ELECTRON-DENSITIES DURING WINTER ANOMALOUS ABSORPTION OF DIFFERENT INTENSITY OBSERVED AT 37.1-DEGREES-N, 6.73-DEGREES-W, DURING WINTER 1975-76 .1.

The results of in situ measurements of electron concentration in the D-region of the ionosphere obtained with different probe techniques during the Western European Winter Anomaly Campaign 1975/76 are described. Electron density profiles were measured on fifteen days when the amount of absorption was low, medium or winter-anomalous at time of launch. In agreement with earlier results and with those gathered by other authors, the electron density was found to be increased between about 75 and 95km when winter-anomaly was present. One probe technique allowed measurements of ionization also at heights well below the D-region where the number density of free electrons is very small compared with that of negative ions. On four flights of this probe, electrons could be distinguished from ions in the probe records at heights between 58 and 62 km, in one case even from 38 km upwards. This means that λ, the ratio between the number density of negative ions and that of free electrons had changed in favour of electrons in this height region when these launches took place. Further, significant increases of ion density by a factor of 14-20 at a height of 40 km and a different scale height of ionization between 40 and 70 km was observed on three flights. Two events of this kind were observed when λ was small at heights below 70 km. This increase of ionization at 40 km was neither related to air temperatures measured with a different instrumentation flown on small meteorological rockets shortly after the launch of the guard ring probes, nor to cosmic ray intensity measured on the ground. © 1979.