Auroral electrojet AL index and polar magnetic disturbances in two hemispheres

Abstract

[1] Earlier we found that the auroral electrojet AL index, indicating substorm activity in the northern hemisphere, in local summer months correlates much better with geomagnetic activity not in the nearby polar cap but in the opposite polar cap; we explained this effect as a result of interhemispheric field-aligned currents, which suppress substorm field-aligned currents in the summer hemisphere but increase these currents in the winter hemisphere. In the present paper, we took into account this effect and examined a method for reliably monitoring the substorm auroral electrojet, measured with the auroral electrojet AL index, by using hourly averages of geomagnetic field from two polar observatories (Thule and Vostok) in two hemispheres. We tested this method for 3 years. The correlation between the predicted and actual AL indices for these years was stable and very high, and it showed no significant dependence on season and a relatively weak UT variation. The correlation coefficient between the predicted and actual AL indices for these three years was about 0.89. The proposed method, based on using magnetic field data from two polar geomagnetic observatories in two hemispheres, not only significantly improves the reliability of monitoring the westward auroral electrojet in the northern hemisphere but it may also be used for monitoring the westward auroral electrojet in the southern hemisphere where no AL index is available because a significant portion of the southern auroral zone is located over the oceans. The results of this paper show that measurements of geomagnetic field in two hemispheres are of high importance for reliably monitoring the geomagnetic activity and related events in each hemisphere.

Seasonal variations in diffuse, monoenergetic, and broadband aurora

“…Although previously studied at lower latitudes, interhemispheric currents have more recently been considered for their effects on the aurora. Lyatskaya et al [2008, 2009] have presented evidence from geomagnetic indices that substorm current signatures are much more pronounced in the winter hemisphere, an effect they attribute to these interhemispheric currents. The sense of interhemispheric currents is such as to enhance the substorm current wedge in the winter hemisphere and to oppose substorm currents in the summer hemisphere.…”

Effect of interhemispheric currents on equivalent ionospheric currents in two hemispheres: Simulation results

“…Similar equatorward displacement of the convection reversal relatively to the polar cap boundary was earlier reported by Sotirelis et al . []. Despite the low ionospheric conductivity in the area between the R1 FACs and the region of enhanced ionospheric conductivity, the magnitude of the ionospheric and equivalent currents in the nightside auroral electrojet (measured with the auroral electrojet AL index) is approximately the same in two hemispheres [e.g., Lyatskaya et al ., , ], which also is a result of the IHCs, branching from the sunlit summer hemisphere and closing the Westward Auroral Electrojet in winter hemisphere. In this case, the IHCs are a substitute for the R1 currents in the winter hemisphere.…”

“…These IHCs were discussed by Rishbeth [], Lyatskaya et al . [, ], Yamashita and Iyemori [], Østgaard et al . [], and other authors, and were computed numerically by Benkevich et al .…”

Effect of Interhemispheric Field-Aligned Currents on Region-1 Currents

“…More recent studies [ Benkevich et al, ; Benkevich and Lyatsky , ; Ohtani et al, ; Ohtani et al, ; Østgaard et al , ; Lyatskaya et al, , ; etc.] showed that an important role in the global 3‐D current system can be played by the interhemispheric currents (IHCs).…”

“…showed that an important role in the global 3‐D current system can be played by the interhemispheric currents (IHCs). The IHCs redistribute ionospheric currents between two polar ionospheres in the regions of closed magnetic field lines in case of asymmetry of ionospheric conductivity between two polar ionospheres, which may happen during unequal illumination of polar ionospheres and other effects [e.g., Richmond and Roble , ; Kozlovsky et al, ; Atkinson and Hatchinson , ; Rishbeth , ; Benkevich et al , ; Benkevich and Lyatsky , ; Yamashita and Iyemori , ; Lyatskaya et al, , ; Ohtani et al , , ; Østgaard et al , , and references therein]. However, since it is difficult to separate the IHCs from other FACs (especially when they flow in the same region), despite the important role of the IHCs in dynamics of the global 3‐D current system, they have not been sufficiently investigated.…”