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19 February 2005 | During intense geomagnetic storms, very bright auroras are often observed to low latitudes. Occasionally, these bright displays of the "northern lights" are strong enough to be observed even into the tropical latitudes. Auroral activity is well known for being associated with strong magnetic perturbations. This is one of the reasons why a global network of magnetic observatories has been constructed at locations around the world - to measure the effects of space weather storms on the Earth's magnetic field.
During intense magnetic storms, the Earth's magnetic field can fluctuate strongly, resulting in large deviations in quantities as fundamental as the magnetic declination used by navigators to establish the direction to the true north/south poles. The fluctuations are even strong enough to be discerned in compass needles where they might not point toward true north with customary accuracy. The source of these magnetic fluctuations is an intense ring of electrical current that courses through the ionosphere. This ring of electrical current was coined the auroral electrojet because of its association with auroral activity and the strong magnetic fluctuations that it produces on the ground.
During quiet times, the northern lights (and the auroral electrojet) are always confined to the high latitude regions near and north of Alaska, northern Canada and northern Europe (Finland). Over the southern polar regions, they are always located well south of New Zealand, Australia and South America. They generally occur near and north of approximately 67 degrees corrected geomagnetic latitude during quiet conditions.
During strong space weather storms, the location of the auroral activity and the auroral electrojets moves equatorward. This explains why sometimes, observers throughout the United States and Europe can see the northern lights.
For years, many scientists believed that the location of the brightest auroral activity coincided with the strongest magnetic perturbations, since this is the area were large quantities of energy are being routed through the Earth's ionosphere. In other words, it was believed that the location of the auroral electrojet coincided with the location of the brightest auroras. However, that opinion is now changing, particularly with the publication of new research that quantitatively answers this question.
An international team of scientists led by Dr. B.-H. Ann[1] of the Department of Earth Science, Kyungpook National University (Daegu, Korea) examined the magnetic records obtained from many magnetic observatories around the world and correlated those records with the observation of bright auroral activity from the International Monitor for Auroral Geomagnetic Effects (IMAGE) spacecraft, and found that the brightest regions of auroral luminosity do not coincide with the location of the auroral electrojet. Instead, it was found that the auroral electrojet was located on the poleward boundary separating the brightest auroral regions from the less bright auroral emissions (see the arrows in the illustration above that is based upon an image from the NASA Polar spacecraft). Furthermore, they found that although the brightest auroral emissions can migrate significantly equatorward (even into the central United States), the auroral electrojet appears to suffer from a saturation effect where it is unable to migrate equatorward of approximately 60 degrees in corrected geomagnetic latitude. Thus, even during the most intense geomagnetic storms, the main belt of strong magnetic perturbations appears to remain generally (over time-averaged intervals) situated near the higher latitudes. The researchers admit that the auroral electrojet can (and does) migrate equatorward of 60 degrees for relatively brief intervals of time, but on the whole tends to stall in its migration equatorward near 60 degrees. This does not preclude the lower latitudes from suffering from the effects of intense magnetic fluctuations, but it does suggest that the strongest magnetic perturbations (which occur nearest to the auroral electrojet) remain confined to those regions near 60 degrees of magnetic latitude.
The researchers explain that the reason why the auroral electrojet is not colocated with the brightest areas of auroral luminosity is because the regions of brightest luminosity are associated with the strongest ionospheric conductivities. Hence, strong electrical currents are unable to flow there because the strong conductivities short-circuit the electric fields (which are required to produce electrical currents). Lower electric fields imply smaller electrical currents (even though conductivities are large). Thus, a requirement for strong magnetic perturbations within the auroral electrojet is the presence of strong electric fields, and those fields are only present poleward of the area of strong auroral luminosity.
These results suggest that a common measure of geomagnetic activity (the geomagnetic K-index) might not be the best measure of auroral luminosity, particularly if only a local geomagnetic K-index value is consulted. For example, if the geomagnetic observatory at Boulder, Colorado reports a local K-index of 9 (the most disturbed level measured), it does not mean that auroral activity will be strongly luminous and visible from Boulder. It simply means that the auroral electrojet (most likely positioned far north of Boulder and certainly north of the brightest visible auroral emissions) is abnormally strong. A better tool would probably be the planetary geomagnetic K-index, which removes the effects of localized perturbations and should better reflect the overall level of global auroral activity. Thus, high planetary geomagnetic K-indices should generally coincide with an expanded auroral oval (not necessarily auroral luminosity).
So the answer to the question, "Where do the Brightest Auroras Occur?" can now be answered in several ways: equatorward of the strongest magnetic fluctuations, or in other words, equatorward of the auroral electrojet - not colocated with it; or in the region of highest ionospheric conductivity.
This research was published in the Journal of Geophysical Research, volume 110 on 15 January 2005 (A01305, doi:10.1029/2004JA010553, 2005) by the American Geophysical Union.
[1]B.-H. Ahn, G. X. Chen, W. Sun, J. W. Gjerloev, Y. Kamide, J. B. Sigwarth, and L. A. Frank (2005), Equatorward expansion of the westward electrojet during magnetically disturbed periods, Journal of Geophys. Res., vol. 110, A01305, doi:10.1029/2004JA010553, 2005.
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