Measurements of the Net Charge Density of Space Plasmas

Space plasmas are composed of charged particles that play a key role in electromagnetic dynamics. However, to date, there has been no direct measurement of the distribution of such charges in space. In this study, three schemes for measuring charge densities in space are presented. The first scheme is based on electric field measurements by multiple spacecraft. This method is applied to deduce the charge density distribution within Earth's magnetopause boundary layer using Magnetospheric MultiScale constellation (MMS) four‐point measurements, and indicates the existence of a charge separation there. The second and third schemes proposed are both based on electric potential measurements from multiple electric probes. The second scheme, which requires 10 or more electric potential probes, can yield the net charge density to first‐order accuracy, while the third scheme, which makes use of seven to eight specifically distributed probes, can give the net charge density with second‐order accuracy. The feasibility, reliability, and accuracy of these three schemes are successfully verified for a charged‐ball model. These charge density measurement schemes could potentially be applied in both space exploration and ground‐based laboratory experiments. Key Points Charge densities in geomagnetopause have been calculated using MMS electric field measurements A method for extracting the charge density from 10‐point electric potential measurements is presented An additional scheme to measure the charge density using seven or eight electric potential probes is explored