A Method of Measuring the Collisional Frequency of Electrons in the Ionosphere

Abstract

I READ Mr. Eckersley's interesting letter¹ concerning the measurement of the collisional frequency of electrons in Region F of the ionosphere, but feel very doubtful whether, as he says, his measurements refer to Region F₁. It seems to me that they must refer to Region F₂, the highest and most densely ionised level of the ionosphere. At the time of his measurements (1605 on October 1, 1934), other observations, carried out at the Radio Research Station of the National Physical Laboratory, show that the extraordinary ray critical frequency for Region F_l was 4.2 me./sec. which is different from the value of 5 mc./sec. indicated by Mr. Eckersley. Moreover, since the ionisation in Region F₁ exhibits a most regular type of seasonal and diurnal variation, with no abnormalities, it does not seem possible to account for the discrepancy as a local effect observed only at Chelmsford. It must also be pointed out that the formula previously given in a discussion of long-delay echoes for the ionospheric reflection coefficient, used by Mr. Eckersley, is an approximate one and its use in connexion with this particular problem does not seem justifiable. For the ordinary ray reflection coefficient we have, more accurately, where P and P are respectively the group and optical paths of the waves, is the electron collisional frequency and c the velocity of light. Since Mr. Eckersley's measurements yield values of P only, he has neglected entirely the value of P in the above formula, although it is known that P and P are of the same order of magnitude.