Generation of field-aligned currents by solar wind impulse

S. Fujita

Meteorological College

Background of this study

• Tamao [1964a,b] studied generation of the preliminary impulse (PI or SC*) in terms of MHD wave theory. In this paper, he discussed conversion from the compressional disturbance (isotropic mode) generated in the magnetopause to the field-aligned current (transverse mode). Now we can follow his theory in a realistic global MHD simulation.

Tamao’s SC model (1964)Tamao [1964a,b]

• The wave modes in 3D configuration in uniform plasmas– Pure transverse mode (Alfven mode) → no spatial attenuation– Isotropic mode (compressional mode) → spatial attenuation– Converted transverse mode deformed shape (mixture of transverse and isotropic)

01

2

2

22

2

EtVz A

0ˆ1

2

2

22

EetVA

Tamao’s wave path(uniform plasma)

Tamao [1964a]

Tamao’s description of SC signals

compression

C-F current

Pure transverse wave

Isotropic wave

Converted transverse wave

ionosphere

Low-latitude MI

Hall current

High-lat. SC*

Pedersen current

Low-lat. SC*

ionosphere

(localized compression)

uniform magnetosphere

Tamao [1964b]

Empirical model of SC* (PI)

Araki [1994]

Polarization current along the wave front of the compressional wave

PI

MI

FAC (the Aflven mode)

Where and how is the FAC generated?

PI signature of FAC (ionosphere)

t=1.7min t=3.3min t=5.0min

Downward FAC (R2-type) in the afternoon shifts toward evening.

Cross-field current (C-F current)

Field-aligned current

conversion

sunward

Enhanced C-F current is converted to FAC in the inner magnetosphere

Generation of FAC

pdt

duBJ

conversion area

Inertia is dominant in the conversion region

conversion is occurred in the region with large Va gradient

sunward

dt

du

p

Electric current in 15h meridian and contour of Va

FAC is generated in the region where spatial gradient of Va is steep.

FAC is generated in the region where spatial gradient of Va is steep.

Arrows indicate current vectors tangential to this plane. Color shows intensity of FAC (red for earthward, blue for upward in the northern hemisphere). Contours are for Va.

Arrows indicate current vectors tangential to this plane. Color shows intensity of FAC (red for earthward, blue for upward in the northern hemisphere). Contours are for Va.

Equatorial planeEquatorial plane

northward

The compressional wave plays an important role?

Araki [1994]

PI

MI

If this is the wavefront of the compressional wavfe, this moves at the magnetosonic speed (>1000km/sec)

FAC (ionosphere)t=1.7min t=5.0min

5h/3.3min → 320km/sec ~ solar wind speed << sound speed

Nightward shift of FAC conversion region

Conversion to FACs does not occur in the wavefront of the compressional wave. FACs are generated in the region with enhanced magnetospheric current.

t=1.7mint=5.0min

Nightward propagation of the compressional wave front

Wave front of the compressional wave passes far away from the conversion site.

t=1.7mint=5.0min

Electromagnetic variations of FLRs around the resonance latitude(x0)

dx

Ed

kkV

ik

B

B

EkxVdx

E

mkxV

kxV

dx

y

yA

y

x

y

yAy

A

A

ln

)/(

0)(/)(/

)(/

22||

22

2||

22

22||

22

2||

22

ｄｄ

Amplitude enhancement at resonance latitude(I0, K0 : the 1st and 2nd modified Bessel functions)

Hughes [1994]))(())(()(

01

)(/forxaround

0000

2

02

2

0||0

ixxmDKixxmCIxE

Emdx

E

ixxdx

E

xVk

y

yyy

ｄｄ

Polarization reversal at the resonance latitude

Ground magnetic variations from the MHD simulation: (Tsw=20min, 0.8mHz)

D

H

12h 15h 18h

0min 100min 0min 100min 0min 100min

90

60

latitude

90

60

latitudePhase reversal

SW pressure: 16.7-3.3nPa, Vx=350km/sec, IMFBz=4.33nT, IMFBy=2.5nT

Ionospheric potential (20min, 0.8mHz)

negative positive

60o

60o 60o

60o

Downward/upward FAC positive/negative potential

convectionvortex

Motoba et al. (2007)

conclusion

• The MHD simulation is capable of reproducing PI signature observed. The FACs of PI are generated by mode conversion from the compressional wave to the Alfven wave due to non-uniformity of Va. This is a refinement of the Tamao-Araki current model.

• The FAC of PI is not generated in the wavefront of the compressional wave. It is effectively converted in the region with enhanced magnetospheric current associated with the compressional disturbance.

------------------------------------------------------------------• (addition) Polarization reversal itself does not always indicate the

field-line resonance.