma outflow constitutes a major part of the net loss from the Earth – 10 26 protons/s are lost from the planet through high-latitude low-energy outflow processes. + recent results from Mars Express Cold plasmas around planetary bodies much more important than previously thought

17. Erik Engwall May 15, 2008 Outflows from the ionosphere Chappell et al. [1987,2000] <8 RE Cluster Solar cycle Satellite missions

18. Erik Engwall May 15, 2008 Previous detections in magnetotail Cold ions in plasma sheet visible due to high flow speed on Cluster (Sauvaud et al. [2001]) Acceleration makes ions visible Other examples: GEOTAIL: Hirahara et al. [1996], Mukai et al. [1994] Polar:Liemohn et al. [2005] E (eV)

19. Erik Engwall May 15, 2008 Previous detections in magnetotail Observations of cold plasma sheet ions when Geotail in eclipse (Seki et al. [2003]). Detection of low- energy ions (<50 eV) 9 eV Cold ions in geomagnetic tail lobes (Engwall et al. [2006], Paper 3)

20. Erik Engwall May 15, 2008 Geographical distribution cm -3 06 18 1200 Cross-polar cap potential [Haaland et al., 2007]

21. Erik Engwall May 15, 2008 Geomagnetic activity DE-1 Akebono Cluster

22. Model for flow velocity from wake Unmagnetized ions on wake length scale ⇨ Spurious field is in flow direction ⇨ E wake = g(u,…) u Frozen-in conditions apply EDI data are good ⇨ u ┴ = E EDI × B / B 2 We get g and u // can now be obtained from the electric field components of EDI and EFW. E wake = E EFW - E EDI = g u ┴ + g u // B/B

23. Comparing flow velocities from particle and electric field data The derived velocity from the wake (red) shows good agreement with the corrected velocity for H + from CODIF (black). Thus, the wake method to derive flow velocity of cold plasma works and can be used in regions where ions are inaccessible to particle detectors.

24. Measurements from SC1 and SC3 High s/c potential will shield out the plasma ions. Few ions will thus reach CIS and the density is underestimated At the same time E- field measurements differ from EDI and EFW. Why? Because of spacecraft wake.

25. Measurements from SC4 Artificial spacecraft potential control reduces the potential to +7 V, and some of the H+ ions will become visible! Flow aligned with B, which is expected for outflows in the polar wind. The velocity of the H + is possible to measure due to low s/c potential. (The lowest velocities are mis- sing due to the instrument low-energy cutoff.)