Formation of the Sporadic E Layer in the Temperate Zones

Abstract

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.