The Earth's Ionospheric Electric Field - Measurements with Double Probes on Satellites and Sounding Rockets

Electric fields play fundamental roles in energy and momentum transfer, heating, and particle acceleration in space physics and astrophysical contexts. They are critically important in the environments of all planets in the solar system, including those with and without a magnetic field, as well as exoplanets. While it has been known for centuries that the Earth has a magnetic field, its electric field has been explored only relatively recently, with the advent of direct measurements in space.

For over half a century, electric fields have been measured using the highly-capable "double probe" technique that was pioneered at the NASA/Goddard Space Flight Center, the University of California at Berkeley, and the Royal Institute of Technology in Sweden. The double probe provides potential difference measurements between two electrodes separated in space. Vector configurations provide highly accurate measurements of the instantaneous, high-time resolution DC and structured electric fields and their associated E x B plasma drifts, as well as AC electric fields or plasma waves, extending to several MHz.

This talk provides an overview of electric field measurements in the ionosphere carried out with double probes on satellite and sounding rocket platforms. We demonstrate how these measurements have provided fundamental knowledge of the Earth's ionosphere including surprising new insights in a wide range of plasma domains. At high latitudes, these include DE-2 satellite observations of structured electric fields that can reach V/m amplitudes, corresponding to plasma drifts of 20 km/s. Near the equator, we present important new results from the C/NOFS satellite, such as E x B shears that drive Kelvin-Helmholtz waves that may subsequently seed scintillation-causing plasma structures with wavelengths from 1000's of km to meters, all measured by the same double probes! We conclude with key examples from sounding rockets that provide vertical profiles of electric fields and their role in ion-neutral coupling and heating as well as focused measurements of processes at low altitudes often inaccessible to satellite probes. We use the electric field double probe measurements to underscore their great promise for future ionosphere, magnetosphere, and planetary missions in the decades -- indeed centuries -- to come.