Space physics

An auroral signature decoded

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It has been hard to relate the activity of the northern lights to specific events in space. But the latest data show that 'auroral streamers' can be matched to bursts of ionized particles from Earth's magnetotail.

Aurorae — the northern and southern lights — occur high in the sky in polar regions. They result from the impact of energized electrons in near-Earth space with atoms and molecules in the upper atmosphere. It has been frustratingly difficult to ascribe particular auroral behaviour to events in near-Earth space. But four papers1,2,3,4, published last month in Geophysical Research Letters, do just that.

Study of the near-Earth space environment originated with investigations of the aurora, which has remained central to space- physics research. This is in part because, in principle, the dynamics of ionized particles (plasma) in space are reflected in auroral behaviour. Just as a television screen glows when beams of electrons, accelerated by electric fields in near-vacuum conditions, strike its surface, so the upper atmosphere glows with shifting aurorae in response to accelerated beams of electrons from space.

This analogy is far from easy to apply in practice. But using coordinated data sets from spacecraft and ground observations made by the International Solar–Terrestrial Program (ISTP), the four papers1,2,3,4 identify a link between aurorae and events in the Earth's magnetotail — the long tail of plasma that extends far downstream from the Earth, on the night side facing away from the Sun (Fig. 1). They show that 'auroral streamers', which are fingers of light moving from the poleward edge of the aurora towards lower latitudes, exhibit a one-to-one correspondence with bulk flows of plasma that burst out of the magnetotail and are convected back towards the Earth. Debate is now underway as to whether these 'bursty bulk flows' are a key part of large auroral and magnetic disturbances. Such disturbances create the spectacular multicoloured curtains and rays of light associated with strong geomagnetic perturbations ('auroral substorms'), which, in extreme circumstances, can disrupt high-latitude communications, knock out power stations, and even harm satellites.

Figure 1: Sun, Earth and the magnetotail.
figure1

An artist's view, not to scale, of the Sun and the magnetic bubble around Earth, the magnetosphere, which the Earth's magnetic field creates in the solar wind. On the side facing away from the Sun the magnetic field lines are stretched into a long magnetotail. (Image from ref. 8, courtesy of David Stern.)

These developments in understanding aurorae owe much to the use of coordinated data sets. Key elements of the ISTP programme include the NASA Polar satellite, which can produce global images of the aurora in ultraviolet and X-ray wavelengths, and the Japanese Geotail satellite, which explores the magnetotail. Figure 2 shows one auroral streamer as imaged by the Polar satellite. The Geotail satellite is the primary source of identifying bursty bulk flows, and there has been increasing evidence that such flows constitute most of the plasma and energy convection to the near-Earth region from the magnetotail.

Figure 2: An auroral streamer, orientated north–south, indicated by the arrow.
figure2

NASA

This ultraviolet image was taken by the Polar satellite. New work1,2,3,4 shows that auroral streamers stem from bursts of plasma that swirl back to the near-Earth space environment from the magnetotail.

Auroral streamers are a relatively little studied phenonemon. They are unusual in that, unlike most auroral activity, they extend north–south rather than east–west. They propagate southwards from the poleward (northern) edge of the oval-shaped torus of auroral activity (the 'auroral oval'). Because magnetic field lines closer to the pole extend out further into space, this direction of motion corresponds to plasma motion in the magnetotail towards Earth.

In 1998, Henderson et al.5 suggested that bursty bulk flows of plasma from the magnetotail might correspond to auroral streamers. That work included no direct measurement of whether bursty bulk flows were actually occurring at the same time as auroral streamers. An association between the two came in work6,7 published last year, which combined global observations of the aurora from Polar with magnetotail flow data from Geotail. These results intrigued the space-physics community, although doubts remained — some even wondered whether a clear connection between auroral and magnetotail dynamics was theoretically possible.

In one of the new papers, Zesta et al.1 use high-resolution ground-based measurements of the aurora to establish a detailed one-to-one correspondence between auroral streamers and the bursty bulk flows observed by Geotail. These results show that it is possible, while watching the aurora from the ground, to observe a specific behaviour (auroral streamers moving north–south) and associate it with a specific magnetotail phenomenon.

Also using ISTP data (from Geotail and a Canadian chain of ground-based observations called CANOPUS), Lui2 shows that each new auroral streamer creates an auroral electrojet in the ionosphere. Auroral electrojets are electric currents driven longitudinally across the ionosphere. They are associated with auroral activity, and large ones may give rise to substorm magnetic activity lasting a few hours. Lui suggests that there is a continuum of behaviour, with full substorms at one end and isolated auroral streamers at the other.

Lyons and colleagues3 use solar-wind data from WIND (a NASA satellite, also part of ISTP), Geotail and CANOPUS to argue that, although auroral streamers are indeed caused by bursty bulk flows, the two are directly driven by variations in the solar wind. This idea runs counter to the more popular belief that such flows are part of internal magnetospheric dynamics. Finally, Anderson et al.4 use X-ray images of the aurora from Polar to conclude that changes in auroral luminosity are correlated with flows of plasma towards the Earth, but not particularly with the onset of substorms.

Auroral streamers are the first auroral signature to be clearly associated with a specific plasma process in the Earth's magnetotail. The bursty bulk flows that cause them may or may not prove to be an essential part of understanding intense auroral and magnetic disturbances. If they are, the long-debated trigger of auroral substorms may finally be understood. However that debate turns out, physicists can now decode at least one aspect of what aurorae are telling us.

References

  1. 1

    Zesta, E., Lyons, L. R. & Donovan, E. Geophys. Res. Lett. 27, 3241– 3244 (2000).

  2. 2

    Lui, A. T. Y. Geophys. Res. Lett. 27, 3245–3248 (2000).

  3. 3

    Lyons, L. R., Zesta, E., Samson, J. C. & Reeves, G. D. Geophys. Res. Lett. 27, 3237–3240 ( 2000).

  4. 4

    Anderson, P. C., McKenzie, D. L., Lyons, L. R. & Hairston, M. Geophys. Res. Lett. 27, 3233–3236 (2000).

  5. 5

    Henderson, M. G., Reeves, G. D. & Murphree, J. S. Geophys. Res. Lett. 25, 3737 –3740 (1998).

  6. 6

    Sergeev, V. A. et al. Geophys. Res. Lett. 26, 417– 420 (1999).

  7. 7

    Fairfield, D. H. et al. J. Geophys. Res. 104, 355– 370 (1999).

  8. 8

    http://www-spof.gsfc.nasa.gov/Education/Intro.html

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Correspondence to Patrick T. Newell.

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