Comment on “The detection of long‐term trends in the ionosphere”

Abstract

I wish to congratulate Henry Rishbeth on his illuminating Forum piece on long‐term ionospheric studies (Eos, December 7, 1999, p. 590). I appreciate his cautious evaluation and, in particular, his arguments in favor of uninterrupted long‐term measurements. Before expressing my comments, I would like to take the opportunity to explain the ionospheric standard parameters upon which the studies rely: M(3000) is read from the main echo trace obtained by vertical pulse sounding of the ionosphere (when varying the radio frequency in a large range). A standard “transmission curve” is shifted along the frequency axis so that it just contacts the trace. The magnitude of the shift determines the factor by which the cut‐off frequency at normal incidence must be multiplied to give the cut‐off at oblique incidence for communication with a receiver distant by 3000 km. This parameter, currently evaluated on ionograms and used in short wave propagation computations, is known to be narrowly (inversely) connected with hm, the height above ground of the ionospheric electron density peak.