On the existence of a “Q.L.”-“Q.T.” “transition-level” in the ionosphere and its experimental evidence and effect

Abstract

The magneto-ionic theory shows that when the effects of collisions and of the earth's magnetic field are taken into consideration, the propagation is “quasi-longitudinal” in the lower part of the ionosphere, where the collision frequency is high and “quasi-transverse” in its upper part, where the collision frequency is low. For vertical incidence, the altitude at which the transition between the Q.L. and Q.T. regions occurs is that corresponding to the critical collision frequency v c , which is a function of the gyrofrequency and of the magnetic dip angle only. For France, where this angle is 64° to 65°, the transition level is located at about 95 km, i.e. within the E region. At midday, when the maximum electron density of the E layer occurs at the lowest heights, it may happen that this maximum penetrates the Q.L. region. The evaluation of N max should therefore be made with caution, for in the latter case, the critical frequency observed (corresponding to the ordinary trace vertical tangent), when used in the classical formula, only valid in the Q.T. region, leads to a serious underestimate of N max . When N max is located somewhat lower than the transition level, or exactly at this altitude, the h′ƒ record should show either a crochet (in the first case), or a hollow, i.e. a complete absence of trace (in the second case). Crochets and hollows of this sort are indeed quite often found, in the h′ƒ records of the Poitiers and Domont stations. It is shown how these hollows and crochets can be fully explained on the basis of the magneto-ionic theory and that they are in fact an excellent evidence of the existence in the E region of the Q.L.-Q.T. transition level. Finally, attention is drawn to the criteria now in use to determine the ƒ 0E , which often leads to an apparent flattening of the N max diurnal curve, and appropriate formulas and methods are suggested to find the correct values of N max .