Low‐latitude ELF‐whistlers observed in Taiwan

Abstract

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.

Multiflash whistlers in ELF-band observed at low latitude

“…Two and more whistlers closely associated in time, but having different sources are known as multiflash whistlers while a whistler with two or more components, each of which have traversed a different path through the ionosphere is known as multipath whistler (Helliwell, 1965). These events are similar to those observed previously at mid and low latitude by Sentman and Ehring (1994) and Wang et al (2005) in some aspects but no observation report multiflash nature of ELF whistlers. Also they are very short in duration and have a dispersion resembling terrestrial (VLF) whistlers contrary to the previously reported ELF whistlers.…”

“…Sentman and Ehring's (1994) measurements at California reported whistler-like events with frequencies laying in ELFband, along with the dispersion characteristics and the local time occurrence distribution similar to the events detected by Heacock (1974). Recently, Wang et al (2005) and Kim et al (2006) also reported a similar type of ELF phenomena.…”

“…Extremely low frequency (ELF: 3-3000 Hz) waves have been associated with a range of magnetosphere-ionosphere coupling phenomena as well as meteorological phenomena (Magunia, 1996;Cummer et al, 1998). Heacock (1974), Sentman and Ehring (1994), Wang et al (2005) and Kim et al (2006) have reported ELF emissions/whistlers at much lower frequencies between 40-200 Hz. Heacock (1974) discovered dispersive emission in the ELF-band near Fairbanks, Alaska (geomag.…”

The Characteristics of Magnetic Signals Observed at Lulin ELF Station and Their Association with Earthquakes in Taiwan

“…The system is also equipped with a GPS clock and all the data were sampled continuously at 5 kHz. In the past, in addition to performing routine recording of lightning sferics, intriguing ELF-whistlers in the frequency range between 60 and 100 Hz had also been observed by this system (Wang et al 2005.…”

“…With the goal of monitoring global lightning activity, a magnetic ELF station located at the Lulin ObservatoryYushan National Park (23.47°N, 120.87°E; 2862 m altitude; magnetic inclination: 16.76°N) has been installed and operated by the NCKU ISUAL team in Taiwan since August 2003 (Wang et al 2005;Huang et al 2011). The system comprises a pair of EMI-BF4 magnetic coils that are sensitive in the radio frequency band between 0.3 and 500 Hz.…”