Variazione stagionale del sistema di correnti Sq

Abstract

For this study, days with A p < 10 were chosen. However, the reading taken on some of these days had to be discarded, owing to the presence of disturbances during the hours under examination. In all, 283 days were taken, out of a total of 945. For determining the position of the «focus», the S q current system above the Italian region was constructed for each of the selected days on the basis of the following two hypotheses: 1) the current have zero intensity around midnight; 2) the current system neither shifts nor changes shape during the interval between 0900 and 1400 local time. On the basis of the hypothesis 1), the zero level of the horizontal component H and declination D is taken as the average of the four hours 0000-0200 and 2200-2400 of the day in question. The differences between the hourly values of ΔH and ΔD between 0900 and 1400 local time and their corresponding values at zero level were then calculated. Finally, from the pairs of values of ΔH and ΔD the current arrows were constructed in the usual way. According to hypothesis 2), these arrows represent the configuration of the current system during the central hours of the day under consideration. Figures 1-5 represent the current systems deduced from the average daily variation S q of H and D for every month of the period considered. They show that the centre of the current system shifts continually from one month to another. However, during the various years considered, between the same months there is a marked correspondence of behaviour, with the exception of January and December. Figure 6 represents the position of the «focus» for each of the selected quiet days in the