PerspectivePlanetary Science

A Dynamic Twist in the Tail

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Science  04 May 2012:
Vol. 336, Issue 6081, pp. 548-549
DOI: 10.1126/science.1221805

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All planetary bodies with intrinsic magnetic fields or atmospheres possess very long magnetic tails (1, 2). If they possess the same intrinsic magnetic field dipole orientation as Earth, then the northern half of the tail is composed of open magnetic fields oriented back toward the planet, and the southern half has the opposite orientation (see the figure, panel A). Sandwiched between these two lobes of the tail is a sheet of hot plasma derived from the solar wind and the atmospheres of the planet and its moons. The cross-tail electric current sheet that separates the sunward magnetic field in the northern lobe from the antisunward flux in the southern lobe lies at the center of the plasma sheet. The intrinsic magnetotails of Mercury and Earth store magnetic energy in their tail lobes and then rapidly convert it into plasma heating and high-speed jetting (3, 4) through the process of magnetic reconnection (5). Venus and comets lack intrinsic magnetic fields, but magnetotails still form as a result of the heliospheric magnetic field lines carried by the solar wind draping about these bodies (see the figure, panel B). The Venera 9 and 10 and Pioneer Venus Orbiter missions to Venus found that the structure of its draped field magnetotail has many similarities to those of magnetized planets, but found no evidence for magnetic reconnection (6, 7). Now, on page 567 of this issue, Zhang et al. (8) report observations of clear magnetic reconnection signatures in the Venus Express data, representing an unexpected discovery with important ramifications for our understanding of Venus.