During the period of high solar and geomagnetic activity in late May 1967 the Air Force satellite OV1-9 detected energetic proton fluxes coming up the magnetic field lines from the earth at high magnetic latitudes (λ⩾65–70 deg). Ludwig Katz and Paul L. Rothwell of Air Force Cambridge Research Laboratories suggest that, because the flux is highly collimated along magnetic-field lines, there may be a radial polar electric field of about 0.14 volts/meter (Phys. Rev. Letters 21, 1764, 1968). Electric fields had been suggested earlier as a possible explanation for aurora or other polar-cap particle precipitation.