Interchange instability

Interchange instability is the radial interchange of two adjacent magnetic flux tubes without significant disturbance of the background magnetic field geometry [Gold 1959].

In the 1950s, the field of theoretical plasma physics emerged. The confidential research of the war time years became declassified and allowed the publication and spread of very influential papers. The world rushed to take advantage of the recent revelations on nuclear energy. Although never fully realized, the idea of controlled thermonuclear fusion motivated many to explore and research novel configurations in plasma physics. Instabilities plagued early designs of artificial plasma confinement devices and were quickly studied partly as a means to inhibit the effects. The analytical equations for interchange instabilities were first studied by Kruskal and Schwarzschild [1954]. They investigated several simple systems including the system in which an ideal fluid is supported against gravity by a magnetic field (the initial model described in the last section). In Bernstein et al. [1958], Bernstein derives an energy principle that rigorously proves that the change in potential must be greater than zero for a system to be stable. This energy principle has been essential in establishing a stability condition for the possible instabilities of a specific configuration. In 1959, Thomas Gold attempted to use the concept of interchange motion to explain the circulation of plasma around the Earth, using data from Pioneer III published by James Van Allen. Gold also coined the term “magnetosphere” to describe “the region above the ionosphere in which the magnetic field of the Earth has a dominant control over the motions of gas and fast charged particles.” Marshall Rosenthal and Conrad Longmire described in their 1957 paper how a flux tube in a planetary magnetic field accumulates charge because of opposing movement of the ions and electrons in the background plasma. Gradient, curvature and centrifugal drifts all send ions in the same direction along the planetary rotation meaning that there is a positive build-up on one side of the flux tube and a negative build-up on the other. The separation of charges established an electric field across the flux tube and therefore adds an E x B motion, sending the flux tube toward the planet. This mechanism supports our interchange instability framework, resulting in the injection of less dense gas radially inward. Since the Kruskal and Schwarzschild’s paper a tremendous amount of theoretical work has been accomplished that handle multi-dimensional configurations, varying boundary conditions and complicated geometries. The motivation for new technologies in World War II and the Cold War also directly led to the advanced rocketry and satellite technology developed in the space race. This boded well for experimentalists who now had the means and support to build satellites that could orbit the Earth. Pioneers like James Van Allen, Louis Frank and others in space physics were finally able to acquire in situ data from Earth’s magnetosphere. Ever since the establishment of NASA in 1958, the program has overseen the bulk of American space exploration. Dozens of unmanned missions have been completed and many are still ongoing. From inner earth orbit to the interstellar medium, these missions have become essential in our understanding of the planets and the properties of their magnetospheres. Without them, the study of interchange instability would be much more brief and much less developed. These unmanned missions, that have now reached every planet in our solar system, have enabled a more comprehensive understanding of interchange motions in Jupiter and Saturn’s magnetospheres. Jupiter has had two major orbital missions: Galileo (launched in 1989) and Juno (launched in 2011 and currently operating in orbit). Saturn currently has the Cassini-Huygens probe in orbit. Cassini was launched in 1997 and has been at Saturn since 2004.

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