A new reconstruction of the D st index for 1932 – 2002

We have reconstructed a new, homogeneous geo-magnetic D st index for 1932–2002, thus extending the original D st index by 25 years, i.e. by more than one full solar magnetic cycle. The extension was done by using data from the original set of four low-latitude stations for 1941– 1956, and by using the nearby CTO station as a predecessor of the HER station for 1932–1940. Despite some open questions related to the composition of the original D st index, the reconstructed index is quite similar to the original one during the overlapping time interval (1957–2002). However, the reconstructed D st index corrects for some known errors in the original D st index, such as the erroneously large daily UT variation in 1971. Also, despite the overall agreement, the reconstructed index deviates from the original index even on the level of annual averages for several years. For instance , all annual averages of the reconstructed index are negative, and for 1962–1966 they are systematically lower (more stormy) than those of the original index. Accordingly, we disagree with the uniquely positive annual average of the original index in 1965, which most likely is erroneous. We also find somewhat higher (less stormy) values than in the original D st index for the three lowest annual averages in 1960, 1989 and 1991, out of which the lowest annual average is found in 1989 rather than in 1991. The annual averages of the geomagnetic A p index and the reconstructed D st index correlate very well over this time interval, except in the beginning of the series in 1932–1940 and in the declining phase of solar cycles 18, 20 and 21, where high speed solar wind streams cause enhanced geomagnetic activity. Using the superposed epoch method we also find that, on average, the storms in the early extended period (1932–1956) are less intense but tend to have a longer recovery phase, suggesting that there are more HILDCAA-type medium activity intervals during the early period than more recently. We also study the annually averaged storm structure over the 71-year time interval and find that the most stormy years occur during the declining phase of solar cycles 17 and 21 and around the solar maxima of cycles 19 and 22.