Abstract. Long-term geomagnetic activity presented by the aa index has been used to show that the heliospheric magnetic field has more than doubled during the last 100 years. However, serious concern has been raised on the long-term consistency of the aa index and on the centennial rise of the solar magnetic field. Here we reanalyze geomagnetic activity during the last 100 years by calculating the recently suggested IHV (Inter-Hour Variability) index as a measure of local geomagnetic activity for seven stations. We find that local geomagnetic activity at all stations follows the same qualitative long-term pattern: an increase from early 1900s to 1960, a dramatic dropout in 1960s and a (mostly weaker) increase thereafter. Moreover, at all stations, the activity at the end of the 20th century has a higher average level than at the beginning of the century. This agrees with the result based on the aa index that global geomagnetic activity, and thereby, the open solar magnetic field has indeed increased during the last 100 years. However, quantitatively, the estimated centennial increase varies greatly from one station to another. We find that the relative increase is higher at the high-latitude stations and lower at the low and mid-latitude stations. These differences may indicate that the fraction of solar wind disturbances leading to only moderate geomagnetic activity has increased during the studied time interval. We also show that the IHV index needs to be corrected for the long-term change of the daily curve, and calculate the corrected IHV values. Most dramatically, we find the centennial increase in global geomagnetic activity was considerably smaller, only about one half of that depicted by the aa index.

Abstract. We have reconstructed a new, homogeneous geomagnetic D st index for , 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][1942][1943][1944][1945][1946][1947][1948][1949][1950][1951][1952][1953][1954][1955][1956], and by using the nearby CTO station as a predecessor of the HER station for [1932][1933][1934][1935][1936][1937][1938][1939][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 . 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 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.

The Dst index has been one of the most important solar-terrestrial indices for decades, and it is used in numerous studies as a measure of the temporal development and intensity of magnetic storms and the ring current. Here we discuss two issues related to the relative and absolute normalization that are problematic to the Dst index. We show for the first time quantitatively that the magnetic disturbances at the four Dst stations are ordered according to the latitudinal projection of an equatorial disturbance upon the local horizontal component of the geomagnetic field. Therefore, the disturbances observed at each station should be first normalized by the cosine of the geomagnetic latitude of the station before they are averaged to form the Dst index. Perhaps surprisingly, the recipe to calculate the Dst index does not include this normalization and, therefore, must be revised on this part. We also discuss the effects of correcting the quiet-time seasonal variation, the so called "non-storm component" in the Dst index. This correction is seasonally varying, being largest around equinoxes and smallest at solstices, leading to an average correction (increase) of about 6 nT, i.e. about 25-30%, for annual averages of the Dst index. This increase also leads to significantly improved correlations between the corrected Dst index, the so called Dcx index, and many other indices of solar-terrestrial disturbance. We show here in detail that the correlation between the geomagnetic Ap index and the Dcx index (cc = 0.83) is much higher than between Ap and Dst (cc = 0.60). These results give further evidence that the Dcx index is a more truthful measure of magnetic storminess than the original Dst index.

This study examines how the asymmetry in the low‐latitude geomagnetic field evolves during sheath and magnetic cloud domains of interplanetary coronal mass ejections using the Dst, SYMH and ASYH indices. For all investigated storms the station that contributed most to Dst was located in the dusk sector while the smallest disturbance field was concentrated around dawn. We found that sheath region storms are associated with larger morning/afternoon asymmetry than magnetic cloud storms. For sheath storms the station in the dusk sector contributed almost four times as much to Dst as the station in the dawn sector. Furthermore, the disturbance field is strongly variable during sheath storms. The results of this study suggest that for magnetic cloud storms the asymmetry arises mainly from ions drifting on open trajectories whereas in a case of a sheath driven storm the sudden intensifications of the substorm associated current systems add significantly to the asymmetry.

[1] It is known that the semiannual variation in the Dst index is excessively large compared to all other indices of geomagnetic activity. This has been interpreted in terms of a separate ''non-storm component'' which forms roughly one half of the whole semiannual variation in the Dst index. Since this component is not related to storms or the ring current it should be removed from the Dst index. We show how the ''non-storm component'' arises from the seasonal variation of the magnetic field at the Dst stations and from the erroneous treatment of the quiet-time curve during the construction of the index. Moreover, we reconstruct a corrected Dst index which is purified from the non-storm component and show that then the semiannual variation indeed attains the same level as in other geomagnetic indices. This correction will greatly affect earlier estimates of the physical causes of semiannual variation based on the Dst index. Since the correction will reduce the power at periods even longer than the semiannual period, in particular the annual variation, it has important consequences also on other types of studies based on the Dst index. Citation: Mursula, K., and A. Karinen (2005), Explaining and correcting the excessive semiannual variation in the Dst index, Geophys.