Long-term changes and trends in the lower ionosphere

Abstract

A brief overview of long-term trends in the lower ionosphere ( h<100 km, ionized component) is presented. The trends have been studied with the use of several different types of data sets: A3 radio wave absorption (LF–HF, oblique incidence on the ionosphere, continuous wave), A2 (cosmic radio noise) riometric absorption, IPHA (indirect measurements of phase reflection height), rocket measurements of electron density and ion composition. The A3 radio wave absorption in the lower ionosphere over Europe has been analyzed for trends in: (1) absolute values of absorption; (2) amplitudes of annual and semiannual variations; (3) inferred planetary wave activity. Trends in absolute values of absorption do not provide a consistent pattern, which may be due to their sensitivity to changes in the shape of electron density profile and to instrumental problems. Similarly, there is no consistent pattern in trends in relative amplitudes of annual variations. However, there appears to be a systematic tendency to the positive trend in amplitudes of semiannual variations. Some increase of the absorption-inferred planetary wave activity was found in 1970s and 1980s, whereas no change was found in 1960s and 1990s. Analyses of cosmic noise absorption measured by a chain of riometers in Finland do not reveal an evident trend in absorption. IPHA analyses in mid-latitudinal Europe point to a systematic decrease of the reflection height (level of constant electron density) near 80 km. This has been interpreted in terms of decreasing atmospheric pressure due to decreasing columnar temperature in 50–80 km. Rocket measurements of electron density display for non-winter electron density at 75 and 80 km an increase at middle latitudes (contrary to IPHA results). Rocket measurements of ion composition in the lower ionosphere are too rare, but models reveal dramatic changes with a large decrease of NO + concentration. Since the lower ionosphere is substantially affected by the 11-year solar cycle, data sets length of more than 20 years is very desirable in long-term trend studies. Results based on various methods and various data sets do not provide sufficiently consistent observational pattern of trends in the lower ionosphere. Therefore, further investigations, tests of data quality and homogeneity, and careful application of various methods are necessary.