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ATMOSPHERIC PHYSICS: AURORAS

The term "aurora borealis" was introduced in 1621 by the French philosopher Pierre Gassendi (1592-1655). The apparent first observation of the magnetic storms associated with the aurora borealis was made by the English physicist John Canton (1718-1772) between 1756 and 1759. In 1867, the Swedish physicist Anders Angstrom (1814-1874) was the first to study the spectrum of the aurora borealis.

The following points are made by Patrick T. Newell (Nature 2003 424:734):

1) "Aurora" is the term given to the glow created when energetic particles from space strike Earth's upper atmosphere, or ionosphere. Aurora -- also known as the northern and southern lights (aurora borealis and aurora australis, respectively) --usually occur within rings around the magnetic poles. Each ring is up to about 3500 km in diameter and a few hundred kilometres deep. This "auroral oval" contains discrete aurora, which form curtain or ray shapes that are often easily visible with the naked eye, and diffuse aurora, which are fainter and lack sharp spatial boundaries. Kubota et al(1) have reported new auroral observations that fit neither category and lie several degrees below the auroral oval. The sub-oval aurora appears to co-rotate with the Earth. These observations suggest that an old idea(2) about how aurora get their shapes may yet prove to be true.

2) Kubota et al(1) used high sensitivity all-sky cameras at Poker Flat, 30 miles north of Fairbanks, Alaska. Much of the time, the solar wind drives a flow of ionized particles, or plasma, around the Earth; as a result, electric current flows from space into the upper atmosphere, creating moderate or high geomagnetic activity and bright auroras. Under such conditions, the Poker Flat observing station lies within the auroral oval, and fulfils its intended function of studying discrete and diffuse auroral forms. When geomagnetic conditions are quiet, however, the position of the auroral oval lies polewards of Poker Flat, and no auroras are expected at this site. Yet under these conditions, Kubota et al discovered faint auroral structures, with a distinctly different phenomenology. These sub-oval aurora have the ribbon-like appearance typical of discrete auroras within the auroral oval. But they also have the quiescence, persistence and low intensity that are more typical of diffuse auroras. Data from an overflying US Air Force satellite showed no acceleration of the electrons impacting on the atmosphere -- as is the case for conventional discrete auroras.

3) The sub-oval auroras also move with the Earth, at about 70% of its velocity, drifting at 140 km/s in the direction of Earth's spin. This is a surprise, and a suggestive one. Auroras tend to be positioned such that an observer on Earth sees auroral patterns apparently drifting westwards, as Earth's spin actually rotates him eastwards. The newly observed faint co-rotating aurora must therefore be more closely linked to the near-Earth region than are previously known auroras.(2-5)

References (abridged):

1. Kubota, M., Nagatsuma, T. & Murayama, Y. Geophys. Res. Lett. doi:10.1029/2002GL016652 (2003)

2. McIlwain, C. E. in Physics of Auroral Arc Formation (eds Akasofu, S.-I. & Kan, J. R.) 173-174 (Am. Geophys. Union, Washington DC, 1981)

3. Wallis, D. D. et al. J. Geophys. Res. 84, 1347-1360 (1979)

4. Anderson, P. C. et al. J. Geophys. Res. 106, 29585-29599 (2001)

5. Foster, J. C. & Vo, H. B. J. Geophys. Res. doi:10.1029/2002JA009409 (2002)

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