Version definitions of AE and Dst geomagnetic indices
Original version Oct. 17, 2022
WDC for Geomagnetism, Kyoto
Note: These definitions can be applied to current procedures for the index calculation but may differ
from those in the early years.
Version definition of the auroral electrojet (AE) Index
The production method of the AE index is the same among the versions, but the primary differenceis the implementation of correction with visual inspection for higher-grade versions. The following
link shows the calculation
u.ac.jp/aedir/ae2/onAEindex.html).

methodoftheAEindex(https://wdc.kugi.kyoto-Real-time AE index
The real-time AE index is an AE version calculated using real-time geomagnetic field data. The
calculation is performed automatically every 20 min (as of October 17, 2022) for all periods up to thecurrent month. We do not remove artificial noise and baseline offset, while we perform manual
corrections when these errors are significant so that the calculated index is useless, even for monitoring
purposes. Specifically, if a large baseline offset occurs in a specific station, the biased value could
affect the AL or AU, resulting in inadequate AE values.
We used a baseline calculated from the international five quietest days (5QDs) data of the previousmonth instead of the current month for real-time AE index production. We recalculated the index of
the previous month with a baseline value calculated from the data on 5QDs of the same month. It wasnoted that real-time AE values may often be modified. The real-time index is used for monitoring but
not in the scientific analysis, and digital data are not publicly available. On our webpage, the plot of
the real-time AE for each month disappears when the provisional AE index of the corresponding monthis released.
Provisional AE index
The provisional AE index is calculated with a lag of several months, and during the calculation
period, we use field data with the highest processing level (provisional/quasi-definitive/definitivevalues). The calculation method is the same as for the real-time AE index, except that manual
correction of artificial noises and baseline offset are performed on the original field data. Recently,
there has been an attempt to automate this process as much as possible. The provisional AE index
values can be used for scientific analysis, and the digital data are publicly available. However, theprovisional AE index may also be modified if the field data are updated. At that time, we publish thenews of the modification on our website. Currently, the provisional AE since January 1990 and inMarch 1989 are available.

Final AE index
The final AE index is a version for which no modification will be made in the future. The final
values for every minute are available only from January 1977–June 1988 and those for every hourfrom July 1957–June 1988. During these periods, the geomagnetic field data were digitalized fromanalog magnetograms for the AE index. As such, the field data used will not be updated in the future.
Since 1988, the final AE value calculation has been on hold. Later, the field data were transferred toour center in digital format, and it is necessary to calculate the final AE values from INTERMAGNETapproved definitive geomagnetic data. However, definitive geomagnetic data are not available forsome AE stations. Thus, we probably have to redefine the final AE index. Note that the AE index
before 1976 had been calculated at the Geophysical Institute of the University of Alaska and WDC-Afor STP in Boulder not at WDC for Geomagnetism Kyoto (see https://wdc.kugi.kyotou.ac.jp/aedir/ae2/onAEindex.html for more detail).
Version definition of the disturbance storm time (Dst) index
There is an essential difference between the methods used to calculate the real-time/provisional
Dst and final Dst. The following link shows the calculation method for the Dst index
https://wdc.kugi.kyoto-u.ac.jp/dstdir/dst2/onDstindex.html).
Real-time Dst index
The real-time Dst index calculation using the real-time geomagnetic field data was performed
automatically every 30 min (as of October 17, 2022) for all periods up to the current month. We do
not remove artificial noise and baseline offset, while we perform manual corrections when these errors
are significant so that the calculated index is useless, even for monitoring purposes. Specifically, if alarge baseline offset occurs at a specific station, the biased value would affect Dst, in inadequate Dstvalues.
For the real-time Dst index production, we used the baseline value and Sq variation calculated
from the data on the international five quietest days (5QDs) of the previous month instead of thecurrent month. We recalculated the previous month’s index using the baseline value and the Sqvariation from the data on the 5QDs of the same month. Note that the real-time Dst values may often
be modified. The real-time Dst index is approved only for monitoring purposes and not for scientificanalysis, but digital data are publicly available on our webpage. The plot of the real-time Dst for eachmonth on our webpage disappears when provisional Dst values for the corresponding month arereleased.
Note that the method to determine the baseline and Sq variation for real-time Dst differs
significantly from the method proposed by Sugiura and Kamei (1991). They suggested modeling Sqvariations by the double-Fourier transformation with data in one year (January–December) andmodeling the long-period secular variation by the second-order polynomial function with the datafrom the past five years. However, we cannot use these methods for real-time Dst calculations, but weused another, which uses only the field data and 5QDs in the current month (or the previous month’s
5QDs). The sum of Sq variation and baseline is determined by averaging the daily variation (as afunction of the local time) over 5QDs and removing the linear trend. Therefore, the real-time Dstvalues may be significantly different from the final Dst values, which can be determined using themethod proposed by Sugiura and Kamei (1991).
Provisional Dst index
Generally, the provisional Dst index is calculated with a lag of several months. During thecalculation period, we used field data with the highest processing level (provisional/quasidefinitive/definitive values). The calculation method is the same as for the real-time Dst index, exceptthat the manual correction of artificial noises and baseline offsets is performed on the original fielddata. We can use the provisional Dst values for scientific analysis, and the digital data are publicly
available.
Final Dst index
The Dst final value is the Dst index with the highest processing level calculated with a lag ofseveral years, and in the future, no further changes will be made. Currently, we use the definitive
geomagnetic field data provided by INTERMAGNET. These data are reliable and have been corrected
automatically and manually, and our center has not made additional corrections. Unlike the real-timeand provisional Dst values, the calculation method of baseline (secular variation) and Sq variation
used the Sugiura and Kamei (1991) method. Therefore, calculations are made from January toDecember (annually), and no updates are made unless INTERMAGNET updates the definitive values.
Appendix: Examples of differences in values between versions of the Dst index
We compared the three-version Dst values in 2014 (Figure 1) as an example. We observed thatthese variations were consistent with each other. The real-time and provisional values appear to be ingood agreement because the baseline and Sq variations were determined using the same method.
Though there are some minor differences due to the differences in noise removal, this depends on theextent to which real-time geomagnetic field data had intense noises, and the errors were corrected (thesame applies to the difference between the real-time and provisional AE). In September and December,there was a substantial discrepancy of about 20–30 nT between the provisional and final values, which
may be because the baseline for the provisional value was determined using only the data for thatmonth. If the horizontal component of the geomagnetic field is smaller than usual for a month due toa series of magnetic storms. The magnetic field average on the 5QDs was small, resulting in a lower
baseline value, which may lead to an underestimation of the strength of magnetic storms. However,
we determined the final Dst baseline from a longer-term trend, so even when the geomagnetic field
declines temporarily, such as for a month or more, it does not cause a high baseline deviation.
Reference
Sugiura, M., and T. Kamei (1991), Equatorial Dst-index 1957–1986, in IAGA Bulletin No. 40, ed.
by A. Berthelier, M. Menvielle (ISGI Publ. Off., Saint. Maur-des-Fosses, 1991)

Figure 1: Comparison between final (red), provisional (blue), and real-time (green) Dst values in 2014.
Data for each month are plotted in each panel.