Detections of ELF whistler‐like events at a low latitude location are reported. Events with frequencies between 60 and 100 Hz were recorded by the ELF station at the Lulin Observatory in Taiwan from August 26, 2003 to July 13, 2004. The most distinguished feature for these events is the frequency descent in the frequency‐time spectrograms, resembling terrestrial whistlers. Other notable features include (a) a long event duration averaging up to two minutes, (b) a daytime diurnal maximum occurring around 10 am, (c) a dominant magnetic field polarization in the north‐south direction with strength of a few to tens of pT, and (d) no detection of vertical electric fields. Similar events were only reported twice for the past thirty years: one at an auroral latitude site in Alaska and the other at a mid‐latitude site in California. Possible source mechanisms including magnetosheath lion roars and lightning‐generated whistlers are discussed.
[1] Detection of 60 to 100 Hz ELF-whistlers in Taiwan was previously reported in 2005. These ELF-whistlers are distinct in two aspects: (1) daytime occurrence from 5 am to 9 pm; (2) average event duration of two minutes. In this paper, the wave modes responsible for these features are examined in multiion ionospheric plasma with the composition inferred from the IRI model. The Class III right-hand ion cyclotron wave mode between the lower cutoff frequencies near the oxygen ion gyrofrequency f cO + and the helium ion gyrofrequency f cHe + is found to be the most probable candidate to account for both features. The diurnal variation of the dominant ion species from O + to H + at the high altitude ionosphere could also explain why these events are only detected during daytime at the observational site. The long event duration is likely related to propagating waves with lower group velocities in multiion plasma. The event duration, which was inferred from the relation between wave frequency and the inverse group velocity, is found to increase with the electron density; a feature which also is consistent with the observed ionospheric NmF2 variations. Wave modes for the other higher latitudinal ELF-whistler detection sites were also checked and the aforementioned wave mode was also found to be the most probable mode.
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