An investigation of the diurnal and seasonal characteristics of irregular magnetic activity in northern Canada, using two indices of disturbance, shows that in general the K index provides a reliable but smoothed measure of short period activity. Two peaks of activity are evident, a day-time peak, dominant inside the polar cap, and a night peak, dominant south of the auroral zone. The latter occurs within one hour of local geomagnetic midnight at four Canadian observatories, whereas the day-time peak occurs progressively later at higher latitudes. The maximum activity occurs in the equinoxes at latitudes near the auroral zone and during the summer solstice at very high latitudes. Measurements of range at the most disturbed observatory suggest that aeromagnetic surveying in high latitudes is in general feasible, but that for accurate reconnaissance work it is very advantageous to plan operations, so far as conditions permit, taking into account the systematic diurnal variation of disturbance. It is shown that errors of about one half the assumed diurnal change between base lines will occur and sufficient data is given to allow approximate predictions of the optimum length of flight lines.