The size of the auroral belt during magnetic storms

Abstract

Using the auroral boundary index derived from DMSP electron precipitation data and the Dst index, changes in the size of the auroral belt during magnetic storms are studied. It is found that the equatorward boundary of the belt at midnight expands equatorward, reaching its lowest latitude about one hour before Dst peaks. This time lag depends very little on storm intensity. It is also shown that during magnetic storms, the energy of the ring current quanti®ed with Dst increases in proportion to v À3 e , where v e is the vvalue corresponding to the equatorward boundary of the auroral belt designated by the auroral boundary index. This means that the ring current energy is proportional to the ion energy obtained from the earthward shift of the plasma sheet under the conservation of the ®rst adiabatic invariant. The ring current energy is also proportional to i mg , the total magnetic ®eld energy contained in the spherical shell bounded by v e and v eq , where v eq corresponds to the quiet-time location of the auroral precipitation boundary. The ratio of the ring current energy i to the dipole energy i mg is typically 10%. The ring current leads to magnetosphere in¯ation as a result of an increase in the equivalent dipole moment.

Temporal and Spatial Evolutions of a Large Sunspot Group and Great Auroral Storms Around the Carrington Event in 1859

“…Having presented an updated view of the spatial evolution of the auroral ovals during the stormy interval around the Carrington event, we can categorize the Carrington event not as an exceptionally outstanding event but as one of the most extreme events by comparison with the spatial evolution of the auroral oval for other extreme magnetic storms. Note that the spatial extent of the equatorward boundary of the auroral oval has a good empirical correlation with the storm intensity as indicated by the Dst index (Yokoyama et al, ).…”

“…Given the empirical correlation between the equatorward boundary of the auroral oval and the storm intensity in Dst value (Yokoyama et al, ), it seems that the Dst value of the Carrington event (September storm) is more likely to be ≈−900 (+50, −150) nT as an hourly average (Cliver & Dietrich, ; Siscoe et al, ), whereas the maximal amplitude of horizontal force at Bombay was reported 1760 nT (Tsurutani et al, ). This hourly value is comparable to that of the May 1921 storm (see Love et al, ).…”

“…The survey on the spatial evolution of the auroral oval benefits a comparison of the intensity of the extreme space weather events, due to the empirical correlation between equatorward boundary of auroral oval and storm intensity in Dst index (Yokoyama et al, ). While the Carrington event is certainly a benchmark, extreme space weather events such as those in 1872, 1909, and 1921 have been suggested as comparable in terms of the equatorward boundary of auroral visibility (Chapman, ; Hapgood, ; Silverman, , ; Silverman & Cliver, ).…”

Time Domain Simulation of Geomagnetically Induced Current (GIC) Flowing in 500‐kV Power Grid in Japan Including a Three‐Dimensional Ground Inhomogeneity

“…The ionospheric current is enhanced almost simultaneously from the pole to the equator in response to the variation of the interplanetary magnetic field (Kikuchi et al, ; Kobea et al, ; Nishida, ). During magnetic storms, the auroral oval expands to lower latitudes (e.g., Yokoyama et al, ). The equatorward boundary of the auroral oval expanded to ~40° MLAT during the 13–15 March 1989 storm (Yokoyama et al, ).…”

“…During magnetic storms, the auroral oval expands to lower latitudes (e.g., Yokoyama et al, ). The equatorward boundary of the auroral oval expanded to ~40° MLAT during the 13–15 March 1989 storm (Yokoyama et al, ). If auroral electrojets are accompanied with the auroral oval, the auroral electrojets may have some influence on the midlatitude GIC for extreme magnetic storms.…”