Space Sciences Laboratory, University of California, Berkeley
Because the Earth's magnetic field is largely dipolar and symmetric, many auroral features are conjugate between the northern and southern hemispheres. However, previous work has shown that the aurora can be nonconjugate. Most previous optical conjugacy studies have been limited to ground based observations or ground based and space based imager comparisons. In both cases at least one observation point is restricted to viewing only small scale features. Conjugate observations of the large scale behavior of the aurora have been confined to rare, serendipitous space based imager conjunctions and single spacecraft viewing of both hemispheres which is tempered by large oblique viewing angles. As the Polar spacecraft's apogee has descended in the the southern hemisphere, the Polar Ultraviolet Imager (UVI) has spent an increasing amount of time viewing the southern aurora. Simultaneously, the Wideband Imaging Camera (WIC) onboard the IMAGE spacecraft has been observing the northern aurora. By using image data from both spacecraft, we are able to systematically analyze the degree of auroral conjugacy on synoptic scales and over a wide variety of geomagnetic conditions. In this work, we focus on the conjugacy of the region of intense auroral emission during substorm onset and expansion. We find a persistent displacement in local time of the region of active aurora during substorm expansion; features in the northern hemisphere are shifted westward (duskward) relative to the conjugate point of the southern hemisphere aurora by up to a few thousand kilometers. Often there is a shift in the opposite direction observed before and at substorm onset. The same sense of displacement has been reported in earlier work. The sense or magnitude of the displacement showns no obvious correlation with the interplanetary magnetic field orientation or strength as suggested in previous studies. Therefore, we suggest that the displacement in local time of the region of intense auroral emission is a result of the asymmetric distortion of the magnetic field by the large scale field-aligned currents associated with substorms.
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