Fig. 1

a Depicts an equatorial plasma depletion (EPD) as a density-depleted magnetic flux tube intercepted from south to north by a polar-orbiting satellite. The sketch shows eastward zonal currents \(\mathbf {j}_{\perp }\) diverging as field-aligned currents \(\mathbf {j}_{\parallel }\) at the walls of the EPD. Weak zonal currents \(\mathbf {j}_{\perp }^{*}\) are shown inside the depletions together with an eastward polarization electric field \(\delta \mathbf {E}_{\perp }\). b Is a zenithal view of the flux tube in a. It additionally describes the Poynting flux \(\mathbf {S}_{\parallel }\) directed away from the dip equator, and the magnetic field \(\delta \mathbf {B}_{\perp }\) associated with the currents \(\mathbf {j}_{\parallel }\) in the northern magnetic hemisphere. Panel (c) illustrates the expected plasma density n, disturbed magnetic field \(\delta \mathbf {B}_{\perp }\), polarization electric field \(\delta \mathbf {E}_{\perp }\), and associated Poynting flux \(\mathbf {S}_{\parallel }\) observed by a polar-orbiting satellite intercepting an EPD—as in a