T. Karlsson

Alfvén Laboratory, Royal Insititute of Technology, Stockholm

High-altitude signatures of ionospheric modification by

field-aligned currents.

BACKGROUND

0 0

1

1 1

Pn

P P

tt

P P

JE j dn

Bdn B

n

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

En

Bt

2003-12-25 - Correlation between E and B

Northern hemisphereMLT ~ 04

EY

,ME

E (mV

/m)B

Z,M

EE (

nT)

s/c 1

s/c 2

s/c 3

s/c 4

t (s)

t (s)

BX

,GS

EB

X,G

SE

BX

,GS

EE

Y,G

SE

RS

eparation

MLTILAT

2002-05-19 Overview Plot

2002-05-19 E and BB

t (nT

)

t (s)

Bn (

nT)

Et (

mV

/m)

En (m

V/m

)Southern hemisphereMLT ~ 20

2002-05-19 Enormal and j//

t (s)

j // (A

/m2 )

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

Enormal

j//

En (m

V/m

)

Observations 2002-05-19

B S/C 4

-80 º

-70 º

-60 º

24

18 • 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

2002-04-27 Enormal and j//

t (s)

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

Southern hemisphereMLT ~ 20

j // (A

/m2 )

En (m

V/m

)

t (s)

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

2002-05-12 Enormal and j//j //

(A

/m2 )

En (m

V/m

)

Northern hemisphereMLT ~ 19

2003-11-15 Enormal and j//

t (s)

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

Northern hemisphereMLT ~ 07

j // (A

/m2 )

En (m

V/m

)

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.

Rejected explanations for correlation between E and j//

U-shaped potential

Eperp

j//

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

Eperp

j//

S-shaped potential

E//

E//

Eperp

j//

Eperp

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

Alfvén wave

Bperp

Eperp Bperp

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.

600

Modelj// carried by e - jP carried by ions z

j

qt

ne

//1

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.

Model – ionospheric modification by

Iono

sphe

reM

agne

tosp

here j//

JP

P

EP

0 0

//

,

//

1 1tP t

up

P P background

down

Pn

P

BJ j dn dn B

n

k j

k j

JE

upward j//

downward j//

downward FACs

t (s)

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

2002-05-19 Model results (E)j //

(A

/m2 )

En (m

V/m

)

2002-05-19 Enormal and j//

t (s)

j // (A

/m2 )

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

Enormal

j//

En (m

V/m

)

t (s)

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

2002-04-27 Model results (E)j //

(A

/m2 )

En (m

V/m

)

2002-04-27 Enormal and j//

t (s)

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

Southern hemisphereMLT ~ 20

j // (A

/m2 )

En (m

V/m

)

data

data

modelled

modelled

En (

mV

/m)

P (

S)

j // (A

/m2 )

J P (

mA

/m)

2002-04-27 Model results S/C 3

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

CG

Lat

MLT

v

+ indicates position of maximum FAC

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

Model - moving current systemIo

nosp

here

Mag

neto

sphe

re

j//

P

v

• 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).

data

data

modelled

modelled

En (

mV

/m)

P (

S)

En (

mV

/m)

J P (

mA

/m)

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

modelled

j// (

A/m

2)

1

4

2 3

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.

t (s)

downward current

upward current

s/c 1

s/c 2

s/c 3

s/c 4

2003-12-30 Enormal and j//j //

(A

/m2 )

En (m

V/m

)