Observation of magnetic flux ropes in the Venus ionosphere

Abstract

MAGNETIC field measurements made by the Pioneer Venus Orbiter spacecraft reveal a very low average field strength within the dayside ionosphere of Venus, typically only a few nanoteslas, in contrast to fields of several tens of nanoteslas just outside the ionosphere¹. Thus, at least in the range of solar zenith angles (65–90°) initially probed by the orbiter, the compressed interplanetary magnetic field of the shocked solar wind plasma (the magnetosheath) is effectively shielded from the ionosphere by currents flowing on the ionopause, the boundary between the ionosphere and the magnetosheath. In addition, the magnetic field pressure just outside the ionopause approximately balances the ionosphereic thermal plasma pressure^2,3. However, within this generally low-field region the spacecraft occasionally passes through regions of very large field strength which can sometimes exceed that observed external to the ionosphere. These intense, short-lived enhancements are described here and interpreted to be due to the passage of the spacecraft through ‘flux ropes’, bundles of twisted magnetic field lines surrounded by ionospheric plasma.