Statistical Comparison of a Southern Auroral Electrojet Index with Northern Hemisphere AE Indices as a Function of Solar Wind and IMF

Abstract:

A Southern Auroral Electrojet (SAE) index has been recently constructed using seven Antarctica magnetometer stations. It has been compared for case studies with the standard Auroral Electrojet (AE) index, and a near-conjugate to the southern stations Northern Auroral Electrojet (NAE) index. Both similarities and differences with the Northern Hemisphere indices have been detected, and they reveal information about the conjugacy of geomagnetic disturbances. In this work we compare the three indices statistically as a function of the accompanying solar wind (SW) and Interplanetary Magnetic Field (IMF) conditions to further explore conjugacy issues. We use 274 days of common north/south data presence between December 2005 and August 2010. We calculate the cross correlation coefficients and differences between all three pairs, AE-SAE, NAE-SAE, and AE-NAE. We estimate the effect of the SW/IMF conditions on the index correlations and differences using three groups of data: 1) the entire data set, 2) periods when there is no station in the Southern Hemisphere located within the 20-02 Magnetic Local Time (MLT) sector where substorms occur, and 3) separately for the four different seasons. We consider the following SW/IMF quantities: IMF B_y, B_z, clock angle θ = tan^-1(|B_y|/B_z), coupling parameter sin²(θ/2), SW dynamic pressure, density, velocity, and electric field. We find that high north-south correlation coefficients are more common during strong SW/IMF driving, e.g., southward IMF, high IMF |B_y|, high SW dynamic pressure, high SW electric field, and high θ and sin²(θ/2). All the above studies are also conducted for the index differences instead of their correlations. We find that the index differences are higher for higher SW/IMF driving, suggesting that the SAE index follows the northern indices trend, but has in general lower values than either the standard AE or the conjugate NAE index. The MLT study shows that the number of high AE/SAE correlations is slightly lower at all clock angles and dynamic pressure levels when no southern station is within 20-02 MLT. Of the four seasons, spring and winter had enough data for their results to be statistically significant. The results show that the number of high correlations is greater during the spring period than the winter period, for various levels of SW pressure, IMF B_y and IMF B_z.