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Cluster Reveals Properties of Cold Plasma Flow. May 15, 2009 Erik Engwall. Outflows from the ionosphere. Vsc ~ 20-40 V > E ion ~ 0-10 eV. Chappell et al. [1987,2000]. Outflows from the ionosphere. Chappell et al. [1987,2000]. Model. Wake formation in flowing plasmas.

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Cluster Reveals Properties ofCold Plasma Flow

May 15, 2009

Erik Engwall


Outflows from the ionosphere
Outflows from the ionosphere

Vsc ~ 20-40 V > E ion ~ 0-10 eV

Chappell et al. [1987,2000]


Outflows from the ionosphere

Chappell et al. [1987,2000]


Model
Model

Wake formation in flowing plasmas


Studies of wake formation
Studies of wake formation

Ion density (normalized)

150 m

1

0.8

100 m

100 m

0.6

0.4

50 m

0.2

0 m

0 m

50 m

200 m

250 m

300 m

100 m

150 m

(Engwall et. al, 2006)

Potential [V]

150 m

100 m

50 m

0 m

0 m

50 m

200 m

250 m

300 m

100 m

150 m


Model1
Model

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

Wake formation in flowing plasmas

EFW & EDI

EDI

FGM

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


Model2
Model

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

Wake formation in flowing plasmas

EFW & EDI

EDI

FGM

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

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+)


Statistical study
Statistical study

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


Statistical study1
Statistical study

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



Geographical distribution1
Geographical distribution

cm-3

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



Solar and magnetic activity
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
Comparison to previous results

Cluster study

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


Comparison to previous results1
Comparison to previous results

(Engwall et al., 2009)

  • 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


Conclusions
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 ionosphere1
Outflows from the ionosphere

Solar cycle

<8 RE

Cluster

Satellite missions

Chappell et al. [1987,2000]


Previous detections in magnetotail
Previous detections in magnetotail

E (eV)

Acceleration makes ions visible

Cold ions in plasma sheet visible due to high flow speed on Cluster (Sauvaud et al. [2001])

  • Other examples:

  • GEOTAIL: Hirahara et al. [1996], Mukai et al. [1994]

  • Polar: Liemohn et al. [2005]


Previous detections in magnetotail1
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)


Geographical distribution2

18

00

12

06

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

Geographical distribution

cm-3


Geomagnetic activity

Cluster

Akebono

DE-1

Geomagnetic activity


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 = gu┴+ 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 data

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 data

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.)


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