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.