t. karlsson alfvén laboratory, royal insititute of technology, stockholm
DESCRIPTION
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 // :. - PowerPoint PPT PresentationTRANSCRIPT
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
)