A 10‐cps periodicity in the precipitation of auroral‐zone electrons

The observation was made, using rocket‐borne radiation detectors, of a purely temporal 10‐cps periodicity in the flux of auroral electrons in the energy range 1–120 kev. The periodicity was biased toward the higher energy electrons in the sense that it was these that underwent the greatest relative fluctuation. A cross‐correlation analysis revealed that there was no significant displacement in time between peaks at 16 kev and at 120 kev thereby localizing the mechanism causing the modulation to within about 1000 km of the earth. At the same time that this periodicity was observed, a determination of the electron energy spectrum showed that a strong monoenergetic electron component ( E ≈ 6 kev) was present in the influx. A process in analogy with that process that produces runaway electrons in laboratory plasmas is proposed to account for both the presence of 100‐kev electrons in the beam and the appearance of 10‐cps periodicity in their flux. The beam of 6‐kev electrons as it entered the increased electron concentration of the upper F layer may have excited a variety of plasma oscillations. Conditions may have been proper for one of these waves to grow in amplitude. A resonance between the frequency of this wave and the gyrofrequency of the 6‐kev electrons could then lead to the rapid acceleration of certain favored electrons to very high energy. The time constant of the excitation, growth, and subsequent damping of this wave is then reflected in the low‐frequency periodicity observed in these accelerated electrons.