Seasonal and diurnal variations in the direction of
propagation of medium-scale travelling ionospheric disturbances (MSTIDs) have
been investigated by analyzing sea echo returns detected by the TIGER
SuperDARN radar located in Tasmania (43.4° S, 147.2° E
geographic; –54.6°Λ). A strong dependency on local
time was found, as well as significant seasonal variations. Generally, the
propagation direction has a northward (i.e. equatorward) component. In the
early morning hours the direction of propagation is quite variable
throughout the year. It then becomes predominantly northwest and changes to
northeast around 09:00 LT. In late fall and winter it changes back to
north/northwest around 15:00 LT. During the other seasons, northward
propagation is very obvious near dawn and dusk, but no significant northward
propagation is observed at noon.
It is suggested that the variable propagation direction in the morning is
related to irregular magnetic disturbances that occur at this local time.
The changes in the MSTID propagation directions near dawn and dusk are
generally consistent with changes in ionospheric electric fields occurring
at these times and is consistent with dayside MSTIDs being generated by the
Lorentz force.
Key words. Ionosphere (ionospheric disturbances; wave
propagation; ionospheric irregularities; signal processing) |