Directly observing the magnetic rope contraction and expansion in space

One of the most fundamental hypotheses proposed to explain the electron acceleration in astrophysical and space plasmas is that the magnetic rope can contract and expand during a short period. However, such contraction and expansion of magnetic rope have never been directly evidenced hitherto. Targeting this longstanding problem, here we provide direct evidence for the magnetic rope contraction and expansion by utilizing the first-order Taylor expansion method and the magnetospheric multiscale measurements. The contraction and expansion of magnetic ropes happen during a few seconds in high-speed plasma flows, with the contraction related to an increase of pressure inside the rope and the expansion related to a decrease of pressure. Excitingly, during the magnetic rope contraction we observe electron acceleration, whereas during the magnetic rope expansion we observe electron deceleration. These findings have robustly validated the fundamental hypothesis in astrophysics, i.e., electrons can be accelerated by contracting magnetic ropes.