Electric fields play a fundamental role in space physics. Yet, experimentally, they are difficult to observe. Closest to Earth, in the inner belt and slot region, electric field measurements are impeded by spacecraft motion, for instance. When a spacecraft passes through perigee, it does so very fast, travelling tens of kilometers per second. As a result, the sensors inevitably detect a large motional electric field; but in reality this is an illusion. For the Van Allen Probes, the motional electric field constitutes at least 95% of the electric fields measured close to perigee. A measurement accuracy much better than 95% is therefore required to observe the electric field in the inner magnetosphere. In an article which has just been published in Geophysical Research Letters, it is shown that the Van Allen Probes achieve such accuracy.
Together with Pr. Forrest Mozer, we present an analysis of two years of electric and magnetic measurement at one altitude chosen to enable comparisons with ground observations (L=1.4). The measurements of electric drift (ExB/B2) revealed departures from the traditional motion of corotation with the Earth in 2 ways.
- We found that the electric fields lead to a rotational angular speed 10% smaller than the rotational angular speed of the Earth.
- We detected the ionosphere dynamo electric fields, which lead to a magnetic local time dependence of the electric fields, in both radial and azimuthal directions.
Such pieces of information are important when discussing, for instance, the structure and dynamics of the plasmasphere or the radial transport of trapped particles at lowest altitudes.