IN a recent communication Heisler and Whitehead1 noted that the fraction of time the critical frequency of the sporadic E layer (f 0 E s) exceeded 5 Mc./s. depended on the horizontal component of the Earth's magnetic field, increasing as the horizontal component increased. This relationship is consistent with sporadic E formation arising from a vertical gradient of horizontal movement of the neutral air. In a region where the gyro-frequency of the ions, ωHi, is less than their collision frequency, v i, it is possible to show that, if there is no vertical electric current and the net horizontal current is zero (a horizontal current at one height being cancelled by a reverse current at some other height), the vertical velocity of ions and electrons is given approximately by: where θ is the dip angle and ΔU y is the departure of the horizontal wind velocity at right angles to the Earth's magnetic field from the mean value in the region. A thin layer of ionization will form where V z is zero but where the vertical gradient of V z is large. The probability of such formation depends on ωHi cos θ, which is proportional to the horizontal component of the Earth's magnetic field.
Heisler, L. H., and Whitehead, J. D., Nature, 187, 676 (1960).
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