Global characteristics between the equatorial electrojet and neutral wind in the Mesosphere-Thermosphere-Ionosphere region

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The equatorial electrojet (EEJ) is a huge eastward current which flows at the
dayside equatorial region of the Earth's ionosphere, in a narrow channel
(±(3 ∼ 5)^◦ in latitudinal range). The EEJ current is observed as an
enhanced magnetic variation of the horizontal component of geomagnetic field at
the dayside magnetic dip equator. The main mechanism of EEJ is an effect of
polarization electric field in the E region of the ionosphere at the dip
equator caused by the horizontal magnetic field at the magnetic equator [e.g.,
Forbes, 1981]. In a recent study, many researchers show the results which comes
to relationship the neutral wind and EEJ [e.g., Fang et al., 2008, Aveiro et
al., 2009]. However, lack of the long-term comparison analysis of geomagnetic
field and wind data obtained from ground magnetometer and atmospheric radars,
the detailed relationship between the EEJ and neutral wind fluctuations in the
mesosphere and lower thermosphere (MLT) regions has not yet been revealed.

We compared the long-term variation of geomagnetic field data obtained from
ground magnetometers which belong to MAGDAS managed by International Center for
Space Weather Science and Education, Kyushu University and neutral wind data
obtained from medium frequency (MF) radar which operated by Research Institute
for Sustainable Humanosphere, Kyoto University. These instruments were located
at the equatorial region. As a result, we found that the relationship between
the variations of zonal wind and the residual-EEJ showed a clear inverse
correlation. Here, the residual-EEJ is defined as the deviation from the second
order fitting curve between the EUV flux and the EEJ amplitude. These results
suggest that the vertical current (J_z), which is generated by the dynamo
action due to the zonal wind perpendicularly across to the background magnetic
field, changes the Cowling conductivity derived under the condition of J_z=0.
This trend is observed in not only the Asia Pacific region (close to the radar)
but also the South Africa region (far from the radar site). We also performed
the frequency analysis to quantitatively define the relationship of zonal wind
and residual-EEJ, and found that both of the neutral wind and residual-EEJ have
almost the same dominant frequency with a small difference. In addition, we
perform the comparative analysis with neutral wind data observed from the
satellite, and found the neutral wind disturbance has the same dominant
frequency around the equatorial region.