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16 February 2005 | Dr. J. A. Cumnock of the Center for Space Sciences at the University of Texas at Dallas has examined the occurrence of transpolar arcs (TPA) in an attempt to determine whether solar wind conditions have any impact on the formation and longevity of these unusual auroral forms.
Theta auroras are aptly named after the greek letter theta that is distinctively represented when the aurora is viewed from a distance above our polar caps. They are also known as sun-aligned arcs. TPAs are typically about 100 or more kilometers in width and can span from the midnight-sector all the way across the polar cap to the noon sector of the polar cap, as the image at the left illustrates. Auroral luminosity along the TPA is usually less intense than the average luminosity elsewhere along the auroral oval.
Dr. Cumnock confirmed a previously held belief that the occurrence of this type of aurora requires a solar wind interplanetary magnetic field (IMF) that is oriented northward for a period of hours together with a change in the direction of the By (east-west) component of the IMF (from positive to negative or negative to positive). There is no apparent relationship between theta aurora and the Bx (toward/away) component of the IMF. In virtually every case examined, theta aurora formed under these conditions.
Theta aurora are the most stable of several different types of "moving auroral arcs." Theta auroras can be observed at very weak intensities without a By sign change (provided the Bz component is strongly northward), but are most dynamic and luminous during a By sign change.
This research also found that theta aurora are brighter during the summer months and weaker during the winter months, at ultraviolet wavelengths.
Research results were published in the Journal of Geophysical Research / Space Physics, volume 110 on 11 February 2005 (doi:10.1029/2004JA010867, 2005) by the American Geophysical Union.
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Terrestrial Dispatch
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