An ‘Ionospheric Weather’ Index

Abstract

This note explains a method for deriving a daily ' ionospheric weather ' index for a given geographic location, and displays the behaviour of this index for the Boulder ionosphere during 8 months of 1968. ' Ionospheric weather ' means the dynamic state of the ionosphere at a particuIar time and place, as influenced by the number and intensity of ionization ' irregularities ' present. Such an index should be useful in attempts to associate short-term fluctuations in ionospheric electron density with other geophysical events, and ultimately, if such associations can be found, in forecasting the dynamic properties of ionospheric telecommunication circuits. The index is derived from continuous, steep-incidence, HF Doppler ionospheric soundings of the type described by Davies & Baker (1966). Ionospheric irregularities cause changes in the radio reflection height, which are recorded as fluctuations in the received frequency (Doppler shifts) of a stable 5 MHz radio wave reflected from heights of about 200 km in the F region. A great variety of irregularity ' signatures ' appear on Doppler-shift-vs-time records, many of which are attributable to the passage of atmospheric waves, whose properties can be measured with spaced sounders (Georges 1968; Davies & Jones 1971). At other times, the fluctuations are nearly random in appearance, defying most quantitative analyses. It was quickly learned, however, that the intensity of the fluctuations seemed to wax and wane on time scales of a few days, and also that intensity bore no obvious relation to other geophysical phenomena (except that high Doppler activity characteristically accompanies geomagnetic storms). Fig. 1 shows representative samples of ' quiet ' and ' disturbed ' Doppler records, as well as one containing wavelike frequency fluctuations. The Boulder data cited here were made available through the courtesy of Dr K. Davies of the NOAA Space Environment Laboratory. The unanswered question is: what causes the variations in ionospheric weather that are not associated with magnetic disturbances? In order to examine possible connections with other geophysical events (specifically, tropospheric weather) it was necessary to find a way to quantify the degree of ionospheric activity represented by a given Doppler record. Although it is often easy to subjectively classify a given record as ' quiet ' or ' disturbed ', many fall in between, and it seemed desirable to develop a more quantitative and objective indicator of' ionospheric activity. After several attempts that need not be discussed here, it was found that measuring the total length of the Doppler trace for a fixed time interval, though physically meaningless, provided a single number that agreed well with subjective feelings about the degree of activity present. To quantify one day's activity in a single number, the line length for the 8-hr interval centred on local noon was measured and divided by the length of the time