Influence of imf sector polarity on annual and diurnal variations of absorption in the auroral zone

The effect of the polarity of the interplanetary magnetic field (IMF) on the absorption in and near the auroral zone has been studied using measurements from a meridional chain of riometers in Finland in the years 1973–1978. The measured absorption values were divided into two groups, corresponding to away and towards polarities of the IMF, and the values of absorption were examined for the two groups separately. The annual means of absorption were found to be independent of sector polarity in the whole latitude range. In the annual and diurnal variations a polarity effect could be found down to L =4.4, in the annual variation some remnants of it down to L = 3.4. In the annual variation, the towards (T) polarity is associated with a strong spring maximum and the away (A) polarity with a fall maximum in absorption. The diurnal variations are most pronounced during T polarity in the spring and during A polarity in the fall, with high daytime maxima in both cases, suggesting that the loss processes of trapped electrons in the day sector might be modulated by IMF polarity. Our analysis reveals the effect of the different components of the IMF; The annual variations show the effect of the vertical component (varying energy transfer from the solar wind into the magnetosphere) and the diurnal variations in the equinoxes the effect of the azimuthal component (shift of the absorption zone along the dawn-dusk meridian).