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Generation of field-aligned currents by solar wind impulse

Generation of field-aligned currents by solar wind impulse. S. Fujita Meteorological College. Background of this study.

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Generation of field-aligned currents by solar wind impulse

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  1. Generation of field-aligned currents by solar wind impulse S. Fujita Meteorological College

  2. 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.

  3. 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)

  4. Tamao’s wave path(uniform plasma) Tamao [1964a]

  5. Tamao’s description of SC signals Pure transverse wave ionosphere (localized compression) Hall current compression High-lat. SC* Converted transverse wave Isotropic wave ionosphere Pedersen current Low-lat. SC* C-F current Low-latitude MI Tamao [1964b] uniform magnetosphere

  6. Empirical model of SC* (PI) Polarization current along the wave front of the compressional wave PI FAC (the Aflven mode) Where and how is the FAC generated? MI Araki [1994]

  7. PI signature of FAC (ionosphere) t=1.7min t=3.3min t=5.0min Downward FAC (R2-type) in the afternoon shifts toward evening.

  8. Field-aligned current Cross-field current (C-F current) conversion sunward Enhanced C-F current is converted to FAC in the inner magnetosphere

  9. Generation of FAC conversion area sunward conversion is occurred in the region with large Va gradient Inertia is dominant in the conversion region

  10. Electric current in 15h meridian and contour of 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. northward FAC is generated in the region where spatial gradient of Va is steep. Equatorial plane

  11. The compressional wave plays an important role? If this is the wavefront of the compressional wavfe, this moves at the magnetosonic speed (>1000km/sec) PI MI Araki [1994]

  12. FAC (ionosphere) t=1.7min t=5.0min 5h/3.3min → 320km/sec ~ solar wind speed << sound speed

  13. Nightward shift of FAC conversion region t=5.0min t=1.7min Conversion to FACs does not occur in the wavefront of the compressional wave. FACs are generated in the region with enhanced magnetospheric current.

  14. Nightward propagation of the compressional wave front t=5.0min t=1.7min Wave front of the compressional wave passes far away from the conversion site.

  15. Electromagnetic variations of FLRs around the resonance latitude(x0) Polarization reversal at the resonance latitude Hughes [1994] Amplitude enhancement at resonance latitude (I0, K0 : the 1st and 2nd modified Bessel functions)

  16. Ground magnetic variations from the MHD simulation: (Tsw=20min, 0.8mHz) 12h 15h 18h 90 D latitude 60 90 latitude H Phase reversal 60 100min 0min 100min 100min 0min 0min SW pressure: 16.7-3.3nPa, Vx=350km/sec, IMFBz=4.33nT, IMFBy=2.5nT

  17. Ionospheric potential (20min, 0.8mHz) 60o 60o convection vortex Downward/upward FAC  positive/negative potential 60o 60o Motoba et al. (2007) positive negative

  18. 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.

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