SOlar Polar Flyby Mission

The field-aligned current density J∥(r) in the solar polar region has never been measured in situ at heliocentric distances below approximately 1.3 AU. Two competing theoretical frameworks — the standard quasi-neutral solar wind model and Alfvén’s unipolar inductor circuit model — make predictions for this quantity that differ by four to five orders of magnitude. We derive the expected J∥(r) profile under each framework from first principles, using Ampère’s law applied to the Parker spiral field and the Spitzer conductivity of the polar corona. We then describe a one-way solar polar flyby mission architecture, using a Jupiter gravity assist to achieve high heliographic inclination and a targeted perihelion of 10–15 solar radii, that would make this measurement for the first time. The required instrument suite has direct flight heritage on Parker Solar Probe and Ulysses; no new technology development is required. The mission fits within the NASA Discovery program cost envelope at an estimated $500–900M. The measurement is decisive in either direction: a null result strongly constrains the Alfvén circuit model; a positive detection would fundamentally revise understanding of the heliospheric current system.