perturbation ionosphérique migratoire

CONT: Travelling ionospheric disturbances ... are probably a cause of focusing and defocusing of radio waves. The disturbance usually appears at higher frequencies, gradually affecting lower frequencies. Some are associated with magnetic storms and originate in the auroral zone. These may travel great distances. Others appear to be more localised, originating in the troposphere in local weather.
CONT: Travelling Ionospheric Disturbance (TID) is a ripple or wave in the electron density structure that propagates horizontally which affects the refraction of radiowaves and so degrades directional finders. There are two main types: .large-scale TID with 1000 km or more between wave crests usually moving from pole to equator and generated by auroral activity; .medium-scale TID with hundreds of km wavelengths thought to be generated by thunderstorm activity.