A New Mechanism for the Field Line Twisting in the Ionospheric Magnetic Flux Rope

Magnetic flux ropes (MFRs) with twisted magnetic field lines are common in the solar system, most of which can be caused by magnetic reconnection or velocity shear. However, these two mechanisms have difficulties in explaining the formation of MFRs observed in the planetary ionosphere. Here we find that a dynamo process can be caused by the differential compression at different parts of a flux tube when it is sinking into the partially ionized ionosphere, leading to an increase in the magnetic energy. The differential compression also produces an azimuthal component of ▽×E, which generates azimuthal magnetic fields with twisted magnetic field lines and gives rise to an ionospheric MFR. Key Points The differential compression of flux tube at different altitudes provides a dynamo process to increase the tube magnetic energy Azimuthal magnetic fields can be generated by (▽×E)ϕ associated with the differential compression The flux tube is twisted in different directions with opposite helical pitch angles on the two sides of the flux tube