Midlatitude detection of ELF whistlers

Abstract

Narrow‐band, whistlerlike magnetic events distinguished by nearly monochromatic signals decreasing in frequency with time have been observed for the first time at midlatitudes in the ELF band. Measurements performed during September 3 to October 5, 1985 at Table Mountain, California (34.4°N, 117.7°W), show that the frequency and dispersion characteristics of these events are similar to events detected at auroral latitudes (Heacock, 1974), including a narrow‐band magnetic signal monotonically decreasing in frequency from 120 to 60 Hz over a 40 s interval with a mean center frequency of approximately 90 Hz. No echoes were observed. Maximum amplitudes of the magnetic signals ranged from just above the approximately 1 pT Hz−1/2 floor of the ambient background to roughly 20 pT Hz−1/2. The polarization was predominantly linear in the geographic east‐west direction. The midlatitude ELF whistlers reported here have a significantly lower average daily rate of occurrence than those reported for auroral latitudes. However, as with the high‐latitude events, they displayed an occurrence rate that is maximum during local daytime. Following Heacock (1974), it is suggested that a possible source for these events is whistler mode lion roars occurring in field‐aligned ducts of enhanced cold plasma densities in the distant magnetosheath. For favorable configurations of these ducts extending from the magnetosheath into the polar cusp, the waves may propagate to the Earth through the cusp acting as a waveguide. Although lightning is usually considered to be the dominant source of ELF noise in the Earth ionosphere cavity, magnetosheath ELF noise coupled into the cavity at high latitudes may represent an additional source. The fractional intensities of the natural ELF noise power within the cavity that are generated by this mechanism are presently unknown. However, a determination of these fractional intensities would be useful in the interpretation of the Schumann resonances as a measure of the totality of global lightning, particularly in the case of measurements made at high latitudes in the vicinity of the cusp‐ionosphere coupling region. Similarly, high‐latitude detection of very weak, artificially generated ELF signals, such as by HF heating of the polar electrojet, may be affected by the proximity of such a local ionospheric ELF noise source.

Multiflash whistlers in ELF-band observed at low latitude

“…The maximum number of events observed around 09:00 to 10:00 h (IST) which is consistent with the low latitude observation by Wang et al (2005). There appears to be a night to dawn gap, the same as that reported by Heacock (1974), Sentman and Ehring (1994) or Wang et al (2005). There also appears an increased rate of occurrence from 20:00-23:00 h (IST), but no other peak is observed as reported by Wang et al (2005) from 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.…”

“…This peculiar event with a multiflash nature occurred only once during the whole period of observation. Time duration for various ELF whistlers and this unique multiflash ELF whistler event ranges from 0.5 s to 3.5 s much smaller than those in previous studies (Heacock, 1974;Kim et al, 2006;Sentman and Ehring, 1994;Wang et al, 2005) which were around 40 s to 120 s.…”

“…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.…”

“…These emissions were detected to possess a frequency range similar to magnetosheath lion roar (Smith et al, 1967) Correspondence to: A. K. Singh (abhay s@rediffmail.com) but their spectral forms are quite different (Heacock, 1974). 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.…”