They suggested that the asymmetries likely result from interhemispheric asymmetries a rising under a strongly southward interplanetary magnetic field (IMF), or when the IMF B y is large. However, they also found significant north-south asymmetries that were not related to this gap. They found that many of the differences between the SAE and the standard AE can be explained by the gap in the southern magnetometer array. They also constructed a northern AE index (NAE) of the stations conjugate to the southern sites. Weygand and Zesta ( 2008) compared the standard auroral electrojet (AE) index to the southern AE (SAE) index derived from the data of seven stations located in the southern auroral region during 3–10 December, 2005. They suggested that this effect may be caused by interhemispheric field-aligned currents flowing from the summer high-latitude ionosphere and closing through the ionosphere in the opposite auroral zone. In other words, substorm activity in the summer correlates much better with the geomagnetic activity in the opposite polar cap. Using the data of four years (1990–91, 1997–98) they found that while, in the northern winter, the correlation between AL and the northern PC/PM indices is very good, in the northern summer, AL correlates much better with the southern PC/PM indices. ( 2008) considered the correlation between the AL index (showing substorm activity in the northern hemisphere), and the Polar Cap (PC) and Polar Magnetic (PM) indices measuring magnetic activity in the polar caps. They found that there was a substantial fraction of days during which there was a significant temporal disagreement between the hemispheres, but the solar wind velocity still appears to control the overall daily intensity. ( 1999) studied the characteristics of broadband ULF magnetic pulsations at two Arctic stations and their conjugate stations at the South Pole. The average daily curves of the geographic north and east field components are very similar at corresponding sites in both hemispheres, except for a sign reversal in the east component between north and south. ( 2011) investigated the diurnal variation during the quiet year 2006 as observed at six polar observatories, three in the northern, and three in the southern, hemisphere in the geomagnetic latitude range of 77–89 deg. For example, Hajkowicz ( 2006) considered the largest negative value of the daily horizontal field at Macquarie Island in the southern hemisphere and at six auroral and subauroral sites in the northern hemisphere in 2003 for 193 disturbed days. Previous statistical investigations based on high-latitude ground magnetometer recordings simultaneously in both hemispheres have been quite rare and limited by spatial and temporal coverage. An increasing number of facilities have been installed in the Antarctic, which makes it now possible to perform extensive interhemispheric comparisons. Ground-based magnetometers play a central role, since they provide long-term continuous and homogeneous data sets. Studies of the high-latitude ionosphere have previously been concentrated on the northern hemisphere, due to a better coverage of ground-based observations. The dipole axis of the geomagnetic field is tilted from the Sun-Earth line which causes seasonal variations and hemispherical asymmetries attributed to the solar radiation, and the solar wind, input into the magnetosphere.
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