Long-Term Dependencies of Annual and Semiannual Components of NmF2 Over Concepción

Abstract

In this work an attempt is made to determine whether the annual and semiannual components of the NmF2 observed over Concepcion (36.8°S; 73.0°W), Chile, exhibit long-term trends. Monthly median hourly values of NmF2 determined from ionosonde observations are analyzed. Although the assessed trends for NmF2 and for the semiannual and annual components of NmF2, are hardly significant by any statistical test, they exhibit a systematic diurnal evolution. NmF2 trends are almost null during night time and negative from around noon to the early evening. The same can be said for trends of the amplitudes of the semiannual component. On the contrary, trends for the annual component are always positive and exhibit a rather irregular diurnal pattern. It seems reasonable then to particularly look for an explanation of the semiannual component long-term changes when concerned with long-term changes of NmF2. The semiannual variation is linked to composition and thermospheric wind effects, and solar zenith angle changes which may be connected to changes of the strength of the global circulation. But, more study is needed to envisage the changes on the semiannual component of NmF2 which may arise from eventual long-term changes of the global circulation.