IT has been suggested recently1 that the drift of ionization in the F2 region of the ionosphere, known to exist from measurements made at middle latitudes2, is caused directly by the electric field associated with the S system of currents in the ‘dynamo’ region of the atmosphere. The presence of these currents was originally invoked to explain the daily variations in the geomagnetic elements; recent ionospheric analysis3,4 and direct experiments using rockets have shown that this dynamo region corresponds in height with the ionospheric E region. According to Martyn, magnetic lines of force linking the E and F regions represent directions of high electric conductivity and can be considered as equipotentials. Thus a horizontal north–south electric field in the E region can give rise to a vertical component of electric field in the F2 region which is greatest at the magnetic equator, where it may exceed by several times the horizontal E region field. If this field exists it must give rise to a horizontal drift of ionization in the F region in an east–west direction. Martyn5 has proposed on theoretical grounds that, except in the immediate vicinity of the magnetic equator, the eastward drift velocity resulting from the diurnal potential in the dynamo region should be given approximately by where θ is the geomagnetic co-latitude of the point in the F region, ϕ is longitude measured positively eastward from the midnight meridian and H is the vertical geomagnetic field component. This equation predicts for high and middle latitudes a drift toward the east by day and to the west by night, which is in agreement with observations. At geomagnetic latitudes less than 35°, however, a phase reversal of this drift should take place, and near the magnetic equator the westward drift by day should reach large values of the order of 200 m./sec.
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