On the temporal fluctuations of pulsating auroral luminosity

Abstract

From a study of all‐sky TV records, it is shown that the luminosity fluctuations of pulsating auroras can be understood in terms of a series of pulses with rapid on‐off switchings in burstlike fashion and that the widths of successive pulses (pulsation “on” times) are fairly constant. This is common even when luminosity fluctuations consist of complex‐irregular variations, in contrast to the pulsation “off” time that is significantly variable. Complex‐irregular variations are found to be due to simultaneous appearance of more pulsating patches that exhibit movements eastward and westward over the site, and each of the patches shows primarily isolated luminosity pulses. Several examples are presented and described in detail. The pulsation “on” time is found to statistically concentrates in two small ranges (0.2–0.5 s for flash including 3‐Hz modulation and 2–6 s for pure‐expansion and streaming‐propagating modes), in spite of variations in appearance. On the other hand, the pulsation “off” time is widely scattered over a range from nearly zero to longer than 1 min. A natural consequence of these observations is that the classical concept of “period” does not mean much and the luminosity fluctuations should be treated as a series of individual isolated pulses where the pulsation “on” time is the most essential quantity. These characteristics are briefly discussed in relation to VLF/ELF wave‐particle interactions in the magnetosphere. Then a new interpretation of the nonlinear relaxation oscillation model is proposed, where the propagation effect of VLF/ELF waves in low energy plasma irregularities near the magnetospheric equatorial plane plays an essential role to produce rapid on‐off switchings of precipitating energetic electron fluxes. Both electromagnetic and electrostatic waves are possibly related to the precipitation pulsations.