Variable frequency VLF signals in the magnetosphere: Associated phenomena and plasma diagnostics

Abstract

Coherent variable frequency signals (ramps) extending from 1 to 8 kHz, injected into the magnetosphere from Siple Station, Antarctica (L=4.3) exhibit upper and lower cutoffs when received at the conjugate station, Roberval, Quebec. Ramp group delay measurements and ionospheric sounding data are used for the first time to determine the cold plasma density and L shell of the propagation path. Relationships among f, df / dt, and the “phase equator” for gyroresonance are calculated using second‐order resonance equations generalized to relativistic electrons. Observed upper cutoff characteristics are interpreted in terms of off‐equatorial gyroresonant interaction regions and ducted propagation limited to frequencies below half the local gyrofrequency. The observed lower cutoff frequencies varied systematically with transmitted ramp slope, suggesting a threshold in the resonant electron number density above which rapid temporal wave growth and saturation can occur. This concept is used to develop a hot plasma diagnostic technique which, for an assumed g(α)υ−n electron distribution, provides an estimate of the energy dependence n. A test of this technique is given using the data and a simplified wave‐particle interaction simulation. Additional aspects of the magnetospheric response to ramp injection, including emission triggering, are discussed.

Propagation of whistler mode waves with a modulated frequency in the magnetosphere

“…The results from this experiment and from our numerical simulations are consistent with experiments conducted at Siple Station, Antarctica. In these earlier experiments, it was shown that signal amplification and triggering were not observed when two signals with a frequency difference less than 20 Hz were launched together, yet signals with a frequency difference of 100–200 Hz were amplified [ Carlson et al , 1985; Helliwell , 1988]. The mechanism causing this amplification of the monochromatic signals was called the coherent wave instability (CWI) [ Helliwell et al , 1980].…”

Preferential amplification of rising versus falling frequency whistler mode signals

“…Historical observations of Siple Station transmissions [Mielke and Helliwell, 1993;Carlson et al, 1985], along with more recent cases from HAARP wave injection experiments [Gołkowski et al, 2008[Gołkowski et al, , 2011 and satellite recordings of discrete chorus elements [Burtis and Helliwell, 1976;Li et al, 2011], indicate a natural preference for the amplification of rising frequency signals over falling frequency signals. Furthermore, the importance of the time and location of the transition from linear to nonlinear growth was postulated from wave injection observations made by Gołkowski et al [2010].…”

“…Burtis and Helliwell [1976] reported that 77% of chorus samples consisted of primarily rising frequency elements, and Li et al [2011] showed that rising frequency chorus elements tend to have higher average amplitude (30-100 pT) than falling frequency chorus elements (<30 pT). Experiments involving the injection of waves into the magnetosphere from ground-based ELF/VLF transmitters [Mielke and Helliwell, 1993;Carlson et al, 1985] and from modulated ionospheric heating experiments [Gołkowski et al, 2008[Gołkowski et al, , 2010[Gołkowski et al, , 2011 have also noted the tendency for rising frequency ramps to be amplified to a higher degree while falling frequency ramps are weaker or absent in recordings made on the ground in the conjugate hemisphere.…”

An explanation of non-observation of transmitted VLF signals on ground

“…Whistler mode waves originate in atmospheric lightning, VLF transmitters, power grid discharges and spontaneous emissions within the magnetosphere. Their central properties are used for diagnostics of both the thermal plasma (Helliwell, 1965) and the energetic electrons that are trapped in the radiation belts (Carlson et al, 1985;Singh, 1994). A lot of work has been done in this field by Stanford group using not only natural whistlers (generated by lightning discharges), ELF/VLF/plasmaspheric hiss but also man-made signals (Helliwell et al, 1986, Sonwalkar et al, 1997.…”