“Electron conic” signatures observed in the nightside auroral zone and over the polar cap

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A preliminary search of the Dynamics Explorer 1 high‐altitude plasma instrument data base has yielded examples of “electron conic” signatures. The three example passes show an association with regions of downward electron acceleration and upward ion beams, but this is not true of all the electron conic events. The electron conic signatures are clearly discernible on energy‐flux‐versus‐time color spectrograms as pairs of discrete vertical bands which are symmetric about a pitch angle of approximately 180°. One of the examples is a polar cap pass with electron conic signatures observed at invariant latitudes from 84° to 75°. The other two cases are nightside auroral zone passes in which the regions of detectable electron conies are spatially more confined, covering only about 1° in invariant latitude. The conic signatures have been found at energies that range from 50 eV < E < 2 keV, with the lowest‐energy examples found in the polar cap. The distribution of the electrons about a pitch angle of 180° is larger than expected for a loss cone feature. If the electrons conserve the first adiabatic invariant in a dipole magnetic field, and in some cases a parallel electric field, the mirroring altitude varies between about 500 km and 8000 km, which is above the atmospheric loss region. For this reason, and in analogy with the formation of ion conics, we suggest that the conic signatures are produced by heating of the electrons perpendicular to the magnetic field.