Comparison of auroral electrojet indices in the Northern and Southern Hemispheres

Abstract

[ 1 ] The auroral electrojet (AE) index is traditionally calculated from as et of about 10 to 13 ground magnetometer stations located around the typical northern auroral oval location. Similar coverage in the Southern Hemisphere does not exist, so the AE calculation has only been performed using Northern Hemisphere data. In the present paper,w eu se seven southern auroral region ground magnetometers as well as their conjugate Northern Hemisphere data to calculate conjugate AE indices during the Northern Hemisphere winter using the standard method. The correlation coefficient between the northern and southern AE indices for many of the intervals is above 0.7, but in one interval, it is close to 0. We compare our conjugate AE indices with the standard AE index and find an umber of asymmetries because of station coverage gaps both in our conjugate indices and in interplanetary magnetic field (IMF) conditions. When the southern AE index is compared with the standard AE index, we find that for most intervals the correlation is less than 0.7, and in one interval, the correlation is about 0. The correlation between the conjugate northern AE index and the standard AE index is somewhat better,s uggesting true interhemispheric asymmetries. The mean difference between the southern and northern AE indices is largest during southward IMF and for large values of IMF j B y j (>5 nT). This is most likely due to the increased activity levels during southward IMF and the greater twisting of the magnetic field lines during strong IMF B y .I MF B x seems to have no effect on the interhemispheric differences of the AE index. The mean differences between the southern and conjugate northern H component are of the order of 35 nT,with the largest differences occurring in the midnight magnetic local time (MLT) sector.W es uggest that these differences may be related to seasonal effects, ionospheric effects, and MLTeffects.

Auroral electrojet indices in the Northern and Southern Hemispheres: A statistical comparison

“…However, good correlations have been shown in previous studies pertaining to conjugate ground magnetometer data [e.g., Maclennan et al, 1991;Saroso et al, 1992;Ballatore et al, 1998;Ballatore and Maclennan, 1999;Weygand and Zesta, 2008;Troshichev and Janzhura, 2012] and conjugate auroral observations [Belon et al, 1969;Stenbaek-Nielsen et al, 1972;Craven et al, 1991;Weygand, 1998;Hajkowicz, 2006]. The previous studies indicate that differences between the southern and northern AE indices are most likely due to interplanetary magnetic field (IMF) conditions, which we will not focus on in this study, or the results of seasonal or hemispheric asymmetries in the ionosphere.…”

“…The study done by Weygand and Zesta [2008] is the most similar to that of Maclennan et al [1991]. The Weygand and Zesta study examined just 7 days of data from December 2005 using seven Southern Hemisphere magnetometers at magnetic latitudes between À60°and À71°, from which they constructed a SAE index.…”

“…Additionally, they indicated that some but not all of the differences between the standard AE and SAE indices could be explained by gaps in the magnetometer arrays (larger gaps in the southern set of stations). Weygand and Zesta [2008] also suggested that other differences in the SAE and northern indices were due to a difference in ionospheric conductivity at the two hemispheres and localized ground conductivity gradients, which can influence the signals in the ground magnetometers. Finally, unlike the Maclennan et al [1991] study, the Weygand and Zesta [2008] study did not find systematic difference between the southern and northern AE related to UT.…”

“…However, each one of the seven southern stations used for SAE index does not have an exact conjugate northern station, due to limited magnetometer coverage in the Northern Hemisphere locations. To remedy this Weygand and Zesta [2008] averaged data from the 2-3 northern stations nearest the northern conjugate location of the southern station to attempt to construct a near conjugate H component for four of the southern stations. Despite the limitations associated with conjugate location, Weygand and Zesta [2008] determined that the southern AE index is well correlated with the WDC AE index and the correlation improved with their near conjugate NAE index.…”

“…To remedy this Weygand and Zesta [2008] averaged data from the 2-3 northern stations nearest the northern conjugate location of the southern station to attempt to construct a near conjugate H component for four of the southern stations. Despite the limitations associated with conjugate location, Weygand and Zesta [2008] determined that the southern AE index is well correlated with the WDC AE index and the correlation improved with their near conjugate NAE index. Furthermore, they determined that the correlation improved significantly with increasing geomagnetic activity, but the absolute difference between the two indices then increased.…”