Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere

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Science  19 Dec 2014:
Vol. 346, Issue 6216, pp. 1506-1510
DOI: 10.1126/science.1257377

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The structure of Earth’s magnetosphere is poorly understood when the interplanetary magnetic field is northward. Under this condition, uncharacteristically energetic plasma is observed in the magnetotail lobes, which is not expected in the textbook model of the magnetosphere. Using satellite observations, we show that these lobe plasma signatures occur on high-latitude magnetic field lines that have been closed by the fundamental plasma process of magnetic reconnection. Previously, it has been suggested that closed flux can become trapped in the lobe and that this plasma-trapping process could explain another poorly understood phenomenon: the presence of auroras at extremely high latitudes, called transpolar arcs. Observations of the aurora at the same time as the lobe plasma signatures reveal the presence of a transpolar arc. The excellent correspondence between the transpolar arc and the trapped closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs.

How trans-polar arcs transpire above

Auroral arcs within the polar cap are a visual marvel, and they may also indicate trapped energetic plasma in Earth's magnetosphere. Fear et al. combined simultaneous observations of both the aurora and signatures of the trapped plasma in Earth's magnetotail to demonstrate one recent instance of this phenomenon. Some researchers have proposed that flux generated by magnetic reconnection might get trapped in the magnetotail lobe, but the standard magnetosphere model does not predict it. This study confirms the idea by taking advantage of a period when the interplanetary magnetic field points northward, a state not yet well understood.

Science, this issue p. 1506

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