The high latitude sources are evidently of two types depending upon whether the observation is made in the neighborhood of the auroral oval or poleward of it (i.e.., in the polar cap region). It has been found that auroral Es is basically a nocturnal phenomenon, it is associated with the optical aurora, and is due to auroral electron precipitation. Because of its proximity to the seat of auroral substorm activity, it is not surprising to find some correlation between auroral Es and some appropriate magnetic index. Indeed, it has been found that auroral Es is positively correlated with magnetic activity. On the other hand, polar cap Es may be relatively weak, and is negatively correlated with substorm activity.
Turning equatorward, it has been found that equatorial Es is most pronounced during daylight hours, and evidence points to formation of ionization irregularities within the equatorial electrojet as the responsible agent at low latitudes.
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