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Planetary science

An ill solar wind

On 1 November last year, my father phoned me from Tenerife to ask why he wasn't able to receive the BBC World Service. His concern was not at missing the news bulletins, but rather at what might have happened to his beloved transmitters at Rampisham in the south of England or on Ascension Island in the mid-Atlantic (my father is a chartered electrical engineer). In fact, it was unusually high sunspot activity that was upsetting his reception. Elsewhere in this issue, Daniel Baker and colleagues report exactly how dramatic were the effects on Earth's atmosphere of the solar winds from those sunspots (Nature 432, 878–881; 2004).

Short-wave radio signals are transmitted around the world by refraction and reflection from a layer of ions in the atmosphere; this Appleton layer (named after Edward Appleton, who discovered it in 1926) lies between 150 and 1,000 km above Earth's surface. Higher still, at 3,000–6,000 km and 20,000–25,000 km above the Equator, are the Van Allen belts — two toroidal regions of high-energy ions, mainly electrons and protons, trapped by Earth's magnetic field. The relatively calm region between the belts is a future orbit of choice for artificial satellites. The inner edge of the outer belt also corresponds to the limit of Earth's plasmasphere, which is dense with ions.

Around Halloween in 2003, two spacecraft — the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) and the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) — detected massive distortions in the Van Allen belts and the plasmasphere as the solar winds hit. The gap between the belts was obliterated throughout November, as electrons were ‘blown’ through into the inner belt, increasing its electron density 50-fold — a situation that has persisted ever since. The effect on the plasmasphere (pictured) was even more dramatic, if shorter-lived. On 28 October, it extended as usual to about 19,000 km above Earth's surface; by 31 October it had contracted to 6,000 km, and in places to as low as 3,000 km, returning to close to normal within a few days.

These changes greatly reduced Earth's natural shields against solar and cosmic radiation. Contact was lost with satellites; astronauts on the International Space Station took cover in a heavily shielded service module; the US Federal Aviation Administration issued its first-ever radiation alert to passengers flying above 7,500 m; and the power system in Malmö, Sweden, failed.

Small wonder, then, that the Appleton layer could not relay the World Service to my father in the Canaries.

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Surridge, C. An ill solar wind. Nature 432, 814 (2004). https://doi.org/10.1038/432814b

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