Wednesday

Communication, power grids, aviation and satellite systems may be impaired by severe space weather events. Geomagnetic indices are frequently used in the characterization of space weather events. The auroral electrojet (AE) index [1] and the disturbance storm time (Dst) index [2] are two such indices which have been recorded almost continuously for multiple solar cycles. SuperMAG [3], provide a collated full set of ground based magnetometer observations and have produced analogues to AE and Dst which span the last four solar cycles. SME is an electrojet index which shares methodology with AE. SMR is a ring current index which shares methodology with Dst. SME and SMR are both at higher spatial and temporal resolution than AE and Dst. We compare the statistical distribution of AE with SME and Dst with SMR for the last four solar maxima. We carry out a quantitative statistical characterization of how well the indices track large excursions in geomagnetic activity. The construction of the indices and differences in sampling rates lead to subtle differences in their performance. We find that SMR and DST track each other reasonably well over the range of observed Dst values, and SMR captures larger excursions than Dst. However, AE begins to depart from SME at values ~1000 nT and significantly underestimates the ground magnetic perturbation at higher values, that correspond to more severe space weather events.
A simple model is used to illustrate how the differences between AE and SME or Dst and SMR arise, and we discuss their significance with respect to the construction of the indices.

[1] Davis, T. N., Sugiura, M. (1966) Auroral electrojet activity index AE and its universal time variations, Journal of Geophysical Research; Vol. 71 Issue 3, p785-801, 17p
[2] Sugiura, M. (1964), Hourly values of equatorial Dst for the IGY, Ann. Int. Geophys., 35, 9, Pergamon Press, Oxford.
[3] Gjerloev, J. W. (2012), The SuperMAG data processing technique, J. Geophys. Res., 117, A09213, doi:10.1029/2012JA017683.