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)

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=1.7mint=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 conversion area Inertia is dominant in the conversion region conversion is occurred in the region with large Va gradient sunward

Electric current in 15h meridian and contour of Va 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. Equatorial 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=1.7mint=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=1.7min t=5.0min

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

Electromagnetic variations of FLRs around the resonance latitude(x 0 ) Amplitude enhancement at resonance latitude (I 0, K 0 : the 1st and 2nd modified Bessel functions) Hughes [1994] Polarization reversal at the resonance latitude

Ground magnetic variations from the MHD simulation: (Tsw=20min, 0.8mHz) D H 12h15h18h 0min 100min 0min 100min 0min 100min latitude latitude Phase reversal SW pressure: nPa, Vx=350km/sec, IMFBz=4.33nT, IMFBy=2.5nT

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