Wave mode of the low-latitudinal ELF-whistlers

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.