The size of the auroral belt during magnetic storms

Abstract

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.

Influences on the radius of the auroral oval

“…8 they demonstrated a decrease in the latitude of the onset arcs of substorms for more negative Sym-H, indicating that substorms occur on an expanded auroral oval when the ring current is intensified. Yokoyama et al (1998) also showed similar variations in the location of the equatorward edge of the auroral with D st . In addition, Milan et al (2008) showed that the polar cap expands and contracts during the substorm cycle, as also supported by our Fig.…”

“…8 can be understood in terms of a hysteresis in the motion of point P during a storm, and the changing conditions through the storm. In a similar vein, Yokoyama et al (1998) also noted that the latitude of the lower edge of the auroral oval tended to reach its maximum equatorward excursion somewhat before the minimum in the D st main phase. Examination of Fig.…”

“…Milan et al (2008) demonstrated a clear dependence of the latitude of substorm onset on the Sym-H index, a measure of the magnetic perturbation associated with the ring current, indicating that the auroral oval expands to low latitudes when the ring current is enhanced. Before this, Meng (1982Meng ( , 1984 and Yokoyama et al (1998), amongst others, showed a link between the latitude of the auroral oval and D st , another measure of ring current intensity. As suggested by Nakai and Kamide (2003), and reiterated by Milan et al (2008), the enhanced ring current found during geomagnetic storms could alter the magnetic field geometry of the nearEarth magnetotail, delaying substorm onset until the open flux content of the magnetosphere grows unusually large.…”

“…As early as the Carrington Storm of 1859 a link between extreme solar activity and the appearance of aurora at unusually low latitudes was established (e.g., Cliver, 2006). Subsequently, Feldstein and Starkov (1967) showed that the changing latitudinal location and extent of the auroral oval could be parameterized by the Q index of geomagnetic activity, and several authors demonstrated a link between the location of the oval and the D st index (e.g., Akasofu and Chapman, 1963;Meng, 1982Meng, , 1984Yokoyama et al, 1998). The interplanetary magnetic field (IMF), and especially its north-south component IMF B Z , is also known to control the location of the aurora (e.g., Holzworth and Meng, Correspondence to: S. E. Milan (steve.milan@ion.le.ac.uk) 1975; Hardy et al, 1981;Nakai et al, 1986), though as D st is itself related to IMF B Z (e.g., Russell et al, 1974;Burton et al, 1975) there is still some debate over which ultimately controls the oval radius.…”

Extreme geomagnetically induced currents

“…The local area of influence of an AE-type GIC with large-amplitude dB/dt can extend to a latitude 20°lower than the equatorward boundary of the auroral oval, even though the dB/dt amplitude is only an order of magnitude smaller than its amplitude in the high latitudes (Ngwira et al 2013(Ngwira et al , 2014. By extending the empirical relationship of Yokoyama et al (1998), the equatorward boundary of the auroral oval during the Carrington storm can be extrapolated down to 30°magnetic latitude (peak Dst = −900 nT), whereas during most superstorms with peak Dst < −300 nT, the boundary is at 36-43°magnetic latitude. More studies are needed to investigate how such an extreme equatorward boundary of the auroral oval can occur because recent simulations have pointed to a saturation level of aurora oval expansion (Ngwira et al 2013), although there are contemporary reports of a low-latitude aurora during the Carrington event (Kimball 1960).…”