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High-altitude signatures of ionospheric modification by field-aligned currents. T. Karlsson Alfvén Laboratory, Royal Insititute of Technology, Stockholm. E n. B t. BACKGROUND. Expected correlation with constant ionospheric conductivity and no E // :.

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slide1

High-altitude signatures of ionospheric modification by field-aligned currents.

T. Karlsson

Alfvén Laboratory, Royal Insititute of Technology, Stockholm

slide2

En

Bt

BACKGROUND

Expected correlation with constant ionospheric conductivity and no E// :

slide3

2003-12-25 - Correlation between E and DB

s/c 1

Northern hemisphereMLT ~ 04

s/c 2

BZ,MEE(nT)

EY,MEE(mV/m)

s/c 3

s/c 4

t (s)

slide4

BX,GSE

BX,GSE

BX,GSE

EY,GSE

R

Separation

ILAT

MLT

t (s)

2002-05-19 Overview Plot

slide5

DBt(nT)

DBn (nT)

Et(mV/m)

En(mV/m)

t (s)

2002-05-19 E and DB

Southern hemisphereMLT ~ 20

slide6

2002-05-19 Enormal and j//

j//

s/c 1

Enormal

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

s/c 4

upward current

downward current

t (s)

slide7

18

DB S/C 4

-80 º

-70 º

-60 º

24

Observations 2002-05-19

  • Spatial separations small (~100 km) (not visible in the CGLAT-MLT plot)
  • Large electric fields correlated with large downward currents instead of with B.
  • But there also exists regions of large downward current where there is no large electric field.

CGLAT-MLT plot

slide8

2002-04-27 Enormal and j//

Southern hemisphereMLT ~ 20

s/c 1

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

s/c 4

upward current

downward current

t (s)

slide9

2002-05-12 Enormal and j//

Northern hemisphereMLT ~ 19

s/c 1

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

upward current

s/c 4

downward current

t (s)

slide10

2003-11-15 Enormal and j//

s/c 1

Northern hemisphereMLT ~ 07

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

s/c 4

upward current

downward current

t (s)

slide11

Observations

  • Several examples of large electric fields correlated with large downward currents.
  • Currents stable on time scale of 40 s, whereas electric field changes appreciably.
  • The observed current sheets of large downward current have a width of the order of 10 km
  • All observations (found by manual inspection) from non-sunlit ionospheric footpoints.
slide12

Rejected explanations for correlation between E and j//

U-shaped potential

S-shaped potential

Alfvén wave

Eperp

Eperp

E//

Bperp

j//

E//

Eperp

Eperp

Bperp

Eperp

j//

j//

A (partially) standing AW could give observed phase shift between E and B, but unlikely on these scales, and would give no preference of any sign of of dB/dt.

If j// is proportional to U//, then the maximum of j// coincides with a minimum of Eperp.

Modelling shows that maximum of j// can be close to region of large Eperp, but never ‘inside’ it.

slide13

600

j// carried by e -

jP carried by ions

Model

Consequence: Outflow of electrons from ionosphere in downward current region, with subsequent cavity formation in ionosphere. E region evacuated in ~10 s for large currents.

slide14

JP

SP

EP

Model –ionospheric modification by

downward FACs

j//

Magnetosphere

upward j//

Ionosphere

downward j//

slide15

2002-05-19 Model results (E)

s/c 1

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

s/c 4

upward current

downward current

t (s)

slide16

2002-05-19 Enormal and j//

j//

s/c 1

Enormal

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

s/c 4

upward current

downward current

t (s)

slide17

2002-04-27 Model results (E)

s/c 1

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

upward current

s/c 4

downward current

t (s)

slide18

2002-04-27 Enormal and j//

Southern hemisphereMLT ~ 20

s/c 1

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

s/c 4

upward current

downward current

t (s)

slide19

2002-04-27 Model results S/C 3

j//(mA/m2)

data

SP(S)

modelled

En(mV/m)

modelled

JP(mA/m)

data

slide20

Observations 2002-04-27

  • Minimum variance analysis yields an angle for the current sheet from E-W direction of ≈ 36º
  • Time delay information gives direction of motion of current sheet
  • Scale size of current sheet ~10 km (at ionosphere).

Current sheet

v

CGLat

S/C 1S/C 2S/C 3S/C 4

+ indicates position of maximum FAC

MLT

slide21

Model - moving current system

  • A moving current system leaves a ‘trail’ of low conductivity behind itself.
  • This widening of the low-conductivity region accounts for e.g. the appearance of the electric field minimum at the high-latitude end of the current sheet at point 4 at the 2002-04-27 observation. Also this model remarkably well reproduces some other features (marked by 1,2, and 3).

j//

v

Magnetosphere

Ionosphere

SP

slide22

2002-04-27 Model results S/C 4 (MOVING CURRENT SYSTEM)

1

2

3

4

j//(mA/m2)

En(mV/m)

modelled

data

SP(S)

modelled

En(mV/m)

modelled

JP(mA/m)

data

slide23

CONCLUSIONS

  • At times, the perpendicular electric field normal to a current sheet may be correlated to the field-aligned current, rather than the magnetic field.
  • This happens for large FACs and electric fields.
  • A simple model of ionospheric modification by downard FACs reproduces these findings well.
  • This mechanism represents a way of generating large electric fields (which may map out to the magnetosphere) of the order of several hundreds of mV/m, even when there is no associated parallel potential drop.
slide25

2003-12-30 Enormal and j//

s/c 1

s/c 2

j//(mA/m2)

En(mV/m)

s/c 3

s/c 4

upward current

downward current

t (s)

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