Wave mode of the low-latitudinal ELF-whistlers

Abstract

[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.

A statistical study on ELF‐whistlers/emissions and M ≥ 5.0 earthquakes in Taiwan

“…Similar events had also been observed in Alaska, California, and the South Pole; several mechanisms including the tail of lightning-generated VLF whistlers have been suggested, but there are no solid conclusions [Heacock, 1974;Sentman and Ehring, 1994;Kim et al, 2006]. Recently, Wang et al [2011] had proposed the wave mode of these observed whistler-like events to be Class III ion-cyclotron ELF-whistlers, yet the source mechanism is still not determined. Therefore, the source of these events has remained unsolved.…”

“…Recently, Wang et al . [] had proposed the wave mode of these observed whistler‐like events to be Class III ion‐cyclotron ELF‐whistlers, yet the source mechanism is still not determined. Therefore, the source of these events has remained unsolved.…”

ELF Whistler Dependence on a Sunlit Ionosphere

“…A time series of ionospheric parameters at 750 km and 1,250 km, respectively, are shown in Figure . Model output at 750 km above Lulin Observatory serves as a comparison to the calculations in Wang et al (), but the date is chosen for a time when observations at Lulin Observatory and the South Pole overlap. IRI‐2016 values were obtained through the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) Virtual Ionosphere, Thermosphere, Mesosphere Observatory.…”

“…For comparison, the low‐latitude data set from Wang et al () is also analyzed, which has an average cutoff frequency range of 62–95 Hz. The upper average cutoff frequency is lower than that observed at the South Pole, though it was acknowledged that a lower system sensitivity above 100 Hz may have resulted in missed signals.…”

“…ELF whistlers are unique, however, in that they typically last on the order of a minute and span less than 100 Hz in frequency. Wang et al () concluded that the most likely wave mode for the ELF whistlers is right‐hand polarized ion cyclotron waves.…”