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

Cluster Reveals Properties of

Cold Plasma FlowMay 15, 2009

Erik Engwall

Erik EngwallMay 15, 2008

Outflows from the ionosphereVsc ~ 20-40 V > E ion ~ 0-10 eV

Chappell et al. [1987,2000]


Outflows from the ionosphere



Model

Wake formation in flowing plasmas


Studies of wake formationIon density (normalized)

Potential [V]

100 m

(Engwall et. al, 2006)

200 m150 m100 m

50 m

150 m

0 m

100 m

50 m

150 m

0 m

100 m

250 m 300 m50 m0 m

200 m150 m100 m 250 m 300 m50 m0 m

0.8

0.6

0.4

1

0.2


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Model




EFW & EDI

EDI

FGM

Wake field assumed to be in flow direction: Ewake = g(u,…) u

Velocity calculation cross-validated with CIS in low-energy mode and ASPOC operating.


Model




EFW & EDI

EDI

FGM

Wake field assumed to be in flow direction: Ewake = 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.


Statistical study1 s/c, July - October 2001-2005, 765.000 data points






Geographical distribution





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cm-3





(Engwall et. al, [2006], Paper V)




Outflow properties

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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, nmv2




Comparison to previous results



Polar @ 8 RE(Su et al. [1998])

Cluster study




Comparison to previous results

Very good agreement to previous values:• Confirms continuation of ionospheric outflows farout in the magnetotail lobes• Ionosphere supplies plasma to magnetosphere

• Cold ion outflow dominates

(Engwall et al., 2009)


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

–1026 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




Outflows from the ionosphere




<8 RE

Cluster

Solar cycle

Satellite missions


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)


Previous detections in magnetotailObservations of cold plasma sheet ions when Geotail in eclipse (Seki et al. [2003]).


krävs för att kunna se bilden.Detection of low-energy ions (<50 eV)

9 eV

Cold ions in geomagnetic tail lobes (Engwall et al. [2006], Paper 3)









cm-3

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Cross-polar cap potential[Haaland et al., 2007]


Geomagnetic activity



DE-1Akebono

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Model for flow velocity from wake

• Unmagnetized ions on wake length scale ⇨ Spurious field is in flow direction ⇨

Ewake = g(u,…) u • Frozen-in conditions apply• EDI data are good

⇨ u┴

= EEDI × B / B2

We get

g and u// can now be obtained from the electric field components of EDI and EFW.

Ewake = EEFW - EEDI = g u┴

+ g u// B/B

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.

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.

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.)

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

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