Magnetic‐quadrupole source of Neptune “high‐latitude” radio emission

We study the occultation of the dominant X-mode component of Neptune's smooth “high-latitude” emission. A dipole magnetic model relates the frequency dependence of observed times of maximum emission to radio-source locations. Dipole-based analysis leads to seemingly convincing auroral sources. Yet multipole models of Neptune's complex magnetic field yield quite different results. Steep slopes on each constant-field radio-source surface explain the radio cutoff at the corresponding gyrofrequency. With the observed sense of circular polarization, the modeled radio horizon at occultation locates the radio source at a low-latitude magnetic quadrupole rather than in the expected auroral region.