Mesospheric planetary wave signatures in the equatorial electrojet

Using five years (1994-1998) of simultaneous and collocated measurements of horizontal winds associated with mesosphere and lower thermosphere (MLT) region and geomagnetic field measurements in the Indian geomagnetic dip equatorial region, an extensive experimental study has been made on the influences of mesospheric planetary waves on the equatorial electrojet (EEJ). The winds are measured using medium-frequency (MF) radar (1.98 MHz) located at Tirunelveli. The EEJ strength is determined by measuring the geomagnetic field strength at both the geomagnetic dip and off-dip equatorial stations, Trivandrum and Alibag, India. By noting the simultaneous time evolution of spectral signal (Morlet wavelet analysis) in both the EEJ strength and mesospheric winds at 88 km, it is found that mesospheric planetary waves with periodicities of 2-25 days often do propagate up to the ionospheric dynamo region and have their influences on EEJ strength. Sometimes only one of the parameters shows strong spectral signal in this periodicity range. It is also observed that often the diurnal oscillation in EEJ strength and mesospheric winds exhibits coherent variations in amplitude when the signature of mesospheric planetary-scale oscillations is found in the EEJ strength. Also, it is found that variations in high-energy solar radiations (F10.7 is used as a proxy) can influence the EEJ strength at planetary timescales. This leads to an interesting question. What actually causes the vertical coupling between ionosphere and atmosphere through upward propagating planetary waves to be operative only during some (and not all) of the episodes?