Time derivative of the horizontal geomagnetic field as an activity indicator

Abstract

Geomagnetically induced currents (GICs) in technological conductor systems are a manifestation of the ground effects of space weather. Large GICs are always associated with large values of the time derivative of the ge-omagnetic field, and especially with its horizontal component (dH/dt). By using the IMAGE magnetometer data from northern Europe from 1982 to 2001, we show that large dH/dt's (exceeding 1 nT/s) primarily occur during events governed by westward ionospheric currents. However, the directional distributions of dH/dt are much more scattered than those of the simultaneous baseline subtracted horizontal variation field vector ∆H. A pronounced difference between ∆H and dH/dt takes place at about 02-06 MLT in the au-roral region when dH/dt prefers an east-west orientation, whereas ∆H points to the south. The occurrence of large dH/dt has two daily maxima, one around the local magnetic midnight, and another in the morning. There is a single maximum around the midnight only at the southernmost IMAGE stations. An identical feature is observed when large GICs are considered. The yearly number of large dH/dt values in the auroral region follows quite closely the aa index, but a clear variation from year-to-year is observed in the directional distributions. The scattering of dH/dt distributions is smaller during descending phases of the sunspot cycle. Seasonal variations are also seen, especially in winter dH/dt is more concentrated to the north-south direction than at other times. The results manifest the importance of small-scale structures of ionospheric currents when GICs are considered. The distribution patterns of dH/dt cannot be explained by any simple sheet-type model of (westward) ionospheric currents, but rapidly changing north-south currents and field-aligned currents must play an important role.

Evaluation of the geometry of ionospheric current systems related to rapid geomagnetic variations

“…They all show two occurrence maxima in the pre-midnight and morning hours, respectively. This agrees with the results by Viljanen et al (2001), who analysed the occurrence of events with dH /dt>1 nT/s (60 nT/min). We, however, also see interesting differences.…”

“…3.3 Diurnal and latitudinal variation Viljanen et al (2001) showed that the occurrence of large dB/dt events displays the strong MLT variation, so we start with a study of these distributions. When doing statistical analysis it is worth separating continuous series of large dB/dt timesteps from isolated timesteps.…”

“…However, a recent survey of dB/dt occurrence and their directional distribution (Viljanen, 1997;Viljanen et al, 2001) indicated that many large GIC events display a large contribution from the east-west dB/dt component, especially in the morning LT hours at auroral latitudes, which cannot be explained by AEJ effects. The detailed study of the April 2000 storm by Pulkkinen et al (2003) also demonstrated the complexity of ionospheric currents during an extreme GIC event.…”

Influence of spatial variations of the geoelectric field on geomagnetically induced currents

“…There is a maximum close to the local midnight (about 22 UT), which is expected (cf. Viljanen, 2001). The effect of the ground conductivity model on the diurnal distribution is very small, i. e. the single-block model gives a nearly identical result.…”

Power grid disturbances and polar cap index during geomagnetic storms

“…However, it is possible (but complicated) to model GIC effects in real power systems with adequate accuracy (e.g., Viljanen & Pirjola 1989;Pulkkinen et al 2005;Wik et al 2008Wik et al , 2009. For the present purpose it suffices to state that the geoelectric field strengths and thus the intensities of GIC depend on the strength and variability of the ionospheric currents -in the simplest approximation expressed through the time derivative dB/dt of the variable magnetic field generated at ground level by the ionospheric currents (e.g., Viljanen et al 2001).…”