Science Nuggets: Magnetospheric Physics

Observations of Plasma Bubbles in the Magnetotail

The nightside of the Earth's magnetosphere is stretched into a long tail-like structure called the magnetotail. The magnetotail is where energy that is added to the magnetosphere from the solar wind is stored. This energy is released in an event known as a substorm and it is substorms that are responsible for the spectacular auroral displays seen at higher latitudes on Earth. Transient high speed flows of plasma in the magnetotail, known as Bursty Bulk Flows, play an important role in transporting energy towards the Earth during substorms and one explanation for how these work is that they are lower density bubbles of plasma that can move faster than their surroundings.

In a recent paper, Walsh et al. (2009), described how they used the European Space Agency's four formation-flying Cluster spacecraft and the joint Chinese-European Double Star spacecraft to study one of these plasma bubbles in unprecedented detail. They took advantage of Cluster's tetrahedron formation to measure the size of a plasma bubble and found it to be three Earth radii (about 19,000km) in diameter and also found that bubbles have wakes of plasma that flow away from the Earth, even as they move towards it. These wakes had been predicted by theory but have never before been directly observed. Another new discovery in the paper was that plasma bubbles can reach as close to the Earth as Geostationary orbit before they are stopped by the Earth's magnetic field .

For more details, see:

Walsh, A. P.; Fazakerley, A. N.; Lahiff, A. D.; Volwerk, M.; Grocott, A.; Dunlop, M. W.; Lui, A. T. Y.; Kistler, L. M.; Lester, M.; Mouikis, C.; Pu, Z.; Shen, C.; Shi, J.; Taylor, M. G. G. T.; Lucek, E.; Zhang, T. L.; Dandouras, I.
Cluster and Double Star Multipoint Observations of a Plasma Bubble
Ann. Geophys., 27, 725-743 (2009) [Online]