The magnetic field separation is performed using the Siebert-Kertz equations. A superposed epoch analysis of all events clearly shows that the internal contribution peaks strongly at substorm onset, when the internal contribution is -• 40% of the total field. After the substorm peak intensity, the internal contribution decreases almost linearly to the quiet time value of 10-20%. The induction effects are largest during the times of rapid changes and at stations located over the Arctic Ocean.
The K indices are to be hand-scaled from analogue magnetograms following morphological criteria expressed in the so-called Mayaud rules. All observatories used to follow these rules in the routine hand-scaling of K indices from analogue magnetograms. As a result of digitalization and automation of observatories, however, observers in charge who are experienced in K scaling from analogue magnetograms are becoming increasingly rare. Therefore the organizations running observatories were forced to find compensating machine methods for the production of K indices. The basic features of the K indices and the Mayaud rules are first reviewed, and then the methods developed for computer derivation of K indices are discussed.In order to decide which of the proposed algorithms was suitable, a comparison among them was organized by the IAGA Working Group on geomagnetic indices. The comparison was made with a common data set, using the same statistical tests. The results are summarized and discussed. Further comparisons between the K indices produced by computer with the different methods are presented in this paper. Four methods were acknowledged by the Working Group during the Vienna IUGG general Assembly in 1991. The results confirm that these methods provide acceptable results in comparison to the average quality of hand-scaled indices.A comparison between computer-produced K indices and reference K indices handscaled following the Bartels-Mayaud rules is also presented. It appears that only two acknowledged methods provide computer-produced K in good agreement with handscaled K. The best computer method (the FMI method) is found to be good enough to allow the continuation of the long tradition of producing K indices without any serious jump in the statistics.
[1] Daily and seasonal variability of long time series of magnetometer data from Dst stations is examined. Each station separately shows a local minimum of horizontal magnetic component near 18 local time (LT) and weakest activity near 06 LT. The stations were found to have different baselines such that the average levels of activity differed by about 10 nT. This effect was corrected for by introducing a new ''base method'' for the elimination of the secular variation. This changed the seasonal variability of the Dst index by about 3 nT. The hemispheric differences between the annual variation (larger activity during local winter and autumn solstice) were demonstrated and eliminated from the Dst index by addition of two Southern Hemisphere stations to a new index termed Dst 6 . Three external drivers of geomagnetic activity were considered: the heliographic latitude, the equinoctial effect, and the Russell-McPherron effect. Using the newly created Dst 6 index, it is demonstrated that these three effects account for only about 50% of the daily and seasonal variability of the index. It is not clear what drives the other 50% of the daily and seasonal variability, but it is suggested that the station distribution may play a role.
[1] Magnetic variations observed at Earth's surface are primarily caused by magnetospheric and ionospheric currents and secondarily affected by currents induced within Earth. For studies of space processes it is necessary to separate the internal contribution from the external one. In this paper we consider the Dst index which reflects the properties of the ring current. A spherical harmonic analysis is applied, using the axisymmetric assumption, to make the separation of magnetic data to external and internal parts. By examining 12 storms in 1997 and 1998, our results show that during the storm main phase the internal contribution to Dst is roughly 30%, after which it decreases to about 20% during the recovery phase. This is supported by an approximate model calculation of the induction in Earth. We also consider H variations at the four Dst observatories (Honolulu, San Juan, Hermanus, Kakioka) separately and at a typical continent station (Boulder) for comparison. It is seen that Kakioka systematically has the largest internal contribution during the storm main phase, while Hermanus has only a very small internal part at that time. The three other stations are closer to the ideal case (i.e., the internal part is roughly 1/3). As the anomalous behavior at Kakioka is thus opposite to that at Hermanus, their effects approximately average out in the computation of Dst. The differences between the stations are obviously due to differences in local ground conductivity structures. This conclusion is supported by investigating the Parkinson induction vectors which are larger at Kakioka and Hermanus than at the other observatories.
The usual plane wave assumption of a sufficiently uniform primary field is questionable at auroral and equatorial latitudes near localized ionospheric currents. Besides attempting to select only events with distant sources, it is reasonable to try to eliminate the distortion due to source effects by taking averages over several events. However, it is difficult to determine the best averaging method in the calculation of the apparent resistivity, the impedance phase, or the induction vector. In this paper, we use synthetic models to demonstrate the differences in averaging methods.
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