In its infancy, Mars may have generated a strong magnetic field, much like Earth’s. Today, all that remains is a far less extensive magnetosphere that surrounds Mars. This magnetosphere is shaped, in part, by Mars’s crustal magnetic fields and, in part, by the Sun’s interplanetary magnetic field (IMF) as it drapes around the Red Planet, carried by the solar wind.
Mars’s lack of a strong global magnetic field makes it vulnerable to the atmosphere-stripping effects of the solar wind. Temporary structures observed in the magnetosphere, known as flux ropes, may contribute significantly to this loss of atmospheric ions into space. New research from Hara et al. provides fresh clues into the formation of these distinctive structures, which extend outward from Mars as twisted, helical filaments of plasma.
The research team used measurements from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission to investigate flux ropes in Mars’s magnetotail, a portion of the magnetosphere that extends into space on the opposite side of the planet from the Sun. Flux ropes are just one of several features and processes found in the magnetotail that are thought to entrain ions in Mars’s ionosphere and carry them away into the solar wind.
Magnetic field and plasma observations from MAVEN revealed that flux ropes in Mars’s magnetotail exist at a wide variety of orientations with respect to the planet. Of 23 flux ropes observed, only three pointed at an angle similar to that of electric currents in the magnetotail itself. This suggests there are at least two different formation mechanisms for the flux ropes.
The researchers conducted a mathematical analysis to determine where and how the 23 flux ropes must have formed in order to end up at their observed positions. The results suggest that the ropes oriented at shallower angles probably formed by a process in which reconnection of field lines in the magnetotail current sheet creates twisted rope structures and also converts magnetic energy into plasma flows. These are analogous to the “plasmoids” that form in Earth’s magnetotail.
The steeper angles of the other flux ropes are more consistent with a different origin. These features likely formed in the ionosphere on the side of Mars that faces the Sun and later slipped around the planet into the magnetotail. They may have arisen from instabilities of plasma flow at the boundary between the solar wind and the ionosphere, ionospheric effects on the penetrating IMF, or magnetic reconnection between the IMF and the Martian crustal fields.
MAVEN may have missed some flux ropes in its observations, so these findings do not necessarily imply that most flux ropes form on Mars’s Sun-facing side. Also, some of the observed ropes don’t neatly fit into either formation scenario. Further research could reveal more about the complex origins of these plasma features. (Geophysical Research Letters, https://doi.org/10.1002/2017GL073754, 2017)
—Sarah Stanley, Freelance Writer
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