Description: Much of the outstanding physics yet to be learned about the coupling of the magnetosphere-ionosphere (M-I) system resides in the small- and mesoscale structures of the aurora, field aligned currents, plasma convective motions, and wave activity. Untangling these spatial and temporal structures with in situ measurements will require a constellation of spacecraft. For cost-effectiveness, such a constellation would consist of CubeSats. To study M-I coupling, one of the essential measurements these CubeSat will need to make is of the DC electric field. The electric field plays a crucial role in M-I coupling from driving convective drifts in the magnetosphere and ionosphere, Joule heating, Poynting flux, waves, and auroral particle precipitation. The environment on the magnetospheric side of these processes is the tenuous plasmas of the earthward plasma sheet, plasma sheet boundary layer, ring current, and auroral zone. In such environments, the sunlit electric field sensors are surrounded by high plasma sheath impedance which causes large errors on the measurement. To reduce the sheath impedance, a bias current is driven from the spacecraft to the sensors. This current is large relative to the plasma thermal current to CubeSat’s small surface area, causing the CubeSat to charge to large potentials, contaminating the measurement. By effectively increasing the spacecraft surface area of a CubeSat, enough thermal plasma current can be collected to offset the effect of the bias current. STEPHEI, the Small TEnuous Plasma Heliophysics Electric field Instrument, is an electric field double probe for CubeSats in tenuous plasmas with a deployable electron collector that will prevent the spacecraft charging. The electron collector consists of a metallized polymer sheath deployed along either the sensor booms or on its own boom. STEPHEI will allow the measurements to be made that are needed to address fundamental problems in M-I coupling.

Benefits: DC electric field measurements have been very valuable in space physics investigations. The science return on Heliphysics missions have been enhanced by electric field measurements. The electric field is used in such investigation to explain particle transport and energization, wave properties, Poynting flux, auroral dynamics, magnetic reconnection, and other processes. STEPHEI technology thus has NASA application by enabling further investigation of suck processes.

Constellations of CubeSats with STEPHEI technology could be used to make multi-point electric field measurements for use in a space weather observatory. Such electric fields map into the ionosphere and directly drive currents and plasma drifts. These processes include those giving rise to GNSS scintillations and thus have application for forecasting PNT conditions.