The boundary of Earth’s magnetic field can be a turbulent, chaotic mess. However, a new analysis of decade-old satellite observations is putting Xs and “islands” on the map.
These strange magnetic field shapes are a result of the interaction between the magnetic fields of the Earth and the Sun. Drawn schematically, the Earth’s magnetic field is coiled around the planet, with field lines sprouting from the poles, and is swept back into a long tail by the solar wind. Everything might stay simple and orderly, but the Sun has a magnetic field of its own, and the same solar winds send it careening toward Earth.
According to theory, when these field lines meet, they snap together in a process called magnetic reconnection that releases bursts of energy. When the solar wind is particularly stormy, reconnection injects showers of energetic particles from the Sun into Earth’s magnetic field, which can wreak havoc on satellite communications and power grids.
When the field lines touch, they resemble an X, like rubber bands being pulled taut toward each other. Then, suddenly, the field lines from the Sun break apart and link up with field lines from the Earth, like two pairs of approaching square dancers switching partners. This process releases the built up tension, and the X lines quickly snap back, away from the original site of reconnection—at least according to theory. In practice, the snap back is difficult to observe and quantify in detail.
However, using the European Space Agency’s Cluster satellites—a quartet of spacecraft that orbit Earth in formation—Wilder et al. have observed a retreating X line and measured its velocity for the first time. As the Cluster craft soared over the Southern Hemisphere in 2005, two of them roughly 5500 kilometers apart encountered a sharp reversal in the magnetic field direction and the flow of ions within 40 seconds of each other. The authors conclude that both craft flew right through an X line retreating down the Earth’s magnetic tail.
The data also hint at a second reconnection event forming another X line behind it. This event would have isolated the patch of field lines caught between these two reconnection events, creating a “magnetic island” sliding down the tail of Earth’s magnetic field.
The 40-second delay between the X line encounters also allowed the team to make the first direct measurement of an X line’s velocity, clocking it at 136 kilometers per second, which is roughly the same speed as the flow of the ions just outside the Earth’s magnetic field in the boundary region called the magnetosheath, where the solar wind slips past the Earth. The authors say this observation lends credence to one version of magnetic reconnection theory that predicts that the magnetosheath dominates the boundary region because of its much higher density of ions and that the ion flow carries the X line away with it. (Journal of Geophysical Research: Space Physics, doi:10.1002/2014JA020453, 2014)
—Mark Zastrow, Freelance Writer
Citation: Zastrow, M. (2015), X marks the spot of magnetic islands in space, Eos, 96, doi:10.1029/2015EO031641. Published on 17 June 2015.