Ion temperature in the polar cap observed by supra-thermal ion mass spectrometer on Akebono satellite

概要

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Mass resolved ion temperatures in the polar cap at altitudes between 2000 and 10000km are studied statistically based on observations made by Suprathermal ion Mass Spectrometer (SMS) on the Akebono satellite. Ion temperature is obtained by calculating the zeroth, first, and second order moments of two-dimensional ion distribution functions. The method provides not only ion temperature but also parallel and perpendicular temperatures with respect to the magnetic field, by assuming a bi-Maxwellian distribution. Also, the moment calculation provides other plasma quantities such as density and drift velocities. The statistical analysis uses the SMS data acquired over the northern polar region for almost three years. Average altitude profiles of ion temperature for H^+, He^+, and O^+ ions decrease with the increase of altitude up to about 6000km, and above this altitude ion temperature becomes constant or shows a slight increase with altitude. The ratio T_<⫽>/T_<⊥> is almost 1.0 for H^+, while this number increases with ion mass below 6000km. The difference is due to the higher T_<⊥> for H^+ than other ions, and suggests strong frictional heating of H^+ ions under the polar wind condition.
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