Further study of flickering auroral roar emission: 1. South Pole observations

摘要

Hughes and LaBelle (2001) reported a single example of a new geophysical phenomenon: ～10 Hz modulation of auroral radio emissions near twice the auroral ionospheric electron gyrofrequency. They called this phenomenon flickering auroral roar and suggested that it is related to flickering aurora, which results from ～10 Hz modulation of the precipitating auroral electrons. Observations at South Pole Station during 2003 using a new high-bandwidth receiving system have yielded 10 examples of flickering roar emissions. Although 10 examples is still a small number for statistics, these observations considerably extend previous knowledge of this phenomenon which was based on a single example. On the basis of the 2003 South Pole data set, flickering auroral roar accounts for only about 1–2% of auroral roar in number of seconds, but ～20% of auroral roar events have some flickering feature. The observed modulation frequencies range from ～3 to 30 Hz. The 10- to 20-Hz modulations, which correspond to about 65% of the time when flickering roar occurs, are much more common than the higher-frequency 20- to 30-Hz modulations, which correspond to about 20% of the time when flickering roar occurs. These frequencies compare favorably with optical observations of auroral emissions, rocket observations of electron flux modulations, and modeling results. If these frequencies correspond to the oxygen gyrofrequency where electrons and ion cyclotron waves interact, the 3- to 30-Hz frequency range would imply sources at altitudes of 1500–10,500 km. Six of the ten flickering roar examples occurred during substorm expansion phase as defined from local magnetometer data, in contrast to flickering aurora, which is usually observed after the onset of auroral breakup.