VLF wave stimulation experiments in the magnetosphere from Siple Station, Antarctica
Authors: Helliwell R. A. 
Published: February 2008

The Earth's magnetosphere is host to remarkable very low frequency (VLF) electromagnetic signals of natural origin. One of these, called a whistler, originates in lightning. Others, such as hiss and chorus, originate within the plasma itself. They are important for at least three reasons. First, they reveal the properties of the plasma through which they travel and thus can be used as remote sensing tools. Second, their high intensity and narrow bandwidths indicate the presence of a previously unknown kind of wave particle interaction that converts the kinetic energy of charged particles to coherent electromagnetic radiation. This process is called the coherent wave instability (CWI). Third, energetic charged particles are precipitated into the ionosphere through resonant scattering by these same waves, causing enhanced thermal ionization, X rays, light, and heat. To better understand and use the CWI, controlled VLF signals have been injected into the magnetosphere from Siple Station, Antarctica and received on satellites and near the conjugate point in Quebec, Canada. In addition to reproducing many puzzling natural phenomena, these experiments have provided critical new data on the CWI, laying a foundation for various theories and computer simulations. Key findings are as follows: 1) Coherent VLF signals often exhibit exponential temporal growth (∼30 dB) and saturation at levels estimated to be of order 5 pT. 2) Temporal growth requires that the input signal exceed a threshold that varies widely with time. The probable cause of the growth threshold is in situ background noise that reduces the efficiency of phase bunching by a coherent input signal whose intensity is comparable to the noise level within the frequency band of the interaction (∼100 hz). 3) Narrowband triggered emissions can be entrained by Siple frequency ramps of different slope but of much lower (−20 dB) amplitude. The mechanism of entrainment is not yet understood. 4) For two equal amplitude input waves spaced 20 Hz apart, the temporal growth of each component is almost totally suppressed. For larger spacings, 40–100 Hz, the lower frequency is more suppressed than the upper. For 10 < Δƒ < 100 Hz, unsymmetrical sidebands at integer multiples (up to seventh order) of Δƒ are created, along with subharmonics. The integer sidebands are attributed to emission growth triggered by one beat and suppressed by the next. Taken together, the spectrum of the stimulated sidebands and sub‐harmonics is thus more noise‐like than the transmitted spectrum. 5) Simulated hiss shows coalescence of selected noise wavelets into longer and stronger chorus‐like emissions, suggesting that chorus and hiss originate in the same mechanism. Future objectives of a VLF wave injection facility include (1) new experiments on the physics of wave growth and wave‐induced particle scattering and precipitation, (2) testing of the predictions of theories of VLF wave‐particle interaction, (3) development of new techniques for remote sensing and control of space plasmas using VLF techniques, and (4) improvements in the design and operation of VLF communication and navigation systems.