Asymmetric auroral electrojet intensity at geomagnetic conjugate stations

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The diurnal and seasonal variations of auroral electrojet perturbations at conjugate stations are investigated to find the dipole tilt angle effect and geomagnetic field strength effect on the auroral ionospheric conductivity, with the data of the H component fluxgate magnetometer recorded at Syowa-Aedey geomagnetic conjugate observatories in the auroral zone. The positive H perturbation in the afternoon to evening sector is 3-5 times larger at Aedey than at Syowa for low and moderate magnetic activity in the northern summer and reaches its maximum around 18 MLT. In contrast, in the southern summer the positive H perturbation at Syowa is less than 3 times that observed at Aedey. It is also shown that the magnitude ratio of ΔH_<Syowa>/ΔH_<Aedey>=1 occurs more frequently in February and October than in the equinox months. This remarkable north-south asymmetry in the intensity of the eastward electrojet indicates that the summertime ionospheric conductivity is larger at Aedey than at Syowa, because Aedey is located at lower geographic latitude than Syowa by 3 degrees and that geographic local time at Aedey is 4 hours earlier than at Syowa. The seasonal change of the ratio for the positive perturbation tends to decrease as the geomagnetic activity increases in the late afternoon hour of 15-18 MLT, suggesting a contribution from precipitating particles. For negative magnetic perturbations, it is shown that a seasonal change is similar to that for positive perturbations during a quiet time, whereas the ratio is smaller on average than that for the positive one. During a disturbed period, it is found that ΔH_<Syowa> tends to exceed ΔH_<Aedey> by a factor of 1.2 on average, regardless of seasons. These features suggest that the effects of the particle precipitation is more significant at Syowa than at Aedey because of the weaker geomagnetic field intensity at Syowa by 20% (8000nT). On the basis of these results, it is pointed out that the difference in geographic latitude and local time among the AE (auroral electrojet) stations could cause a change of up to two times in the auroral electrojet indices, particularly the AU index. Furthermore, the difference in the local geomagnetic field strength among the stations could also cause a change of 18% in the AL index.
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Asymmetric auroral electrojet intensity at geomagnetic conjugate stations

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