Explaining and correcting the excessive semiannual variation in the Dst index

Abstract

[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.

Non-storm irregular variation of the <I>D</I><sub>st</sub> index

“…7 Seasonal variations are another consideration. The quiettime D st exhibits seasonal dependence (Cliver et al, 2001;Mursula and Karinen, 2005), although its mechanism has not yet been clarified. On the other hand, the solar-wind conditions around Earth might exhibit seasonal dependence.…”

“…This non-storm long-term variation appears to be roughly periodic. In fact, the quiet-time D st index exhibits seasonal variations (Cliver et al, 2001;Mursula and Karinen, 2005). However, it does not seem that this periodic variation explains all the features of the non-storm long-term variation, which implies that the non-storm variation also includes some non-periodic irregular variation.…”

Evolution of Solar and Geomagnetic Activity Indices, and Their Relationship: 1960 – 2001

“…Dcx index 8 has recently been proposed (Mursula and Karinen, 2005;Mursula, Holappa, and Karinen, 2008) as a corrected and extended version of Dst index. The corrections regard the seasonally varying quiet-time level of Dst, the so-called "nonstorm component" which is unrelated to magnetic storms.…”

Energetic electron flux behavior at low L-shells and its relation to the South Atlantic Anomaly

“…Overview of energetic electron fluxes at the inner radiation belt Fig. 2 displays the total trapped and precipitating electron fluxes in the energy range from 30 keV to 2.5 MeV at Lo2 from both NOAA-15 and 16 satellites, the Dcx (corrected Dst) index (in which the Sq-variation of the magnetic X-component is properly subtracted away, Mursula and Karinen, 2005;Karinen and Mursula, 2006) as well as solar wind speed and dynamic pressure in one panel. The trapped flux seems to follow the storm activity closely during the relatively quiet time when the Dcx4 À 100 nT (from February until mid-March), returning nearly back to the quiet time level after each enhancement.…”